Enhanced superconductivity and coexisting ferroelectricity at oxide interfaces

Domain: physics
Published: 2025-12-01
Editorial outcome: minor_revision

Abstract
The coexistence of superconductivity and ferroelectricity is rare due to their conflicting requirements: superconductivity relies on free charge carriers, whereas ferroelectricity typically occurs in insulating systems. At LaAlO3/KTaO3 interfaces, we demonstrate the coexistence of two-dimensional superconductivity and ferroelectricity, enabled by the unique properties of KTaO3 as a quantum paraelectric. Systematic gating and poling experiments reveal an enhancement of the superconducting transition temperature (Tc) by ~0.2–0.6 K and bistable transport properties, including hysteresis, strongly suggesting the existence of switchable ferroelectric polarization in the interfacial conducting layer. Raman scattering measurements and hysteresis loops indicate robust ferroelectricity below 50 K. The Tc enhancement is attributed to ferroelectric polarization-induced reduction in dielectric constant, which narrows the interfacial potential well, confining carriers closer to the interface. The bistability arises from switchable ferroelectric polarization, which modulates the potential well depending on polarization direction. These findings establish a straightforward mechanism coupling ferroelectricity and superconductivity, providing a promising platform for exploring their interplay.

Introduction
The coexistence of superconductivity and ferroelectricity represents a long-standing challenge due to their fundamentally conflicting requirements. Superconductivity demands on a high density of free charge carriers, while ferroelectricity is generally found in insulating materials. The introduction of free charges into such materials screens long-range Coulomb interactions, suppressing ferroelectricity. Although the coexistence of free carriers and polarization has been proposed1 and observed2,3 in polar metals, including polar superconductors4,5, the polarization in these systems is non-switchable. Recently, however, superconductivity and ferroelectricity were observed to coexist in two-dimensional (2D) van der Waals heterostructures6, where ferroelectricity arises from mechanisms distinct from conventional Coulomb interactions.

Oxide interfaces, particularly those based on SrTiO3 (STO)7–9 and KTaO3 (KTO)10–15, offer a compelling alternative for exploring the interplay between superconductivity and polarization. Both STO and KTO are wide-gap semiconductors (3.2 eV for STO and 3.6 eV for KTO) and quantum paraelectric16,17. Electron-doped STO was the first oxide superconductor discovered18, and its proximity to ferroelectricity inspired the discovery of high-temperature cuprate superconductors19. Quantum ferroelectric fluctuations have been proposed as a potential pairing mechanism in these systems20–27, and a ferroelectric quantum phase transition was observed inside the superconducting dome of STO28. Additionally, when STO is combined with insulating oxides such as LaAlO3 (LAO), 2D superconductivity emerges at the interface8. In STO-based interfaces, superconducting-ferroelectric coexistence has been achieved via 18O substitution19, and ferroelectric-switchable 2D electron gases have been realized through Ca alloying29. Furthermore, the effects of ferroelectric/ferroelastic domains on both conductivity30,31 and superconducting pairing32 have been suggested. However, the full integration of superconductivity with electrically switchable ferroelectric polarization, despite being conceptually proposed29, has remained experimentally unrealized.

In contrast, while superconductivity has not been observed in electron-doped bulk KTO33, recent studies reveal that 2D superconductivity can emerge at KTO surfaces34,35 and interfaces11–13. Notably, KTO-based systems exhibit significantly higher superconducting transition temperature (Tc) than STO-based interfaces, making them more promising for exploring the interplay between ferroelectric polarization and superconductivity. Here, we demonstrate the coexistence of superconductivity and ferroelectricity at LAO/KTO interfaces, providing a novel platform for studying this unconventional phenomenon.

Results
Universally enhanced superconductivity and bistability
A noteworthy feature of KTO interface superconductivity is its tunability via gate bias (VG) across KTO13,36–38, as illustrated in Fig. 1a. Over the past several years, we have gated more than 100 LAO/KTO samples and identified a universal phenomenon: after gating experiments (where a VG of up to ±200 V was applied and subsequently removed, referred to as “poling”), the Tc of LAO/KTO exhibited a significant enhancement of ~0.2–0.6 K (see Fig. 1b for selected results). Additionally, the normal-state sheet resistance (Rs) exhibits a bistable characteristic after poling with different bias polarities (Fig. 1c), indicating the presence of ferroelectricity.

Ferroelectric hysteresis under gating cycles
To further investigate these phenomena, we examined the transport behaviors of a typical LAO/KTO (111) Hall bar device under continuous gating cycles. For each VG value, both the temperature dependence of R(T) and the Hall effect (measured at T=4.5 K) were recorded. Figure 2a shows the R(T) curve before any VG was applied (denoted as “origin”). A metallic behavior, followed by a superconducting transition at Tc = 1.89 K (defined as the temperature where Rs drops to 50% of its normal-state value at 4.5 K), was observed. Figures 2b, c display the R(T) curves during a cycle where VG was swept from +180 V to −180 V and then back. In both sweeping directions, a clear overall tuning effect consistent with previous studies13,38 was observed: a positive (negative) VG decreases (increases) the normal-state Rs and lowers (raises) Tc. However, in addition to this overall tuning effect, the R(T) curves exhibit a strong dependence on the gating history. For example, the three different VG = 0 states (“origin”, “0+”, and “0−”, where “0+” and “0−” represent the VG = 0 state after removing positive or negative VG, respectively) yielded significantly different R(T) curves.

As summarized in Fig. 2d–f, two notable and correlated features emerge beyond the overall tuning effect. First, during the initial run starting from the “origin” state (indicated by the blue lines), the device undergoes an irreversible change likely associated with polarization formation, corresponding to the universal Tc enhancement. Second, after the initial run, the device exhibits repeatable and pronounced hysteresis in both VG-Rs (Fig. 2d) and VG-Tc (Fig. 2e) loops. Analysis of the VG-Rs loops at different temperatures, measured during both decreasing (Fig. 2g) and increasing (Fig. 2h) temperature orders, reveals that hysteresis begins above 50 K and becomes pronounced below 30 K. This temperature-dependent behavior closely matches previous observations39–41 of weak spontaneous polarization (0.04 μC/cm−2) in KTO, where defect-induced polar nanoregions develop macroscopic coherence within this specific temperature range.

Effects of poling magnitude, temperature, and time
Further investigations of the 0± states after poling with VG (denoted as V0±) were conducted to gain deeper insights into the observed ferroelectric behaviors. Since LAO/KTO is sensitive to gating history, we ensured an “origin” state in different experiments by using fresh samples when necessary. Figure 3a–d show the effects of poling VG polarity and magnitude. Two identical Hall bar devices (S2_A# and S2_B#) cut from the same LAO/KTO sample were used. Similar experiments were also conducted on a single device (S7#, Supplementary Fig. 1), whose “origin” state was regenerated through a refreshing process (described below).

For each polarity, the VG was gradually increased from 0 to ±180 V. At each VG, we applied the poling VG at 4.5 K for 3 min, followed by R(T) and Hall effect measurements after removing VG to 0. As shown in Fig. 3a, b, and Supplementary Fig. 1a, the Tc increases with the magnitude of poling |VG| for both polarities, and Tc for “0−” is consistently higher than for “0+”. Poling up to ±180 V, corresponding to ±3.6 kV/cm, did not saturate Tc enhancement, suggesting that the ferroelectric polarization remains unsaturated. As shown in Fig. 3c, d, and Supplementary Fig. 1b, the increase in Tc was accompanied by a clear decrease in mobility μ (with a slight increase in carrier density ns; however, the change in ns was much smaller than that of μ). This indicates that, as discussed below, the ferroelectric polarization primarily modulates the interfacial potential well rather than directly altering ns. It should be noted that the transport coefficients, ns and μ, derived from Hall measurements on Hall bar devices, represent spatially weighted averages of all contributing layers, as these parameters are inherently depth-dependent along the z-direction.

Poling temperature also plays a critical role. As shown in Fig. 3e, Tc begins to increase sharply when poling temperatures dropped below 30 K, matching the temperature at which VG-Rs hysteresis becomes pronounced (Fig. 2g, h). Notably, significant Tc enhancement was observed after just one sec of poling, with saturation occurring after 10–15 sec of cumulative poling time (Fig. 3f and Supplementary Fig. 2). The tuned states remain nonvolatile at low temperatures (Fig. 3g). Full recovery to the “origin” state can be achieved by leaving the samples at ambient conditions for several days (Fig. 3h and Supplementary Fig. 3).

Induced ferroelectricity in KTO
All these experimental observations support that ferroelectricity emerges in the KTO side of LAO/KTO interfaces. This is unsurprising, as ferroelectricity has previously been suggested in KTO through minimal chemical doping (e.g., Nb, Li)42, oxygen deficiency43, strain44, or randomly distributed defects40,45,46. Notably, even nominally pure KTO samples typically contain substantial defect concentrations (~1017 cm3). These defects promote the formation of polar nanoregions, which collectively drive KTO into a weak ferroelectric state at low temperatures40,46. Further evidence for low-temperature ferroelectricity in LAO/KTO comes from our Raman scattering measurements (Supplementary Note 1 and Supplementary Fig. 4), where the TO2 and TO4 optical modes emerge below ~40 K (~60 K in poled samples) – closely coinciding with the onset of hysteresis in the VG-Rs loops (Fig. 2g, h). The similarities between the observed hysteresis and Raman features in LAO/KTO and those in the intentionally ferroelectric Al0.99Sr0.01TiO3 interface29 provide additional support for ferroelectricity in LAO/KTO. We attribute the induced ferroelectricity to a field-induced ordering of the polar nanoregions that are present in KTO, which are inherently disordered and lacking long-range correlation in the pristine state.

While VG-induced irreversibility in Rs has been previously observed in SrTiO3-based heterostructures and attributed to charge trapping/detrapping effects47–49, the behavior we observe in LAO/KTO is fundamentally distinct. Our system exhibits reproducible and switchable hysteresis that is independent of the initial VG sweep direction – a hallmark of ferroelectricity (see Supplementary Fig. 5). Moreover, the limited magnitude of carrier density modulation in LAO/KTO (Supplementary Fig. 6) is incompatible with charge trapping, which would require significantly larger ns variations47,48. Although ionic migration could theoretically produce switchable polarization, this process is typically slow and thermally activated50–52, conflicting with our low-temperature results. Taken together, the observed hysteresis and switching behaviors in LAO/KTO cannot be explained by charge trapping or ionic migration, providing compelling evidence for ferroelectricity as the governing mechanism.

Coexistence of ferroelectricity and superconductivity in the conducting layer
As illustrated in Fig. 1a, superconductivity at KTO interfaces is confined within a thin KTO layer (~5–10 nm thick11–13,38), corresponding to the width of the interfacial potential well. The remaining KTO bulk serves as a thick insulator across which VG is applied. While ferroelectric polarization is expected in the insulating bulk, we propose that it also occurs in the conducting layer. If polarization were confined solely to the insulating bulk, as in conventional ferroelectric transistors, its effects would primarily manifest in ns rather than μ, inconsistent with our observations. Moreover, the bistability in transport properties suggests a switchable polarization within the conducting layer, whose built-in field modulates the interfacial potential well. These results lead us to conclude that ferroelectric polarization coexists with 2D superconductivity in the interfacial conducting layer. Although ferroelectricity and conductivity are traditionally considered mutually exclusive, polarization in the conducting KTO layer could emerge via coupling with the ferroelectric KTO bulk, mediated by their shared lattice bonding, while potentially developing distinct interfacial behavior.

Ferroelectric polarization modulating the interfacial potential well
We now address how the presence of ferroelectricity in the conducting KTO layer can explain our experimental observations. Before delving into the specific phenomena associated with ferroelectricity, we reinforce that the transport properties of LAO/KTO are largely governed by the interfacial potential well profile, with μ serving as a key indicator. Previous studies13,36,38 have shown that, particularly when ns is relatively large, gating at KTO interfaces primarily modulates μ rather than ns. As shown in Fig. 2f and Supplementary Fig. 6a, sweeping VG from −180 V to 180 V caused μ to vary from ~20 cm²V⁻¹s⁻¹ to ~90 cm²V⁻¹s⁻¹, while ns changed only slightly (and in this device, in a manner opposite to that expected from a simple capacitance effect. See Supplementary Figs. 6b-d for more information). This behavior can be attributed to VG-induced modulation of the interfacial potential well, which alters the spatial distribution of carriers13,53,54. Such modulation influences the “effective disorder”, thereby affecting μ (a narrower potential well reduces μ)13,53,54.

Notably, the hysteresis observed in VG-Rs (Fig. 2d) and VG-Tc (Fig. 2e) loops is mirrored in VG-μ loops (Fig. 2f) but not in VG-ns loops (Supplementary Fig. 6a). Furthermore, the universal Tc enhancement is accompanied by a significant decrease in μ (Figs. 2f, 3c, d, and Supplementary Fig. 1b), which can be attributed to narrowing of the interfacial potential well13,36,38. Therefore, in LAO/KTO interfaces, unlike typical ferroelectric effects, ferroelectricity primarily affects transport by modulating the interfacial potential well profile rather than directly altering ns. We note that the enhancement of Tc with potential well narrowing is a well-established experimental phenomenon in KTO-based interfaces13,38, yet the underlying mechanism remains elusive. Several potential mechanisms may be at play: (i) fractal superconductivity induced by strong scattering55, (ii) enhanced spin-orbit coupling energy38, (iii) increased local three-dimensional carrier density, and (iv) modifications to electron-phonon coupling56. Further studies are needed to discern which of these mechanisms, if any, contribute to the observed enhancement.

Ferroelectric polarization in the interfacial KTO layer has two key effects: (1) it reduces the dielectric constant (ε) (Fig. 4a) and (2) introduces a built-in electric field due to polarization and the associated screening charges (insets of Fig. 4b). KTO, as an quantum paraelectric, exhibits a large ε of up to 4500 at low temperatures17. However, ε decreases dramatically under applied fields57–59 and induced polarization39,41. As illustrated in Fig. 4a, after poling, ferroelectric polarization, regardless of its direction, lowers ε39,41, thereby narrowing potential well (solid blue and red lines vs. dashed line, Fig. 4b), which explains the universal Tc enhancement. Bistability naturally arises from switchable polarization, which can either narrow or widen the potential well depending on its direction. As shown in Fig. 4b, the built-in electric field (Ebuilt-in) forming the potential well results from the superposition of two competing contributions: EP from ferroelectric polarization (opposing the gate field direction) and ES from surface screening charges. After VG removal, the persistent residual field – dominated by ES which opposes and exceeds EP – creates two distinct states: (1) In the “0−” state, the net field Ebuilt-in = EP + ES narrows the potential well, enhancing carrier confinement and increasing Tc; (2) conversely, in the “0+” state, the net field broadens the well, reducing Tc.

As shown in the VG = 0 region of Fig. 2e, the Tc enhancement after initial run is over 3 times larger than the Tc difference between the “0+” and “0−” states in subsequent runs. This indicates that the reduction in ε due to polarization has a stronger influence on the potential well profile than built-in field of the polarization alone. The inability to fully recover the origin state by warming the samples above the ferroelectric onset temperature (even to room temperature; see Supplementary Fig. 3a) is likely due to unbalanced screening charges60,61 at the insulating LAO surface persisting in the cryostat environment. Upon cooling, these charges act as a poling field, reinducing polarization.

Our work demonstrates the coexistence of superconductivity and ferroelectricity at LAO/KTO interfaces, offering a unique platform for studying their interplay. The switchable polarization modulates the interfacial potential well by reducing ε and altering built-in field, driving Tc enhancement and bistable transport properties. These findings deepen our understanding of the mechanisms coupling superconductivity and ferroelectricity, and open new possibilities for designing multifunctional quantum devices. Prospectively, the doping tunable ferroelectricity in KTO42 may enable LAO/KTO heterostructures to serve as a programmable quantum platform62,63 for controlling superconducting-ferroelectric coupled states.

Methods
Hall bar devices
Hall bar structures were patterned onto KTO substrates using standard optical lithography and lift-off techniques, with ~200 nm thick AlOx films serving as a hard mask35. The AlOx films were deposited by PLD at room temperature under base pressure, with a laser fluence of 2.5 J/cm2. To ensure high insulation in the AlOx-covered areas, the patterned substrates were annealed at 300 °C for 2 h in a flow of 1 bar O2. Subsequently, LAO films were deposited onto these pre-patterned KTO substrates, forming conducting LAO/KTO interfaces exclusively in the uncovered regions. Each sample contained four identical Hall bar devices. Across different samples, the central Hall bar bridges were fabricated in two sizes: 20 μm in width and 100 μm in length, or 100 μm in width and 500 μm in length.

Electrical contact
Electrical contacts to the conducting LAO/KTO interfaces were established using ultrasonic bonding with Al wires.

Gating and poling
As illustrated in Fig. 1a, a back-gating voltage (VG) was applied between the conducting interface and the bottom silver electrode, with the polarity defined relative to the interface. The value of VG represents the bias applied to the bottom silver electrode. Throughout the gating process, the leakage current was consistently below 10 nA.

The process of applying VG and subsequently setting it to 0 is referred to as “poling”, similar to operations in conventional ferroelectrics. The states “0+” and “0−” represent the VG = 0 state after removing positive and negative VG, respectively. For clarity, we also denote the VG = 0 state after poling with VG as V0±. For example, “1200+” indicates the VG = 0 state following an applied VG = +120 V. This nomenclature is extended to all cases accordingly.

Transport measurements
Low-temperature transport measurements were performed using a commercial 4He cryostat equipped with a 3He insert (Cryogenic Ltd.). A four-probe DC technique was employed, utilizing a Keithley 6221 current source and a Keithley 2182A nanovoltmeter. Carrier mobility (μ) and density (ns) were obtained from Hall effect measurements conducted on Hall bar devices.

Raman spectroscopy
The low-temperature Raman measurements were performed in a cryostat (Attocube, attoDRY2100) equipped with a commercial confocal microscope (Horiba, LabRam Odyssey). The sample temperature was controlled in the range of 1.6 K to 300 K. A linearly polarized 532 nm laser was used as the excitation source, with a ∼1 μm beam radius and ∼3.6 mW power at the sample position.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript by Zhang et al. delves into two unusual phenomena occurring during the gate-voltage tuning process in the\nLaAlO3/KTaO3(111) (LAO/KTO) heterostructure: (i) After cycles of gating and poling, the superconducting transition\ntemperature T constantly becomes higher than the original value. (ii) The normal-state resistance changes accordingly in a\nc\nbistable manner that relies on the polarity of voltage. The authors attribute these observations to the gate-voltage\nmanipulated ferroelectricity of KTO near the interface, which determines the width of electron wavefunction distribution for\nthe interfacial 2D electron gas (2DEG).\nI found this manuscript to be of sufficiently high quality. The analysis and discussion are well-reasoned, supplied by an\nimpressive amount of experimental data. Nonetheless, I hope the author can provide some reassurances about the general\ninterest of these results. Otherwise, I still hesitate to recommend publication.\nIt has been repeatedly stressed that the controllable ferroelectric/ferroelastic properties of SrTiO3 (STO) [e.g., B. Kalisky, et\nal. Nat. Mater. 12, 1091 (2013); C. Cen, et al. Nat. Mater. 7, 298 (2008); Y.-Y. Pai, et al. Phys. Rev. Lett. 120, 147001 (2018);\nM. Yu, et al. Phys. Rev. X 15, 011037 (2025)] and KTO [M. Yu, et al., Nano Lett. 22, 6062-6068 (2022)] have significant\nimpact on the behavior of interfacial 2DEG, especially the superconducting pairing. Likewise, it is also well known that the\ninterfacial superconductivity can be effectively tuned by changing the width of interfacial potential well (Refs. [13], [48]). The\npresent work establishes a link between these two understandings. Such a link is, however, not very surprising or\nunexpected; at this stage, I am not sure if it represents a substantial achievement in this field.\nThere are several further questions that probably lead to new insights. First, I wonder what is the underlying mechanism for\nthe dependence of superconducting properties (T , superfluid density, etc) on the width of interfacial potential well (or depth\nc\nof the electron wavefunction's extension from interface to the KTO side). Why the superconductivity changes substantially ---\nbecomes more “unconventional” with an increase of T --- upon squeezing the itinerant electrons towards the interface?\nc\nSince the width is supposed to be a few nanometers, it cannot be a simple dimension effect. Then what indeed matters? Is it\nthe spin-orbit coupling effect at the interface that serves as the crucial factor? Clarification of this point is of primary\nimportance.\nMy second question concerns the statement that ferroelectric polarization and metallic conductivity (as well as\nsuperconductivity) coexist in the top layers of KTO in proximity to the interface. It undoubtedly means that the carrier density\nis not uniform in the conducting layer, but develops a gradient along the normal direction of the interface. In this sense, how\nmeaningful are the parameters such as Hall carrier density n and Hall mobility μ ? Indeed, these parameters can be\ns H\nfunctions of the distance z from the interface and are thereby spatially varying; the high mobility can potentially stem from the\ncarriers farther from the interfacial scattering centers, yet this does not mean the absence of slower electrons in the vicinity of\ninterface (they are simply short-circuited by their faster counterparts in deeper positions). Consequently, the interfacial\nsuperconductivity may also be nonuniform and z-dependent? What is the best picture describing the superconducting\nsystem in the oxide heterointerfaces?\nI am looking forward to seeing the authors' reply to my questions and my final evaluation would be made based on those\nresponses.\nA relatively minor point: for the difference between 0+ and 0- states, seems the interpretations are mainly presented in the\ncaption of Fig. 4b; the main text contains very limited contents of corresponding discussion. Probably this can be balanced to\nsome extent.", "reviewer_2": "The manuscript claimed about the coexistence of superconductivity and ferroelectricity in KTaO3 (111) based\nheterostructure. The authors further attribute the enhancement of superconducting Tc to the ferroelectric polarization induced\nreduction in dielectric constants. While the authors have demonstrated superconductivity, there is no direct experimental\nevidence about ferroelectricity. All claims about ferroelectricity are rather speculative, which may be true or may not be true as\nalternative scenario can also be attributed to the experimental claims.\nAnother major issue I found that the authors have completely ignored the issue of charge trapping/detrapping under\nelectrical gating. This has been demonstrated extensively in context of SrTiO3 based heterostructrues [Sci. Rep. 4, 6788\n(2014); Phys. Rev. Lett. 124, 017702 (2020); Phys. Rev. Applied 15, 054008 (2021)]. Such charge trapping phenomena also\ngives rise to hysteresis in resistance vs. gate voltage measurement. Similar observations have been reported in Fig. 2 of this\nmanuscript but attributed to the ferroelectricity. The authors need to demonstrate that their observations are not linked with\ncharge trapping phenomena.\nBecause of these two significant issues in the manuscript, I do not recommend the publication of this paper in Nature\nCommunications."}, "author_response": "Dear Reviewers,\nThank you for your thoughtful feedback on our manuscript. Your comments have been\ninvaluable in strengthening this work. We have carefully addressed all points through\nadditional experiments, enhanced discussions, and textual revisions.\nThis response letter is organized as follows:\n• Part A: Summary of major revisions\n• Part B: Point-by-point responses to reviewer comments\nAll changes are highlighted in the revised manuscript for ease of review. We appreciate\nthe opportunity to improve this work and are happy to provide further clarification if\nneeded.\nSincerely,\nYanwu Xie and Meng Zhang\nOn behalf of all the authors\n1. Divided the original manuscript into the main text and Supplementary Information.\n2. Added Prof. Yi Zhou (theoretical physicist) to the author list for his substantial\ncontribution in revising the manuscript and addressing reviewer comments,\nparticularly regarding the mechanism of T enhancement with the potential well\nc\nnarrowing.\n3. Revised the Introduction and Conclusion sections to better highlight the\nsignificance of this work.\n4. Added Raman spectroscopy measurements (Supplementary Note 1 and\nSupplementary Fig. 4), with corresponding revisions to the main text and Methods\nsection.\n5. Performed additional experiments tracking the evolution of R with V under\ns G\ncontinuous gate voltage sweeps (shown in Supplementary Fig. 5).\n6. Substantially revised the “Induced ferroelectricity in KTO” section (Pages 5&6) to\nstrengthen evidence for ferroelectricity while explicitly ruling out charge trapping\nas the primary mechanism.\n7. Revised the concluding sentence of the “Coexistence of ferroelectricity and\nsuperconductivity in the conducting layer” section (Page 6) to clarify the\nrelationship between interfacial and bulk ferroelectricity.\n8. Added a discussion paragraph (Page 7) exploring potential mechanisms underlying\nT enhancement induced by potential well narrowing.\nc\n9. Reallocated the 0+/0- state difference interpretation between the main text and Fig.\n4b legend.\n10. Updated references, with the following additions (in the main text):\n[1] Kalisky, B. et al. Nat. Mater. 12, 1091-1095 (2013)\n[2] Cen, C. et al. Nat. Mater. 7, 298–302 (2008)\n[3] Pai, Y.-Y. et al. Phys. Rev. Lett. 120, 147001 (2018)\n[4] Biscaras, J. et al. Sci. Rep. 4, 6788 (2014)\n[5] Yin, C. et al. Phys. Rev. Lett. 124, 017702 (2020)\n[6] Ojha, S. K. et al. Phys. Rev. Appl. 15, 054008 (2021)\n[7] Feigel'man, M. V. et al. Ann. Phys. 325, 1390–1478 (2010)\n[8] Chen, X. et al. Nat. Commun. 15, 7704 (2024)\n[9] Yu, M. et al. Nano Lett. 22, 6062–6068 (2022)\n[10] Yu, M. et al. Phys. Rev. X 15, 011037 (2025)\nThis manuscript by Zhang et al. delves into two unusual phenomena occurring during\nthe gate-voltage tuning process in the LaAlO /KTaO (111) (LAO/KTO) heterostructure:\n3 3\n(i) After cycles of gating and poling, the superconducting transition temperature\nT constantly becomes higher than the original value. (ii) The normal-state resistance\nc\nchanges accordingly in a bistable manner that relies on the polarity of voltage. The\nauthors attribute these observations to the gate-voltage manipulated ferroelectricity of\nKTO near the interface, which determines the width of electron wavefunction\ndistribution for the interfacial 2D electron gas (2DEG).\nReply: We sincerely thank the reviewer for their careful reading and insightful\nsummary of our work.\nI found this manuscript to be of sufficiently high quality. The analysis and discussion\nare well-reasoned, supplied by an impressive amount of experimental data. Nonetheless,\nI hope the author can provide some reassurances about the general interest of these\nresults. Otherwise, I still hesitate to recommend publication.\nReply: We sincerely thank the reviewer for their positive assessment of our\nmanuscript’s quality and experimental rigor. Below we address the broader significance\nof these findings (additional details are provided in our response to the reviewer’s\nsubsequent comment):\n(1) Scientific novelty:\nOur work reports two unprecedented phenomena in oxide interfaces:\n• First demonstration of electric-field-controlled non-volatile T\nc\nenhancement\n• First observation of polarity-switchable bistable states with coupled\nsuperconducting properties.\n(2) Potential impact:\n(i) Fundamental advance\nWhile superconductor-ferroelectric coexistence is uncommon, electrically\nswitchable systems are exceptionally rare (the only comparable reports\ninvolve non-oxide systems, Ref. [6]). Our findings establish LAO/KTO as a\npromising platform for studying intrinsic superconductor-ferroelectric coupled\nstates.\n(ii) Tunability prospects:\nBeyond impurity-induced ferroelectricity, KTO exhibits doping-tunable\npolarization (Ref. [42]), suggesting that the superconducting-ferroelectric\ncoupling in LAO/KTO heterostructures can be systematically controlled.\nthe introduction and conclusion sections to better articulate these scientific implications.\nIntroduction (revised, Page 2):\n“In STO-based interfaces, superconducting-ferroelectric coexistence has been achieved\nvia 18O substitution19, and ferroelectric-switchable 2D electron gases have been realized\nthrough Ca alloying29. Furthermore, the effects of ferroelectric/ferroelastic domains on\nboth conductivity30,31 and superconducting pairing32 have been suggested. However,\nthe full integration of superconductivity with electrically switchable ferroelectric\npolarization, despite being conceptually proposed29, has remained experimentally\nunrealized.’’\nConclusion (added, Page 9):\n“Prospectively, the doping tunable ferroelectricity in KTO42 may enable LAO/KTO\nheterostructures to serve as a programmable quantum platform62,63 for controlling\nsuperconducting-ferroelectric coupled states.”\nIt has been repeatedly stressed that the controllable ferroelectric/ferroelastic properties\nof SrTiO (STO) [e.g., B. Kalisky, et al. Nat. Mater. 12, 1091 (2013); C. Cen, et al. Nat.\nPhys. Rev. X 15, 011037 (2025)] and KTO [M. Yu, et al., Nano Lett. 22, 6062-6068 (2022)] have significant\nimpact on the behavior of interfacial 2DEG, especially the superconducting pairing. Likewise, it is also well known that the\ninterfacial superconductivity can be effectively tuned by changing the width of interfacial potential\nwell (Refs. [13], [48]). The present work establishes a link between these two\nunderstandings. Such a link is, however, not very surprising or unexpected; at this stage,\nI am not sure if it represents a substantial achievement in this field.\nReply: We thank the reviewer for recognizing how our work bridges the understanding\nof ferroelectricity and superconductivity in oxide interfaces. Below we further clarify\nits significance:\n(1) First demonstration of ferroelectric-switched superconductivity\nThe quantum paraelectric nature of STO and KTO indeed suggests ferroelectric\nfluctuations as a mechanism for superconducting pairing, making their coexistence an\nimportant research focus. As the reviewer pointed out, previous studies have suggested\nthat the tunable ferroelectric/ferroelastic domains in STO may influence interfacial\nconductive behavior, potentially offering insights into superconducting pairing\nmechanisms. However, the experimental realization of electrically\ncontrolled superconducting states via ferroelectric switching has remained elusive due\nto two critical gaps:\n(i) Coexistence without control: While STO-based systems have shown\nferroelectricity/superconductivity coexistence (Refs. [19,28]), the essential\nfunctionality of electrically switching superconductivity via ferroelectric\npolarization has never been achieved.\n4\n(ii) Switching without superconductivity: While STO interfaces have\ndemonstrated ferroelectric-switchable conduction (Ref. [29]; Noël\net al., Nature 580, 483–486 (2020)), these studies either (a) did not achieve\nsuperconducting states or (b) did not explore the superconducting regime.\nOur work bridges these gaps by providing the first experimental demonstration of\npolarity-switchable superconductivity in oxide interfaces.\n(2) Further justification of significance\nAchieving ferroelectric-controlled superconductivity is extremely challenging and\nrequires overcoming non-trivial material constraints:\n(i) STO’s inherent limitations: The remanent polarization (~1 μC/cm² = 6×10¹²\ncm⁻²) (Ref. [29]; Noël et al., Nature 580, 483-486 (2020)) is significantly\nsmaller than the required superconducting carrier density. This mismatch,\ncombined with STO’s low T , makes switchable control very challenging.\nc\nThus, while previous studies in STO-based interfaces have revealed key\naspects of ferroelectricity and superconductivity, achieving ferroelectric-\ncontrolled superconductivity in STO-based interfaces remains elusive.\n(ii) KTO’s advantages. In contrast, KTO interfaces exhibit both higher T and\nc\na distinct tuning mechanism primarily governed by interfacial potential\nmodulation rather than carrier density. These intrinsic properties enable the\nswitchable ferroelectric control reported here.\n(iii) Wider significance: Across all known condensed matter systems,\nelectrically switchable ferroelectric control of superconductivity remains\nexceptionally rare - highlighting both the fundamental challenge and the\nimportance of our results.\nRevision summary: To address the reviewer’s comment, we have modified the\nintroduction in the revised manuscript (Page 2) as follows:\n“In STO-based interfaces, superconducting-ferroelectric coexistence has been achieved\nvia 18O substitution19, and ferroelectric-switchable 2D electron gases have been realized\nthrough Ca alloying29. Furthermore, the effects of ferroelectric/ferroelastic domains on\nboth conductivity30,31 and superconducting pairing32 have been suggested. However,\nthe full integration of superconductivity with electrically switchable ferroelectric\npolarization, despite being conceptually proposed29, has remained experimentally\nunrealized.’’\nThere are several further questions that probably lead to new insights. First, I wonder\nwhat is the underlying mechanism for the dependence of superconducting properties\n(T , superfluid density, etc) on the width of interfacial potential well (or depth of the\nc\nelectron wavefunction's extension from interface to the KTO side). Why the\nsuperconductivity changes substantially --- becomes more “unconventional” with an\nincrease of T --- upon squeezing the itinerant electrons towards the interface?\nc\nSince the width is supposed to be a few nanometers, it cannot be a simple dimension effect.\nas the crucial factor? Clarification of this point is of primary importance.\nReply: We thank the reviewer for raising this critical question regarding the mechanism\nlinking interfacial potential well width (i.e., carrier confinement) to unconventional\nsuperconductivity enhancement in LAO/KTO interfaces. While it is still an open\nquestion in the field, our manuscript attributes the phenomenon primarily\nto ferroelectricity-induced reduction in dielectric constant, which modulates the\npotential well profile and carrier distribution. Below we clarify the key points raised:\n(1) Mechanism for T enhancement and unconventionality:\nc\n(i) As detailed in Sections \"Ferroelectric polarization modulating the interfacial\npotential well\" (Page 7) and Fig. 4, ferroelectric polarization in the interfacial KTO\nlayer reduces the dielectric constant (ε). This reduction occurs because polarization\nsuppresses quantum paraelectric fluctuations inherent to bulk KTO (Refs. [39, 41,\n57-59]).\n(ii) A lower ε directly narrows the interfacial potential well (Fig. 4b), confining the\nelectron carriers closer to the interface. Crucially, this confinement increases\ncarrier scattering (evidenced by decreased mobility μ, Figs. 2f, 3c-d,\nSupplementary Fig. 1b) but paradoxically enhances T .\nc\n(iii) This counterintuitive result — where increased disorder (lower μ) correlates\nwith higher T — signals a departure from conventional disorder-sensitive\nc\nsuperconductivity.\n(iv) This seeming paradox can be resolved via the fractal superconductivity\nscenario: In disordered systems, fractal geometry can enhance superconducting T\nc\nby (a) creating localized superconducting \"puddles\" with higher local T , and (b)\nc\nenabling percolative phase coherence. [See the famous paper: “Fractal\nsuperconductivity near localization threshold” M.V. Feigel'man, L.B. Ioffe, V.E.\nKravtsov, and E. Cuevas, Annals of Physics 325 (2010) 1390-1478]\n(v) We note that the percolative nature of polar nanodomains in KTO (Refs. [40, 46])\nmay lead to the formation of fractal-like superconducting pathways.\n(2) Role of potential well width vs. simple dimensionality:\n(i) The reviewer correctly notes that the well width (~5-10 nm) is too large for pure\n2D quantum size effects to dominate T enhancement. Our data support that it is\nc\nnot the width per se, but the change in the electrostatic environment (specifically\nε reduction) caused by ferroelectricity that is key.\n(ii) The evidence lies in the correlation between polarization-induced ε reduction, well\nnarrowing, mobility decrease (indicating modified scattering), and T increase.\nc\nSimple dimensional confinement without this dielectric change would likely\nsuppress T via enhanced disorder without the compensating unconventional\nc\npairing mechanism.\n6\n(3) Alternative explanations for T enhancement:\nc\nWhile we ascribe the T enhancement primarily to ferroelectricity-induced dielectric\nc\nconstant reduction, we acknowledge several alternative mechanisms requiring further\ninvestigation.\n(i) Spin-orbit coupling (SOC). The reviewer correctly notes SOC’s potential\ninvolvement. KTO’s intrinsically strong bulk SOC likely contributes to the\ninterfacial superconducting state, as shown by gate-modulation experiments [Hua\net al., npj Quantum Mater. 7, 97 (2022)] where suppression of the superconducting\nlayer thickness occurs together with increases in both T and spin-orbit energy.\nc\nHowever, while these phenomena are clearly correlated, we cannot establish a direct\ncausal relationship between SOC enhancement and T elevation, leaving this as an\nc\nopen question.\n(ii) Three-dimensional carrier concentration (n ) effects. A straightforward\n3d\nconsideration suggests that n increases with potential well narrowing, which might\n3d\nelevate T .\nc\n(iii) Electron-phonon coupling modification. Recent work identifies surface\nelectron-phonon coupling as crucial for KTO superconductivity [Chen et al., Nat.\nCommun. 15, 7704 (2024)]. This coupling may strengthen under progressive\nelectron confinement near the interface.\nRevision summary: To address the reviewer’s question, in the revised manuscript on\nPage 7 we have added the following statement:\n“We note that the enhancement of T with potential well narrowing is a well-established\nc\nexperimental phenomenon in KTO-based interfaces13,38, yet the underlying mechanism\nremains elusive. Several potential mechanisms may be at play: (i) fractal\nsuperconductivity induced by strong scattering55, (ii) enhanced spin-orbit coupling\nenergy38, (iii) increased local three-dimensional carrier density, and (iv) modifications\nto electron-phonon coupling56. Further studies are needed to discern which of these\nmechanisms, if any, contribute to the observed enhancement.”\nMy second question concerns the statement that ferroelectric polarization and metallic\nconductivity (as well as superconductivity) coexist in the top layers of KTO in\nproximity to the interface. It undoubtedly means that the carrier density is not uniform\nin the conducting layer, but develops a gradient along the normal direction of the\ninterface. In this sense, how meaningful are the parameters such as Hall carrier density\nn and Hall mobility μ ? Indeed, these parameters can be functions of the distance z\ns H\nfrom the interface and are thereby spatially varying; the high mobility can potentially\nstem from the carriers farther from the interfacial scattering centers, yet this does not\nmean the absence of slower electrons in the vicinity of interface (they are simply short-\ncircuited by their faster counterparts in deeper positions). Consequently, the interfacial\nsuperconductivity may also be nonuniform and z-dependent? What is the best picture\ndescribing the superconducting system in the oxide heterointerfaces?\nquestion about oxide interfaces. We fully agree with the key points raised about the z-\ndependent nature of both transport parameters and superconductivity\n(1) Regarding the parameters such as Hall carrier density and Hall mobility:\nWe note that this z-dependence is characteristic of nearly all heterointerfaces, including\nconventional semiconductor systems, when charge transfer occurs. The carrier\ndistribution profiles can in principle be modeled via self-consistent Poisson-\nSchrödinger simulations. Importantly, despite this inherent z-dependence, Hall\nmeasurements remain the standard experimental technique for characterizing such\nheterointerfaces (including oxide interfaces), as they effectively capture the\nmacroscopic transport behavior through weighted spatial averaging of all contributing\nlayers.\n(2) Concerning the z-dependence of superconductivity\nWe fully concur with the reviewer regarding the inherent depth dependence of\ninterfacial superconductivity. While systematic z-resolved studies remain challenging\nfor STO- and KTO-based interfaces (due to limitations in current experimental\ncapabilities for growing atomically-resolved, high-quality doped STO and KTO films),\ncompelling evidence from cuprate heterostructures demonstrates clear z-dependence,\nwith the overall superconducting properties being governed by the optimally\nsuperconducting sublayer.\n(i) In model La CuO /La Sr CuO bilayers studied via atomic-layer-resolved\n2 4 1.55 0.45 4\ndelta-doping, the optimal superconducting state occurs in the second CuO plane\n2\nfrom the interface [Logvenov et al., Science 326, 699 (2009)].\n(ii) Our recent work (in review) demonstrates tunable depth profiles in La\n2-\nSr CuO /LaSrCuO bilayers, where the optimal superconducting CuO plane\nx x 4 4 2\nsystematically shifts from the 3rd (x = 0) to the 6th plane (x = 0.15) with Sr doping.\nThus, returning to STO- or KTO-based oxide interfaces, we speculate that––precisely\nas the reviewer suggests––the interfacial superconducting state exhibits z-dependence,\nwith the measured properties dominated by the optimal superconducting sublayer.\nI am looking forward to seeing the authors' reply to my questions and my final\nevaluation would be made based on those responses.\nReply: We sincerely appreciate the reviewer’s thoughtful suggestions. We’re pleased\nto confirm that all comments have been fully addressed through extensive responses\nand thorough manuscript revisions. These improvements now better reflect the\nsignificance of our findings. We’d be delighted to provide any additional information\nthe reviewer may require.\nA relatively minor point: for the difference between 0+ and 0- states, seems the\ninterpretations are mainly presented in the caption of Fig. 4b; the main text contains\n8\nvery limited contents of corresponding discussion. Probably this can be balanced to\nsome extent.\nReply: We thank the reviewer for this constructive suggestion. The recommended\nmodifications have been implemented accordingly in the revised manuscript.\nRevision summary:\nIn the revised manuscript, we have added the following discussion regarding the “0+”\nand “0-” states on Page 8:\n“As shown Fig. 4b, the built-in electric field (E ) forming the potential well results\nbuilt-in\nfrom the superposition of two competing contributions: E from ferroelectric\nP\npolarization (opposing the gate field direction) and E from surface screening charges.\nS\nAfter V removal, the persistent residual field––dominated by E which opposes and\nG S\nexceeds E ––creates two distinct states: (1) In the “0-” state, the net field E = E +\nP built-in P\nE narrows the potential well, enhancing carrier confinement and increasing T ; (2)\nS c\nconversely, in the “0+” state, the net field broadens the well, reducing T .”\nc\nWe have also refined the caption of Fig. 4b for conciseness:\n“b, Potential well modulation (thick lines) and electron envelope wavefunction (thin\nlines) for original (dashed), “0-” (blue), and “0+” (red) states. Insets show competing\nfields from E (ferroelectric polarization) and E (screening charges). The net field E\nP S built-\n= E + E modulates the well profiles. Post V removal, persistent tuning (aligned\nin P S G\nwith V direction) suggests that | E | > | E |”\nG S P\nThe manuscript claimed about the coexistence of superconductivity and ferroelectricity\nin KTaO (111) based heterostructure. The authors further attribute the enhancement of\n3\nsuperconducting T to the ferroelectric polarization induced reduction in dielectric\nc\nconstants. While the authors have demonstrated superconductivity, there is no direct\nexperimental evidence about ferroelectricity. All claims about ferroelectricity are rather\nspeculative, which may be true or may not be true as alternative scenario can also be\nattributed to the experimental claims.\nReply: We sincerely appreciate the reviewer’s critical comment regarding the evidence\nof ferroelectricity in our system. In response, we have performed additional\nexperiments (Raman scattering and additional transport) and strengthened our\ndiscussion to rigorously establish the ferroelectric origin of the observed phenomena.\nAs detailed in our response to the reviewer’s second comment, alternative\ninterpretations like charge trapping/detrapping can be excluded. For clarity, we organize\nthis reply into three sections (Section 2 contains the key experimental results).\n(1) Established understanding of polar nanoregions in KTO\nWhile KTO remains quantum paraelectric down to the lowest attainable temperatures\nin its pure form, this low-temperature paraelectric state is rather unstable. The\nferroelectric phase can readily be induced through several well-documented approaches\nincluding (i) chemical doping, (ii) oxygen deficiency, (iii) applied strain, and (iv)\nnaturally occurring defects (even nominally pure KTO samples contain substantial\ndefect concentrations of ~1017 cm3). These defects generate polar nanoregions (PNRs)\nthat have been extensively characterized in previous studies (Refs. [40,45-46]).\nImportantly, as demonstrated in previous observations (Ref. [40]), these PNRs undergo\ncooperative ordering below a critical temperature, transforming KTO into a weak\nferroelectric state with small but measurable spontaneous polarization (~0.04 μC/cm-2)\n(Ref. [40]). The ferroelectric behavior we observed in our LAO/KTO heterostructures\ndirectly originates from this PNR-induced ferroelectricity that has been firmly\nestablished in the literatures (Refs. [40,45-46]).\n(2) Raman spectroscopy evidence for polar nanoregions in LAO/KTO\nheterostructures\nTo directly demonstrate the existence of PNRs in our LAO/KTO heterostructures, we\nconducted Raman spectroscopy measurements—a well-established technique for\ndetecting PNRs in KTO. For comprehensive comparison, we examined (i) as-grown\nLAO/KTO, (ii) poled LAO/KTO (following our defined protocol in the manuscript),\n(iii) KTO single-crystalline substrate, and (iv) commercial ferroelectric KTa Nb O\n0.65 0.35 3\n(KTN ) single crystal. All spectroscopic data are presented in Figure R1 with detailed\n0.35\nanalysis in subsequent subsections.\nTemperature dependence of normalized Raman spectra of (a) ferroelectric KTN (111) single\n0.35\ncrystal, (b) KTO(111) substrate, and LAO/KTO(111) heterostructures in both (c) original and (d) 0-\nstate (poled at -180 V). Bold red curves highlight T* (temperature where transverse polar optical\nphonon modes TO and TO emerge), with mode frequencies indicated by arrows. (e) Back-ground-\n2 4\nsubtracted TO peak intensity versus temperature (inset shows saturation below 10 K). Comparison\n4\nof original and 0- state spectra at temperatures at (f) 60 K and (g) 1.6 K, with TO regions magnified\n4\nin insets.\n(2.1) Background on KTO phonon modes\nThree transverse polar optical phonon modes (TO , TO , and TO ) are theoretically\n1 2 4\npredicted and experimentally observed in perovskite KTO systems, including single\ncrystals [H. Uwe et al. Phys. Rev. B 33, 6436-6440 (1986)] and ceramics [S. Glinsek et\nal. J. Appl. Phys. 111, 104101 (2012)]. The TO mode corresponds to the relative\n1\nvibration of Ta5+ ions against rigid O octahedra (Slater mode), TO reflects K+ ion\n6 2\nmotion against a rigid TaO framework (Last mode), and TO involves the bending of\n6 4\nO octahedra (Axe mode) [X. Wen et al. Appl. Phys. Lett. 122, 232905 (2023); S.\n1413 (1967)]. While the soft TO mode strongly softens upon cooling and exhibits low\n1\nintensity, TO and TO show stable frequencies but gradually diminishing intensities\n2 4\nwith increasing temperature. Notably, these Raman-inactive modes in ideal KTO are\nconsistently observed and attributed to PNRs [X. Wen et al. Appl. Phys. Lett. 122,\n232905 (2023); S. Kojima et al. Jpn. J. Appl. Phys. 57, 11UB05 (2018); S. Glinsek et\nal. J. Appl. Phys. 111, 104101 (2012); H. Uwe et al. Phys. Rev. B 33, 6436-6440 (1986);\nO. Aktas et al. Phys. Rev. B 90, 165309 (2014)].\n(2.2) Raman spectroscopic measurements\n11\n(2.2.1) Experimental results\nWe conducted temperature-dependent Raman spectroscopy on LAO/KTO(111)\nheterostructures (both before and after electrostatic poling), using KTa Nb O\n0.65 0.35 3\n(KTN )(111) and KTO(111) single-crystal substrates as controls (Fig. R1).\n0.35\nElectrostatic poling was performed at -180 V and 4.5 K in a vacuum cryostat. Following\npoling, the sample was rapidly warmed to room temperature for Raman measurements\nto minimize relaxation effects (Supplementary Fig. 3 confirms that brief ambient\nexposure does not fully erase the poled state).\nAs shown in Fig. R1a, the commercially available ferroelectric KTN (111) single\n0.35\ncrystals exhibit prominent TO and TO modes, indicative of symmetry breaking. These\n2 4\nmodes persist up to ~ 175 K, confirming the ferroelectric character. In the KTO(111)\nsubstrates, weaker TO (~ 198 cm-1) and TO (~ 544 cm-1) peaks are observed until ~40\n2 4\nK (Fig. R1b), consistent with previous studies attributing them to PNRs. Crucially, the\nLAO/KTO(111) heterostructures exhibit similar features (Fig. R1c), with the TO peak\n4\npersisting to ~ 60 K post-poling (Fig. R1d).\nTo enhance the visibility of these features, we performed background subtraction (using\nhigh-temperature references) and data normalization. The processed TO peak\n4\nintensities versus temperature are shown in Fig. R1e. At 60 K, the poled sample shows\na distinguishable TO peak (Fig. R1f), absent in the unpoled case; at 1.6 K, the TO\n4 4\npeak is stronger in the poled sample than the unpoled one (Fig. R1g). The enhanced\nintensity and elevated T* demonstrate improved PNR alignment after poling.\n(2.2.2) Conclusions\n(i) PNR-induced weak ferroelectricity exists in both KTO substrates and\nLAO/KTO heterostructures. This polarization originates primarily from the bulk\nKTO substrate (not exclusively interfacial).\n(ii) Switchable R serves as a polarization indicator. The temperature range of hysteresis\ns\nin R -V occurs matches the Raman-derived T* where the transverse polar optic phonon\ns G\nmodes (TO and TO ) appear, confirming R as a reliable ferroelectric probe––\n2 4 s\nconsistent with reports on Ca-doped SrTiO interfaces [J. Brehin et al. Phys. Rev. Mater.\n3\n4, 041002(R) (2020); P. Noël et al. Nature 580, 483–486 (2020)].\nThese results, combined with transport data, provide conclusive evidence for\nferroelectricity in LAO/KTO.\n(3) Connecting PNR-induced substrate ferroelectricity to the interfacial\nferroelectricity coexisting with superconductivity\nIn the following discussion, we elucidate how switchable substrate ferroelectricity\nenables the switchable interfacial ferroelectricity in LAO/KTO heterostructures:\n(i) The conductive interfacial KTO layer remains crystallographically continuous\nwith the bulk substrate. While E-fields cannot directly polarize this conductive\nlayer, gating through insulating bulk KTO modulates bulk polarization, which\n12\ncoherently alters interfacial polarization (enabling switchable ferroelectric control\nof superconductivity).\n(ii) The ferroelectric tuning of interfacial conduction cannot be explained by\nconventional capacitance effects (which would require only bulk KTO\nferroelectricity). Beyond arguments in the main text, we emphasize that PNR-\ninduced ferroelectric polarization–as established in previous studies–is\nexceptionally weak (~0.04 μC/cm-2), ruling out purely capacitive coupling.\n(iii) Importantly, we clarify a subtle but crucial point: While the interfacial KTO\nlayer’s polarization may differ quantitatively from bulk KTO’s, their inherent\nlattice bonding ensures that bulk polarization switching induces interfacial\nswitching.\nRevision Summary:\n(1) We have added Raman scattering measurements as Supplementary Note 1 and\nSupplementary Fig. 4 in the revised manuscript. The corresponding text in the main\ntext now reads (Page 5):\n“Further evidence for low-temperature ferroelectricity in LAO/KTO comes from our\nRaman scattering measurements (Supplementary Fig. 4), where the TO and TO\n2 4\noptical modes emerge blow ~40 K (~60 K in poled samples) – closely coinciding with\nthe onset of hysteresis in the V -R loops (Figs. 2g-h).”\nG s\n(2) On page 4, we have rewritten the last sentence in the “Ferroelectric hysteresis\nunder gating cycles” section as:\n“This temperature-dependent behavior closely matches previous observations39–41 of\nweak spontaneous polarization (0.04 μC/cm-2) in KTO, where defect-induced polar\nnanoregions develop macroscopic coherence within this specific temperature range.”\n(3) On Page 5, we have added the following discussion paragraphs:\n“Notably, even nominally pure KTO samples typically contain substantial defect\nconcentrations (~1017 cm3). These defects promote the formation of polar nanoregions,\nwhich collectively drive KTO into a weak ferroelectric state at low temperatures 40,46.”\nand\n“The similarities between the observed hysteresis and Raman features in LAO/KTO\nand those in the intentionally ferroelectric Al/Sr Ca TiO interface29 provide\n0.99 0.01 3\nadditional support for ferroelectricity in LAO/KTO.”\n(4) On Page 6, we have refined the concluding statement in the “Coexistence of\nferroelectricity and superconductivity in the conducting layer” section to read:\n“Although ferroelectricity and conductivity are traditionally considered mutually\nexclusive, polarization in the conducting KTO layer could emerge via coupling with\nthe ferroelectric KTO bulk, mediated by their shared lattice bonding, while potentially\ndeveloping distinct interfacial behavior.”\ncharge trapping/detrapping under electrical gating. This has been demonstrated\nextensively in context of SrTiO based heterostructures [Sci. Rep. 4, 6788 (2014); Phys.\ntrapping phenomena also gives rise to hysteresis in resistance vs. gate voltage\nmeasurement. Similar observations have been reported in Fig. 2 of this manuscript but\nattributed to the ferroelectricity. The authors need to demonstrate that their observations\nare not linked with charge trapping phenomena.\nReply: We thank the reviewer for raising this important point regarding charge trapping\neffects, as documented in SrTiO -based systems [Sci. Rep. 4, 6788 (2014); Phys. Rev.\nkey lines of evidence and reasoning that indicate charge trapping is unlikely to be the\ndominant mechanism in our LAO/KTO results.\nLine 1: Distinct phenomenological differences\nTo better address the reviewer’s concern, we performed additional experiments to\nsystematically trace the evolution of R with V under continuous V sweeps. The\ns G G\nresults are summarized in Fig. R2.\nFig. R2 (also presented as Supplementary Fig. 5 in the revised manuscript) | Ferroelectric-like\nhysteresis independent of first sweep polarity in back-gated LAO/KTO. a-c, V -R\nG s\ncharacteristics of LAO/KTO heterostructures exhibit stable bistable states (a) and a systematic\nincrease in hysteresis width with increasing maximum gate voltage (V max = 60, 120, 180 V),\nG\nirrespective of the polarity of the initial voltage sweep (b, c). The hysteresis loops ultimately\nstabilize into a reproducible double-state configuration at zero bias (0±), consistent with robust\nferroelectric polarization switching behavior. FS: forward sweep; BS: backward sweep.\nFor comparison, we reproduced in Fig. R3 the typical R -V curves from previous\ns G\nstudies on SrTiO -based systems [adapted from Phys. Rev. Lett. 124, 017702 (2020)]\n3\nand Sci. Rep. 4, 6788 (2014)].\ns G 3\nfirst backward sweeps. Adapted from Refs. [J. Biscaras et al. Sci. Rep. 4, 6788 (2014); C. Yin et al.\nPhys. Rev. Lett. 124, 017702 (2020)].\nWe can clearly see that the R -V curves for our LAO/KTO heterostructure are very\ns G\ndifferent from those of SrTiO -based systems, exhibiting three features inconsistent\n3\nwith charge trapping but consistent ferroelectric hysteresis.\n(i) Reproducible and switchable hysteresis\n• Observation: After the initial forward sweep, we observe a pronounced,\nrepeatable hysteresis in the R -V curve.\ns G\n• Contrast with SrTiO : In SrTiO -based heterostructures, charge trapping\n3 3\ncauses an irreversible resistance change during the first sweep. Subsequent\nsweeps (for V < V max) exhibiting overlapping trajectories.\nG G\n(ii) Independent of initial V direction\nG\n• Observation: Regardless of whether the initial V is positive or negative, the\nG\nsystem stabilizes into a robust hysteretic loop.\n• Contrast with SrTiO : SrTiO -based heterostructures exhibit direction-\n3 3\ndependent reversibility (e.g., irreversible forward sweeps vs. reversible\nnegative sweeps).\n(iii) Non-volatile binary states at zero voltage\n• Observation: The hysteresis stabilizes into two well-defined R states at V =\ns G\n0 (e. g., 0± sates), a hallmark of ferroelectric polarization switching.\n• Contrast with SrTiO : Charge trapping in SrTiO does not produce such\n3 3\ndeterministic, non-volatile states at zero bias.\nThese behaviors are signatures of ferroelectricity and cannot be explained by charge\ntrapping mechanisms reported in SrTiO -based systems.\n3\n(Sr,Ca)TiO interfaces (with isovalent Ca2+ substitution) demonstrate clear switchable\n3\ntwo-dimensional electron gas and hysteresis in R -V curves [J. Brehin et al. Phys. Rev.\ns G\nour LAO/KTO interface reproduces this hysteretic behavior, including the same\ncharacteristic loop shape and non-volatile states. The similarity to this known\nferroelectric interface provides compelling evidence for ferroelectric modulation in\nLAO/KTO.\nLine 3: Carrier concentration stability\nIn charge trapping scenarios, electron trapping leads to substantial carrier density\nreduction after bias removal [e.g., up to 43% (J. Biscaras et al. Sci. Rep. 4, 6788 (2014))\nor even inducing insulating behavior (C. Yin et al. Phys. Rev. Lett. 124, 017702 (2020)].\nIn striking contrast, our LAO/KTO heterostructures retain nearly unchanged carrier\nconcentration in the \"0+\" state, with some samples even showing slight increases (Figs.\n3c-d; Supplementary Fig. 1b) - demonstrating mobility-dominated behavior rather than\ncarrier density changes, which further contracts charge trapping mechanisms.\nWe note that KTO’s dielectric constant (ε ≈ 5000 at low T) is only ~25% of SrTiO ’s,\n3\nfundamentally limiting its carrier density modulation capability. Even under upper-\nbound estimates, a 180 V V variation would induce Δn ≤ 1×1013 cm-2 (with actual\nG\nvalues further suppressed by field-induced ε suppression).\nTogether, these experimental observations and material constraints provide further\ncompelling evidence against charge trapping as the dominant mechanism.\nLine 4: Temperature-dependent hysteresis correlates with Raman spectra\nThe R -V hysteresis emerges only below ~50 K (Fig. 2), mirroring the onset of\ns G\ntransverse polar optic modes observed by Raman spectroscopy (Fig. R1).\nThis\nsynchronized temperature dependence between transport and spectroscopic data\nstrongly implicates ferroelectricity as the dominant mechanism.\nRevision summary:\nWe have incorporated Fig. R2 as Supplementary Fig. 5 in the revised manuscript.\nCorrespondingly, we have added the following discussion paragraph on Page 6:\n“While V -induced irreversibility in R has been previously observed in SrTiO -based\nG s 3\nheterostructures and attributed to charge trapping/detrapping effects47-49, the behavior\nwe observe in LAO/KTO is fundamentally distinct. Our system exhibits reproducible\nand switchable hysteresis that is independent of the initial V sweep direction – a\nG\nhallmark of ferroelectricity. Moreover, the limited magnitude of carrier density\nmodulation in LAO/KTO is incompatible with charge trapping, which would require\nsignificantly larger n variations47,48. …… Taken together, the observed hysteresis and\ns\nswitching behaviors in LAO/KTO cannot be explained by charge trapping or ionic\nmigration, providing compelling evidence for ferroelectricity as the governing\nmechanism.”\nBecause of these two significant issues in the manuscript, I do not recommend the\n16\npublication of this paper in Nature Communications.\nReply: We sincerely appreciate the reviewer’s constructive critique which has\nsubstantially improved our manuscript. To address the two key concerns raised:\n1. We have performed new experimental characterizations (Raman spectroscopy\nand systematic transport measurements) that provide compelling support for our\nconclusions.\n2. We have thoroughly revised our discussion to deliver more rigorous analysis\nand strengthened interpretations.\nWhile we understand the reviewer’s initial reservations, we believe the enhanced\nexperimental evidence and improved discussion now comply with the journal’s\nrigorous criteria. We are truly grateful for the reviewer’s insightful comments and time\nspent evaluating our work and hope the revised manuscript merits reconsideration for\npublication.\n17"}, {"round": 2, "reviewer_comments": {"reviewer_1": "I have read the authors' response and the revised manuscript. I am impressed by the scientific care and scrutiny the authors show in addressing all concerns and questions raised in the first round of review. In general, I am satisfied with the responses and revisions. The manuscript has been extensively improved, particularly with the scientific significance clearly stressed and validity of main conclusions strengthened by the newly-supplied Raman data. I also appreciate that the authors mentioned Feigel’man’s fractal superconductivity as a potential origin of T enhancement. Taking c all these together, my recommendation would be to publish this work in Nature Communications.\nMeanwhile, I have one remaining suggestion. As elucidated by the authors in the reply file, the transport coefficients derived from the Hall measurement, including n and μ, are weighted spatial average of all contributing layers since these parameters are inherently dependent on the depth along z direction. This point is not mentioned in the manuscript, while I think it is helpful to the reader to see it addressed. Probably a brief note at the end of the paragraph introducing these parameters (the first paragraph on page 5) can resolve this matter. Also, n and μ are only defined in the Methods but not in the main text.", "reviewer_2": "I have gone through the revised manuscript along with the response letter and supplemental materials. I am still not convinced by the claim about the ferroelectricity. The authors have performed Raman spectroscopy, and the polar mode TO2 and TO4 are present in every sample they examined, including a bare KTO (111) substrate. While the mode confirms the presence of polar distortion, it does not confirm ferroelectricity. The polar distortion may not be uniform and can be confined within a few unit cells. It is well known that depending on the concentration of polar nano regions and temperature, one can get several phases, including paraelectric, ferroelectric, and dipolar glass phase [see J. Phys. Condens. Matter 15, R367–R411(2003)]. In fact, in one recent article [Nature Communications 15, 3830 (2024)], it has been demonstrated the electron dynamics of metallic KTaO3-x sample behaves like a glass, which is related to the dipolar glassy phase of polar nano regions. Overall, the experimental results reported in this paper do not confirm ferroelectricity, although that may be a possibility. So, I do not recommend the publication of this manuscript in Nature Communications in its present form."}, "author_response": "Responses to the comments from Reviewer #1\nI have read the authors’ response and the revised manuscript. I am impressed by the scientific care and scrutiny the authors show in addressing all concerns and questions raised in the first round of review. In general, I am satisfied with the responses and revisions. The manuscript has been extensively improved, particularly with the scientific significance clearly stressed and validity of main conclusions strengthened by the newly-supplied Raman data. I also appreciate that the authors mentioned Feigel’man’s fractal superconductivity as a potential origin of T enhancement. Taking c all these together, my recommendation would be to publish this work in Nature Communications.\nReply: We sincerely thank the reviewer for the positive assessment of our work and greatly appreciate the time and effort devoted to improving the quality of this manuscript.\nMeanwhile, I have one remaining suggestion. As elucidated by the authors in the reply file, the transport coefficients derived from the Hall measurement, including n and μ, s are weighted spatial average of all contributing layers since these parameters are inherently dependent on the depth along z direction. This point is not mentioned in the manuscript, while I think it is helpful to the reader to see it addressed. Probably a brief note at the end of the paragraph introducing these parameters (the first paragraph on page 5) can resolve this matter. Also, n and μ are only defined in the Methods but not in the main text.\nReply: We thank the reviewer for these constructive suggestions. To address these concerns, we have made the following revisions:\n(1) We have added the following clarification at the end of the first paragraph on page 5: “It should be noted that the transport coefficients, n and μ, derived from Hall measurements on Hall bar devices, represent spatially weighted averages of all contributing layers, as these parameters are inherently depth-dependent along the z-direction.”\n(2) We have explicitly defined n and μ in the first paragraph of page 5: “...the increase s in T was accompanied by a clear decrease in mobility μ (with a slight increase in carrier c density n ; however, the change in n was much smaller than that of μ) ...”\nI have gone through the revised manuscript along with the response letter and supplemental materials. I am still not convinced by the claim about the ferroelectricity. The authors have performed Raman spectroscopy, and the polar mode TO and TO are 2 4 present in every sample they examined, including a bare KTO (111) substrate. While the mode confirms the presence of polar distortion, it does not confirm ferroelectricity. The polar distortion may not be uniform and can be confined within a few unit cells. It is well known that depending on the concentration of polar nanoregions and temperature, one can get several phases, including paraelectric, ferroelectric, and dipolar glass phase [see J. Phys. Condens. Matter 15, R367–R411(2003)]. In fact, in one recent article [Nature Communications 15, 3830 (2024)], it has been demonstrated the electron dynamics of metallic KTaO sample behaves like a glass, which is related 3-x to the dipolar glassy phase of polar nanoregions. Overall, the experimental results reported in this paper do not confirm ferroelectricity, although that may be a possibility. So, I do not recommend the publication of this manuscript in Nature Communications in its present form.\nReply: We thank the reviewer for their detailed comments and for acknowledging the evidence of polar distortions. We understand the reservation regarding the confirmation of ferroelectricity and offer the following clarification to align our interpretation with the reviewer’s perspective.\nFirstly, we fully agree that the Raman modes indicate polar distortions which could correspond to various phases, including a dipolar glass as the reviewer rightly points out. Our claim is not that the material is intrinsically ferroelectric in its pristine state.\nSecondly, our key argument is that the application of an electric field aligns these polar nanoregions (PNRs), inducing a switchable ferroelectric state. The hysteresis loops we observe are direct evidence of this field-induced ferroelectricity. We believe this effect occurs primarily within the insulating bulk KTO substrate, as this is the region across which the gate voltage is effectively applied. The ferroelectric properties observed at the conducting interface are thus a consequence of this effect propagating from the adjacent bulk region.\nTherefore, our results demonstrate how the inherently disordered PNRs in KTO can be electrically manipulated to exhibit functional and switchable ferroelectric properties.\nTo address the reviewer’s concerns directly, we have added a clarifying statement at the end of the first paragraph on page 6: “We attribute the induced ferroelectricity to a field-induced ordering of the polar nanoregions that are present in KTO, which are inherently disordered and lacking long-range correlation in the pristine state.”\n2"}]

Decision letter