Strongly coupled interface ferroelectricity and interface superconductivity in amorphous LaAlO_3/KTaO_3(111)

Domain: neuroscience
Published: 2026-02-14
Editorial outcome: major_revision

Abstract
Interfaces can differ from their parent compounds in charge, spin, and orbital orders, providing fertile ground for emergent phenomena, strongly correlated physics, and novel device applications. Here, we present evidence of a ferroelectric order at the interface of two oxides, amorphous LaAlO3/KTaO3 (111), where two seemingly mutually exclusive orders—ferroelectricity and superconductivity—coexist. Ferroelectricity is confirmed through scanning transmission electron microscopy (STEM), second harmonic generation (SHG) microscopy, and piezoelectric force microscopy (PFM). STEM reveals a displacement of K atoms relative to Ta atoms, facilitated by oxygen vacancies at the LAO/KTO interface. The resulting ferroelectric polarization is locally switchable by applying a voltage between the PFM tip and the LaAlO3 film. Flipping ferroelectric polarization reduces interfacial conductivity by more than 1000 times, simultaneously suppresses superconductivity. Moreover, the ferroelectric hysteresis correlates with hysteretic changes in interfacial conductivity and the superconducting transition temperature (Tc), providing clear evidence of coupling between ferroelectricity and superconductivity. These findings open a pathway to ferroelectric superconductivity with broken inversion symmetry and non-volatile control of superconductivity.

Introduction
To overcome the hurdle that the screening of itinerant electrons effectively suppresses the long-range Coulomb forces between electric dipoles and consequently the ferroelectric order, Anderson and Blount proposed in 1964 that ferroelectric metal might exist if itinerant charges are weakly coupled to transverse optical phonons1. Since then, experimental evidence of this novel state remained elusive until the ferroelectric metal, WTe2, with a switchable polarization axis, was discovered2,3. So far, the coexistence of electric polarization and metallic conductivity4 has been observed in several materials, including perovskite oxides5–7, layered perovskites8, and two-dimensional materials2,3,9, although the polarization switchability for many of them has yet to be demonstrated.

The lower dimensionality and carrier density reduce electron screening and thus are beneficial for stabilization of ferroelectric order. Besides two-dimensional materials, interfaces of heterostructures, which host a two-dimensional electron gas10,11, represent another conducting material system with lower dimensionality and carrier density. It remains an intriguing question whether ferroelectric order might also emerge in these systems, a topic that warrants in-depth theoretical and experimental investigation. Even more attractively, at low temperatures, interface superconductivity has been discovered in heterointerfaces such as La2−xSrxCuO4/La2CuO412–14, LaAlO3/SrTiO315,16, monolayer FeSe/SrTiO317, EuO/KTO18,19, LAO/KTO16,20,21, and other KTO-based heterointerfaces22–27. Here, SrTiO3 and KTO are both dielectric materials with large dielectric constants (~20,000 and 4000 at low temperature28,29, respectively), indicating that they are on the verge of a paraelectric to ferroelectric phase transition. However, signatures of ferroelectricity in SrTiO3 or KTO-based interfaces remain elusive till now. Ferroelectric polarization at the interface, if existing or induced, may be intertwined with superconductivity. Ferroelectric superconductors are exceptionally rare in single-phase materials, with the only known examples being Sr1-xCaxTiO3−δ bulk30 and bilayer MoTe231. In contrast, heterointerfaces offer infinite combinations of parent compounds, potentially opening the door to a new family of ferroelectric superconductors.

We deposited amorphous LaAlO3 films onto KTaO3(111) substrates for the study of interface ferroelectricity. For simplicity and readability, the abbreviation “LAO/KTO” is used throughout the paper for “amorphous LaAlO3/KTaO3”. Both LAO and KTO are insulators, and the electrical conductivity of LAO/KTO(111) comes entirely from its interface. Superconductivity with a transition temperature of approximately 1.7 K resides on the KTO side of the interface18,20,24, and the thickness of the (super)conducting layer is about 4 nm18,20, which is less than the coherence length of 18.8 nm18,20, illustrating that the superconductivity is two-dimensional. KTO-based interface superconductivity shows an intriguing dependence on substrate orientation; for example, LAO/KTO(111) and LAO/KTO(110) are superconducting, while LAO/KTO(100) is not18,19,21. This is attributed to pairings mediated by transverse optical phonons26. Superconductivity in LAO/KTO(111) can be controlled by back-side gating the KTO(111) substrate, illustrating the dominant role of carrier mobility and interfacial disorder20. Thus, the properties at the interface, such as lattice, phonons, carrier density, and mobility, are crucial for understanding interface superconductivity.

Results
A 10 nm LAO film was deposited onto a preconditioned KTO(111) substrate by pulsed laser deposition. During deposition, the substrate was stabilized at 300 °C and the oxygen partial pressure was kept at 1×10−5 mbar. Water vapor with a partial pressure 1×10−7 mbar was added to the growth chamber, a method that has been proven to improve film quality and enhance the superconducting Tc. The details of the synthesis are included in the “Methods” section, and the growth recipe has been continuously optimized through many rounds of synthesis-characterization cycles.

The integrated differential phase contrast (iDPC) STEM images of the LAO/KTO(111) heterointerface clearly show the coexistence of K atom displacement and oxygen vacancies right at the interface (Fig. 1). The top LAO film is amorphous, lacking spatial periodicity, in contrast to the regular lattice formed by K, Ta, and O atoms in the KTO substrate (Fig. 1b). The cation interdiffusion across the interface is limited to within 1 nm, as determined by energy-dispersive spectroscopy (EDS) mapping of the La, Al, K, and Ta atoms (see Fig. S1 of Supplementary Information for details).

K atoms in the vicinity of the LAO/KTO interface are displaced relative to the lattice center formed by Ta and O atoms (Fig. 1c–e). The K displacement, δ, is along the KTO [110] direction and depends sensitively on its distance from the interface. In contrast, the iDPC-STEM image taken from the KTO 112 plane (Fig. S2 of Supplementary Information), which is perpendicular to the KTO 110 plane in Fig. 1, shows no appreciable K displacement along the KTO 110 direction. This verifies that the K displacement is precisely along the KTO [110] direction. The retrieved δ (N) relation (Fig. 1f) shows that δ is huge (~0.54 Å) at the interface and decreases rapidly with N, where N is the number of TaO2 planes along the KTO [001] direction counting from the interface. The identified positions of oxygen atoms show that a significant number of oxygen atoms in the K–O plane are missing in the KTO lattice close to the LAO/KTO interface, contrasted by the KTO lattice far away from the interface (Fig. 1c–e). This is corroborated by the change in electron energy loss spectroscopy (EELS) spectra on oxygen K edge taken from areas close and far away from the LAO/KTO interface (Fig. S3 of Supplementary Information). From the line profile (Fig. 1g), the KTO layers accommodating oxygen vacancies range from N=1 to N=6. Thus, the critical thickness at which oxygen atoms are restored to the lattice coincides with the critical thickness at which δ diminishes (Fig. 1f). This clearly demonstrates that the K displacement is associated with oxygen vacancy.

It should be mentioned that exposure to a high-energy electron beam quickly degrades LAO/KTO samples, especially near the interface, which is susceptible to beam damage, as oxygen vacancies may be created, moved, or removed during STEM imaging. Thus, acquiring atom-resolved images within a very limited timeframe is challenging. The STEM images shown in this work are all taken from as-grown samples before severe beam damage occurs.

The ferroelectric polarization and inversion-symmetry breaking originate from the displacement of the K atom and the presence of oxygen vacancies that give rise to the nonlinear optical effect detected by the SHG microscopy (Fig. 2a). With both the incident laser and the generated second harmonic light normal to the film surface, and their polarizations in-plane, the SHG signal is sensitive only to the in-plane component of electric polarization. Since charge transfer between LAO and KTO produces an out-of-plane electric polarization, the SHG signal primarily originates from the in-plane component of ferroelectric polarization (Fig. 1). At 2 K—below the onset temperature of superconductivity and the lowest temperature accessible to our setup—a linearly polarized 1064 nm laser incident normally on the LAO/KTO(111) sample generates a strong second-harmonic signal at 532 nm (Fig. 2b). This signal is absent in the KTO(111) substrate alone (Fig. S5 of Supplementary Information). The SHG intensity scales quadratically32 with the laser power P, ISHG / P2 (Fig. 2c). Temperature-dependent measurements of ISHG(T) show no abrupt transition as T increases from 2 K to 300 K (Fig. 2d), indicating that ferroelectric order persists to room temperature. At 300 K, rotating the half-wavelength plate allows in-plane rotation of the incident light polarization. The measured SHG intensity, I2ωx and I2ωy, with polarization parallel and perpendicular to that of the incident light, respectively, show angular dependence on ϕ (Fig. 2e, f), where ϕ denotes the angle between the polarization of the incident light and the KTO crystallographic 110 direction. Both I2ωx(ϕ) and I2ωy(ϕ) display 6 peaks evenly spaced by ~60° over 360°, reflecting the symmetry of the KTO(111) surface. The data are simultaneously fitted by expressions based on m polar group, yielding a ferroelectric polarization oriented along the KTO [110] direction, in agreement with STEM results (see the Methods section for details). To unambiguously demonstrate ferroelectricity in the superconducting state, we further replaced the picosecond laser with a femtosecond laser and lowered the laser power to only 1.5 mW for the measurements, compared with 83 mW in Fig. 2e, f. With these improvements, the base temperature of the LAO/KTO(111) film reaches 1.62 K—well below its superconducting temperature. Despite this drastically reduced laser intensity, the SHG signal remains significant and approximately temperature-independent (Fig. S12 of Supplementary Information), confirming that ferroelectric polarization is robust from room temperature all the way down to the superconducting regime.

The switchability of ferroelectric polarization is investigated using the PFM method. On a flat area of LAO/KTO(111), in response to the voltage applied to the PFM tip, we observed a “butterfly” curve in the switching-spectroscopy PFM (SS-PFM) amplitude, along with a hysteresis loop in the PFM phase, which are both characteristic signatures of ferroelectric order33–35 (Fig. 3a). It should be emphasized that the application of PFM tip voltage and measurements were carried out in a closed chamber filled with N2 gas. This removes water vapor from environments that breaks down under applied tip voltage and introduces H+ or OH− ions into the sample36,37. Moreover, any ions induced would have to migrate through the 10 nm-thick LAO layer to reach the interface, making their influence even less likely. In this way, we eliminate the effects due to ion implantation and the measured ferroelectric signal is intrinsic to the LAO/KTO(111) heterostructure.

No ferroelectric domain has been found in the as-grown LAO/KTO(111) under PFM imaging, consistent with the absence of domains in SHG images. However, ferroelectric domains can be created and manipulated by locally switching the ferroelectric polarization with the PFM tip33–35. By applying a +10 V voltage on the PFM tip, which is sufficient to flip the ferroelectric polarization (Fig. 3a), a 4×4 μm² area with polarization opposite to the surrounding as-grown area was created, featuring clear domain boundaries (Fig. 3b–d). Within this square, a smaller concentric 2×2 μm² square was created by applying a −10 V voltage on the PFM tip to flip back the ferroelectric polarization. This clearly demonstrates the switchability of ferroelectric polarization. Note that the outside as-grown area has the same contrast as the inner smaller square created by the −10 V tip voltage, implying that the as-grown LAO/KTO(111) is in a single domain state, so no domain contrast is present in the pristine state for PFM and SHG imaging.

Beyond ferroelectric order, redistribution of oxygen vacancies may also contribute to the PFM response, as previously noted in LaAlO3/SrTiO338. To distinguish between these two contributions, we measured the time decay of the PFM domain contrast, which typically decays within hours when induced by oxygen-vacancy redistribution39, in contrast to the stability of ferroelectric domains arising from intrinsic ferroelectric order. After the concentric square pattern was written by the PFM tip, the PFM amplitude decreased rapidly from 2.7 to 1.5 in the first 1 h, then more gradually from 1.5 to 0.5 between 1 and 12 h, after which it stabilized (see Fig. S7 of Supplementary Information). In stark contrast, the concurrently acquired electrostatic force microscopy (EFM) images showed a rapid contrast decay, vanishing within 12 h. These observations suggest that the decaying components of the PFM and EFM signals likely relate to oxygen-vacancy redistribution, while the persistent PFM signal originates from ferroelectric polarization. Moreover, we obtained SHG images after pattern writing (Fig. 3e), which are sensitive to the in-plane component of ferroelectric polarization and thus complement the PFM measurements. As the out-of-plane component was flipped during pattern writing, the coupled in-plane component was switched simultaneously, producing a contrast between regions written with +15 and −15 V applied to the PFM tip. Remarkably, this SHG contrast persisted after 70 h (Fig. 3f), consistent with the sustained PFM amplitude, thereby confirming its origin in ferroelectric polarization.

For comparison, an LAO/MgO(001) film was grown under conditions identical to the superconducting LAO/KTO(111) samples. Applying ±30 V on the PFM tip—well above the voltage applied for LAO/KTO(111)—we wrote a concentric square pattern on LAO/MgO. While both PFM and EFM contrast appeared, they vanished within 48 h (Fig. S9 of Supplementary Information), indicating that these signals likely arise from surface charging or oxygen-vacancy effects38–40. No SHG signal was detected from LAO/MgO, even immediately after pattern writing, indicating that surface charging or oxygen vacancies do not generate SHG signals. These results demonstrate that neither the amorphous LAO film itself nor its interface with MgO produces the PFM and SHG signals observed in LAO/KTO(111).

Combining the results from STEM, SHG, and PFM methods, we conclude that ferroelectricity resides at the interface of LAO/KTO(111), and that this interface ferroelectricity can be switched by an external electric field. Next, we will show the coupling between interface ferroelectricity and interface superconductivity.

We ramp up the PFM tip voltage step-by-step to drive the LAO/KTO through a complete ferroelectric hysteresis loop, while simultaneously measuring the corresponding SHG signal and longitudinal resistance to elaborate on the one-to-one correspondence between ferroelectricity and superconductivity. To do this, a designated area between two voltage contacts on a Hall bar is scanned by the PFM tip at a fixed voltage (Fig. 4a), allowing ferroelectricity in this area to be modulated41 (Fig. 4b). By ramping down the tip voltage Vtip to −21 V, then up to +50 V and eventually back to 0 V, the SHG signal I2ωx(ϕ) and I2ωy(ϕ) evolves with Vtip accordingly (Fig. 4c, d). Consistent with Fig. 2d, e, I2ωx(ϕ) and I2ωy(ϕ) both manifest six peaks with a 60° interval. The SHG intensities at the peaks change with Vtip, but the peak angles remain the same. Since ferroelectricity has both in-plane and out-of-plane components, and the tip writing only modulates the out-of-plane component, the fittings of I2ωx(ϕ) and I2ωy(ϕ) require modeling of sophisticated ferroelectric domain distributions in the intermediate states of the hysteresis loop. Nevertheless, putting these details aside, the evolution of I2ωx(ϕ) and I2ωy(ϕ) with Vtip, corroborated with PFM imaging of ferroelectric domains (Fig. 4b), undoubtedly further rules out the possibility of ion implantation by the tip writing and verifies its effectiveness in modulating ferroelectric domains.

It should be noted that the effect of tip writing decays over time, particularly when Vtip < −10 V. Using longitudinal resistance R as an indicator, the effect decayed rapidly within the first minute and then gradually relaxed over several hours until stabilization was reached (inset of Fig. 4e). Because transferring the sample and setting up the optics takes time, SHG measurements were performed 30 min after tip writing, when the initial fast decay phase had already ended. By contrast, the R values in Fig. 4 were measured in situ about 3 s after writing, capturing the maximum effect. For the R(T) measurements shown in Fig. 4f, g, we waited 10 h after writing to ensure the effect had stabilized before initiating cooldown. Apparently, the time decay of R parallels the decay of PFM and SHG signals after writing. Once ferroelectric polarization is flipped, oxygen vacancies redistribute accordingly, a process that requires several hours to stabilize. This redistribution accounts for the decaying components observed in the PFM amplitude and R(Time). After the decay phase, the remaining PFM and SHG contrasts, together with the residual change in R, reflect intrinsic effects directly associated with ferroelectric polarization switching.

As Vtip ramps, the resistance R shows clear hysteresis behavior (Fig. 4e) that is directly linked to the ferroelectric hysteresis loop. The sharp increase or decrease of R at Vtip ~ −17 and 6 V, respectively, corresponds to critical voltages to flip ferroelectric polarization. Remarkably, the change in R reaches more than 105 times for two saturated states of the hysteresis loop at 300 K, implying a drastic effect of ferroelectric polarization orientation on interfacial conductivity. More importantly, compared to the pristine LAO/KTO sample with Tc = 1.8 K (Here, Tc is denoted as the midpoint of the superconductivity transition), superconductivity disappears at Vtip = −17 V. As Vtip reverses its sign and flips the ferroelectric polarization back to the pristine state at Vtip = +50 V, superconductivity reappears (Fig. 4f, g). The writing cycles and consequent modulation of superconductivity are repeatable, which clearly demonstrates the non-volatile modulation of superconductivity by ferroelectricity. The interface ferroelectricity and its strong coupling with interface superconductivity are universal for synthesized LAO/KTO(111) samples. More examples from other samples are shown in Fig. S10 of the Supplementary Information. The coercivity and shape of the hysteresis loop vary slightly from sample to sample, but the conclusions remain the same.

Notably, R(T) for the high-resistance state (Vtip = −21 V) has a positive slope from room temperature down to low temperatures, characteristics of a metallic state. However, the calculated mean free path for this state is shorter than the lattice constant, which is unphysical. This presumably suggests that conduction is highly inhomogeneous and dominated by percolative current paths. Consequently, the resistance is high but retains a metallic temperature dependence. In addition, an upturn in R(T) appears below 30 K, likely arising from weak localization due to reduced dimensionality and enhanced disorder scattering in the high-resistance state.

The modulation of (super)conductivity arises from the combined effects of ferroelectric polarization P, disorder scattering and oxygen vacancies. Firstly, flipping P from its pristine state effectively reduces the interfacial electron density. The work-function imbalance between LAO and KTO drives electron transfer from the LAO film to the KTO substrate, forming a two-dimensional electron gas (2DEG). This interfacial charge transfer creates an internal electric field E due to electron depletion on the LAO side and accumulation on the KTO side (Fig. 4h). Analogous to a p–n junction, electron accumulation strengthens E and suppresses further transfer, with the amount of transferred charge determined by the interfacial chemical-potential balance. P at the LAO/KTO interface introduces an additional field. When its out-of-plane component, P⊥, is antiparallel to E, the threshold E field for halting transfer increases, yielding higher 2DEG density (Fig. 4i). When parallel, the 2DEG density decreases. In pristine LAO/KTO(111), P⊥ is antiparallel to E since this is the lowest energy ground state. Therefore, resistance is low in the pristine state and increases significantly as P is flipped. Secondly, the modification of disorder scattering affects the electron mobility. The flipping of P alters the profile of the potential well confining the 2DEG. Electrons in a narrower potential well experience stronger scattering from disorder, resulting in higher resistance. This is akin to the electric field control of interface superconductivity in LAO/KTO(111) by the backside gating20, where the modulation of superconductivity is also ascribed mainly to a change in charge mobility due to modification of disorder scattering. Thirdly, redistribution of oxygen vacancies affects both electron density and mobility. Ferroelectric polarization is correlated with oxygen vacancies at the interface (Fig. 1), and the flipping of P is inevitably accompanied by the redistribution of oxygen vacancies. This is consistent with literature reports42,43 that oxygen vacancies can enhance electric polarization or piezoelectricity in perovskite oxide films. Additionally, the effect of electron trapping by these vacancies, which was found in LaAlO3/SrTiO344–46, may also play a role by reducing electron density and increasing resistance. Switching experiments, however, provide evidence against charge trapping as the dominant mechanism (Fig. S13 of Supplementary Information). As this work represents an initial step in understanding ferroelectric superconductivity, it is premature to conclude whether there are additional novel features of ferroelectric superconductivity that may contribute to the modulation of (super)conductivity. This remains an open question for future investigation.

To understand why the pristine ferroelectric state is likely a single domain, symmetry-breaking mechanisms must be considered. In the out-of-plane direction, the electric field E, generated by charge transfer across the LAO/KTO interface, breaks the up-down symmetry, making the lowest energy state for P⊥ antiparallel to E (Fig. 4i). For the in-plane component P∥, emergent electronic nematicity spontaneously breaks the rotational symmetry and lifts the energy degeneracy of P∥. Supporting evidence for electronic nematicity in LAO/KTO(111) has been reported27,47, including our recent work46, where we used the angle-resolved resistivity method to measure anisotropy in electrical transport along various in-plane directions. The results provided compelling evidence for electronic nematicity in the normal, superconducting, fluctuating, and quantum metal states.

The discovery of interface ferroelectricity significantly extends the families of materials hosting ferroelectricity, and its easy tunability due to lower dimensionality is beneficial for device applications. The coupling between interface ferroelectricity and interface superconductivity engenders non-volatile modulation of superconductivity and fuels research on superconductivity without inversion symmetry.

Methods
Films synthesis and device fabrication
A 248 nm KrF excimer laser was used with a 0.5–0.7 J cm−2 laser fluence and 10 Hz laser repetition rate to deposit 10 nm LAO films onto KTO (111) substrates by PLD. The sample temperature was maintained at 300 °C during growth, in a mixed atmosphere of 1×10−5 mbar O2 and 1×10−7 mbar H2O vapor. For the Hall bar used for electrical transport measurements in Fig. 4, we employed standard UV-lithography and lift-off techniques to pre-pattern the KTO substrate and then deposit LAO films to form the desired device.

Scanning transmission electron microscopy (STEM)
A dual-beam focused ion beam instrument (Helios 5 UX, Thermo Fisher Scientific) was used to prepare a sample of LAO/KTO (111) approximately 30 nm thick. The sample was cut by 30 kV Ga ions and cleaned by 5 kV and 2 kV Ga ions to minimize the damage. The STEM images were collected by a spherical aberration-corrected STEM (Themis Z, Thermo Fisher Scientific) at 300 kV. The chemical composition was resolved with an energy dispersive X-ray analyzer (UltraX) in STEM spectrum imaging mode. The STEM–EELS experiments were performed using a Gatan GIF Continuum K3 HR/1069 HR system with an accelerating voltage of 300 kV.

PFM and SS-PFM
PFM measurements were carried out using a commercial Environmental Atomic Force Microscopy system (Cypher ES, Oxford Instruments) at room temperature in an N2 atmosphere. The PFM and the switching-spectroscopy piezoelectric hysteresis loops were measured in PFM dual-AC resonance tracking mode. A triangular voltage waveform with a frequency of 0.025 Hz was applied to collect both the amplitude and phase signals of modulated tip vibration as a function of the bias voltage, with a maximum of 20 V. The writing of ferroelectric domains was accomplished by scanning the PFM tip over a designated area with a fixed voltage in a nitrogen gas environment to eliminate ion implantation.

Second harmonic generation
A confocal microscope (WITec, Alpha 300 RAS) equipped with 1064 nm laser excitation (NPI Rainbow 1064 OEM) was employed for non-linear optics measurements (Fig. 2a). The incident light passes through a linear polarizer and a half-wavelength (λ/2) plate before striking the sample normally. The generated second harmonic signal with a 532 nm wavelength passes through the λ/2 plate and then the second linear polarizer (analyzer). The λ/2 plate is rotated in 10° steps for studies of the angular dependence of SHG. For low temperature SHG measurements, the sample is mounted onto a sample holder cooled by an open-cycle liquid nitrogen cryostat capable of reaching temperatures as low as 200 K.

Suppose the polarization of the incident light makes the angle ϕ with respect to the KTaO3 110 direction, i.e., the x-axis, then the electric field is given by
E = E0 (cos ϕ, sin ϕ, 0)

The SHG d matrix for the m point group with electric polarization in 110 plane is
d = 
0 0 0 0 d15 d16
d21 d22 d23 d24 0 0
d31 d32 d33 d34 0 0

Hence, the SHG signal is given by
P2ω = d m (E0^2 cos^2 ϕ, E0^2 sin^2 ϕ, 0, 0, 2E0^2 sin ϕ cos ϕ, 2E0^2 cos^2 ϕ sin ϕ)

Since the SHG light goes through a λ/2 wave plate before it reaches the analyzer, the SHG light intensities corresponding to x- and y-polarization of the analyzer become
I2ωx = |d21 sin ϕ cos^2 ϕ + d22 sin^3 ϕ + 2d16 sin ϕ cos^2 ϕ|^2
I2ωy = |d21 cos^3 ϕ + d22 sin^2 ϕ cos ϕ + 2d16 sin^2 ϕ cos ϕ|^2

The above expression fits nicely with the SHG data shown in Fig. 2.

Electrical transport measurements
Longitudinal resistance and Hall effect measurements were conducted using the Hall bar pattern shown in Fig. 4a. The width of the Hall bar is 30 μm, and the distance between the two contacts for longitudinal resistance measurement is 100 μm. The excitation current is set at 100 nA. All electric measurements were performed using a physical properties measurement system (PPMS DynaCool 12T, Quantum Design) with variable sample temperatures ranging from 1.8 to 300 K. For temperatures below 1.8 K, a dilution refrigerator attachment (Quantum Design) is used to reach temperatures as low as 100 mK.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript titled “Strongly coupled interface ferroelectricity and interface superconductivity in LAO/KTO” reports the observation and manipulation of ferroelectric distortion at the interface between LaAlO3(LAO) and (111)-oriented KTaO3(KTO). The ferroelectricity manifests as relative displacements of K and Ta atoms in KTO near the interface. Application of voltage via an AFM tip flips the polarization of ferroelectricity and gives rise to local domains; such domain formation increases the electrical resistance profoundly and suppresses the superconductivity. Such process is partially reversible with sweeps of the tip voltage, thus guarantees an on-off switch of interfacial charge transport and superconductivity. Some of these results are interesting, pointing towards good promises from the perspective of device applications. However, the manuscript contains many vague arguments and obscure presentations, which inevitably lead to a certain level of confusion. Furthermore, AFM-tip manipulation of the interfacial electronic state between two insulating oxides has been extensively explored [e.g., C. Cen, et al., Nat. Mater. 7, 298 (2008); Y.-Y. Pai, et al., Phys. Rev. Lett. 120, 147001 (2018); A. Nethwewala, et al. Nanoscale Horiz. 4, 1194 (2019); M. Yu, et al., Phys. Rev. X 15, 011037 (2025)]. I doubt the present work has sufficient scientific significance and novelty that can promise its publication in Nature Communications.\nBefore I can provide a final evaluation of this manuscript, a few questions need to be addressed.\n(1) Page 4, lines 94-100. The authors wrote that “The K displacement, δ, is along KTO [110] direction and depends sensitively on its distance from the interface. In contrast, the iDPC-STEM image taken from the KTO(11-2) plane (Extended Data Fig. 2), which is perpendicular to the KTO(1-10) plane in Fig. 1, shows no appreciable Ta displacement. This verifies that the Ta displacement is normal to the KTO(11-2) plane and hence it is precisely along KTO [110] direction.” I found these sentences to be very confusing. First, they initially mentioned “K displacement” but then switched to “Ta displacement” --- are the two terms exchangeable? Or they describe different things? Second, since the whatever displacement is along [110], it is apparently normal to the (110) plane instead of (11-2); hence, I cannot comprehend what the authors meant by saying “This evidences that the K-Ta displacement is entirely along KTO [110] direction and thus has no projection in the (11-2) plane” (caption of Extended Data Fig. 2, line 514-515). The [110] vector definitely has a finite projection in the (11-2) plane.\n(2) Still the contexts mentioned in (1). Let’s acknowledge that the atom displacements that causes ferroelectric distortion is along [110], but there are three equivalent [110] directions at the (111) interface, according to its threefold rotational symmetry. In essence, there could correspondingly be three types of ferroelectric domains with the polarizations changing by 120 degrees between each two, but the authors then noted “No ferroelectric domain has been found in the as-grown LAO/KTO(111)…” (line 150). To me, this means rotational symmetry breaking as the system spontaneously selects one special (110) direction for the distortion. Is this true? If so, what is the underlying mechanism? I suggest that discussions on this point must be added, since such a tendency of emergence nematicity can be crucial for understanding the unique properties of KTO-based heterointerfaces [e.g., G. Zhang, et al., Nat. Commun, 14, 3046 (2023)].\n(3) As shown in Fig. 4h, the resistance decreases remarkably within a time window of 1 h after AFM-tip writing. Therefore, it may keep change during the cooldown process in PPMS. Is this the cause of the “metallic” behavior displayed in Fig. 4f? Have the authors performed measurement of ρ(T) under thermal cycles and testify the data repeatability? Also, what is origin of the resistive upturn below ~ 30 K in the 0 V, 6 V and -21 V curves?\n(4) I have further questions about the model proposed by the authors, that the metallic resistivity stems from either a conducting layer beneath the insulating ferroelectric layer, or percolative paths through it. Indeed, the data presented in Figs. 4h and i point towards an unphysically short electron mean free path of ~ 0.002 nm (~1/200 of the lattice constant) at 300 K. I think this can only be reconciled with the latter (1D filamentary path) scenario but cannot be understood with the presence of a 2D conducting layer, because a parallel connection of an insulating layer and a conducting layer must has a resistance lower than the conducting layer. The problem is, however, if the 1D picture is correct, then the 2D Hall number in this regime is meaningless.\n(5) The authors attributed the suppression of superconductivity in the high resistance state (driven by negative tip voltage) to enhanced disorder. This seems to be inconsistent with the above-mentioned scenarios: if the ferroelectrically flipped layer (thickness ~1.9 nm) is indeed insulating, as implied by the extremely high resistance, the superconductivity is certainly killed therein with no excuse. To me, such an insulating state is more likely driven by the charge depletion related to the change of oxygen vacancies, instead of the disorder-induced localization. It was mentioned in the manuscript that the oxygen vacancies vary with the ferroelectric distortion. This possibility ought to be considered to some extent. Besides, a thickness of 1.9 nm for the dead layer sounds lower than the reported thickness of superconducting layers in KTO-based heterointerfaces [Ref. [24]; X. Hua, et al., npj Quantum Mater. 7, 97 (2022)]. Hence, this is insufficient to explain the absence of superconductivity if the underlying metallic layer is “not directly affected by the ferroelectric flipping” as evidence by the STEM results. In all, the Hall mobility in Fig. 4i for -21 V is too low to be a meaningful parameter, I do not think an argument of disorder effect (in a metallic state) can be made based on it.\nMinor issues\n- I suggest the authors to avoid using abbreviations (“LAO/KTO”) in the title because no one really understand what they are.\n- Figures 1b and c, the red dashed boxes are almost invisible on the dark background. They should be made clearer.\n- The raw data of Hall resistance should be displayed somewhere.", "reviewer_2": "The authors have investigated LaAlO3/KTaO3 (111) interface using a set of techniques and claim that superconductivity and ferroelectricity coexist in the system. While the authors have demonstrated superconductivity and ferroelectricity individually at different temperature, there is no direct experimental evidence to claim they coexist at same temperature. I have several other technical comments also. Based on these, I am unable to judge whether the manuscript is suitable for publication in Nature Communications. I request the authors to provide a thorough response to my questions before I make a decision about the manuscript.\n1. What is the role of water vapor in the growth process? Does it alter local chemistry of the substrate? What characterizations/measurements lead the authors to maintain a partial pressure of water vapor 10^-7 mbar during the growth? What happens is the growth is performed without water vapor?\n2. The authors claim the interface is sharp. While the EELS shows the intermixing of the length scale of 1 nm. Since LAO layer is amorphous, the title of the paper is somewhat confusing. There is no experimental report that the film is LaAlO3, most likely it is La_xAl_yO_z. So, the author should bring this aspect appropriately through out the text including the title.\n3. Regarding SHG: In the method section, the authors have written ‘For low temperature SHG measurements, the sample is mounted onto a sample holder cooled by an open-cycle liquid nitrogen cryostat capable of reaching temperatures as low as 200 K.’ In the main text, SHG data is shown down much lower temperature (till 1.6 K?). How did the authors achieve 1.6K in SHG set up? The polarimetry data is missing at 1.6 K. This is essential as it will show the presence of polar distortion (if present) in the superconducting phase, which is the main claim of the paper. Fig. 2d shows that the SHG signal is actually decreasing below 120 K. Any discussion on this aspect is missing in the text.\n4. The authors have performed resistance measurement by applying a gate voltage through PFM tips and shown the hysteresis in R vs. V_tip. Under a gate voltage, charge trapping can happen due to gate voltage induced band bending, oxygen vacancy migrations etc., which had been demonstrated extensively in context of SrTiO3 based heterostructrues [Sci. Rep. 4, 6788 (2014); Phys. Rev. Lett. 124, 017702 (2020); Phys. Rev. Applied 15, 054008 (2021)]. Such charge trapping phenomena also gives rise to hysteresis in resistance vs. gate voltage measurement. The authors have not considered this aspects at all in the explanation of their observations.\n5. Regarding the data presented in Figure 4(e), was the resistance measured using a two-probe or four-probe technique? If a four-probe method was employed, please explain how such high resistance values could be accurately determined.\n6. Regarding the data plotted in Figure 1f, was it acquired from a specific localized region? Additionally, please clarify the methodology used to determine the error bars.\n7. The introduction of the paper started with the discussion of the polar metal. However, the paper ends with the effect of gate voltage modulation of the carrier density, which controls superconductivity. This has been demonstrated already in earlier literature.\nIn summary, while the experiments confirm the presence of ferroelectricity at 300 K using SHG, PFM, and STEM, the manuscript does not directly show the presence of ferroelectricity in the superconducting state.", "reviewer_3": "The manuscript discusses a putative coexistence of ferroelectricity and superconductivity at the LaAlO3/KTaO3(111) interface. The manuscript is clear and well-written, and if the claim is verified, it certainly merits publication in Nature Communications, as it is at the forefront of current literature in the field of oxide heterointerfaces and related applications in electronics and materials science.\nHowever, I believe the claim is not adequately supported by the evidence presented in its current form. I am particularly concerned about the issue of oxygen vacancies (OxVac) at the interface, as noted by the authors too.\nWhile the superconducting state appears indisputable and is also supported by literature (APL Mater. 11, 121108, 2023), the ferroelectric (FE) ordering is much more subtle. It is well-known that FE measurements are elusive and prone to misleading results (J. Phys.: Condens. Matter 20 (2008) 021001), and in my opinion, the three pieces of evidence provided by the authors (STEM, SHG, and PFM) are insufficient to make a definitive claim.\nThe first strong evidence of FE ordering would be detecting a FE phase-transition critical temperature, which was not explored in this manuscript. Another critical point is the switchability of polarization, claimed in the manuscript through PFM writing. However, regarding the LaAlO3/SrTiO3 interface, Bark et al. write (Nano Lett. 2012, 12, 1765−1771):\n\"The PFM response we have detected in LAO is very similar to that observed in ferroelectric materials. Its most likely origin, however, is a spatial polarization induced in the LAO layer due to the redistribution of oxygen vacancies.\"\nI suspect a similar phenomenon is occurring here. Indeed, other authors (App. Phys. Lett. 103, 062902, 2013) have used the stability of polarization contrast over several hours as proof of the true FE nature of the system:\n\"However, these results represent a necessary but not sufficient proof in favor of room temperature ferroelectricity in our films. As discussed in Ref. 18, there are two possible alternative explanations: (1) simple charging of the surface and (2) ionic displacements of the oxygen ions perpendicular to the surface in a direction defined by the applied electric field. For this reason, we investigated the time dependence of the amplitude of the piezo-response signal, which in the case of ferroelectric materials is proportional to the polarization.\"\nHere Ref. 18 is the aforementioned Nano Lett. paper.\nAccording to the authors of this manuscript: \"It should be mentioned that after the writing process, the effect of tip writing decays over time, quite significantly for Vtip < -10 V.\" Moreover, the authors acknowledge the significant role of OxVac in their observations.\nThe issue is that both PFM and SHG signals are not exclusively related to FE ordering; they can have various other sources. One possible way to investigate this aspect is to anneal the sample in an oxygen-rich atmosphere (even simply in air) to significantly reduce the OxVac density, disentangling intrinsic and extrinsic effects, as reported in Appl. Phys. Lett. 104, 261603 (2014). For a genuine FE ordering claim, the SHG signal should return to (almost) its initial state after the annealing process.\nOne minor but not negligible issue I see, is that the SHG signal of the interface was compared to a pristine substrate. However, this is not the appropriate comparison since the pristine substrate likely has no, or negligible, OxVac density at the surface. The proper comparison would be between the investigated interface and a substrate subjected to the same pressure, temperature, and time in the PLD chamber, but without the laser pulse to induce epitaxy. Alternatively, the comparison should be made with at least a doped substrate with a comparable OxVac density.\nFinally, the issue of FE domain size: SHG and PFM have very different spatial resolutions, with the former being much larger than the latter. It is possible to have domains too small to be seen by SHG but easily detected by PFM. Conversely, if the domains are very large, they could exceed the total scanned area of PFM, but in this case, SHG should detect them. If they are absent in both techniques, it is quite strange. How can a FE material have no spontaneous domain structure? The idea of a single domain state forming spontaneously is quite unlikely, especially in 2D FE, where domain size is known to be of the same order of magnitude as the interface thickness. The PFM writing could be attributed to OxVac accumulation effects. The fact that the tip writing decays over time is a strong indication that the signal results from ion migration rather than FE ordering. Finally, the hysteretic loops could also be due to trapping/untrapping mechanisms at the interface, modulating the interfacial electric field.\nIn summary, I believe the manuscript cannot be accepted in its present form, nor by merely changing its text. It requires further experimental work to confirm the proposed claim. However, I think the importance of the findings, if confirmed, justifies the effort.\nMinor points:\n17 - \"we discover that ferroelectric order resides at the interface...\" -> \"we found evidence of a ferroelectric order at the interface...\"\n22 - Superconductivity cannot \"diminish and reappear\": the critical temperature can be higher or lower with respect to a certain value, the material can undergo a phase transition so that it is not superconductive, even if only for a short period of time. This sentence is quite misleading in its present form.\n25 - \"STEM reveals K ions are displaced relative to\" -> \"STEM reveals the displacement of K ions relative to\"\n26 - \"electric\" -> \"ferroelectric\"\n28 - \"hysteresis change\" -> \"hysteretical change\"\n73 - \"postulated to be related\" -> \"attributed\"\n- Problem of beam damaging in STEM measures, how do they know when it is the correct time to stop?\n- 124: the SHG intensity is proportional to the square of the fundamental intensity (eq16 in Ref36).\n135-137: this statement is correct only if one che link the SHG signal to FE order alone. In case the SHG would be found to be of structural origin (i.e. similar on oxvac samples with no FE order) then the statement is not correct."}, "author_response": "Point-by-point response to the referees’ comments.\nReviewer #1 (Remarks to the Author):\nComment 1: The manuscript titled “Strongly coupled interface ferroelectricity and\ninterface superconductivity in LAO/KTO” reports the observation and manipulation\nof ferroelectric distortion at the interface between LaAlO (LAO) and (111)-oriented\n3\natoms in KTO near the interface. Application of voltage via an AFM tip flips the\npolarization of ferroelectricity and gives rise to local domains; such domain\nformation increases the electrical resistance profoundly and suppresses the\nsuperconductivity. Such process is partially reversible with sweeps of the tip\nvoltage, thus guarantees an on-off switch of interfacial charge transport and\nsuperconductivity.\nSome of these results are interesting, pointing towards good promises from the\nperspective of device applications. However, the manuscript contains many vague\narguments and obscure presentations, which inevitably lead to a certain level of\nconfusion. Furthermore, AFM-tip manipulation of the interfacial electronic state\nbetween two insulating oxides has been extensively explored [e.g., C. Cen, et al.,\nNat. Mater. 7, 298 (2008); Y.-Y. Pai, et al., Phys. Rev. Lett. 120, 147001 (2018); A.\nNethwewala, et al. Nanoscale Horiz. 4, 1194 (2019); M. Yu, et al., Phys. Rev. X 15,\n011037 (2025)]. I doubt the present work has sufficient scientific significance and\nnovelty that can promise its publication in Nature Communications.\nBefore I can provide a final evaluation of this manuscript, a few questions need to\nbe addressed.\nResponse 1: The Referee gives a nice summary of our work. While it is true that\nusing an AFM-tip to manipulate interfacial electronic states is not a new idea—and\nwe have never claimed otherwise—the focus of our study lies in interfacial\nferroelectricity and ferroelectric superconductivity, which are important for both\nfundamental research and potential applications. Reported candidates for\nferroelectric superconductivity are scarce in the literature, and the discovery of\nferroelectric superconductivity at the interface of parent compounds may open new\nopportunities for exploring novel superconducting systems.\nComment 2: Page 4, lines 94-100. The authors wrote that “The K displacement,\nδ, is along KTO [110] direction and depends sensitively on its distance from the\ninterface. In contrast, the iDPC-STEM image taken from the KTO(11-2) plane\n(Extended Data Fig. 2), which is perpendicular to the KTO(1-10) plane in Fig. 1,\nshows no appreciable Ta displacement. This verifies that the Ta displacement is\nnormal to the KTO(11-2) plane and hence it is precisely along KTO [110] direction.”\nI found these sentences to be very confusing. First, they initially mentioned “K\ndisplacement” but then switched to “Ta displacement” --- are the two terms\nexchangeable? Or they describe different things? Second, since the whatever\ndisplacement is along [110], it is apparently normal to the (110) plane instead of\n(11-2); hence, I cannot comprehend what the authors meant by saying “This\n1\nevidences that the K-Ta displacement is entirely along KTO [110] direction and\nthus has no projection in the (11-2) plane” (caption of Extended Data Fig. 2, line\n514-515). The [110] vector definitely has a finite projection in the (11-2) plane.\nResponse 2: We thank the Referee very much for pointing out these discrepancies,\nand have revised the text accordingly to avoid confusion. In the original manuscript,\nthe terms “K displacement” and “Ta displacement” were used interchangeably.\nAlthough the displacement between K and Ta atoms is, in some sense, relative,\nwe now use “K displacement” consistently throughout the revised manuscript. This\nchange reflects the fact that oxygen atoms in the Ta-O plane show no displacement\nrelative to the Ta atoms (the original Extended Data Fig. 2; now Fig. S2 in the\nSupplementary Information); strictly speaking, it is the K atoms that are displaced\nrelative to the Ta and O lattice.\nThe Referee is correct that the [110] vector has a projection onto the (11-2) plane,\nand we apologize for the earlier inaccuracy. A clear K displacement along [110] is\nvisible in the (-110) plane (Fig. 1). We rotated the viewing angle to check for\ncomponents orthogonal to [110], examining the (11-2) plane, which is\nperpendicular to the (-110) plane. No displacement was observed along [-110] (the\noriginal Extended Data Fig. 2; now Fig. S2 in the Supplementary Information),\nconfirming that the displacement lies entirely along [110]. We have revised the text\naccordingly to ensure accuracy.\nComment 3: Still the contexts mentioned in (1). Let’s acknowledge that the atom\ndisplacements that causes ferroelectric distortion is along [110], but there are three\nequivalent [110] directions at the (111) interface, according to its threefold rotational\nsymmetry. In essence, there could correspondingly be three types of ferroelectric\ndomains with the polarizations changing by 120 degrees between each two, but\nthe authors then noted “No ferroelectric domain has been found in the as-grown\nLAO/KTO(111)…” (line 150). To me, this means rotational symmetry breaking as\nthe system spontaneously selects one special (110) direction for the distortion. Is\nthis true? If so, what is the underlying mechanism? I suggest that discussions on\nthis point must be added, since such a tendency of emergence nematicity can be\ncrucial for understanding the unique properties of KTO-based heterointerfaces\n[e.g., G. Zhang, et al., Nat. Commun, 14, 3046 (2023)].\nResponse 3: We greatly appreciate the Referee’s insightful comment. In the out-\nof-plane direction, the electric field 𝐸⃗ , generated by charge transfer across the\nLAO/KTO interface, breaks the up-down symmetry, making the lowest energy state\nfor the ferroelectric polarization antiparallel to 𝐸⃗ (Fig. R1). The in-plane direction,\nretains degenerate energy states if the rotational symmetry is preserved. Emergent\nnematicity provides an intriguing explanation for this puzzle. In addition to the\nreference cited by the Referee, our group recently reported direct evidence of\nelectronic nematicity in LAO/KTO(111) [Cheng, X. B. et al. Electronic nematicity in\ninterface superconducting LAO/KTO(111). Phys. Rev. X 15, 021018 (2025)]. We\nused the angle-resolved resistivity method to measure the anisotropy of electrical\ntransport along various in-plane directions and found compelling evidence for\nelectronic nematicity in its normal state, superconducting fluctuating state, and\nquantum metal state, confirming the absence of ferroelectric domain in pristine\nflipped and domains can be induced (Fig. 3 and 4b). This discussion has been\nincorporated into the revised manuscript.\nComment 4: As shown in Fig. 4h, the resistance decreases remarkably within a\ntime window of 1 h after AFM-tip writing. Therefore, it may keep change during the\ncooldown process in PPMS. Is this the cause of the “metallic” behavior displayed\nin Fig. 4f? Have the authors performed measurement of ρ(T) under thermal cycles\nand testify the data repeatability? Also, what is origin of the resistive upturn below\n~ 30 K in the 0 V, 6 V and -21 V curves?\nResponse 4: Because measuring R(T) from room temperature to 20 mK takes\nseveral hours, we waited 10 hours for the effect to stabilize before initiating\ncooldown. This makes it unlikely that the observed metallic behavior arises from a\ntime-dependent decay of the effect. Repeated measurements of R(T) for the same\nstate yielded overlapping curves. It is likely that the rapid decay of resistance is\nrelated to oxygen vacancy redistribution, while the effect of ferroelectric order\npersists over time.\nThe resistive upturn below 30 K is presumably due to the localization effect,\nconsistent with the increased disorder scattering in these high-resistance states.\nComment 5: I have further questions about the model proposed by the authors,\nthat the metallic resistivity stems from either a conducting layer beneath the\ninsulating ferroelectric layer, or percolative paths through it. Indeed, the data\npresented in Figs. 4h and i point towards an unphysically short electron mean free\npath of ~ 0.002 nm (~1/200 of the lattice constant) at 300 K. I think this can only\nbe reconciled with the latter (1D filamentary path) scenario but cannot be\nunderstood with the presence of a 2D conducting layer, because a parallel\nconnection of an insulating layer and a conducting layer must has a resistance\nlower than the conducting layer. The problem is, however, if the 1D picture is\ncorrect, then the 2D Hall number in this regime is meaningless.\nResponse 5: What we are suggesting here is that both the reduction of the\nconducting layer thickness and percolative conductivity contribute to the high\nresistance state. And the Referee is right that the mean free path for the highest\nresistance state is shorter than the lattice constant, indicating that percolative\nconductivity plays a major role. Thus, for the high resistance state, the retrieved\nHall number is under influence of percolative connectivity and should be\ninterpreted with caution. We have moved Fig. 4h and 4i to the Supplementary\nInformation and made this point explicit there.\nComment 6: The authors attributed the suppression of superconductivity in the\nhigh resistance state (driven by negative tip voltage) to enhanced disorder.\nThis seems to be inconsistent with the above-mentioned scenarios: if the\nferroelectrically flipped layer (thickness ~1.9 nm) is indeed insulating, as implied\nby the extremely high resistance, the superconductivity is certainly killed therein\nwith no excuse. To me, such an insulating state is more likely driven by the\ncharge depletion related to the change of oxygen vacancies, instead of the disorder-\ninduced localization. It was mentioned in the manuscript that the oxygen vacancies\nvary with the ferroelectric distortion. This possibility ought to be considered to some\nextent. Besides, a thickness of 1.9 nm for the dead layer sounds lower than the\nreported thickness of superconducting layers in KTO-based heterointerfaces [Ref.\n[24]; X. Hua, et al., npj Quantum Mater. 7, 97 (2022)]. Hence, this is insufficient to\nexplain the absence of superconductivity if the underlying metallic layer is “not\ndirectly affected by the ferroelectric flipping” as evidence by the STEM results. In\nall, the Hall mobility in Fig. 4i for -21 V is too low to be a meaningful parameter,\nI do not think an argument of disorder effect (in a metallic state) can be made based\non it.\nResponse 6: We appreciate the Referee’s comments and have improved the\ndiscussion on the mechanisms for modulation of conductivity and\nsuperconductivity. Incorporating the valuable suggestions of all three Referees,\nwe now propose that the modulation arises from the combined effects of:\n1. Flipping of ferroelectric polarization 𝑃⃗ . The work-function imbalance\nbetween LAO and KTO drives electron transfer from the LAO film to the\nKTO substrate, forming a two-dimensional electron gas (2DEG). This\ninterfacial charge transfer creates an internal electric field 𝐸⃗ due to electron\ndepletion on the LAO side and accumulation on the KTO side (Fig. R1).\nAnalogous to a p-n junction, electron accumulation strengthens 𝐸⃗ and\nsuppresses further transfer, with the amount of transferred charge\ndetermined by the interfacial chemical-potential balance. Ferroelectric\npolarization 𝑃⃗ at the LAO/KTO interface introduces an additional field.\nWhen its out-of-plane component, 𝑃⃗ , is antiparallel to 𝐸⃗ , the threshold 𝐸⃗\n⊥\nfield for halting transfer increases, yielding higher 2DEG density. When\nparallel, the 2DEG density decreases. In pristine LAO/KTO(111), 𝑃⃗ is\n⊥\nantiparallel to 𝐸⃗ since this is the lowest energy ground state. Therefore,\nresistance is low in the pristine state and increases significantly as 𝑃⃗ is\nflipped.\n2. Modification of effective disorder scattering. Beyond carrier-density effects,\n𝑃⃗ alters the profile of the potential well confining the 2DEG. Electrons in a\nnarrower potential well experience stronger scattering from disorder,\nresulting in higher resistance.\n3. Redistribution of oxygen vacancies. Ferroelectric polarization correlates\nwith oxygen vacancies at the interface (Fig. 1). The flipping of 𝑃⃗ is\ninevitably accompanied by the redistribution of oxygen vacancies. Electron\ntrapping by these vacancies renders electron’s mobility and increases\nresistance.\nAs this study represents an initial step in exploring ferroelectric superconductivity,\nit is premature to conclude whether there are additional novel features of\nferroelectric superconductivity that may contribute to the modulation process.\nWe therefore focus on our experimental findings while leave open possibilities for\nfuture investigation.\nferroelectric polarization ⃗𝑷⃗ . a, 2DEG is formed due to charge transfer from LAO\nto KTO and electrons are confined in a potential well. The interfacial charge\ntransfer creates an internal electric field 𝐸⃗ due to electron depletion on the LAO\nside and accumulation on the KTO side. b, The out-of-plane component 𝑃⃗ ,\nis\n⊥\nantiparallel to 𝐸⃗ , for minimization of electrostatic energy. 𝑃⃗ increase the\n⊥\nthreshold 𝐸⃗ field for halting charge transfer, yielding higher 2DEG density.\nComment 7: Minor issues\n- I suggest the authors to avoid using abbreviations (“LAO/KTO”) in the title\nbecause no one really understand what they are.\n- Figures 1b and c, the red dashed boxes are almost invisible on the dark\nbackground. They should be made clearer.\n- The raw data of Hall resistance should be displayed somewhere.\nResponse 7: We’ve revised our manuscript accordingly by changing “LAO/KTO”\nto “amorphous LaAlO /KTaO (111)” and enhancing the visibility of the yellow boxes.\n3 3\nThe raw Hall resistance data has been added to the Supplementary Information.\nReviewer #2 (Remarks to the Author):\nComment 8: The authors have investigated LaAlO /KTaO (111) interface using a\n3 3\nset of techniques and claim that superconductivity and ferroelectricity coexist in\nthe system. While the authors have demonstrated superconductivity and\nferroelectricity individually at different temperature, there is no direct experimental\nevidence to claim they coexist at same temperature. I have several other technical\ncomments also. Based on these, I am unable to judge whether the manuscript is\nsuitable for publication in Nature Communications. I request the authors to provide\na thorough response to my questions before I make a decision about the\nmanuscript.\nResponse 8: We have added supplementary experimental results and revised\ndiscussions to address the Referee’s comments and concerns, with the aim of\nmeeting the Referee’s high standards. The coexistence of superconductivity and\nferroelectricity is discussed in Response 11.\nComment 9: What is the role of water vapor in the growth process? Does it alter\n5\nlocal chemistry of the substrate? What characterizations/measurements lead the\nauthors to maintain a partial pressure of water vapor 10^-7 mbar during the growth?\nWhat happens is the growth is performed without water vapor?\nResponse 9：During sample growth, the water vapor partial pressure (1×10-7 mbar)\nis two orders of magnitude lower than the oxygen partial pressure (1×10-5 mbar).\nAt this level, hydrogen incorporation into the KTO(111) substrate is negligible and\nbelow the detection limits of techniques such as SIMS.\nAtomic force microscopy (AFM) and X-ray diffraction (XRD) analyses confirm that\ntrace water vapor does not produce detectable changes in surface morphology or\ncrystalline structure (Figs. R2a-c). Nevertheless, it enhances the superconducting\ntransition temperature, as shown by R(T) curves comparing samples grown with\nand without trace water vapor (Fig. R2d). Meanwhile, we also confirmed that a\nbare KTO(111) substrate exposed to identical water vapor and oxygen levels, and\nsubjected to the same thermal processes without actual LAO film deposition,\nremained highly insulating. Thus, the trace water vapor likely acts as a catalyst,\nmodifying local surface chemistry to promote better interface formation.\nIt should also be noted that tiny residual water vapor is commonly present in gas\npipelines of deposition systems (e.g., PLD, sputtering) [P. Scheiderer et al. Phys.\nRev. B 92, 195422 (2015)], and is known to influence oxide film growth, with\ncontrolled introduction improving film quality [S. Ishizuka et al. J. Appl. Phys. 100,\n096106 (2006); S. Ishizuka et al. Jpn. J. Appl. Phys. 44, L679 (2005); T. Li et al.\nACS Appl. Mater. Interfaces 16, 31237-31246 (2024); N. Oka et al. Appl. Phys.\nExpress 5, 075802 (2012)]. Wet oxidation methods are also routinely used in\nsemiconductor processing to enhance oxide film properties.\nTherefore, incorporating ~ 1 % partial pressure of water vapor (1×10-7 mbar) during\ngrowth improves superconducting performance (T ~ 2.1 K) [Z. Chen et al. Science\nc\n372, 721–724 (2021)]. While further investigation into the surface chemistry\nevolution would be valuable to elucidate the underlying mechanisms, such work\nlies beyond the scope of this study.\ngrown with and without water vapor. a and b, AFM images showing the surface\nmorphology of 10 nm LAO/KTaO(111) samples grown without and with water vapor,\nrespectively. c, Corresponding XRD spectra are similar for both cases. d, Sheet\nresistance R (T) measurements indicate enhanced superconductivity when water\ns\nvapor was introduced into the growth chamber. The inset amplifies R (T) for\ns\ntemperatures below 5 K.\nComment 10: The authors claim the interface is sharp. While the EELS shows the\nintermixing of the length scale of 1 nm. Since LAO layer is amorphous, the title of\nthe paper is somewhat confusing. There is no experimental report that the film is\nLaAlO3, most likely it is La_xAl_yO_z. So, the author should bring this aspect\nappropriately through out the text including the title.\nResponse 10: We followed the convention established in previous publications,\ne.g., the first report of KTO-based interface superconductivity (Ref. 22) used the\nnotation LAO/KTO(111) (Ref. 22). We agree that this notation might cause\nconfusion and have therefore changed the title to “a-LaAlO /KTaO (111)”, where\n3 3\n“a” denotes amorphous. For any film deposition method, interfacial atom\ninterdiffusion is inevitable due to entropy. To avoid controversy, the claim of a sharp\ninterface has been removed.\nRegarding composition, the PLD target is polycrystalline LaAlO bulk. Although the\n3\ndeposited film’s composition may deviate from that of the target, it is standard\npractice in the PLD community to refer to the nominal composition. In keeping with\nthis convention, and with previous work on this subject, we denote the film as\n“amorphous LaAlO ”. For simplicity and readability, we use the abbreviation “LAO”\n3\n7\nfor “amorphous LaAlO ”, which is explicitly stated in the revised manuscript.\nComment 11: Regarding SHG: In the method section, the authors have written\n‘For low temperature SHG measurements, the sample is mounted onto a sample\nholder cooled by an open-cycle liquid nitrogen cryostat capable of reaching\ntemperatures as low as 200 K.’ In the main text, SHG data is shown down much\nlower temperature (till 1.6 K?). How did the authors achieve 1.6K in SHG set up?\nThe polarimetry data is missing at 1.6 K. This is essential as it will show the\npresence of polar distortion (if present) in the superconducting phase, which is the\nmain claim of the paper. Fig. 2d shows that the SHG signal is actually decreasing\nbelow 120 K. Any discussion on this aspect is missing in the text.\nResponse 11: We employed two SHG instruments installed in two separate\ncryogenic systems (Fig. R3). One is installed in a WITec, Alpha300RAS\ncryogenic system (Fig. R3a), which operates down to 77K and features a rotatable\npolarizer for measuring I2ω(ϕ) and I2ω(ϕ) This served as our primary SHG\nx y\ninstrument. The other is installed in a HORIBA LabRam Odyssey and Attocube\nattoDRY2100 system (Fig. R3b) that is currently still under development and\ncurrently has limited functionality. It can only record SHG signals with the polarizer\nfixed at an angle, but the HORIBA system can reach temperatures as low as 2 K.\nSHG data in Fig. 2d of the original manuscript were obtained using the HORIBA\nsetup, while all other SHG data came from the WITec setup.\nAlthough SHG or PFM measurements in a dilution refrigerator would be ideal, we\nlake access to such facilities and are unaware of commercial systems that offer\nthem. Nevertheless, by combing results from both SHG setups, we obtained\ncompelling evidence for the coexistence of superconductivity and ferroelectricity.\n1. I is substantial at 2 K and scales with P2 (Fig. R3d and R3e), confirming\nSHG\nthat ferroelectricity is present at 2 K.\n2. Superconductivity onsets at 3.5 K (Fig. R3c).\n3. I (T) shows no sign of transition during cooldown (Fig. 2d), indicating that\nSHG\nferroelectricity persists to low temperatures.\n4. The orientation of ferroelectric order, determined from I2ω(ϕ) and I2ω(ϕ),\nis\nx y\nconsistent with TEM and PFM results.\n5. Ferroelectric order shall become stronger as thermal fluctuations become\nweaker at lower temperatures. The possibility of a ferroelectric-to-\nparaelectric phase transition is further ruled out by the R(T) measurement\n(the pristine curve in Fig. 4f), which shows no kink indicative of such a\ntransition.\nThese results and the accompanying discussion have been added to the revised\nmanuscript, further reinforcing our conclusions.\noptics installed in the WITec’s cryogenic system, with temperature limited to 77 K.\nb, SHG optics installed in the HORIBA LabRam Odyssey and Attocube\nattoDRY2100 cryogenic system, capable of reaching ~2 K. c, R(T) of\nLAO/KTO(111), showing the onset of superconductivity ~ 3.5 K. d, I at 2 K as a\nSHG\nfunction of P, the laser power at 1064 nm wavelength. e, Quadratic dependence\nof I on P at 2 K.\nSHG\nComment 12: The authors have performed resistance measurement by applying\na gate voltage through PFM tips and shown the hysteresis in R vs. V_tip. Under a\ngate voltage, charge trapping can happen due to gate voltage induced band\nbending, oxygen vacancy migrations etc., which had been demonstrated\nextensively in context of SrTiO based heterostructrues [Sci. Rep. 4, 6788 (2014);\ncharge trapping phenomena also gives rise to hysteresis in resistance vs. gate\nvoltage measurement. The authors have not considered this aspects at all in the\nexplanation of their observations.\nResponse 12: We thank the Referee for highlighting the possible role of charge\ntrapping effect in the observed resistance hysteresis. In LAO/STO, such effect\nmanifests as hysteresis in resistivity and carrier density when the backside gating\nvoltage is ramped. A similar mechanism could occur in LAO/KTO and contribute\nto the resistance hysteresis observed during tip gating. Motivated by the Referee’s\nsuggestion, we plan to conduct systematic studies of charge trapping, an intriguing\ntopic but beyond the scope of the present work.\nexperiments and analyses, as detailed in Response 19. We have added these\nuseful references and revised the manuscript to incorporate this discussion.\nComment 13: Regarding the data presented in Figure 4(e), was the resistance\nmeasured using a two-probe or four-probe technique? If a four-probe method was\nemployed, please explain how such high resistance values could be accurately\ndetermined.\nResponse 13: All resistance measurements were performed using the four-probe\nmethod for better accuracy.\nA Keithley 2450 sourcemeter supplied a dc current and simultaneously measured\ndc voltages while in-situ flipping the ferroelectric polarization with the PFM tip.\nThe square-wave method, alternating the current polarity and measuring the\ncorresponding voltage difference, was employed to eliminate contact-voltage\noffsets. For low resistance samples (≤105 Ω), the excitation current was 300 nA;\nfor higher resistance, a lower current ~10 nA was used. Although the noise is\nlarger with the lower current, the resistance measurement remained reliable with\nlonger averaging times.\nComment 14: Regarding the data plotted in Figure 1f, was it acquired from a\nspecific localized region? Additionally, please clarify the methodology used to\ndetermine the error bars.\nResponse 14: The iDPC-STEM imaging at all examined locations yielded\nconsistent results, including the relative displacement between Ta and K atoms,\nand the presence of oxygen vacancies. The data in Fig. 1f are representative and\ngeneric to LAO/KTO(111).\nIn the iDPC-STEM images, one pixel corresponds to a physical length of 0.0085\nnm. For clearly resolved atomic columns, the error bar was estimated as 2×0.0085\nnm; for columns with tails, the estimate was 4×0.0085 nm.\nComment 15: The introduction of the paper started with the discussion of the polar\nmetal. However, the paper ends with the effect of gate voltage modulation of the\ncarrier density, which controls superconductivity. This has been demonstrated\nalready in earlier literature.\nResponse 15: The focus of this work is to report the discovery of interfacial\nferroelectricity and ferroelectric superconductivity in LAO/KTO(111)\nheterointerface. The modulation of superconductivity via tip gating demonstrates\nthat interfacial ferroelectricity can be switched by an external electric field—an\neffect still elusive in most of the polar metals (hence the term “polar” rather than\n“ferroelectric” metal, Ref. 8). This also confirms that ferroelectricity and\nsuperconductivity are coupled, enabling non-violate control of superconductivity\nthrough ferroelectricity.\nOur study represents an initial step in exploring ferroelectric superconductivity—a\n10\nnovel state with both fundamental significance and potential for multifunctional\nelectronics. This work easily extends to topics such as the pairing symmetry of\nferroelectric superconductors, where inversion-symmetry breaking rules out\nconventional s-, p-, and d-wave pairing and allows for unconventional pairings.\nGiven the limited current understanding, ferroelectric superconductivity may host\nentirely new phenomena, and we hope this work lays the foundations for future\nadvances.\nComment 16: In summary, while the experiments confirm the presence of\nferroelectricity at 300 K using SHG, PFM, and STEM, the manuscript does not\ndirectly show the presence of ferroelectricity in the superconducting state.\nResponse 16: We conducted comprehensive in situ experiments combining\nresistance measurements and SHG characterization at temperatures down to 2 K.\nAt 2 K, SHG signals from LAO/KTO(111) were detected concurrently with its\nsuperconducting transition, as detailed in Response 11. These results demonstrate\nthat ferroelectricity coexists with superconductivity. We trust that these extensive\nefforts address the Referee’s concerns.\nReviewer #3 (Remarks to the Author):\nComment 17: The manuscript discusses a putative coexistence of ferroelectricity\nand superconductivity at the LaAlO3/KTaO3(111) interface. The manuscript is clear\nand well-written, and if the claim is verified, it certainly merits publication in Nature\nCommunications, as it is at the forefront of current literature in the field of oxide\nheterointerfaces and related applications in electronics and materials science.\nResponse 17: We thank the referee very much for recognition of the value and\nimpact of our work.\nComment 18: However, I believe the claim is not adequately supported by the\nevidence presented in its current form. I am particularly concerned about the issue\nof oxygen vacancies (OxVac) at the interface, as noted by the authors too.\nWhile the superconducting state appears indisputable and is also supported by\nliterature (APL Mater. 11, 121108, 2023), the ferroelectric (FE) ordering is much\nmore subtle. It is well-known that FE measurements are elusive and prone to\nmisleading results (J. Phys.: Condens. Matter 20 (2008) 021001), and in my\nopinion, the three pieces of evidence provided by the authors (STEM, SHG, and\nPFM) are insufficient to make a definitive claim.\nResponse 18: We agree with the Referee that caution is warranted when claiming\nferroelectricity, and we appreciated the amusing reference provided—though we\nare sure that a banana could not produce the STEM, SHG, and PFM signals we’ve\nobserved!\nFollowing the Referee’s suggestions, we conducted supplementary experiments\nto examine alternative explanations of the results. Considering both the original\nand new data, we are confident that switchable ferroelectric order exists at the\nevidence is provided in Response 23.\nComment 19: The first strong evidence of FE ordering would be detecting a FE\nphase-transition critical temperature, which was not explored in this manuscript.\nAnother critical point is the switchability of polarization, claimed in the manuscript\nthrough PFM writing. However, regarding the LaAlO /SrTiO interface, Bark et al.\n3 3\nwrite (Nano Lett. 2012, 12, 1765−1771):\n\"The PFM response we have detected in LAO is very similar to that observed in\nferroelectric materials. Its most likely origin, however, is a spatial polarization\ninduced in the LAO layer due to the redistribution of oxygen vacancies.\"\nI suspect a similar phenomenon is occurring here. Indeed, other authors (App.\nPhys. Lett. 103, 062902, 2013) have used the stability of polarization contrast over\nseveral hours as proof of the true FE nature of the system:\n\"However, these results represent a necessary but not sufficient proof in favor of\nroom temperature ferroelectricity in our films. As discussed in Ref. 18, there are\ntwo possible alternative explanations: (1) simple charging of the surface and (2)\nionic displacements of the oxygen ions perpendicular to the surface in a direction\ndefined by the applied electric field. For this reason, we investigated the time\ndependence of the amplitude of the piezo-response signal, which in the case of\nferroelectric materials is proportional to the polarization.\"\nHere Ref. 18 is the aforementioned Nano Lett. paper.\nResponse 19: We thank the Referee for noting that the PFM writing might arise\nfrom the redistribution of oxygen vacancies and for suggesting a practical method\nto distinguish different mechanisms. Following these valuable suggestions, we\nperformed supplementary measurements of the time decay of domain contrast\nafter writing.\nwritten onto LAO/KTO(111) sample at time zero, and the PFM amplitude at the\ncenter square was monitored over time. A substantial PFM signal persists beyond\n24 hours, indicating a stable ferroelectric polarization. The corresponding PFM and\nEFM images after 10, 720, and 1440 minutes are shown in the upper and lower\ninset panels, respectively. While the PFM contrast remains with time, the EFM\ncontrast diminishes by 720 minutes, supporting the interpretation that the EFM\nsignal arises from surface charge or oxygen vacancy effects, whereas the PFM\nsignal originates from ferroelectricity.\nAs shown in Fig. R4, after writing concentric square patterns with +15 and -15 V,\nthe PFM amplitude decreases rapidly from 2.7 to 1.5 in the first 1 hour, then more\ngradually from 1.5 to 0.5 between 1 and 12 hours, after which it stabilized. PFM\nimages taken at 10, 720, and 1440 minutes (upper inset) corroborate these trends.\nIn stark contrast, EFM images acquired at the same time intervals (lower inset)\nshow rapid contrast decay, vanishing by 720 minutes (note the identical coloring\nof the central square and its surroundings at 720 and 1440 minutes). From these\nobservations and the reference cited by the Referee, we infer:\n1. The persistent PFM signal originates from ferroelectric polarization,\nreversible via tip gating.\n2. The decaying components of the PFM and EFM signals likely relate to\noxygen-vacancy redistribution.\nMore compelling evidence comes from time-decay measurements of SHG signals\nafter writing. The SHG images (Fig. R5) display clear contrast between regions\nwritten with +15 and -15 V applied to the PFM tip. With both the incident laser and\n13\nthe generated second harmonic light normal to the film surface, and their\npolarizations in-plane (Fig. 2a), the SHG signal is sensitive only to the in-plane\ncomponent of electric polarization. Since charge transfer between LAO and KTO\nproduces an out-of-plane electric polarization, the observed SHG contrast mainly\nreflects the in-plane component of ferroelectric order along [110] (Fig. 1). As the\nout-of-plane component was flipped using the tip gating, the in-plane component\nwas coupled with the out-of-plane component and was flipped concurrently,\nresulting in modified contrast in SHG images (Fig. R5a). Importantly, the SHG\ncontrast persists after 70 hours (Fig. R5b), consistent with the PFM results in Fig.\nR4. Together, these findings provide unambiguous evidence that LAO/KTO(111)\npossesses a switchable ferroelectric polarization.\nFigure R5 | SHG images after pattern writing with the PFM tip. Concentric\nsquares were written by applying +15 V and -15 V voltages on the PFM tip. SHG\nimages were acquired within 4 hours (a) and after 70 hours (b) of writing. The\npersistent contrast after 70 hours indicates a stable, switchable ferroelectric order\nin LAO/KTO(111).\nTo strengthen our case, we performed a comparative experiment on\nLaAlO /MgO(001) to verify that the LAO film itself, or its interface with another\n3\noxide, would not trivially produce the PFM and SGH signals observed in\nLAO/KTO(111).\nThe LAO/MgO(001) film was grown under identical conditions to the\nsuperconducting LAO/KTO(111) samples. Applying ±30 V on the PFM tip—well\nabove the voltage applied for LAO/KTO(111)—we wrote a concentric square\npattern on LAO/MgO.\nsample after writing a concentric square pattern with the PFM tip. No SHG\nsignal was detected, and the EFM contrast decayed within 48 hours (d), in sharp\ncontrast to that observed on LAO/KTO(111).\nComparison of the results for LAO/KTO(111) (Figs. R4 and R5) and LAO/MgO(001)\n(Fig. R6) leads to the following conclusions:\n1. PFM and EFM contrast appear in LAO/MgO but vanish within 48 hours,\nindicating that these signals likely arises from surface charging or oxygen-\nvacancy effects. The EFM signal of LAO/KTO also decays over time,\npresumably for the same reason. However, the time-stable component of\nthe LAO/KTO PFM signal points to the presence of switchable ferroelectric\npolarization.\n2. No SHG signal was detected from LAO/MgO, even immediately after\npattern writing, in sharp contrast to the stable SHG signal from LAO/KTO.\nThis indicates that surface charging or oxygen vacancies do NOT generate\nSHG signals, and that the SHG signal from LAO/KTO originates from\nferroelectric polarization.\nComment 20: According to the authors of this manuscript: \"It should be mentioned\nthat after the writing process, the effect of tip writing decays over time, quite\nsignificantly for Vtip < -10 V.\" Moreover, the authors acknowledge the significant\nrole of OxVac in their observations.\nThe issue is that both PFM and SHG signals are not exclusively related to FE\nordering; they can have various other sources. One possible way to investigate\nthis aspect is to anneal the sample in an oxygen-rich atmosphere (even simply in\nair) to significantly reduce the OxVac density, disentangling intrinsic and extrinsic\neffects, as reported in Appl. Phys. Lett. 104, 261603 (2014). For a genuine FE\nordering claim, the SHG signal should return to (almost) its initial state after the\n15\nannealing process.\nResponse 20: We followed the Referee’s suggestion and annealed the samples\nin air. While ramping the temperature from room temperature to 450°C, I\nSHG\nmanifests a pronounced peak around 300°C for both LAO/KTO(111) samples (Fig.\nR7), indicative of a change in the structure or chemical stoichiometry. This behavior\ncomplicates the data analysis and warrants further investigation. Nevertheless, the\nSHG signal survives the annealing process, albeit with reduced intensity, and the\nSHG symmetry remains unchanged upon returning to room temperature.\nWe are not surprised that the ferroelectric order varies with the concentration of\noxygen vacancies. As noted in the original manuscript, ferroelectricity at the\nLAO/KTO interface must be correlated with oxygen vacancies at the interface (Fig.\n1). It is also well established in the literature that oxygen vacancies can enhance\nferroelectric polarization or piezoelectricity in perovskite oxide films (e.g., see Sci.\nAdv. 8, eabm8550 (2022) and Science 375, 653 (2022)). Thus, it is consistent with\nexpectations that the strength of ferroelectric polarization changes with oxygen\nvacancy density. However, the key point here is that the observed SHG signal\noriginates from ferroelectric order, rather than directly from oxygen vacancies.\nUnlike the experimental setup used in the reference paper (Appl. Phys. Lett. 104,\n261603 (2014)), in our SHG optical configuration both the incident laser and the\ngenerated second-harmonic light are normal to the film surface, and the\npolarizations of both are in-plane (Fig. 2a). As a result, our SHG setup is sensitive\nonly to the in-plane component of electric polarization. The charge transfer\nbetween LAO and KTO, and the resultant electric polarization, are orientated along\nthe out-of-plane direction. Therefore, the SHG signal in our measurements is\nmainly contributed by the in-plane component of ferroelectric polarization along the\n[110] direction (Fig. 1). Furthermore, we have already demonstrated in Fig. R6 that\noxygen vacancies in LAO/MgO(001) generate NO detectable SHG signal.\nTherefore, the SHG signal of LAO/KTO(111) can be attributed to ferroelectric order.\nFigure R7 | SHG intensity of LAO/KTO(111) during air annealing for the\nsample #1 (a) and #2 (b). I manifests a pronounced peak during heating,\nSHG\nindicative of a change in the structure or chemical stoichiometry.\nComment 21: One minor but not negligible issue I see, is that the SHG signal of\nthe interface was compared to a pristine substrate. However, this is not the\nappropriate comparison since the pristine substrate likely has no, or negligible,\ninvestigated interface and a substrate subjected to the same pressure,\ntemperature, and time in the PLD chamber, but without the laser pulse to induce\nepitaxy. Alternatively, the comparison should be made with at least a doped\nsubstrate with a comparable OxVac density.\nResponse 21: Our aim in comparing the SHG signals from the LAO/KTO(111)\nsample and the KTO(111) substrate is to demonstrate that the KTO(111) substrate\nitself does not generate SHG signal (Fig. R8), and the measured SHG signal of\nLAO/KTO(111) therefore originates from the heterointerface. This comparison is\nnecessary because the inversion symmetry of KTO(111) is broken at its surface,\nwhich, in principle, could allows SHG to emerge.\nFollowing the Referee’s suggestion, and we carried out the corresponding\nexperiment. The KTO(111) substrate was heated to 300oC under conditions\nidentical to those used for LAO film deposition. The annealed KTO(111) substrate\nproduced only a weak SHG signal (Fig. R8). We also attempted to write patterns\nwith tip voltage ramped all the way up to ±50 V, but observed no contrast in either\nPFM or EFM images after writing, indicating that the ferroelectric polarization was\ntoo weak to be detected by PFM or EFM.\nAs proposed in the manuscript, oxygen vacancies at the LAO/KTO interface\npresumably affect the relative rotation of the oxygen octahedra, as well as the Ta-\nO-Ta bond length and angle, thereby inducing interfacial ferroelectricity. For the\nannealed KTO(111) substrate, the same mechanism may also operate: oxygen\nvacancies generated at the surface during annealing give rise to a weak\nferroelectric polarization. This is consistent with literature reports that oxygen\nvacancies can enhance ferroelectric polarization or piezoelectricity in perovskite\noxide films (e.g., see Sci. Adv. 8, eabm8550 (2022) and Science 375, 653 (2022)).\nFigure R8 | SHG signals for the KTO(111) substrate, annealed KTO(111)\nsubstrate, and LAO/KTO(111) sample. The wavelength of the laser is 1064 nm\nand the SHG light shall have a wavelength of 532 nm.\nComment 22: Finally, the issue of FE domain size: SHG and PFM have very\ndifferent spatial resolutions, with the former being much larger than the latter. It is\npossible to have domains too small to be seen by SHG but easily detected by PFM.\nConversely, if the domains are very large, they could exceed the total scanned\narea of PFM, but in this case, SHG should detect them. If they are absent in both\ntechniques, it is quite strange. How can a FE material have no spontaneous\ndomain structure? The idea of a single domain state forming spontaneously is quite\nunlikely, especially in 2D FE, where domain size is known to be of the same order\nof magnitude as the interface thickness. The PFM writing could be attributed to\nOxVac accumulation effects. The fact that the tip writing decays over time is a\nstrong indication that the signal results from ion migration rather than FE ordering.\nFinally, the hysteretic loops could also be due to trapping/untrapping mechanisms\nat the interface, modulating the interfacial electric field.\nResponse 22: As we’ve demonstrated, ferroelectric domains created by PFM tip\nwriting can be readily resolved by both PFM and SHG methods. Thus, the absence\nof ferroelectric domains is not due to any malfunction of the techniques employed.\nIf the ferroelectric domain size is close to the interface thickness as the Referee\nsuggested, then it would be about 1.9 nm, which is the thickness of the interfacial\nferroelectric phase determined by the iDPC-STEM measurements (Fig. 1). Given\nthe spatial resolution of PFM (~25 nm) and SHG microscopy (~1.5 μm), it’s\nunsurprising that domains of this size cannot be resolved by either method.\nHowever, from the images taken after pattern writing (Fig. 3), the unpatterned\noutside region appears to have the same contrast as the inner square patterned\nwith +10 V tip voltage. This likely implies that the pristine state is indeed a single\ndomain.\nWe are therefore inclined towards the interpretation proposed by the Referee 1\nthat an emergent electronic nematicity spontaneously breaks the in-plane\nrotational symmetry, rendering the three [110] directions at the (111) interface\ninequivalent. This provides a preferred direction for the ferroelectric order. Indeed,\nwe have previously discovered nematic superconductivity and nematic normal\nstate in LAO/KTO(111) [Cheng, X. B. et al. Electronic nematicity in interface\nsuperconducting LAO/KTO(111). Phys. Rev. X 15, 021018 (2025)]. Please refer to\nComment 3 and Response 3 for detailed discussion.\nFor discussions of oxygen vacancies and time decay, see Responses 18,19 and\n20.\nComment 23: In summary, I believe the manuscript cannot be accepted in its\npresent form, nor by merely changing its text. It requires further experimental work\nto confirm the proposed claim. However, I think the importance of the findings, if\nconfirmed, justifies the effort.\nResponse 23: Here, we briefly summarize the key experimental evidence\nsupporting a switchable ferroelectric order in LAO/KTO(111):\n1. STEM images reveal the K atom displacement relative to the Ta-O lattice\n(Fig. R9). This directly demonstrates that the centers of positive and\nnegative ions are displaced relative to each other—the defining\ncharacteristic of ferroelectricity.\n2. STEM images also show that ferroelectric order is coupled to oxygen\nvacancies at the interface. Consequently, flipping the ferroelectric\npolarization also redistributes oxygen vacancies, likely responsible for the\nrapid decaying component of the PFM amplitude and resistance over time.\n3. A substantial portion of the PFM amplitude and resistance change after\n18\nflipping is stable against time decay, indicating that it originates from\nferroelectric polarization.\n4. Due to the configuration of our SHG optics, the SHG signal is sensitive only\nto the in-plane component of ferroelectric polarization. Thus, the measured\nSHG signals and images directly reflect the ferroelectric order.\n5. Taken together STEM, PFM and SHG results confirm that ferroelectric\npolarization emerges at the LAO/KTO(111) heterointerface and it can be\nflipped by poling.\nWe have also revised our discussions on the mechanisms underlying modulations\nof conductivity and superconductivity to explicitly incorporate contributions from\nferroelectric order, oxygen vacancies, and disorder. We are grateful for the\nReferee’s suggestions, and we hope that with these improvements, which required\nsubstantial efforts, this work now meets the Referee’s high standards.\nFigure R9 | Projections of polar KTO lattice in the (1-10) (a), (11-2) (b), and\n(111) (c) planes according to STEM images. The purple solid line represents the\nangle-dependent SHG intensity I2ω(ϕ) at room temperature.\nx\nComment 24: Minor points:\n17 - \"we discover that ferroelectric order resides at the interface...\" -> \"we found\nevidence of a ferroelectric order at the interface...\"\n22 - Superconductivity cannot \"diminish and reappear\": the critical temperature can\nbe higher or lower with respect to a certain value, the material can undergo a phase\ntransition so that it is not superconductive, even if only for a short period of\ntime. This sentence is quite misleading in its present form.\n25 - \"STEM reveals K ions are displaced relative to\" -> \"STEM reveals the\ndisplacement of K ions relative to\"\n26 - \"electric\" -> \"ferroelectric\"\n28 - \"hysteresis change\" -> \"hysteretical change\"\n73 - \"postulated to be related\" -> \"attributed\"\n- Problem of beam damaging in STEM measures, how do they know when it is the\ncorrect time to stop?\n- 124: the SHG intensity is proportional to the square of the fundamental intensity\n(eq16 in Ref36).\n135-137: this statement is correct only if one che link the SHG signal to FE order\nalone. In case the SHG would be found to be of structural origin (i.e. similar on\noxvac samples with no FE order) then the statement is not correct.\nResponse 24: We thank the Referee for pointing out these issues and we have\n19\ncarefully incorporated all the recommended linguistic improvements in the revised\ntext.\nRegarding beam damage, it is a trial-and-error process. We monitored the STEM\nimage during imaging until we saw the damage occurred. Then, next time we\ncontrolled the electron dose to be below the threshold value.\nRegarding SHG signal, please refer to Response 11, 19, 20 and 21 for detailed\ndiscussions. We have included these experimental results and discussions in the\nrevised manuscript and Supplementary Information.\n20"}, {"round": 2, "reviewer_comments": {"reviewer_1": "I appreciate that the authors made comprehensive responses to my questions. After reading the updated version of manuscript, I found that the data presentation and corresponding discussions are sufficiently improved; the potential influence of oxygen vacancies has been carefully analyzed via the performance of supplementary experiments, which allows to distinguish the contributions from ferroelectric polarization and oxygen vacancy redistribution. This information is very helpful to the readers for understanding the mechanism of the PFM-tip manipulation process. Nevertheless, as I mentioned in the first round of review, microscale (or even nanoscale) charge writing and ferroelectric programming exploiting an AFM tip have been extensively realized in oxide interface systems. Modulation of superconductivity via creating ferroelastic domains has also been reported [for instance, Y.-Y. Pai, et al., Phys. Rev. Lett. 120, 147001 (2018)]. In the present work, the writing and erasing of non-superconducting region are achieved by the creation and elimination of ferroelectric domain with flipped polarization vector, respectively; such domains are characterized by reduced carrier density and boosted impurity scattering rate, indeed, they are most likely to be insulating owing to profound localization effect. As such, the control of superconductivity is actually engendered by introducing an insulating phase at the interface. This, in my opinion, can hardly be regarded as a significant breakthrough on the basis of existing results. I tend to suggest that this manuscript is suitable for a more specialized journal.", "reviewer_2": "While the authors have attempted to answer the questions that I asked during the first round of review, two major questions have not been addressed adequately, in my opinion.\nIssue 1: Regarding the coexistence of ferroelectricity at the superconducting phase: The authors have argued by showing the power dependence of SGH data at 2 K that the sample has polar distortion. However, the resistance at 2 K is 180 K, which clearly establishes that the sample is metallic, not superconducting. Thus, the author’s claim of the coexistence of ferroelectricity and superconductivity at the same temperature is not experimentally demonstrated.\nIssue 2: Role of charge trapping/detrapping under electric field: In spite of acknowledging that this is a possible scenario, the authors have not shown any experimental data to rule out this important possibility.\nOverall, I do not think that the reported experimental results directly confirm the author’s claim, and I do not recommend the publication of this in Nature Communications in its present form.", "reviewer_3": "The authors have undertaken a thorough and careful revision of their manuscript, incorporating not only substantial improvements to the text but also presenting several new and compelling experimental findings. Figure R7 (SHG vs T) stands out as particularly intriguing, displaying an unusual and noteworthy behavior. I strongly encourage the authors to further explore this phenomenon in future studies. Given the significant effort invested in strengthening their claims, I believe the paper merits publication in this journal. I also hope it will inspire further research into the role of OxVacs in this and related compounds."}, "author_response": "Point-by-point response to the referees’ comments.\nReviewer #1 (Remarks to the Author):\nComment 1: I appreciate that the authors made comprehensive responses to\nmy questions. After reading the updated version of manuscript, I found that the\ndata presentation and corresponding discussions are sufficiently improved; the\npotential influence of oxygen vacancies has been carefully analyzed via the\nperformance of supplementary experiments, which allows to distinguish the\ncontributions from ferroelectric polarization and oxygen vacancy redistribution.\nThis information is very helpful to the readers for understanding the mechanism\nof the PFM-tip manipulation process. Nevertheless, as I mentioned in the first\nround of review, microscale (or even nanoscale) charge writing and ferroelectric\nprogramming exploiting an AFM tip have been extensively realized in oxide\ninterface systems. Modulation of superconductivity via creating ferroelastic\ndomains has also been reported [for instance, Y.-Y. Pai, et al., Phys. Rev. Lett.\n120, 147001 (2018)]. In the present work, the writing and erasing of non-\nsuperconducting region are achieved by the creation and elimination of\nferroelectric domain with flipped polarization vector, respectively; such domains\nare characterized by reduced carrier density and boosted impurity scattering\nrate, indeed, they are most likely to be insulating owing to profound localization\neffect. As such, the control of superconductivity is actually engendered by\nintroducing an insulating phase at the interface. This, in my opinion, can hardly\nbe regarded as a significant breakthrough on the basis of existing results. I tend\nto suggest that this manuscript is suitable for a more specialized journal.\nResponse 1: We thank the referee for acknowledging the substantial efforts\nwe have devoted to addressing the referees’ comments and improving the\nmanuscript. However, with due respect, the referee appears to have\nmisunderstood the novelty and central focus of our work—points that we have\nconsistently emphasized in the cover letter, abstract, introduction, conclusion\nand response letter.\n1. Our primary discovery is the emergence of ferroelectric superconductivity\nat the interface of LAO/KTO(111). Examples of this novel superconducting state\nare exceedingly rare. To date, the only known candidates are Sr CaxTiO\n1-x 3-\nbulk [Ref. 34] and bilayer MoTe [Ref. 35]. The former discovery was published\n2\nin Nature Physics and the latter was in Nature, underscoring the scientific\nsignificance of this phenomenon and the community’s strong interest in it.\n2. The discovery of Interface Ferroelectricity itself represents an important\nbreakthrough that introduces an entirely new class of ferroelectric materials.\nWhile polar interfaces are not uncommon, their electric polarization is typically\nfixed in space. In stark contrast, the electric polarization at the LAO/KTO(111)\ninterface can be flipped by an external electric field, confirming that it is\ngenuinely ferroelectric. Since interfaces between distinct materials can be\n1\nengineered in innumerable combinations, the demonstration of interface\nferroelectricity at the interface of two oxides opens a vast space for discovering\nnew ferroelectrics.\n3. The technique referenced by the referee concerns only the experimental\nmethod used to demonstrate ferroelectric switchability. Although this method\nhas been applied in other contexts, its prior use dose not diminish the\nsignificance of the phenomena we uncover and therefore is irrelevant to the\nnovelty of our discoveries. The 2018 PRL paper mentioned by the referee\ninvestigates the one-dimensional nature of superconductivity at the LAO/STO\ninterface, interpreting observations in terms of ferroelastic domains. While we\nappreciate the referee drawing our attention to this work and have cited it in the\nrevised manuscript, its focus and conclusions are completely different from ours.\nNeedless to say, ferroelasticity and ferroelectricity are fundamentally different\nphenomena, and the PRL paper has anything to do with interface ferroelectricity\nor ferroelectric superconductivity.\n4. Our findings open exciting avenues for further research of fundamental\nimportance. Theoretically speaking, the breaking of inversion symmetry in a\nsuperconducting state can induce mixing between s, p or d-wave pairings,\npotentially giving rise to novel properties and effects. For ferroelectric\nsuperconductivity, little is known and our study represents an initial step\ntowards understanding this new class of superconductivity.\nAs recognized by the other referees, our discoveries are of broad interest to the\ngeneral readership. Furthermore, presentations of this work at international\nconferences have consistently generated strong enthusiasm within the\ncommunity. We sincerely hope that the referee will reconsider the significance\nof our findings in light of the above clarifications.\nReviewer #2 (Remarks to the Author):\nComment 2: While the authors have attempted to answer the questions that I\nasked during the first round of review, two major questions have not been\naddressed adequately, in my opinion.\nResponse 2: We have now performed additional experiments that directly\naddress both remaining concerns, providing compelling evidence to clarify\nthese issues. We have added these results to the Supplementary Information.\nComment 3: Regarding the coexistence of ferroelectricity at the\nsuperconducting phase: The authors have argued by showing the power\ndependence of SGH data at 2 K that the sample has polar distortion. However,\nthe resistance at 2 K is 180 K, which clearly establishes that the sample is\nmetallic, not superconducting. Thus, the author’s claim of the coexistence of\nferroelectricity and superconductivity at the same temperature is not\nexperimentally demonstrated.\nResponse 3: As explained in our previous response letter, performing SHG\nmeasurements at such a low temperature is technically challenging.\nFerroelectric order is expected to strengthen—not weaken—as temperature\ndecreases, owing to the suppression of thermal fluctuations. Indeed, Fig. 2d\nshows that ferroelectric polarization persists down to 2 K, leaving no reason to\nexpect its disappearance at even lower temperatures. Nevertheless,\nto unambiguously demonstrate the coexistence of superconductivity and\nferroelectricity, we devoted substantial effort to further lowering the base\ntemperature of our SHG measurement setup.\nTo reduce the heating from the laser beam, we replaced the picosecond laser\nwith a femtosecond laser and lowered the laser power to only 1.5 mW for the\nmeasurements shown in Fig. R1, compared with 83 mW in Figs. 2e and 2f.\nDespite this drastically reduced laser intensity, the SHG signal in Fig. R1\nremains significant. With these improvements, the base temperature of the\nLAO/KTO(111) film reaches 1.62 K—well below its superconducting\ntemperature. During the SHG measurements at 1.62 K, we simultaneously\nmonitored the resistance to ensure that the film stayed in the zero-resistance\nstate, thus ruling out any light-induced transition out of superconductivity.\nAs shown in Fig. R1a, the resistance of the LAO/KTO(111) sample drops to\nzero at approximately T = 1.75 K (the R(T) curve was measured both in a\ndilution refrigerator and in the cryostat equipped with SHG optics). Concurrently,\nan SHG signal is clearly detected at T = 1.62 K, when the sample is in the\nsuperconducting state. Here the wavelength of the incident laser is 1031±7 nm\nand a pronounced peak appears near 517 nm, corresponding to the SHG signal.\nThe SHG intensity, 𝐼 (𝑇) , remains roughly the same at elevated\n𝑆𝐻𝐺\ntemperatures (Figs. R1b and R1c), consistent with Fig. 2d. These results\nconfirm that ferroelectric polarization is robust from room temperature all the\nway down to the superconducting regime.\nFigure R1 | Low-temperature SHG experiments confirm the coexistence\nof ferroelectricity and superconductivity in LAO/KTO(111). a, Temperature\ndependence of the resistance R(T), showing the onset of zero-resistance at\n3\n~1.75 K. The arrows indicate the temperatures at which SHG measurements\nwere performed. b, With an incident laser wavelength of 1031± 7 nm, a\npronounced peak appears near 517 nm, corresponding to the SHG signal. SHG\nspectra acquired from T = 1.62 to 2.81 K provide clear evidence of ferroelectric\norder persisting within the superconducting state. c, Temperature dependence\nof the SHG intensity 𝐼 (𝑇), demonstrating that ferroelectricity remains robust\n𝑆𝐻𝐺\ndown to 1.62 K and thus coexists with superconductivity.\nComment 4: Role of charge trapping/detrapping under electric field: In spite of\nacknowledging that this is a possible scenario, the authors have not shown any\nexperimental data to rule out this important possibility.\nResponse 4: We have conducted new switching experiments that provide\nevidence against charge trapping as the dominant mechanism (Fig. R2). The\ncritical distinction lies in the hysteretic behavior: charge trapping or detrapping\nwould produce volatile, non-hysteretic resistance changes that track the applied\nvoltage instantaneously. In contrast, our data reveals a completely different,\nstrongly hysteretic behavior (Fig. R2):\n1. Stable bistates above the coercive voltage. Once the voltage applied\nto the PFM tip during scanning exceeds the coercive voltage, the\nLAO/KTO(111) film switches into one of two stable states—low or high\nresistance—depending solely on the voltage polarity (Fig. R2a).\n2. Non-volatile switching to the high-resistance state. Starting from the\nlow-resistance state at 0 V, stepwise ramping V down to -50 V drives\ntip\nthe film to the high-resistance state. When V is subsequently ramped\ntip\nback to 0 V, the resistance remains locked, demonstrating clear non-\nvolatile memory (Fig. R2c).\n3. Non-volatile switching to the low-resistance state. Starting from\neither a high (Fig. R2d) or low (Fig. R2f) resistance state at 0 V,\nincreasing V stepwise to +50 V forces the film into the low-resistance\ntip\nstate, which remains stable when V is returned to 0 V.\ntip\n4. Retention of intermediate states. When V is swept to a value\ntip\ncorresponding to an intermediate resistance state (Figs. R2b and R2e),\nreducing V back to 0 V preserves this intermediate state as well.\ntip\nThis pronounced hysteresis, robust retention, and non-volatile memory\nobserved here are hallmark signatures of ferroelectric polarization switching.\nCharge trapping, by contrast, would cause the resistance to relax upon field\nremoval—behavior absent in Fig. R2. These results therefore provide direct\nexperimental evidence that ferroelectric switching, rather than charge\ntrapping/detrapping, governs the resistance modulation.\nThe longitudinal resistance manifests clear hysteresis behavior in response to\nPFM tip writings at 300 K. b-f, The tip voltage is ramped from 0 V step-by-step\nto a designated value V and then ramped back to 0 V. The sequence of\nmax\nvoltage ramping is indicated by the arrows and numberings. The value of V\nmax\nfor panels b to f are indicated by the labels from “b” to “f” in panel a, respectively.\nApparently, the hysterical behavior of R(V ) is correlated with ferroelectric\ntip\nhysteresis, rather than charge trapping/detrapping.\nReviewer #3 (Remarks to the Author):\nComment 5: The authors have undertaken a thorough and careful revision of\ntheir manuscript, incorporating not only substantial improvements to the text but\nalso presenting several new and compelling experimental findings. Figure R7\n(SHG vs T) stands out as particularly intriguing, displaying an unusual and\nnoteworthy behavior. I strongly encourage the authors to further explore this\nphenomenon in future studies. Given the significant effort invested in\nstrengthening their claims, I believe the paper merits publication in this journal.\nI also hope it will inspire further research into the role of OxVacs in this and\nrelated compounds.\nResponse 5: We are delighted by the reviewer’s positive and supportive\nassessment of our revised manuscript. We fully agree that this is a fascinating\nphenomenon and are grateful for encouragements to explore it further.\n5"}, {"round": 3, "reviewer_comments": {"reviewer_2": "Based on the new experimental data provided and the comments of other reviewers, I recommed the publication of this manuscript. I appreciate the efforts of the authors in responding to the questions.", "reviewer_3": "In my view, the revised manuscript adequately addresses both my concerns and those of the other reviewers regarding the experimental validation of the paper’s claims. The work now appears sound and well-supported in its conclusions. That said, the issue raised by Reviewer 1 concerning the broader or narrower appeal of the topic is, to some extent, subjective. In my opinion, this matter should be decided by the editors rather than the reviewers, given that all three reviewers have accurately outlined the current state-of-the-art."}, "author_response": "Point-by-point response to the referees’ comments.\nReviewer #2 (Remarks to the Author):\nComment 1: Based on the new experimental data provided and the comments\nof other reviewers, I recommend the publication of this manuscript. I appreciate\nthe efforts of the authors in responding to the questions.\nResponse 1: We are grateful for the referee’s recommendation and\nconstructive suggestions during reviewing process.\nReviewer #3 (Remarks to the Author):\nComment 2: In my view, the revised manuscript adequately addresses both\nmy concerns and those of the other reviewers regarding the experimental\nvalidation of the paper’s claims. The work now appears sound and well-\nsupported in its conclusions. That said, the issue raised by Reviewer 1\nconcerning the broader or narrower appeal of the topic is, to some extent,\nsubjective. In my opinion, this matter should be decided by the editors rather\nthan the reviewers, given that all three reviewers have accurately outlined the\ncurrent state-of-the-art.\nResponse 2: We really appreciate the referee’s positive assessment and\nrecognition of the value of our work. It’s encouraging for us to keep working on\nthis important topic.\n1"}]

Decision letter