The thermal Hall effect is an important tool to study the dynamics of quasi-particles in insulating magnets including both the ordered state and liquid states [1]. The first observation of magnon Hall effect was attributed to the Berry phase induced by the spin-orbit interaction [2]. However, when the non-adiabatic effect in electron-phonon coupling, i.e., the correction to the Born-Oppenheimer approximation, is considered, it is shown that the phonons can also contribute to the thermal Hall effect without the spin-orbit interaction [3]. Recent experiment on YMnO3 and its theoretical analysis have shown that the magnons contribute appreciably but also the phonons contribution cannot be neglected. In this talk, I will discuss the spin-phonon coupling in Mott insulators and consequent thermal Hall effect. The key idea is the scalar spin chirality of spin system, and the experimental results are analyzed for YMnO3 [5] and Kitaev spin liquid [6].

[1] H.Katsura, N. Nagaosa, and P.A.Lee, Phys. Rev. Lett. 104, 066403 (2010).

[2] Y. Onose, Y. et al. Science 329, 297–299 (2010).

[3] T. Saito, K.Misaki, H. Ishizuka, and N. Nagaosa, Phys. Rev. Lett.123, 255901(2019).

[4] H.L. Kim, T. Saito, H. Yang, H. Ishizuka, M.J. Coak, J.H. Lee, H. Sim, Y.S. Oh, N. Nagaosa,

J.-G. Park, Nature Communications 15 (1), 243 (2024).

[5] T. Oh, and N. Nagaosa, arXiv:2408.01671 2024, and to appear in PRX.

[6] T. Oh, and N. Nagaosa, in preparation. 

Heat travels in solids thanks to phonons. In clean non-magnetic crystalline insulators, thermal conductivity is governed by Umklapp phonon-phonon collisions near the Debye temperature. At cryogenic temperatures, phonons become ballistic, travelling between sample boundaries. The thermal conductivity peaks at an intermediate temperature. In a variety of insulators, hydrodynamic regimes of phonon transport have been detected near this peak. A poorly understood phonon thermal Hal signal has been observed in elemental non-magnetic insulators, such as black P, germanium and diamond. Interestingly, the thermal Hall angle is maximum at this peak and its maximum amplitude does not correlate with the phonon mean free path. I will argue that the most plausible source of the latter is the field-induced geometric phase of acoustic phonons, driven by the breakdown of the Born-Oppenheimer approximation in a magnetic field.

Two dimensional (2D) magnets, particularly van der Waals magnets, have been attracting much attention worldwide since they were first reported by a series of papers in 2016 [1]. Over the past few years, much of the attention has been on discovering new materials with novel ground states, and the current focus is on topological magnets. Antiferromagnetic metallic Co1/3-TaS2 is the latest addition to this new class of 2D magnets [2]. It exhibits a considerable anomalous Hall effect (AHE), which was recently assigned to a rather unusual form of 3Q tetrahedral structure, a highest-density Skyrmion phase [3]. We also found that this 3Q phase is extremely sensitive to external variables like Co concentration and drastic Fermi surface change [4, 5]. As another demonstration of this controllability, we demonstrated that the ground state can be changed dramatically by controlling carrier density via gating. 

[1] Je-Geun Park, J. Phys. Condens. Matter 28, 301001 (2016).

[2] Pyeongjae Park, et al., npj Quantum Materials 7, 42 (2022).

[3] Pyeongjae Park, et al., Nature Communications 14, 8346 (2023).

[4] Pyeongjae Park, et al., Phys. Rev. B Letter 109, L060403 (2024).

[5] Han-Jin Noh, et al., (under review).

Since the discovery of superconductivity at 80 K in single crystals of La3Ni2O7 at pressures above 14.0 GPa, extensive efforts have been made to understand the properties of the bilayer nickelate system at both ambient and high pressure [1-3]. Density-wave-like orders, structural transition, strange metal behavior, oxygen vacancies, and orbital-dependent correlations were observed in the pressure-dependent phase diagram of La3Ni2O7[4-9]. Their connections to superconductivity are under debate.

The trilayer nickelates La4Ni3O10 and Pr4Ni3O10 and the intergrowth nickelate La5Ni3O11 also show superconductivity under pressure. Importantly, superconductivity with the transition temperature above 40 K at ambient pressure has been observed in La3Ni2O7 thin film samples. Many puzzles of the superconductivity in the Ruddlesden-Popper nickelates can be explored. 

In this talk, I will review the study of the superconductivity in the Ruddlesden-Popper nickelates and discuss the key parameters for high Tc superconductivity in nickelates. 

[1] H. L. Sun, M. W. Huo, X. W. Hu et al., Nature 621, 493-498(2023)

[2] Y. N. Zhang, D. J. Su, Y. E. Huang et al., Nature Physics, 20, 1269(2024) 

[3] J. Hou, P. T. Yang, Z. Y. Liu et al., Chinese Physics Letters 40, 117302(2023)

[4] Z. Liu, H. L. Sun, M. W. Huo, et al., Sci. China-Phys. Mech. Astron. 66, 217411(2023).

[5] L. H. Wang, Y. Li, S. Y. Xie et al., JACS 146, 7506(2024)

[6] Z. H. Dong, M. W. Huo, J. Li et al., Nature 630, 847 (2024)

[7] J. Yang, L. Zhao, M. Wang, X. J. Zhou et al., Nat. Commnu. 15, 4373(2024)

[8] Z. Liu, M. W. Huo, J. Li et al., Nat. Commnu. 15, 7570(2024)

[9] T. Xie, M. W. Huo, X. S. Ni et al., Sci. Bull. 69, 3221(2024) 

Antiferromagnetic insulators with strong spin-orbit coupling, such as Sr2IrO4, present exciting platform for terahertz magnonics due to their ultrafast spin dynamics and sensitivity to external stimuli [1]. However, achieving precise control of magnon propagation remains challenging. In this talk, I will discuss Sr2IrO4, a quasi-two-dimensional antiferromagnetic Mott insulator with Jeff = 1/2 pseudospins, whose terahertz spin waves respond strongly to adjacent materials. Using resonant inelastic x-ray scattering, we systematically probed the spin-wave dispersion of Sr2IrO4 thin films interfaced with metallic and insulating crystals including 4d ruthenates. Our results reveal a notable softening of single-magnon modes near the (π/2, π/2) zone boundary in films adjacent to metallic crystals, while the magnon spectrum remains unchanged for insulating interfaces. Complementary Raman spectroscopy highlights a softening of two-magnon excitations and a hardening of phonon modes near metallic interfaces, pointing to electron-phonon interactions as the primary mechanism [2]. These findings suggest a novel means to control terahertz magnon propagation, distinct from conventional interfacial effects such as strain or doping. It also underscores the need for further theoretical studies to better understand the underlying microscopic interactions between magnons and phonons.

[1] H.-H. Kim et al. Nat. Commun. 13, 6674 (2022).

[2] S. Shrestha et al., Nat. Commun. 16, 3592 (2025).

We investigate the signatures of fractionalization in quantum spin liquids, explicitly focusing on the Kitaev honeycomb model under an external magnetic field and the Kagome Heisenberg model. We compute the dynamical response functions to identify spectral features associated with fractionalized excitations using the infinite projected entangled pair states ansatz combined with analytical methods. Our analysis shows that these spectral signatures uniquely distinguish distinct fractionalized quantum sectors, providing theoretical benchmarks for experimental verification.

A quantum spin liquid hosts massive quantum entanglement whose identification is one of the most significant problems in physics. Yet, its detection is known to be notoriously difficult because of featureless properties without a symmetry order parameter. Here, we demonstrate dynamic signatures of a quantum spin liquid state by investigating Kitaev's spin model on the hyper-honeycomb lattice, where a quantum spin liquid state is stabilized as a stable thermodynamic phase. The real-time dynamics of spin correlation function is obtained with the large-scale quantum Monte Carlo simulation. We find the onset of a characteristic oscillation in dynamic local spin correlation as entering the quantum spin liquid phase. Our results show that a quantum spin liquid may be characterized by a sharp growth of coherent spin dynamics of the system, which we name as a dynamic order. We further propose that a dynamic-order may naturally detect a featureless thermal phase transition, which has been reported in a class of strongly correlated materials.

    Motivated by the physics of pyrochlore oxides we consider the effect of quantum fluctuations on the most general symmetry-allowed nearest-neighbor Hamiltonian on the celebrated pyrochlore lattice. On the classical level, recent works unveiled a rich landscape of classical spin liquids described by higher-rank gauge theories and associated fracton excitations, as well as a variety of non-conventional magnetic phases. In contrast, much remains unclear for the quantum model, where the introduction of quantum fluctuations has the potential to drastically change the classical landscape and potentially stabilize novel magnetic phases. We address this pressing question by assessing the quantum phase diagram of this model at relevant cross-sections, which at the classical level are host to a triple-point featuring an algebraic nodal spin liquid described by a rank-2 gauge theory, and a phase boundary with spin-nematic order. Employing state-of-the-art pseudo-fermion functional renormalization group calculations for the spin-1/2 model, we find large regions in parameter space where conventional magnetic order is absent. Based on a careful analysis of known fingerprints in the structure factors of different phases, we present evidence that the non-conventional region is composed of an ensemble of distinct phases stabilized by quantum fluctuations. Most interestingly, we hint at a spin nematic phase and identify a quantum triple point featuring pinch-line scattering features as expected from a gauge theoretical description of the classical phase, thus potentially realizing a quantum version of the rank-2 U(1) gauge theory. We highlight the importance of assessing the subtle interplay of quantum and thermal fluctuations in reconciling the experimental findings on the nature of magnetic order in Yb2Ti2O7 with those predicted from theoretical approaches. 

The interplay between electronic correlations and nontrivial topology in condensed matter systems often gives rise to remarkable quantum phenomena. In this regard, the pyrochlore latticeconsidered a three-dimensional (3D) analogue of the kagome latticeemerges as a highly promising material platform, as it can simultaneously host 3D flat bands and Dirac bands, which can give rise to strong correlations and electronic topology, respectively [1,2]. However, previous studies of the electronic structures in pyrochlore lattice compounds have largely been interpreted within a non-interacting single-particle framework. 

In this presentation, I will present our recent studies on the metallic spinel oxide compound LiV2O4, which features a V-pyrochlore sublattice. Using angle-resolved photoemission spectroscopy (ARPES), we observed an electronic flat band at the Fermi level, consistent with its enigmatic d-orbital heavy fermion behavior [3]. Through a collaborative study employing dynamical mean field theory (DMFT) method, we find that the emergence of heavy electronic state in LiV2O4 is driven by strong local electronic correlationprimarily governed by Hund’s couplingand is unrelated to the flat band originating from geometrical hopping frustration. 

I will also discuss how the heavy fermion behavior in LiV2O4featuring the largest Sommerfeld coefficient reported among d-orbital heavy fermion compoundscan be stabilized from the perspectives of crystal field splitting and magnetic frustration. Our findings not only offer crucial insight into resolving a long-standing puzzle regarding the origin of heavy fermion behavior in LiV2O4 but also suggest a new pathway for realizing heavy electronic states in frustrated spinel oxides.

[1] J. P. Wakefield et al., Nature 623, 301 (2023).

[2] D. Oh et al., Phys. Rev. B 110, 205102 (2024).

[3] D. Oh et al., arXiv:2502.07234 (2025).

The physics of flat band materials is a longstanding issue in condensed matter physics. Theorists studied various lattices that realize flat bands for a long time, and recently the presence of a flat band has been experimentally confirmed in several two-dimensional systems and unique electronic properties related to the flat band have been discovered [1]. In contrast, there are few three-dimensional systems having a flat band, whereas the pyrochlore-related materials are rare examples; cubic Laves phase CaNi2 was found to have almost flat bands by ARPES experiments [2] and β-pyrochlore CsW2O6 was predicted to have the perfect flat band arising from the strong spin-orbit coupling [3]. In this talk, I will report that NbSeI is a new system for studying the flat band physics. NbSeI was reported to crystallize in a cubic MoSBr type with the space group of F–43m [4], where each Mo, Se, and I atoms form a breathing pyrochlore structure. This crystal structure can also be understood as consisting of Nb4Se4 cubanoclusters surrounded by I atoms, as shown in Fig. 1(b). We succeeded in synthesizing the NbSeI single crystals, as shown in Fig. 1(a) and performed various experiments on them. First principles calculations based on the structural parameter determined by the synchrotron XRD data showed that a metallic electronic state is realized in NbSeI at room temperature, where the Fermi level is located in an almost flat electronic band. However, the electrical resistivity and magnetic susceptibility data indicated that NbSeI is a nonmagnetic insulator. Synchrotron XRD and heat capacity data showed that NbSeI exhibits a structural phase transition at 110 K. The local disorder above 110 K, which is related to the structural distortion below 110 K, may play an important role in the insulating nature of NbSeI. This work was done in collaboration with K. Kojima, R. Okuma, J. Yamaura, S. Kitou (Univ. Tokyo), and Y. Yamakawa (Nagoya Univ.). 

[1] Y. Cao et al., Nature 556, 80 (2019). 

[2] J. P. Wakefield et al., Nature 623, 301 (2023). 

[3] H. Nakai and C. Hotta, Nat. Commun. 13, 579 (2022). 

[4] V. E. Federov et al., Zh. Neorg. Khim. 26, 2701 (1981). 

TBA

   Kitaev spin liquids (KSLs) are an exotic quantum state of matter that could host elusive Majorana fermions [1]. However, detecting these spin liquids is challenging. Unlike conventional magnets, they lack magnetic order, making them invisible to standard scattering techniques. Additionally, Majorana fermions are electrically neutral, complicating electrical measurements. In our research, we took an alternative approach: leveraging the fact that Majorana fermions contribute to entropy and that their excitation spectra vary with the direction of an applied magnetic field. By measuring how heat capacity changes with the magnetic field angle, we sought to identify KSL behavior.  In the prime candidate material α-RuCl3, we find a distinct closure of the low-energy bulk gap is observed for the fields in the Ru-Ru bond direction, and the gap opens rapidly when the field is tilted. This is the hallmark of an angle rotation–induced topological transition of fermions, providing conclusive evidence for the KSL with Majorana quasiparticles [2,3]. In contrast, in Na2Co2TeO6, which was considered a promising 3d KSL candidate due to potentially stronger Kitaev interactions than the 4d electron systems, the behavior was markedly different; the field-angular dependence of specific heat shows opposite behavior to the expectation in the KSL [4]. This finding suggests that Na2Co2TeO6 is not a KSL, but exhibits topological magnon excitations. Our study demonstrates that angular-dependent heat capacity measurements provide a powerful tool for distinguishing true Kitaev spin liquids.

[1] Y. Matsuda, T. Shibauchi, and H.-Y. Kee, Rev. Mod. Phys. (submitted); arXiv:2501.05608.

[2] O. Tanaka, Y. Mizukami, R. Harasawa, K. Hashimoto, K. Hwang, N. Kurita, H. Tanaka, S. Fujimoto, Y. Matsuda, E.-G. Moon, and T. Shibauchi, Nat. Phys. 18, 429-435 (2022).

[3] K. Imamura, S. Suetsugu, Y. Mizukami, Y. Yoshida, K. Hashimoto, K. Ohtsuka, Y. Kasahara, N. Kurita, H. Tanaka, P. Noh, J. Nasu, E.-G. Moon, Y. Matsuda, and T. Shibauchi, Sci. Adv. 10, eadk3539 (2024).

[4] S. Fang, K. Imamura, Y. Mizukami, R. Namba, K. Ishihara, K. Hashimoto, and T. Shibauchi, Phys. Rev. Lett. (to be published); arXiv:2410.18449.

α-RuCl3 is a leading candidate for the realization of the Kitaev spin liquid in spin-orbit coupled Mott insulators. Experimental evidence of fractionalized excitations suggests that α-RuCl3 is in proximity to the Kitaev spin liquid phase; however, it exhibits the zigzag antiferromagnetic order at low temperature. Despite extensive efforts, the precise nature of the pseudospin interactions responsible for long-range antiferromagnetic order remains an open question. By investigating the momentum dependence of the magnetic dynamics by resonant inelastic x-ray scattering (RIXS) at the Ru L3 edge, we have determined the pseudospin Hamiltonian with an unprecedented precision [1]. Furthermore, the recent synthesis of the sibling compounds RuX3 (X = Br, I) has expanded the set of material candidates. The RIXS spectra of RuBr3 and RuI3 reveal quasi-elastic magnetic correlations and spin-orbit transitions to the J = 3/2 states, establishing the J = 1/2 description of magnetism in the RuX3 family. The evolution of the multiplet structures indicates a gradual suppression of electronic correlation as X changes from Cl to I, which is attributed to an increased bandwidth due to enhanced d-p hybridization [2]. Additionally, we found that the zigzag correlations in RuBr3 persist well above the Néel temperature, in contrast to the fragile zigzag correlations observed in α-RuCl3 [3]. Our findings highlight the crucial role of halogen p orbitals in governing the magnetic properties of RuX3.

[1] H. Suzuki et al., Nat. Commun. 12, 4512 (2021).

[2] H. Gretarsson et al., Phys. Rev. B. 109, L180413 (2024).

[3] H. Suzuki et al., in preparation

Kitaev quantum spin liquids—exotic spin states characterized by macroscopic quantum entanglement and fractionalized excitations—represent one of the most elusive phases of quantum matter. While prior investigations have predominantly focused on bulk materials, our approach leverages heterostructure engineering to directly tune the spin Hamiltonian parameters of candidate systems. In the first part of this talk, I will present our optical spectroscopy results on the two-dimensional honeycomb cobaltate Cu₃Co₂SbO₆ [1], a recently proposed class of materials for realizing Kitaev quantum spin liquid. Our measurements reveal unconventional spin fluctuations persisting well above the Néel temperature (16 K), signaling significant spin frustration driven by competing exchange interactions. In the second part, I will discuss how strain engineering can modulate this frustration and suppress the long-range order in Cu₃Co₂SbO₆ [2]. We find a clear correlation between trigonal crystal distortion and the Néel temperature, confirming that strain could enhance spin frustration. 

[1] B. Kang et al., Nature communications 16, 1323 (2025)

[2] G. H. Kim et al., Science advances 10, eadn8694 (2024)

TBA

Recently, it was observed that many of the superconducting properties of moiré superconductors with ultra-flat bands deviate greatly from conventional BCS theory predictions [1]. In particular, the observed superconducting coherence length is more than 20 times longer than the one estimated from the BCS theory. In this talk, I would like to present a Ginzburg-Landau theory, derived from a microscopic flat band Hamiltonian, which incorporates the quantum metric effects of moiré flat band superconductors. We show that the length scale defined by quantum metric, which we call the quantum metric length (QML), determines the superconducting coherence length of flat band superconductors [2,3]. Moreover, we show that the QML is a fundamental length scale which determines the decay length of the Majorana modes of flat band topological superconductors [4]. We further show that QML also determines the length scale of flat band Josephson junctions [5]. Overall, the QML is a fundamental length scale in flat band materials which is important in other ordered phases as well. 

[1] Haidong Tian, et al. Nature 614, 440 (2023).

[2] Shuai A Chen, KT Law, Phys. Rev. Lett. 132, 026002 (2024).

[3] Jin-Xin Hu, Shuai A Chen, KT Law, Communications Physics 8 20 (2025).

[4] Xingyao Guo, Xinglei Ma, Xuzhe Ying, KT Law, arXiv:2406.05789.

[5] Zhong CF Li, Yuxuan Deng, Shuai A Chen, Dmitri K Efetov, KT Law, To appear at Physical Review Research (2025).

In quantum materials with multiple degrees of freedom with similar energy scales, intertwined electronic orders with distinct broken symmetries often appear in a strongly coupled fashion. Recently, in a class of kagome superconductors represented by CsV3Sb5, experimental reports have suggested rotational symmetry breaking and time reversal symmetry breaking associated with a charge density wave (CDW) order, revealing an exotic nature of this CDW order. In this talk, I will first introduce our recently developed capability of performing angle-resolved photoemission spectroscopy in a tunable magnetic field (magneto-ARPES), then present measurements of magneto-ARPES on CsV3Sb5, from which we reveal momentum-selective response of the electronic structure of CsV3Sb5 to an external magnetic field, directly providing spectroscopic evidence of time reversal symmetry breaking of the CDW order. Our magneto-ARPES work demonstrates a novel tuning knob for disentangling intertwined orders in the momentum space for quantum materials. 

In the early days of high-Tc cuprate research, theorists proposed that chiral spin liquids could strongly compete with the Néel ordered state, particularly when ring-exchange terms or strong frustration are present [1]. Despite intense effort, direct experimental evidence for such chiral spin states has remain elusive. In this talk, I will discuss how Raman interferometry can detect the emergent gauge flux associated with chiral spin liquids, manifesting as a Berry phase acquired by an electron hopping virtually around a plaquette. In the absence of this phase, Raman processes at order t^4/U^3 order cancel out upon summation over all paths. However, when a non-zero flux is present, the interference yields terms proportional to spin chirality. We apply this methodology to square-lattice iridates and find clear signatures for slow spin chirality fluctuations. Upon doping away from half filling, the signal becomes significantly enhanced. These Raman data thus provide the first clear evidence for spin chirality in a S=1/2 square-lattice antiferromagnet, shedding new light on the long-sought chiral spin liquid state.  

[1] X. G. Wen, Frank Wilczek, and A. Zee, Phys. Rev. B 39, 11413 (1989).

Altermagnet [1] is a class of antiferromagnets showing a staggered spin ordering with wave vector q = 0 and its net magnetization is cancelled out in the limit of zero relativistic spin-orbit coupling. The simplest case is when the up and down spins are ordered on two sublattice sites within the unit cell which are not connected by either translation or inversion. Consequently, the system breaks the “macroscopic” time-reversal symmetry and exhibits non-relativistic spin splitting in the energy band [2,3] and characteristic cross-correlation phenomena [3,4]. 

In this talk, I will introduce our theoretical studies based on effective models for typical strongly correlated electron systems, e.g., κ-type organic compounds [3,5] and perovskite-type transition metal compounds [6]. In these systems, the checker-plate-type molecular arrangements and the GdFeO3-type lattice distortions, respectively, play the role of the crystallographic setting for altermagnetism. We show that antiferromagnetic orderings give rise to the non-relativistic spin splitting, owing to anisotropic sublattice dependent (and thus spin dependent) electron hoppings, and its consequent spin current generation (Figure 1), and the anomalous Hall effect in the presence of the spin-orbit coupling.

[1] L. Šmejkal, J. Sinova, and T. Jungwirth, Phys. Rev. X 12, 031042 (2022).

[2] K.-H. Ahn, A. Hariki, K.-W. Lee, and J. Kuneš, Phys. Rev. B 99, 184432 (2019).

[3] M. Naka, S. Hayami, H. Kusunose, Y. Yanagi Y. Motome, and H. Seo, Nat. Comm. 10, 4305 (2019).

[4] L. Šmejkal, R. González-Hernàndez, T. Jungwirth, and J. Sinova, Sci. Adv. 6, eaaz8809 (2020).

[5] M. Naka, S. Hayami, H. Kusunose, Y. Yanagi, Y. Motome, and H. Seo, Phys. Rev. B 102, 075112 (2020); H. Seo and M. Naka, J. Phys. Soc. Jpn. 90 064713 (2021); S. Sumita, M. Naka, and H. Seo, Phys. Rev. Res. 5, 043171 (2023).

[6] M. Naka, Y. Motome, and H. Seo, Phys. Rev. B 103, 125114 (2021); Phys. Rev. B 106, 195149 (2022); npj Spintronics 3, 1 (2025).

Moiré superlattice emerges from the interference between two mismatched atomic lattices, and it has led to tremendous success in designing and tailoring the electronic states in two-dimensional (2D) homo- and hetero-structures. Yet, the power of moiré superlattice in controlling the spin degree of freedom and thus modifying the magnetic states is much less explored. Only very recently after the development of 2D magnet research, there have been a few experimental attempts in realizing moiré magnetism in twisted 2D magnet homo-structures. In this talk, I will show our recent effort in studying magnetic phases in twisted double bilayer chromium triiodide (CrI3) and progressive steps towards realizing moiré magnetism. Noting that bilayer CrI3 is a layered antiferromagnet and that any homogeneous stacking of two bilayers necessarily produces zero magnetization, we have revealed, in twisted double bilayer CrI3, an unexpected net magnetization showing up at intermediate twist angles and its accompanied noncollinear spin textures. I will show the optical spectroscopy signatures of this twist-induced magnetic phase, then discuss its dependence on twist angle, external magnetic field, and temperature [1, 2, 3].

[1] H. Xie et al., Nature Physics 18, 30 (2022)

[2] H. Xie et al., Nature Physics 19, 1150 (2023)

[3] Z. Sun et al, submitted (2025)

I will present experimental studies on the topological and correlated properties of monolayer TaIrTe4. First, I will discuss a dual quantum spin Hall (QSH) insulator, arising from the interplay between its single-particle topology and density-tuned correlations [1]. At charge neutrality, monolayer TaIrTe4 exhibits QSH insulator behavior, characterized by enhanced nonlocal transport and quantized helical edge conductance. Upon introducing electrons from charge neutrality, TaIrTe4 only shows metallic behavior in a small range of charge densities but quickly goes into a new insulating state. This insulating state could arise from a strong electronic instability near the van Hove singularities, e.g., a charge density wave. Within this correlated insulating gap, we observe a resurgence of the QSH state. I will also discuss our recent efforts to study this correlated gap using nonlinear Hall responses. 

[1] J. Tang, et al., Nature, 628, 515 (2024).

An equilateral spin trimer composed of three 𝑆 = 1/2 spins is known to possess magnetic and electric dipoles at low energies, the latter arising from virtual charge fluctuations akin to ring exchange [1]. A weakly-coupled trimer system is then expected to exhibit magnetoelectric (ME) effects due to the interplay between emergent electric dipoles and other low-energy degrees of freedom per trimer [2]. In this talk, we demonstrate a materials design strategy to realize this concept using organic chemistry, which offers remarkably flexible routes [3]. Specifically, our collaborators synthesized single-crystalline TNN·CH3CN, an organic quantum spin trimer antiferromagnet composed of TNN, a perfectly 𝐶₃-symmetric molecular magnet [4]. The ME effects, as revealed by thermodynamic, magnetic, and dielectric measurements, suggest a rich multiferroic phase diagram. Theoretically, we derive low-energy effective models and run semiclassical CP2  and CP3  Monte Carlo simulations for the low- and high-magnetic field regimes, respectively. We find a linear ME effect due to spin-orbital entanglement in the effective Kugel-Khomskii model, orbital ferroelectricity in the effective compass model, and a novel kinetic ME effect in the effective bosonic 𝑡-𝐽 model. Our results thus demonstrate a bottom-up pathway for designing multiferroics and ME effects rooted in quantum spin trimer systems.

[1] L. Bulaevskii et al., Phys. Rev. B 78, 024402 (2008).

[2] Y. Kamiya and C. D. Batista, Phys. Rev. Lett 108, 097202 (2012).

[3] Y. Takano, Y. Hosokoshi, Y. Kamiya, C. D. Batista et al., in preparation.

[4] Y. Nakano et al., Polyhedron 24, 2141 (2005).

Long sought is smoking gun evidence for Chiral Topological Superconductivity via nonlocal responses of its emergent modes. I will discuss the first observation of protected, non-local transport from the edge modes of the potential Weyl-superconductor FeTe0.55Se0.45. Specifically, an anomalous conductance plateau emerges only when topological, superconducting, and magnetic phases coexist, with source-drain contacts coupled via the edge. Moving the drain to the bulk switches the non-local transport process to a local Andreev process, generating a zero-bias conductance peak (ZBCP). The edge mode's topological protection is confirmed by its insensitivity to external magnetic fields and increasing temperatures until the spontaneous magnetization is substantially suppressed. Our findings provide a new methodology to demonstrate TSC via topologically protected non-local transport.

Josephson junctions (JJs) fabricated on a three-dimensional (3D) topological insulator (TI) can be used to probe signatures of Majorana bound states (MBSs) at Josephson vortices. In this talk, I will present our recent advancements for such observations using Corbino-geometry JJs fabricated on a single surface of a bulk-insulating 3D TI. We successfully fabricated high-quality Nb JJs on Sn-doped Bi1.1Sb0.9Te2S single crystals with extremely low bulk carrier concentrations, with their pristine surfaces preserved by Te films grown via molecular beam epitaxy. Utilizing different Corbino-style geometries enabled Josephson interferometry within a single junction, revealing the skewed current-phase relation of our highly transparent junctions. Strikingly, we observed a superconducting diode effect alternating its sign for even- and odd-fluxoid states. Our analysis attributes this phenomenon to a topological phase, with diode polarity directly reflecting the sign reversal in periodic boundary conditions for even/odd numbers of Josephson vortices. Lastly, I will discuss our latest efforts to probe the non-Abelian statistics of MBSs in Corbino–Josephson trijunction hybrid devices, where Josephson vortices can be exchanged, collided, and split.