One important issue in current condensed matter physics is the search of quantum spin liquid (QSL), an exotic magnetic state with strongly-fluctuating and highly-entangled spins down to zero temperature without static order. However, there is no consensus on the existence of a QSL state in any real material so far, due to the inevitable disorders and competing exchange interactions on frustrated spin lattices. Here we report systematic heat transport measurements on a honeycomb-lattice compound BaCo2(AsO4)2, which manifests magnetic order in zero field. In a narrow in-plane field range after the magnetic order is nearly suppressed, in both perpendicular and parallel to the zigzag direction, a finite residual linear term of thermal conductivity is clearly observed, which is attributed to mobile fractionalized spinon excitations. This provides smoking-gun evidence for a gapless QSL state in BaCo2(AsO4)2 [1].

[1] C. P. Tu et al., arXiv:2212.07322 (2022).

Two-dimensional honeycomb lattice is a platform for realizing quantum spin liquid states arising from the bond-dependent Ising-type interaction [1], which is often called the Kitaev-type, or competing nearest and next nearest neighbor interactions. In recent years, transition metal compounds under the effect of spin-orbit coupling including iridate, ruthenate, and cobaltate have been extensively investigated in search for the Kitaev-type quantum spin liquid. While there are several candidate materials, the establishment of the quantum spin liquid state is still underway. In this talk, we would like to introduce our recent development of new honeycomb materials from different material categories. One is the samarium triiodide SmI3, where Sm3+ (4f5) ions form the honeycomb lattice. Our magnetic and structural characterizations revealed that the electronic state of the Sm3+ ions and their geometrical environment in the crystal structure materialize the situation where the emergence of the antiferromagnetic Kitaev-type interaction is theoretically proposed [2]. The second material is [(CH3)2(NH2)]3[Cu3(OH)(SO4)4]·0.24H2O [3], which we call Dimethylammonium Copper Sulfate (DiMACuS). Our magnetization measurements up to 120 tesla clarified its full magnetization process, which indicates the Cu-spin triangles with strong intra-triangle interactions that carry the effective spin-1/2 at low-temperatures are coupled to from the honeycomb lattice on the underlying star lattice. In both materials, the magnetic ordering was not observed down to 0.1 kelvin [4,5].

[1] A. Kitaev, Ann. Phys., 321, 2 (2006).

[2] Y. Motome et al., J. Phys. Condens. Matter, 32, 404001 (2020).

[3] M. Sorolla et al., J. Am. Chem. Soc., 142, 5013 (2020).

[4] H. Ishikawa et al., Phys. Rev. Materials, 6, 064405 (2022).

[5] H. Ishikawa et al., under review.

Recently, quantum many-body calculations based on some specially designed models support the existence of a second order phase transition between two symmetry breaking states, namely, a deconfined quantum critical point (DQCP) [1]. Such a DQCP violates the Landau paradigm of an order-order quantum phase transition and may also be accompanied by novel properties such as enhanced symmetries at the critical point and fractional excitations. However, up to now, no experimental evidence of DQCP has been found. Here I will report our exploration of DQCP on a spin frustrated Shastry-Sutherland material SrCu2(BO3)2 [2], through high pressure, high field, and ultra-low temperature NMR studies. Under high pressure, some thermodynamic studies on the compound reported that its ground state may undergo a phase transition from a dimerized spin state (DS) to a plaquette singlet state (PS) [3,4,5].

We discovered microscopic experimental evidence of symmetry breaking in the PS state in the NMR spectra. Further, at pressures of 2.1 and 2.4 GPa, we found that magnetic fields induced a weak first-order phase transition from the PS state to the antiferromagnetic state (AFM). At the quantum phase transition, the coexistence temperature of the two phases is as low as 0.07 K, and when the pressure rises, the transition becomes more continuous; the (H, T) phase boundaries of both PS and AFM phases follow a duality with the same power-law scaling exponent; the spin lattice relaxation rates also reveal that the low energy spin dynamics at 2.4 GPa exhibit a quantum critical scaling behavior; the numerical simulation of the system shows that an enhanced O(3) symmetry at the transition [6]. These results give an unambiguous proximate DQCP in the system, and provide a concrete platform for studying properties of DQCP.

[1] R. R. P. Singh, Physics 3, 35 (2010).

[2] H. Kageyama et al., Phys. Rev. Lett. 82, 3168 (1999).

[3] M. E. Zayed et al., Nat. Phys. 13, 962 (2017).

[4] J. Guo et al., Phys. Rev. Lett. 124, 206602 (2020).

[5] J. Larrea Jimenez et al., Nature 592, 370 (2021).

[6] Y. Cui et al., arXiv: 2204.08133 (2022).

The spin-1/2 antiferromagnetic Heisenberg model on a Kagomé lattice offers an excellent framework for investigating complex quantum entangled states. In particular, it allows for the exploration of a quantum spin liquid in the absence of an external field, as well as magnetic analogs of liquid, solid, and supersolid phases near magnetization plateaus. In this talk, we will address the experimental realization of these predicted novel phases in the material YCu3(OD)6+xBr3-x (x≈0.5) on a Kagomé lattice.

By combining thermodynamic and Raman spectroscopic techniques, we provide compelling evidence for the existence of fractionalized spinon excitations in YCu3(OD)6+xBr3-x (x≈0.5). These excitations exhibit a Dirac nodal structure, although their spectral weight at low energies is modified by the presence of bond randomness. Our preliminary NMR data reveal an inhomogeneous ground state due to inevitable perturbations. More significantly, we observe the 1/9 magnetization plateau at an accessible field range of μ0H1/9=15-21 T undiscovered hitherto in S=1/2 Kagomé antiferromagnets. Our findings highlight the significance of YCu3(OD)6+xBr3-x as a model material, which enables the investigation of field-induced quantum entangled states within the 1/9 plateau phase.

Kagome superconductor AV3Sb5 (A=K, Cs, Rb) exhibits rich quantum phase transitions, such as bond-order (BO), time-reversal-symmetry-breaking (TRSB) phase, superconductivity. Here, we focus on the TRSB, which was reported by μSR [1], Kerr rotation, field-tuned chiral transport and STM study. Inside/Outside the TRSB phase, emergent of nematic (C2) order was reported and attracts great attention. The nematic transition is clearly observed by the elastoresistance, the scanning birefringence, and the STM studies. More recently, magnetic torque measurement reveals the nematic order with TRSB at 130 [K].

However, the microscopic origin of TRSB and C2-nematicity have been unsolved. To solve this problem, we consider the important roles of bond-order (BO) fluctuations [2,3], which is developed near the quantum-critical point of BO phase. Considering the exchange processes of BO fluctuation, we reveal that the chiral current phase is the most stable phase. This exchange process cause the sizable off-site Umklapp scattering. The order parameter of loop current is expressed by the imaginary hopping in Hermite Hamiltonian. Thus, the chiral current order brings TRSB. Furthermore, we discover that the coexistence of the BO and current order cause the novel C2-nematicity along the three lattice directions (3Q) on kagome lattice. To show this fact, we calculate the Ginzburg-Landau coefficients and find the emergence of C2-nematic state. The present theory reveals the close relationship between the TRSB, BO, C2-nematicity.

In the presentation, we also showed the recently revealed interesting results; (i) the chiral current order is strongly magnified under the magnetic field along c-axis. (ii) Giant magnetization emerges due to the tiny order parameter of loop current phase [4]. These results a consistent with the field-induced enhancement of the loop current observed by μ-SR studies and field-tuned chiral transport study.

[1] C. Mielke, et al., Nature 602, 245 (2022).

[2] R. Tazai, et al., arXiv:2207.08068 (2022).

[3] R. Tazai, et al., Sci. Adv. 8, eabl4108 (2022).

[4] R. Tazai, et al., arXiv:2303.00623 (2023).

The quantum dime models (QDM) with fixed dimer(s) per site have strictly local constraint, which are common low-energy effective description for a class of frustrated magnetisms. We have developed an efficient quantum Monte Carlo method – Sweeping Cluster Algorithm (SCA), which can directly sample in constrained Hilbert space. Then we have carefully studied the fractional excitations and topological phases of QDM on triangular. What’s more interesting, we find a hidden order which has been treated as quantum spin liquid (QSL) for long time. Furthermore, a general QDM with variable dimer density was designed which contains both even and odd Z2 QSLs. The phase diagram and phase transition have been studied. The models are likely to be implemented in the Rydberg array experiment and some Ising magnets.

[1] Z. Yan et al., Phys. Rev. B 99,165135(2019).

[2] Z. Yan et al., npj Quantum Mater. 6, 39 (2021).

[2] Z. Yan, Phys. Rev. B 105,184432(2022).

[3] Z. Yan et al., arXiv: 2205.04472 (2022).

[4] Z. Yan et al., Nat. Commun. 13, 5799 (2022).

     Identifying the magnetic state of materials is of great interest in a wide range of applications, but direct identification is not always straightforward due to limitations in neutron scattering experiments. In this work, we present a machine-learning approach using decision-tree algorithms to identify magnetism from the spin-integrated excitation spectrum, such as the density of states. The dataset was generated by Hartree-Fock mean-field calculations of candidate antiferromagnetic orders on a Wannier Hamiltonian, extracted from first-principle calculations targeting BaOsO3. Our machine learning model was trained using various types of spectral data, including local density of states, momentum-resolved density of states at high-symmetry points, and the lowest excitation energies from the Fermi level. Although the density of states shows good performance for machine learning, the broadening method had a significant impact on the model’s performance. We improved the model's performance by designing the excitation energy as a feature for machine learning, resulting in excellent classification of antiferromagnetic order, even for test samples generated by different methods from the training samples used for machine learning. We apply the model to dynamical mean-field results, which are generated using a completely different computational method, and find that it achieves good accuracy. Our application of the model to dynamical mean-field results, generated through a completely different computational method, yielded good accuracy, suggesting its potential applicability to more realistic data.

This talk aims to introduce orbitronics, the study of electron angular momentum in solids. Starting from the well-established intrinsic spin Hall effect, we argue that the electron orbital angular momentum is crucial for the intrinsic spin Hall effect in centrosymmetric systems. We then discuss the issue of orbital quenching. We demonstrate that the crystal field's suppression of the electron orbital angular momentum is limited to equilibrium, and the orbital angular momentum can easily develop in many materials when an external electric field is applied. We then review recent efforts to probe the orbital dynamics experimentally. The main ideas of the orbital torque and orbital accumulation will be briefly discussed. Formal aspects of the orbital dynamics will also be discussed, which imply important differences between the orbital and the spin dynamics.

Chirality is an important and universal concept in nature, from biology and chemistry to particle physics. In condensed matter physics, chiral (quasi-)particles have attracted considerable attention, e.g., chiral fermions because of their relevance in emergent phenomena and topology [1,2]. On the other hand, however, the experimental verification of chiral phonons (bosons) remains challenging [3,4], despite their expected significant relevance to the fundamental physical properties as chiral fermions. Here we show a proof-of-principle study using resonant inelastic X-ray scattering with circularly polarized X rays to measure chiral phonons [5]. Using the prototypical chiral material, quartz, we disclosed that circularly polarized X rays couple to chiral phonons at general momentum points in reciprocal space, which potentially allows us to determine the chiral phonon dispersion. Our experimental proof of chiral phonons demonstrates a new degree of freedom in condensed matter and opens the door to the exploration of novel emergent phenomena based on chiral phononics.

[1] S. Xu et al., Science 349, 613-617 (2015).

[2] G. Chang et al., Nat. Mater. 17, 978-985 (2018).

[3] H. Zhu et al., Science 359, 579-582 (2018).

[4] S.-W. Cheong et al., npj Quantum Mater. 7, 40 (2022).

[5] H. Ueda et al., accepted in Nature.

The topology of spin texture either in momentum space and real space can generate emergent electromagnetic fields acting on the conduction and valence electrons in solid, producing intriguing properties and responses. One archetypical example is magnetic topological insulators, in which the spin-momentum locking as well as the magnetization-induced mass-gap shows up to form the ideal Weyl (spin-momentum locked) fermion system at surface. With control of the magnetizations on the top and bottom surfaces, quantum anomalous Hall state and quantum magnetoelectric (axion insulator) state can be realized.  One other important example of topological magnets is a magnetic skyrmion, which forms real-space spin-swirling texture with integer winding number. Its real-space topology protects the skyrmion from external perturbations, realizing a robust metastable state. Novel electromagnetic phenomena arise from the topological spin textures, such as huge topological Hall effect (via Berry curvature) and emergent electromagnetic induction (via temporal Berry connection).  These momentum-space or real-space topological spin textures are overviewed with perspectives of exploration for new quantum materials and functions.  

Skyrmion crystals have been recently discovered in several centrosymmetric itinerant magnets, where the long-range, highly frustrated RKKY interaction is believed to be crucial for its stabilization. In this talk, we show that the RKKY interaction mediated by a 2D electron gas at low filling generally has multiple degenerate peaks at 2kF in the susceptibility, that allows single- or multi-Q orderings. The degeneracy of the highly frustrated ground state manifold is further lifted once magnetic field, lattice and single-ion anisotropies are considered. A natural consequence is the emergence of the skyrmion crystal phase, which occupies a sizable region of the phase diagram. The same electrons which mediate the RKKY interaction will be renormalized and show a large topological Hall response. We also show that the skyrmion crystal phase is further stabilized by the higher order spin exchanges, by working directly with the Kondo lattice model. The stabilization of skyrmion crystal in the Kondo lattice was conjectured more than 10 years ago, but was not demonstrated at generic filling fractions before -- the reason will be explained.

The proximity-coupling of a chiral non-collinear antiferromagnet (AFM) [1,2] with a singlet superconductor allows spin-unpolarized singlet Cooper pairs to be converted into spin-polarized triplet pairs, thereby enabling non-dissipative, long-range spin correlations [3,4]. The mechanism of this conversion derives from fictitious magnetic fields that are created by a non-zero Berry phase [5] in AFMs with non-collinear atomic-scale spin arrangements [1,2]. In the first part of my talk, I would like to describe our recent achievement of long-ranged lateral Josephson supercurrents through an epitaxial thin film of the triangular chiral AFM Mn3Ge [6]. The Josephson supercurrents in this chiral AFM decay by approximately one to two orders of magnitude slower than would be expected for singlet pair correlations [3,4] and their response to an external magnetic field reflects a clear spatial quantum interference. Given the long-range supercurrents present in both single- and mixed-phase Mn3Ge, but absent in a collinear AFM IrMn, our results pave a way for the topological generation of spin-polarized triplet pairs [3,4] via Berry phase engineering [5] of the chiral AFMs.

Spin-triplet supercurrent spin-valves [7-9] - switching the spin-polarized triplet supercurrents on and off via a magnetic-field-controlled non-collinearity between the spin-mixer and spin-rotator magnetizations in ferromagnetic Josephson junctions - are of practical importance for the realization of superconducting spintronic logic circuits [3]. In the second part, I would like to present our recent demonstration of an antiferromagnetic equivalent of such spin-triplet supercurrent spin-valves [7-9] in chiral antiferromagnetic Josephson junctions and a direct current superconducting quantum interference device (dc SQUID), where non-collinear atomic-scale spin arrangements in the topological chiral antiferromagnet Mn3Ge [1,2] with fictitious magnetic fields (i.e. Berry curvature)8 accommodate triplet Cooper pairing over a long distance (> 150 nm) [6]. We theoretically reproduce the observed supercurrent spin-valve behaviours under a tiny magnetic field (< 2 mT) for current-biased junctions and a dc SQUID, possessing hysteretic field interference of the Josephson critical current, based on the magnetic-field-modulated antiferromagnetic texture which alters the Berry curvature [6,10]. This provides a topological route for controlling the pair amplitude of spin triplets in the single chiral antiferromagnet [11].

[1] Nature 527, 212 (2015)

[2] Sci. Adv. 2, e1501870 (2016)

[3] Nat. Phys. 11, 307 (2015)

[4] Rep. Prog. Phys. 78, 104501 (2015)

[5] Rev. Mod. Phys. 82, 1959 (2010)

[6] Nat. Mater. 20, 1358 (2021)

[7] Phys. Rev. B 76, 060504(R) (2007)

[8] Nat. Comm. 5, 4771 (2014)

[9] Phys. Rev. Lett. 116, 077001 (2016)

[10] Nat. Comm. 12, 572 (2021)

[11] In press, Nat. Nanotech. (2023).

Topological crystalline insulators (TCI) can host surface states whose anomalous band structure inherits the characteristics of the crystalline symmetry that protects the bulk topology. Especially, in magnetic crystals, the diversity of magnetic crystalline symmetries indicates the potential to achieve novel magnetic TCI with distinct surface characteristics. Here, we propose a new type of magnetic TCI, coined the topological magnetic Dirac insulator (TMDI), whose two-dimensional surface hosts four-fold degenerate Dirac fermions protected by magnetic wallpaper groups. The bulk band topology of TMDIs is protected by diagonal mirror symmetries, which give the chiral dispersion of surface Dirac fermions and mirror-protected hinge modes. We also propose a class of candidate materials for TMDIs including Nd4Te8Cl4O20 and DyB4 based on first-principle calculations, and construct a general scheme to search TMDIs using the space group symmetry of paramagnetic parent states. In the second part, we are going to discuss the anomalous intrinsic surface states of CoS2, which is recently proposed as a ferromagnetic Weyl semimetal. Due to the interplay of the surface termination and bulk band topology, various intrinsic surface states can appear other than the Fermi arcs. We will discuss the intrinsic relation between the spin polarization of the bulk/surface and the intrinsic nature of the surface states

[1] Y. Hwang, Y. Qian, H. C. Choi, B.-J. Yang et al, arXiv:2210.10740

The electron correlation effect in topological semimetals is an important subject in Quantum Matter research. We investigated the quantum transport of Dirac electrons on the verge of a Mott transition in perovskite CaIrO3. The electron correlation renormalizes the Fermi velocity and realizes a Dirac semimetal state with low carrier density (~1017 cm-3) and high carrier mobility (~ 6×104 cm2/Vs)[1]. Giant magnetoresistance (MR) related to the formation of a density wave state appears in the quantum limit of Dirac electrons. We also demonstrate the enhanced Nernst effect of magnetic Weyl semimetal NdAlSi, due to the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction mediated by Weyl electrons [2].

[1] J. Fujioka et al., Nat. Commun. 10, 362 (2019). / R. Yamada et al., npj QM 7, 13 (2022).

[2] J. Gaudet et al., Nat. Mater. 20, 1650-1656 (2021).

We discuss emergent non-Fermi liquid behaviors in multipolar Kondo systems, where conduction electrons interact with the local moments carrying higher-rank multipolar moments such as quadrupolar and octupolar moments. We first show that unexpected non- Fermi liquid states arise in the single impurity multipolar Kondo system using the renormalization group and conformal field theory. Next, we study the competition between the Kondo and RKKY interactions in the Bose-Fermi Kondo systems, where the RKKY interaction between multipolar moments is represented by a bosonic degree of freedom. We present the renormalization group solution of this problem and describe the quantum critical behaviors. We compare the theoretical results with existing experimental data on various f- electron systems such as Pr(Ti,V)2Al20 and Ce3Pd20Si6.

*This work is supported by the NSERC of Canada and the Centre for Quantum Materials at the University of Toronto

[1] A. S. Patri and Y. B. Kim, ”Critical theory of non-Fermi liquid fixed point in multipolar Kondo problem”, Phys. Rev. X 10, 041021 (2021).
[2] S. Han, D. J. Schultz, Y. B. Kim, ” Non-Fermi liquid behavior and quantum criticality in cubic heavy fermion systems with non-Kramers multipolar local moments, Phys. Rev. B 106, 155155 (2022).
[3] D. J. Schultz, S. Han and Y. B. Kim, ” Quantum impurity model for two-stage multipolar ordering and Fermi surface reconstruction”, arXiv:2207:07661

The search for multipolar degrees of freedom of magnetic moments has been actively studied in heavy fermions and quantum magnets. In particular, the hidden order, the non-trivial response to the control parameter and their peculiar coupling to itinerant electrons, give rise to new emergent quantum states and phase transitions. Here, for the first time, we discuss the presence of unique multipolar degrees of freedom in three-dimensional quasicrystals. Focusing on the rare-earth ions forming icosahedral structures, we show that the interplay of crystal field effect and spin orbit coupling results in a low energy doublet that is well separated from other multiplets. Such a doublet is found to describe purely multipolar degrees of freedom in the absence of magnetic dipoles. Considering the Hamiltonian for such doublet on symmetry grounds, we discuss possible frustration between multipoles and state their exotic behavior.

The Kitaev honeycomb model provides a rare example of exact quantum spin liquids in more than one dimension [1]. In the exact solution, the spin degree of freedom is fractionalized into itinerant Majorana fermions and localized Z2 fluxes, whose quantum entanglement could be utilized to realize topological quantum computation [1,2]. Since the proposal by Jackeli and Khaliullin [3], numerous efforts have been devoted to realizing it in materials [4], but further exploration is necessary to clarify how to create, probe, and control the fractional excitations for topological quantum computation. In this presentation, we will discuss our recent theoretical progress on (i) spin-current injection, (ii) inter-edge spin resonance, and (iii) effects of dissipation on the Kitaev spin liquids.

[1] A. Kitaev, Ann. Phys. 321, 2 (2006).

[2] A. Kitaev, Ann. Phys. 303, 2 (2003).

[3] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. 102, 017205 (2009).

[4] For a review, see Y. Motome and J. Nasu, J. Phys. Soc. Jpn. 89, 012002 (2020).

Recent numerical studies [1,2] on the JKΓΓ' Kitaev model, which is relevant for α-RuCl3, under external magnetic fields discovered an intermediate, highly entangled quantum phases. They feature a spontaneous breaking of rotational symmetry and also relatively higher entanglement entropy. This suggests that they may represent long-range entangled, fractionalized states of matter. Motivated from this, we systematically search for the nematic spin liquid wavefunctions via the projective symmetry group analysis [3]. From this analysis, we will also discuss the possible experimental/theoretical signatures of these nematic states such as emergent non-Abelian/Abelian anyons, thermal Hall conductance and spin structure factors.     

[1] Lee, Kaneko, Chern, Okubo, Yamaji, Kawashima, and Kim, Nature Comm. 11, 1639 (2020)

[2] Gohlke, Chern, Kee, and Kim, Phys. Rev. Research 2, 043023 (2020) 

[3] Cheong-Eung Ahn, Yong Baek Kim, and Gil Young Cho, in preparation  

In this talk, we investigate the potential of graphene as a platform for detecting quantum spin liquid behavior. By placing graphene in close proximity to a quantum spin liquid material, RuCl3, we observe significant changes in its electronic properties α-RuCl3. The α-RuCl3 is known to become antiferromagnetically ordered below 10 K. Shubnikov-de Haas oscillations in the longitudinal resistance together with Hall resistance measurements provide clear evidence for a band realignment that is accompanied by a transfer of electrons originally occupying the graphene’s spin degenerate Dirac cones α-RuCl3 band states with in-plane spin polarization. Left behind are holes in two separate Fermi pockets, only the dispersion of one of which is distorted near the Fermi energy due to spin selective hybridization with these spin polarized α-RuCl3 band states. This interpretation is supported by our density functional theory calculations. An unexpected damping of the quantum oscillations as well as a zero-field resistance upturn close to the Néel temperature of α-RuCl3 suggest the onset of additional spin scattering due to spin fluctuations in the α-RuCl3.While we did not observe direct detection of quantum spin liquid behavior, our study highlights the potential of graphene-based heterostructure for investigating and characterizing quantum spin liquid systems. 

Topological defects such as domain walls in systems with broken translation symmetry have various exotic electronic and magnetic properties, which bear a wide range of engineering opportunities. We are interested in domain walls in the correlated electronic system of 1T-TaS2 in its broken symmetry states of charge density waves (CDW). We have characterized the existence and structures of various types of domain walls originating from the highly, 13-fold, degenerate CDW ground state [1, 2]. These domain walls exhibit a wide variety of electronic states, which represent well defined 1D electronic channels of non-trivial electronic states with strong electron correlation in their isolated forms [3] and in networks [4]. The ordering and frustration of spins in this system are even more interesting due partly to the possibility of a gapless quantum spin liquid state in the CDW state. Using spin-polarized scanning tunneling spectroscopy, we observe the ferromagnetic 1D spin ordering along the domain walls, which has intriguing local ordering perpendicular to the walls locally [5]. Implication of this result for the quantum spin liquid phase invite further discussion.  

[1] D. Cho et al., Nature Commun. 7, 10453 (2015).

[2] J. W. Park , J. Lee, and H. W. Yeom, NPJ Quantum Materials 6, 32 (2021).

[3] D. Cho et al., Nature Commun. 10, 1038 (2017).

[4] J. W. Par, G. Y. Cho, J. Lee, and H. W. Yeom, Nature Commun. 10, 4038 (2019).

[5] H. R. Park et al., submitted (2023).

The stability of the spin polaron quasiparticle, well established in studies of a single hole in the 2D antiferromagnets, is investigated in the 1D antiferromagnets using a t–J model. We perform an exact slave fermion transformation to the holon-magnon basis, and diagonalize numerically the resulting model in the presence of a single hole. We prove that the spin polaron is stable for any strength of the magnon-magnon interactions except for the unique value of the SU(2)-symmetric 1D t–J model, proving that the widely accepted spin-charge separation paradigm is in fact an exception, not the rule [1]. I will then briefly discuss how a single spinon can be excited by adding one extra spin to the ground state wave function of the Heisenberg chain [2].

[1] P. Wrzosek et al., cond-mat/2203.01846.

[2] T. Kulka et al., cond-mat/2303.02276.

We report a quantum criticality crossover representing two different universal scaling behaviors in a Kitaev quantum magnetic material α-RuCl3. α-RuCl3 presents both a symmetry-breaking antiferromagnetic order and a long-range entangled topological order of a quantum spin liquid, and thus could be a candidate system for a unique universality class involving deconfined fractionalized excitations of the local Z2 fluxes and itinerant Majorana fermions. Theoretical analyses on the inelastic neutron scattering, ac-magnetic susceptibility, and specific heat results demonstrate that Wilson–Fisher-Yukawa-type ‘conventional’ weak-coupling quantum criticality in high-energy scales crosses over to heavy-fermion-type ‘local’ strong-coupling one in low-energy scales. Our findings provide deep insight on how quantum criticality evolves in fermion-boson coupled topological systems with different types of deconfined fermions.

We report heat capacity measurements of SrCu2(BO3)2 under high pressure along with simulations of relevant quantum spin models and map out the (P,T) phase diagram of the material. We find a first-order quantum phase transition between the low-pressure quantum dimer paramagnet and a phase with signatures of a plaquette-singlet state below T = 2 K. At higher pressures, we observe a transition into a previously unknown antiferromagnetic state below 4 K. Our findings can be explained within the two-dimensional Shastry-Sutherland quantum spin model supplemented by weak interlayer couplings. The possibility to tune SrCu2(BO3)2 between the plaquette-singlet and antiferromagnetic states opens opportunities for experimental tests of quantum field theories and lattice models involving fractionalized excitations, emergent symmetries, and gauge fluctuations.

[1] J. Guo et al., Rhys. Rev. Lett. 124, 206602 (2020)

Tomonaga-Luttinger liquid (TLL) provides the conceptual framework for describing gapless interacting fermions in one-dimensional (1D) systems and further various systems, such as quantum wires, cold atoms, and quantum spin chains. Irrespective of the underlying microscopic Hamiltonian, all the low-energy properties of a TLL are characterized by the excitation velocity u and the interaction parameter K. In particular, a S=1/2 1D quantum Heisenberg antiferromagnet (QHAF) can host either a repulsive (K<1) or an attractive TLL (1<K) that features a quantum spin-liquid ground state. In 1D QHAFs, the TLL undergoes a quantum phase transition by a magnetic field or chemical substitution. Quantum criticality (QC) is solely characterized by temperature and a universal scaling law that hinges on the symmetry and dimensionality of the systems. Till now, the field-induced QC and the finite-temperature scaling relation have been experimentally corroborated in a variety of TLL materials. However, little is known about the QC of TLL that emerges in an anisotropic triangular lattice (ATL) due to the scarcity of model materials. 

In this talk, we present the comprehensive study of 2D ATL Ca3ReO5Cl2 using Raman spectroscopy, 35Cl nuclear magnetic resonance (NMR) and muon spin relaxation (μSR). The Raman data reveal a magnetic continuum consisting of one- and two-pair spinon-antispinon excitations as well as triplon excitations, expected for the 1D QHAF. In addition, the strong thermal evolution of spinon scattering is compatible with the bosonic spinon scenario. Our NMR and μSR results evince that the TLL is driven by anisotropic spin diffusion arising from dimensional reduction. Remarkably, we observe a universal scaling of the 35Cl spin-lattice relaxation rate and the time-field scaling of the muon spin polarization. These findings instantiate that Ca3ReO5Cl2 with intermediate spatial anisotropy realizes a TLL below the temperature scale of the interchain interaction.

Quantum spin liquids (QSLs) have been theoretically shown to exist in the two-dimensional kagome lattice with Heisenberg antiferromagnetic interactions. While many kagome materials have shown interesting properties that may come from the fractionalized excitations in QSLs, the existence of intrinsic magnetic impurities make it hard to further nail down the nature of the ground states. Recently, a new kagome compound YCu3(OH)6Br2[Brx(OH)1-x] has been successfully synthesized and is free of magnetic impurities. Here I will report our specific-heat, magnetic-susceptibility and inelastic neutron scattering results. Our results show strong evidences that support a Dirac QSL ground state in the YCu3(OH)6Br2[Br0.33(OH)0.67], including the quadratic temperature dependence of the specific heat at zero field, the appearance of the linear temperature dependence of the specific heat under magnetic fields [1], and cone-like low-energy spin excitations.

[1] Z. Zeng et al., Phys. Rev. B 105, L121109 (2022).

Transition-metal based materials with kagome lattice offer exciting platforms, where topological states might interplay with geometrically frustrated, magnetic, or superconducting states. The recently discovered Kagome metals AV3Sb5 (A=K, Cs, Rb) have shown rich electronic phases, of which the most intriguing one is a putative chiral charge density wave phase that breaks time reversal symmetry, but without static local moment. In this talk, I will report direct measurements of magnetic anisotropy of CsV3Sb5 using a torque magnetometry based probe. A series of phase transitions have been unveiled with decreasing temperature, and, most strikingly, the sample develops very small magnetic moment with strong magnetic anisotropy below a characteristic temperature of ~30 K, indicating highly two-dimensional magnetization along c-axis of CsV3Sb5. Our observation of a small but anisotropic magnetic susceptibility suggests a novel phase of correlated matter, in which the magnetism might originate from chiral charge motion that beyond the common scheme of spin or orbital magnetism