arXiv subjects

Yong Baek Kim

Publications and source records attributed to Yong Baek Kim.

At least 19 recordsLinked to original sources

arXiv2026

The influence of quantum geometry on the phase boundary and collective excitations of electron liquids and crystals

Recent experiments on multilayer graphene systems have reinvigorated the study of electron crystallization, now with the inclusion of quantum geometry. In this work, we apply time-dependent Hartree-Fock (TDHF) to the $λ$-jellium model to analyze the impact that quantum geometry has on the electronic liquid--crystal phase diagram and how it modifies the collective modes and responses of the liquid and crystal phases. In agreement with recent results utilizing neural quantum states, we find that quantum geometry favours electron crystallization, shifting the transition to higher densities. We also study the instabilities revealed by TDHF in the Fermi liquid ground state at low densities, providing insight into the fluctuations driving the crystallization transition. We further find that quantum geometry reduces the dispersion of the plasmon mode and suppresses Friedel oscillations deep in the liquid phase. Resolving the density response in terms of individual orbitals, we find that this suppression is caused by spectral weight transfer to an out-of-phase inter-orbital mode. Finally, we show that an analogous mode that emerges in the crystal phase corresponds to the breathing mode of an emergent real-space pseudospin skyrmion lattice.

cond-mat.mes-hall↗

arXiv2026

Chiral superconductors from parent states with nonuniform Berry curvature: Momentum-space vortices, Bogoliubov-de Gennes topology, and thermal Hall conductivity

We investigate chiral superconductivity emerging from parent electronic states with non-uniform Berry curvature, motivated by recent experiments in rhombohedral graphene multilayers. Using the continuum $λ_N$-model-a tunable platform with independently controllable Berry curvature profiles-we solve the full BCS gap equation on a continuum Chern band beyond the weak-coupling limit. We find that a non-uniform Berry curvature of the parent band enriches the superconducting order parameter, leading to the formation of momentum-space vortices in the gap function away from high-symmetry points. By tuning the Berry curvature profile, we identify distinct regimes associated with vortex nucleation and vortex number saturation, and show that the nucleation of momentum-space vortices tends to lower the condensation energy. We then show analytically that the parent band Chern number constrains the number of momentum-space vortices that can nucleate in the gap-independent of details of the $λ_N$-model. We also provide a gauge-invariant formulation for computing the Bogoliubov-de Gennes (BdG) Berry curvature for continuum models, and find that it is determined by a momentum-space phase current. The winding of this current around vortices in the occupied region in turn determines the BdG Chern number. Finally, we discuss how thermal Hall measurements can be used to probe the formation of momentum-space vortices. Our results highlight the crucial role of Berry curvature in shaping chiral superconductivity, and offer guiding principles for its identification in systems such as rhombohedral graphene.

cond-mat.supr-con↗

arXiv2026

Thermodynamic Spectroscopy of Emergent Excitations in Quantum Spin Ice

Quantum spin ice (QSI) is a three-dimensional quantum spin liquid where fractionalized spinons interact with emergent photons. The XXZ model on the pyrochlore lattice realizes such a $U(1)$ quantum spin liquid and a number of pyrochlore magnets have been investigated as candidate materials, but the detection of emergent excitations has been a major challenge. The specific heat is expected to show the higher-energy spinon excitations and a lower-energy anomaly at the ring-exchange scale, which sets both the photon bandwidth and the energy of the emergent magnetic monopoles (or visons). In reality, the lower-energy peak is obscured by the nuclear Schottky anomaly in non-Kramers Pr-based systems, while it is not clearly resolved from the spinon contributions in Ce-based dipolar-octupolar systems. In this work, we propose a novel thermodynamic probe of the elusive ring-exchange energy scale. We show that in the presence of a weak perturbation coupled to the transverse component of the pseudospin degrees of freedom, the temperature derivative of an observable conjugate to such a weak perturbing force is highly sensitive to the ring-exchange energy scale. Using this scheme, it is shown that the difference between thermal expansion coefficients along the [100] and [010] directions should show a peak at the ring-exchange energy scale for non-Kramers QSI. Similarly, the temperature derivative of the magnetization, $dM/dT$, of dipolar-octupolar pyrochlores under a weak magnetic field can also detect the same signal. Moreover, the sign of these signatures distinguishes the zero-flux and $π$-flux QSI states.

cond-mat.str-el↗

arXiv2026

Twisted Lattice Gauge Theory: Membrane Operators, Three-loop Braiding and Topological Charge

3+1 dimensional topological phases can support loop-like excitations in addition to point-like ones, allowing for non-trivial loop-loop and point-loop braiding statistics not permitted to point-like excitations alone. Furthermore, these loop-like excitations can be linked together, changing their properties. In particular, this can lead to distinct three-loop braiding, involving two loops undergoing an exchange process while linked to a third loop. In this work, we investigate the loop-like excitations in a 3+1d Hamiltonian realization of Dijkgraaf-Witten theory through direct construction of their membrane operators, for a general finite Abelian group and 4-cocycle twist. Using these membrane operators, we find the braiding relations and fusion rules for the loop-like excitations, including those linked to another loop-like excitation. Furthermore, we use these membrane operators to construct projection operators that measure the topological charge and show that the number of distinct topological charges measured by the 2-torus matches the ground state degeneracy of the model on the 3-torus, explicitly confirming a general expectation for topological phases. This direct construction of the membrane operators sheds significant light on the key properties of the loop-like excitations in 3+1 dimensional topological phases.

cond-mat.str-el↗

arXiv2026

Quantum Fisher Information as a Thermal Probe in Frustrated Magnets through Insights from Quantum Spin Ice

Quantum Fisher information (QFI) is a measure of multipartite entanglement accessible via inelastic neutron scattering. Here we demonstrate that QFI reveals thermal and dynamical properties of quantum spin ice (QSI), a three-dimensional quantum spin liquid with fractionalized excitations. By developing a multi-directed loop update quantum Monte Carlo algorithm, along with exact diagonalization and gauge mean-field theory, we compute the QFI for the pyrochlore lattice. The temperature and momentum dependence of QFI maps the phase diagram, distinguishing the ferromagnetic ordered phase, its critical region, the zero-flux QSI, and the $π$-flux QSI. QFI also captures two crossover scales: from trivial paramagnet to classical spin ice, then to QSI. We discuss the $π$-flux QSI in light of experiments on cerium-based pyrochlores. Our results suggest that QFI not only detects entanglement but also serves as a sensitive thermal and dynamical probe for frustrated quantum magnets.

cond-mat.str-el↗

arXiv2026

Superconductivity in kagome metals due to soft loop-current fluctuations

We demonstrate that soft fluctuations of translation symmetry-breaking loop currents provide a mechanism for unconventional superconductivity in kagome metals that naturally addresses the multiple superconducting phases observed under pressure. Focusing on the rich multi-orbital character of these systems, we show that loop currents involving both vanadium and antimony orbitals generate low-energy collective modes that couple efficiently to electrons near the Fermi surface and mediate attractive interactions in two distinct unconventional pairing channels. While loop-current fluctuations confined to vanadium orbitals favor chiral $d+id$ superconductivity, which spontaneously breaks time-reversal symmetry, the inclusion of antimony orbitals stabilizes an $s^{\pm}$ state that is robust against disorder. We argue that these two states are realized experimentally as pressure increases and the antimony-dominated Fermi surface sheet undergoes a Lifshitz transition.

cond-mat.supr-con↗

arXiv2026

Elastic Response and Instabilities of Anomalous Hall Crystals

Anomalous Hall crystals (AHCs) are exotic phases of matter that simultaneously break continuous translation symmetry and exhibit the quantum anomalous Hall effect. AHCs have recently been proposed to explain the observation of an integer quantum anomalous Hall phase in a multilayer graphene system. Despite intense theoretical and experimental interest, little is known about the mechanical properties of AHCs. We study the elastic properties of AHCs first by using a continuum model with quadratic dispersion and uniform Berry curvature. We find using time-dependent Hartree-Fock that the stiffness of the AHC is an order of magnitude smaller than that of the WC, which we attribute to the finite Chern number of the AHC preventing exponential localization of the charge density. By modifying the dispersion relation to include a local minimum modeled after that of rhombohedral pentalayer graphene (R5G), we find that deformations away from the triangular lattice minimize the AHC's kinetic energy, which overwhelms the small stiffness and triggers a mechanical instability. Using a microscopic model of R5G, we observe a similar mechanical instability over an experimentally relevant parameter regime. We conclude that the topologically limited stiffness of AHCs makes them susceptible to mechanical instabilities, an important consideration when interpreting experiments in terms of AHCs.

cond-mat.str-el↗

arXiv2026

Electric field control of a quantum spin liquid in weak Mott insulators

The triangular lattice Hubbard model at strong coupling, whose effective spin model contains both Heisenberg and ring exchange interactions, exhibits a rich phase diagram as the ratio of the hopping $t$ to onsite Coulomb repulsion $U$ is tuned. This includes a chiral spin liquid (CSL) phase. Nevertheless, this exotic phase remains challenging to realize experimentally because a given material has a fixed value of $t/U$ that can difficultly be tuned with external stimuli. One approach to address this problem is applying a DC electric field, which renormalizes the exchange interactions as electrons undergo virtual hopping processes; in addition to creating virtual doubly occupied sites, electrons must overcome electric potential energy differences. Performing a small $t/U$ expansion to fourth order, we derive the ring exchange model in the presence of an electric field and find that it not only introduces spatial anisotropy but also tends to enhance the ring exchange term compared to the dominant nearest-neighbor Heisenberg interaction. Thus, increasing the electric field serves as a way to increase the importance of the ring exchange at constant $t/U$. Through density matrix renormalization group calculations, we compute the ground state phase diagram of the ring exchange model for two different electric field directions. In both cases, we find that the electric field shifts the phase boundary of the CSL towards a smaller ratio of $t/U$. Therefore, the electric field can drive a magnetically ordered state into the CSL. This explicit demonstration opens the door to tuning other quantum spin systems into spin liquid phases via the application of an electric field.

cond-mat.str-el↗

arXiv2026

Quantum Fisher Information as a Probe of Critical Scaling in Frustrated Magnets: Signatures from Kagome Quantum Spin Liquid

Quantum Fisher information (QFI) is a measure of multipartite quantum entanglement that can be obtained from inelastic neutron scattering data on quantum magnets. In this work, we demonstrate that the QFI can distinguish an unconventional quantum critical point (QCP) with fractionalization and emergent gauge structure from conventional ones within the Landau paradigm. We compute the QFI, via large-scale quantum Monte Carlo (QMC) simulations and exact diagonalization, in a kagome lattice quantum spin liquid (QSL) model with an XY and a cluster-Ising interactions. When the XY interaction is ferromagetic, the QFI obtained by QMC reveals a large anomalous dimension, which is a fingerprint of the (2+1)d XY$^\ast$ universality class for the transition from the ferromagnetic phase to the $\mathbb{Z}_2$ QSL. The investigation of thermal and dynamical properties of QFI is further extended to the case of antiferromagnetic XY interaction via exact diagonalization. In this regime, a transition to a possibly distinct QSL phase is suggested via both entanglement-based probes, such as QFI and genuine multipartite negativity, and analyses of the energy spectrum and structure factors. These results not only demonstrate the versatility of QFI in identifying QSL states and unconventional QCPs but also provide useful guidance for future theoretical and experimental studies of frustrated magnets.

cond-mat.str-el↗

arXiv2026

Topological phonons in anomalous Hall crystals

Recent experiments on few-layer graphene structures have reported indirect signatures of anomalous Hall crystals (AHCs), but the need for a top gate to stabilize the phase precludes direct imaging of the emergent electronic lattice. This situation necessitates the investigation of alternative signatures of AHCs. The gapless phonons of the emergent electronic lattice provide a clear distinction from conventional quantum Hall states, but it may be difficult to disentangle these phonons from the plethora of other possible low-lying modes. Intriguingly, the quantum geometry of the underlying electronic ground state can imprint on the collective modes, possibly leading the phonons themselves to be topological. Were this the case, the resulting neutral chiral edge modes would provide a further signature of an AHC. Using time-dependent Hartree-Fock, we compute the spectra of collective modes of Wigner crystals (WCs) and AHCs arising in minimal models and study the topology of the phonons and low-lying excitons. Across the WC to AHC transition, we observe a series of band inversions among collective modes, producing topological phonons and excitons, and a sharp sign change in the phonon Chern number upon entering the AHC phase. We conclude by discussing the relevance of collective mode topology to experiments on candidate systems for AHCs.

cond-mat.mes-hall↗

arXiv2026

Spectroscopic Demarcation of Emergent Photons and Spinons in a Dipolar-Octupolar Quantum Spin Liquid

The identification of fractionalized excitations in quantum spin liquids (QSLs) remains a central challenge in condensed matter physics. In dipolar-octupolar (DO) pyrochlores, such as $\text{Ce}_2\text{Zr}_2\text{O}_7$, the candidate $π$-flux quantum spin ice (QSI) state is predicted to host both gapless emergent photons and a continuum of spinons. However, resolving these modes at zero field is complicated by their spectral overlap and the presence of nonmagnetic scattering near zero energy. Here, we report neutron scattering experiments on $\text{Ce}_2\text{Zr}_2\text{O}_7$ under a magnetic field along the $[1,1,1]$ direction. In contrast to previous unpolarized studies at zero-field that relied on high-temperature subtraction, we use a same-temperature high-field subtraction protocol to isolate the photon mode. Leveraging the selective coupling of the magnetic field to the dipolar degrees of freedom, we demonstrate the spectroscopic demarcation of these excitations. We observe that weak fields ($\approx 0.15$ T) suppress the low-energy photon weight while leaving the high-energy spinon continuum robust, albeit hardened. Our results, supported by gauge mean-field theory and exact diagonalization calculations, provide strong evidence for the $π$-flux QSI state and introduce a powerful field-tuning protocol for investigating DO-QSLs.

cond-mat.str-el↗

arXiv2025

Quantum Spin Liquids in Weak Mott Insulators with a Spin-Orbit Coupling

The weak Mott insulating regime of the triangular lattice Hubbard model exhibits a rich magnetic phase diagram as a result of the ring exchange interaction in the spin Hamiltonian. These phases include the Kalmeyer-Laughlin type chiral spin liquid (CSL) and a valence bond solid (VBS). A natural question arises regarding the robustness of these phases in the presence of a weak spin-orbit coupling (SOC). In this study, we derive the effective spin model for the spin-orbit coupled triangular lattice Hubbard model in the weak Mott insulting regime, including all SOC-mediated spin-bilinears and ring-exchange interactions. We then construct a simplified spin model keeping only the most relevant SOC-mediated spin interactions. Using infinite density matrix renormalization group (iDMRG) we show that the CSL and VBS phases of the triangular lattice Hubbard model can be stabilized in the presence of a weak SOC. The stabilization results from a compensation between the Dzyaloshinskii-Moriya interaction and a SOC-mediated ring exchange interaction. We also provide additional qualitative arguments to intuitively understand the compensation mechanism in the iDMRG quantum phase diagrams. This mechanism for stabilization can potentially be useful for the experimental realization of quantum spin liquids.

cond-mat.str-el↗

arXiv2025

Quantum Spin Liquids in Pyrochlore Magnets With Non-Kramers Local Moments

Numerous experiments on pyrochlore oxides Pr$_2$(Zr, Sn, Hf, Ir)$_2$O$_7$ with non-Kramers Pr$^{3+}$ ions suggest that they support a quantum spin liquid (QSL) ground state, but the precise nature of the QSL remains unclear. Quantum spin ice with dominant dipolar Ising and smaller quadrupolar transverse exchange interactions is one such candidate, but a dominant inelastic neutron scattering signal suggests that such a picture may not be consistent with experimental results. The microscopic exchange couplings of these compounds are also not known, leaving room for many possible QSL states. In this work, we use Schwinger boson mean-field theory supplemented by a projective symmetry group classification to study possible $\mathbb{Z}_2$ QSLs in pyrochlore magnets with dipolar-quadrupolar non-Kramers local moments. We build a mean-field phase diagram and find four QSLs in the frustrated region of parameter space that are consistent with inelastic signals observed in neutron scattering data on Pr$_2$Zr$_2$O$_7$ and Pr$_2$Hf$_2$O$_7$. Among these, two robust QSLs occur in the regime with dominant transverse exchange rather than Ising exchange. We then compute the static and dynamic spin structure factors for these QSL candidates, which can be used to distinguish them in neutron scattering experiments.

cond-mat.str-el↗

arXiv2025

Spin excitation continuum to topological magnon crossover and thermal Hall conductivity in Kitaev magnets

There has been great interest in identifying a Kitaev quantum spin liquid state in frustrated magnets with bond-dependent interactions. In particular, the experimental report of a half-quantized thermal Hall conductivity in $α$-RuCl$_3$ in the presence of a magnetic field has generated excitement as it could be strong evidence for a field-induced chiral spin liquid. More recent experiments, however, provide a conflicting interpretation advocating for topological magnons in the field-polarized state as the origin of the non-quantized thermal Hall conductivity observed in their experiments. An inherent difficulty in distinguishing between the two scenarios is the phase transition between a putative two-dimensional spin liquid and the field-polarized state exists only at zero temperature, while the behaviour at finite temperature is mostly crossover phenomena. In this work, we provide insights into the finite temperature crossover behavior between the spin excitation continuum in a quantum spin liquid and topological magnons in the field-polarized state in three different theoretical models with large Kitaev interactions. These models allow for a field-induced phase transition from a spin liquid (or an intermediate field-induced spin liquid) to the field-polarized state in the quantum model. We obtain the dynamical spin structure factor as a function of magnetic field using molecular dynamics simulations and compute thermal Hall conductivity in the field-polarized regime. We demonstrate the gradual evolution of the dynamical spin structure factor exhibiting crossover behaviour near magnetic fields where zero-temperature phase transitions occur in the quantum model. We also examine nonlinear effects on topological magnons and the validity of thermal Hall conductivity computed using linear spin wave theory. We discuss the implications of our results to existing and future experiments.

cond-mat.str-el↗

arXiv2025

Disentangling spin excitation continua in classical and quantum magnets using 2D nonlinear spectroscopy

Inelastic neutron scattering (INS) has traditionally been one of the primary methods for investigating quantum magnets, particularly in identifying a continuum of excitations as a hallmark of spin fractionalization in quantum spin liquids (QSLs). However, INS faces severe limitations due to its inability to distinguish between such QSL signatures and similar excitation continua arising from highly frustrated magnetic orders with large unit cells or classical spin liquids. In contrast, two-dimensional coherent spectroscopy (2DCS) has emerged as a powerful tool to probe nonlinear excitation dynamics, offering insights into the underlying mechanisms behind these broad spectral features. In this paper, we utilize classical molecular dynamics (MD) techniques to explore the 2DCS responses of frustrated magnets with dominant Kitaev interactions. Comparing the classical and quantum versions of the pure Kitaev model our results indicate both clear similarities, in the form of sharp line features, and clear distinctions, in the locations of these features and in selection rules. Moreover, in the extended $KΓΓ'$ model, we show that the 2DCS response of the Kitaev spin liquid is completely distinct from that of large unit cell magnetic orders, despite both generating a broad continuum in INS. Additionally, we demonstrate the extreme sensitivity of classical 2DCS to thermal fluctuations and discuss the potential significance of quantum coherence in experimental settings. Overall, our work illustrates the potential of 2DCS in resolving the complex physics underlying ambiguous spin excitation continua, thereby enhancing our understanding of the dynamics in these frustrated systems.

cond-mat.str-el↗

arXiv2025

Fermi Surface Bosonization for Non-Fermi Liquids

Understanding non-Fermi liquids in dimensions higher than one remains one of the most formidable challenges in modern condensed matter physics. These systems, characterized by an abundance of gapless degrees of freedom and the absence of well-defined quasiparticles, defy conventional analytical frameworks. Inspired by recent work [Delacretaz, Du, Mehta, and Son, Physical Review Research, 4, 033131 (2022)], we present a procedure for bosonizing Fermi surfaces that does not rely on the existence of sharp excitation and is thus directly applicable to non-Fermi liquids. Our method involves parameterizing the generalized fermionic distribution function through a bosonic field that describes frequency-dependent local variations of the chemical potential in momentum space. We propose an effective action that produces the collisionless quantum Boltzmann equation as its equation of motion and can be used for any dimension and Fermi surface of interest. Even at the quadratic order, this action reproduces non-trivial results obtainable only through involved analysis with alternative means. By offering an alternative method directly applicable to studying the low-energy physics of Fermi and non-Fermi liquids, our work potentially stands as an important building block in advancing the comprehension of strange metals and associated phenomena.

cond-mat.str-el↗

arXiv2025

Interplay of competing bond-order and loop-current fluctuations as a possible mechanism for superconductivity in kagome metals

The pairing symmetry and underlying mechanism for superconducting state of AV${}_3$Sb${}_5$ (A=K, Rb, Cs) kagome metal has been a topic of intense investigation. In this work, we consider an 8-band minimal model, which includes V, and the two types of Sb, both within and above/below the kagome plane. This model captures the Fermi surface pocket with significant in-plane Sb contribution near the zone center, and also has the two types of van Hove singularities (VHS), one of which has a strong out of plane Sb weight. By including V-V and V-planar Sb nearest-neighbor Coulomb interactions, we obtain the susceptibilities for fluctuating bond-order and loop-current in both charge and spin channels, and examine the resulting superconducting instabilities. In particular, we find that the time-reversal odd (even) charge-loop-current (charge bond-order) fluctuations favor unconventional (conventional) pairing symmetry such as $s_{+-}$ and $d+id$ ($s_{++}$). Recent experimental works have highlighted the presence of $s$-wave pairing with two distinct gaps, one isotropic and one anisotropic. We discuss how this scenario may be compatible with either $s_{++}$ or $s_{+-}$ pairing, with an isotropic gap on the pocket dominated by in-plane Sb, but a highly anisotropic gap on V-dominated bands.

cond-mat.supr-con↗

arXiv2025

Electronic Crystal Phases in the Presence of Non-Uniform Berry Curvature and Tunable Berry Flux: The $λ_N$-Jellium model

Recent experiments on multilayer graphene systems have rekindled interest in electronic crystal phases in two dimensions -- but now for phases enriched by non-trivial quantum geometry. In this work, we introduce a simple continuum model with tunable Berry curvature distribution and total flux, enabling systematic study of crystallization in geometrically nontrivial bands. In the noninteracting limit, the addition of a C6-symmetric periodic potential yields a rich phase diagram, for which we provide several analytical insights. Notably, we derive a general formula for the Chern number in the weak-potential regime that is broadly applicable to single-band projected models. Removing the periodic potential and treating Coulomb interactions self-consistently at the Hartree-Fock level, the resulting phase diagrams host a variety of crystalline states, including anomalous Hall crystals, halo Wigner crystals in which localized electrons spontaneously acquire orbital angular momentum leading to depleted electron occupation at the zone center, and a novel halo anomalous Hall crystal that combines these properties with a finite Chern number. We identify why these phases are energetically favorable through analytical and energetic considerations. Our results provide insight into the interplay between crystallization and band geometry, while also offering a simple toy model amenable to numerical methods beyond mean-field.

cond-mat.str-el↗