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２０２１年７月１３日（火）物 理 学 第 一 分 野

物理学第一分野修士論文発表会
場所：理学研究科５号館 ５階・第４講義室+オンライン
発表：１５分（別に質問時間５分程度）
２０２１年７月１３日（火）１０：００～１１：００

目

次１．Thermal Hall effect in the pseudogap phase of cuprates
Andre de Oliveira Silva（１０：００）
・・・・・ １
２．Mott Transition and Magnetism in a fragile topological insulator
Ashish Joshi（１０：２０）
・・・・・２３．Ultrastrong coupling between phonons in halide perovskites and terahertz vacuum photons
Zhenya Zhang（１０：４０）
・・・・・３

Thermal Hall effect in the pseudogap phase of cuprates
Quantum Condensed Matter

Andre de Oliveira Silva

Abstract We have performed measurements of thermal Hall in underdoped La2-xSrxCuO4. Compared tothe previous report, the thermal Hall signal show a similar field and temperature dependences but its
magnitude is four times smaller. Further measurements are needed to clarify the origin of the thermal Hall
effect in the pseudogap phase.
© 2021 Department of Physics, Kyoto University

High-Tc cuprate superconductors display a rich variety of electronic states in the temperature-doping
phase diagram. The parent compound is an antiferromagnetic Mott insulator and a Fermi liquid state appears
with high enough doping. In between these two states, there exist for example charge-density wave,
superconductivity and in particular the pseudogap states. In the pseudogap state, it has been discussed thatmany symmetry breaking phenomena appear, including broken in-plane rotational and time-reversal
symmetries. The pseudogap state holds the key for understanding high temperature superconductivity but its
nature is controversial. Therefore both experimental and theoretical efforts are still being made even 30 years
after its discovery.
Recently, a giant thermal Hall effect (THE) was reported in the parent compound and underdoped
cuprates [1, 2]. The THE signal becomes negative inside the pseudogap state and its magnitude increases as thetemperature decreases. Remarkably, the magnitude of the THE measured in the direction perpendicular to theCuO2 plane (κzy) is comparable to that measured parallel to the planes (κxy) despite its layered crystal structure.
The most plausible origin is phonons which are the only known quasiparticle able to propagate along the c-axis(out-of-plane). Although it is proposed that the phonon would be chiral inside the pseudogap, understanding thespecific mechanism by which phonons couple with a magnetic field is a challenge with our current perspectiveof the cuprates. Therefore, elucidating the origin of the giant negative THE is expected to give further insight
into the mysterious pseudogap phase. Previous measurements used thermocouple to measure the temperature
gradient but it has large field dependence at lower temperatures, leading to unreliable estimates for determining
the THE signal [3]. Here we measured thermal Hallconductivity κxy in underdoped La2-xSrxCuO4 inapplied heat current along the a-axis and magnetic
field along the c-axis, using Cernox thermometerswhich show negligible field dependence above 20 K.
The observed THE shows a nonzero negative value
at lower temperatures, which is consistent with theprevious reports [1]. Although the field andtemperature dependence looks similar, we find thatits magnitude is much smaller than the previously
reported values. Moreover, at high temperatures
above 80 K we observed a sign change of κxy, which
has not been reported in Ref. [1]. We will discuss
Fig. 1: Temperature dependence of the thermal
the possible reasons for the discrepancies.
Hall conductivity κxy/T in La2-xSrxCuO4. Red andgreen circles represent our data for x = 0.07 at 12
T and Sherbrooke’s data for x = 0.06 at 15 T [1],respectively.

References[1] G. Grissonnanche et al., Nature 571, 376-380 (2019).
[2] G. Grissonnanche et al., Nat. Phys. 16, 1108-1111 (2020).
[3]Y. Nakamura, et al., Physica C. 185, 1409-1410 (1991).

Mott Transition and Magnetism in a fragile topological insulator
Condensed matter theory group

Ashish Joshi

Abstract We study the effects of electronic correlations on fragile topology using dynamical
mean-field theory. Discovered recently, fragile topological insulators offer Wannier obstructions
that can be removed without closing the bulk gap. Here, we analyze the Mott-insulator transition
due to strong correlations and show a topological phase transition to stable topology in presenceof a magnetic field.
© 2021 Department of Physics, Kyoto University

Topological insulators are characterized by a gapped bulk band structure with gapless edge states crossing thebandgap. This is known as the 'bulk-boundary correspondence'. A new type of topological insulator has been
recently discovered that does not show this bulk-boundary correspondence. Dubbed as fragile topologicalinsulators (FTIs), these insulators offer obstruction to the formation of exponentially localized Wannier functions,just like the conventional stable topological insulators do [1]. But this obstruction in FTIs can be removed byadding certain trivial degrees of freedom. For the same reason, FTIs do not host symmetry-protected flow of edge
states between bulk bands but are expected to have a spectral flow between the fragile bands and other bands
under certain twisted boundary conditions. Thus, showing a new form of 'twisted bulk-boundary correspondence'[2]. In this work, we study the effects of electronic correlations on the fragile topology and the twisted boundary
states using dynamical mean-field theory. We analyze commonly observed effects of strong correlations, such as
the Mott-insulator transition and magnetism, on a known model hosting fragile topology. We show that in thenonmagnetic case, fragile topology, along with the twisted boundary states, is stable with interactions below a
critical interaction strength. Above this interaction strength, a transition to the Mott insulating phase occurs, andthe twisted boundary states disappear. Furthermore, by applying a homogeneous magnetic field, the fragile
topology is destroyed. However, we show that a magnetic field can induce a topological phase transition whichconverts a fragile topological insulator to a Chern insulator (Fig. 1, C denotes the Chern number). Finally, we
study ferromagnetic solutions of the fragile topological model and find that a stable ferromagnetic phase exists athigh interaction strengths which is topologically stable [3].

Fig 1. Band structure of our fragile topological insulator model under cylindrical boundary conditions in presenceof a magnetic field. The fragile topology changes to stable topology, which results in the edge states crossing thebulk band gap. The inset shows the edge states near ky=0 and C denotes the Chern number of the individual bands.
References[1] H. C. Po, H. Watanabe, and A. Vishwanath, Physical Review Letters 121, 126402 (2018).
[2] Z.-D. Song, L. Elcoro, and B. A. Bernevig, Science 367, 794 (2020).
[3] A. Joshi and R. Peters, Physical Review B 103, 165130 (2021).

Ultrastrong coupling between phonons in halide perovskites andterahertz vacuum photons
Nanophotonics

Zhenya Zhang

Abstract We demonstrate ultrastrong coupling between vacuum photons and phonons of perovskite
semiconductors at the terahertz frequencies in nanometer-sized cavities. We show that the enhancement ofthe vacuum field in a smaller-gap split ring resonator overcomes the decrease of dipole number, resulting
in the ultrastrong coupling.
© 2021 Department of Physics, Kyoto University

When a matter is strongly coupled to the light field, a hybridized state emerges and behaves differently from theoriginal state. As the coupling strength increases, the hybrid system reaches the regime of ultrastrong coupling(USC) which is attracting a rising attention in the field of materials science and quantum optics. In particular, thecoupling of the vacuum photons to vibrational modes in molecules or phonons in solids opens up unique materialengineering methods, such as modification of chemical reaction rates [1] and a novel way to induce phasetransitions in solids [2]. Because stronger coupling strength is expected to lead to more drastic changes in material
properties, it is necessary to understand how the coupling strength between photons and phonons can be enhanced.
While the coupling strength in a cavity structure depends on the vacuum field amplitude, the dipole moment andthe number of dipoles, previous studies mainly focused only on the role of dipoles. In this work, from a systematic
study on the light-matter coupling in the cavities of metamaterials with different designs in the terahertz (THz)frequency region, we highlight the important role of the vacuum field strength.
Here, we demonstrate that USC between vacuum photons and phonons (oscillation of Pb-I-Pb bonds) ofperovskite CH3NH3PbI3 (MAPbI3) at 0.95 THz can be realized in the nanostructured split ring resonators (SRR).
In order to evaluate the coupling strength, we performed THz spectroscopy on the SRR coated with perovskite
films as shown in Fig. 1. When the resonance frequency p of SRR is equal to the phonon frequency  TO1, theabsorption peak splits as shown in Fig. 1(b). The Rabi splitting represented by R is induced not by the incident
THz field but by the coupling between vacuum photons and phonons. We find that R increases as the gap size
decreases. The coupling strength defined as  = R/(2p) becomes 0.24 for a gap size of 100 nm, which is well in
the USC regime (  ). Our analysis of the gap-size dependence shows that the enhancement of vacuum fieldoccurs significantly at the nanometer-scale gaps and more than compensates for the reduction in the dipole
number (number of Pb-I-Pb bonds) inside the cavity, thus leading to the phononic ultrastrong coupling [3].

Fig. 1. Transmission spectra of the MAPbI3 film and the bare SRR (a), and the coupled system (b).

References[1] A. Thomas et al., Science 363, 615 (2019).
[2] Y. Ashida et al., Phys. Rev. X 10, 41027 (2020).
[3] Z. Y. Zhang et al., under review.