arXiv subjects

Masafumi Udagawa

Publications and source records attributed to Masafumi Udagawa.

At least 19 recordsLinked to original sources

arXiv2026

Entropic crystallization of geometrically frustrated magnets on 1/1 approximant Tsai-type quasicrystal

We have studied the antiferromagnetic Ising model on the icosahedral bcc lattice, as a model system of 1/1 approximant Tsai-type quasicrystals. We addressed thermal equilibrium properties of this system with Markov-chain Monte Carlo simulation supplemented with the parallel tempering technique to accelerate the relaxation dynamics. As a result, we found a second-order phase transition takes place to the magnetic ordered phase with ${\mathbb Z_3}\times {\mathbb Z_2}$ symmetry breaking. Despite the ordering, the low-temperature phase keeps macroscopic degeneracy as identified by finite residual entropy, $\mathcal{S}\sim0.1767/{\rm spin}$. Remarkably, the existence of residual entropy turns out to play a major role in the formation of magnetic order. Generation of domain wall is suppressed, as it reduces the residual entropy locally stored in icosahedra, beyond the gain of configurational entropy due to domain wall patterns. Magnetic order arises out of this competition as entropic crystallization, which manifest universal mechanism of strongly frustrated systems with large geometrical units.

cond-mat.str-el↗

arXiv2025

Charge glass from supercooling topological-ordered liquid

Topological order characterizes a class of quantum and classical many-body liquid states that escape the conventional classification by spontaneous symmetry breaking. Many properties of the topological-ordered states still await a clear understanding, and nature of phase transition dynamics is one of them. Normally, when a liquid freezes into a solid, crystallization starts with nucleation and a solid domain quickly grows on the surface of the expanding nucleus, and the domains evolve into macroscopic size. In this work, we reveal that the crystallization of the topological-ordered liquid proceeds in a fundamentally different way. The topological-ordered phase is characterized by a global conserved quantity and its conjugate fractional charge, which we call a flux and a triplet in our working system of the charge Ising model on a triangular lattice. In contrast to the normal crystallization process, the phase transition is driven by the diffusive motion of triplets, which is required to change the value of conserved fluxes to exit the topological-ordered phase. In order to complete crystallization, triplets must spend a divergently long time to diffuse over a macroscopic distance across the system, which results in glassy behavior. Reflecting the diffusive motion of triplets, the initial crystallization process shows slowing down with unusually small Avrami exponent $\sim0.5$. These anomalous dynamics are specific to the crystallization from topological-ordered liquid, and well account for the main features of charge glass behavior exhibited by the organic conductors, $θ$-(BEDT-TTF)$_2$X(SCN)$_4$.

cond-mat.stat-mech↗

arXiv2025

Spinon band flattening by its emergent gauge field in quantum kagome ice

Fractional excitations provide a key to identifying sought-after topological quantum spin liquid states in realistic materials. Their single-particle dynamics already presents a challenging many-body problem on account of the coupling to their emergent gauge field. Here, we study the spinon excitations of kagome ice, realized at the $2/3$ magnetization plateau of spin ice, by combining up-to-$63$-site exact diagonalization with an analytical state graph mapping. We find a macroscopically degenerate mode in the spinon spectrum. It originates from the destructive interference due to the interaction with surrounding gauge fields, a form of many-body caging. We explicitly construct, and count, the concomitant many-body wave functions. Finally, we discuss the possible role of these flat modes in the magnetization process of kagome antiferromagnets, in particular with regard to the asymmetric termination of the kagome ice magnetisation plateau.

cond-mat.str-el↗

arXiv2025

Hollow Lattice Tensor Gauge Theories with Bosonic Matter

Higher rank gauge theories are generalizations of electromagnetism where, in addition to overall charge conservation, there is also conservation of higher rank multipoles such as the total dipole moment. In this work we study a four dimensional lattice tensor gauge theory coupled to bosonic matter which has second rank tensor electric and magnetic fields and charge conservation on individual planes. Starting from the Hamiltonian, we derive the lattice action for the gauge fields coupled to $q=1,2$ charged scalars. We use the action formulation to carry out Monte Carlo simulations to map the phase diagram as a function of the gauge ($β$) and matter ($κ$) couplings. We compute the nature of correlators at strong and weak coupling in the pure gauge theory and compare the results to numerical simulations. Simulations show that the naive weak coupling regime (small $κ$, large $β$) does not survive in the thermodynamic limit. Instead, the strong coupling confined phase, spans the whole phase diagram. It is a proliferation of instantons that destroys the weak coupling phase and we show, via a duality transformation, that the expected strong confinement is present in the analog of Wilson line correlators. For finite matter coupling at $q=1$ we find a single thermodynamic phase albeit with a first order phase transition terminating in a critical endpoint.For $q=2$ it is known that the the X-cube model with $\mathbb{Z}_2$ fractonic topological order is recovered deep in the Higgs regime. The simulations indeed reveal a distinct Higgs phase in this case.

hep-lat↗

arXiv2025

Spinor ice correlation in flat-band electronic states on kagome and pyrochlore lattices with spin-orbit coupling

We investigate the emergence and transformation of pinch-point singularities in the excitation spectrum of electronic flat band systems on kagome and pyrochlore lattices with spin-orbit coupling (SOC) and Coulomb interactions. While pinch points are widely recognized as signatures of classical spin liquids, they also appear in electronic flat-band systems when there exists a singular band-touching point to dispersive bands. We explore how SOC modifies the pinch-point structure in the chiral spin flat-band metallic state, which we term spinor-ice. The pinch point profile can rotate or redistribute its spectral weight, governed by a prefactor in the spectral function that primarily depends on the direction of the ground-state spin polarization, where we show that SOC flat bands could be experimentally probed by rotating the spin polarization of the injected electron to infer internal magnetic structures. These observations are discussed in conjunction with the angle-resolved photoemission spectroscopy (ARPES) and the application to the potential SOC flat-band material $\rm CsW_2O_6$. We also demonstrate the persistent residual pinch-point features under Coulomb interactions and deviations from the ideal flat-band limit.

cond-mat.str-el↗

arXiv2025

Sign-reversal and non-monotonicity of chirality-related anomalous Hall effect in highly conductive metals

The non-monotonic temperature dependence and sign reversal of chirality-related anomalous Hall effect in highly conductive metals are studied. Through the analysis of scattering rate, we find that the non-monotonicity and sign reversal have two major origins: (1) competition between the contribution from short-range and long-range spin correlations and (2) non-monotonic spin correlation in the high field. The former mechanism gives rise to non-monotonic temperature dependence in a wide range of electron density and, in some cases, a sign reversal of Hall resistivity as the temperature decreases. On the other hand, the latter mechanism is responsible for the sign reversal of Hall conductivity in the high field, which sign reversal generally occurs in magnets with antiferromagnetic interactions. The results demonstrate how the Hall effect reflects local spin correlation and provide insights into the mechanism of non-monotonicity and sign reversal of the anomalous Hall effect by spin chirality.

cond-mat.mes-hall↗

arXiv2024

Dynamic orders of a Quantum Spin Liquid at Non-zero Temperatures

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.

cond-mat.str-el↗

arXiv2024

Pinch-point spectral singularity from the interference of topological loop states

Pinch point is a spectral discontinuity found in the neutron diffraction image of spin ice. Similar spectral singularity is commonly observed in a broad range of systems that have a close connection with flat bands. We focus on the electron flat band and its two topologically distinct classes of wavefunction: the compact localized state (CLS), and the non-contractible loop state (NLS). We establish their simple mathematical relationship, showing that different Bloch NLSs can be derived as momentum derivatives of a Bloch CLS, depending on the approaching direction toward the singular point. This CLS-NLS correspondence helps visualize the pinch point as an interference pattern among NLSs through a ``polarizer", which encodes the information about the location of singular momentum and the experimental techniques like spin-polarized photoemission spectroscopy. It helps extract topological information knit to microscopic electronic and magnetic structures.

cond-mat.mtrl-sci↗

arXiv2024

From chiral spin liquids to skyrmion fluids and crystals, and their interplay with itinerant electrons

The physics of skyrmions, and in particular the issue of how to isolate and manipulate them individually, is a subject of major importance nowadays in the community of magnetism. In this article we present an in-depth extension of a study on this issue that was recently proposed by some of the authors [H. D. Rosales, et al. Phys. Rev. Lett. \textbf{130}, 106703 (2023)]. More precisely, we analyse the competition between skyrmions and a chiral spin liquid in a model on the kagome lattice. We first present an analytical overview of the low-energy states using the Luttinger-Tisza approximation. We then study the effect of thermal fluctuations thanks to large-scale Monte-Carlo simulations, and explore the entire parameter space with a magnetic field $B$, in-plane $D^{xy}$ and out-of-plane $D^z$ Dzyaloshinskii-Moriya interactions. While skyrmions and the chiral spin liquid live in different regions of the parameter space, we show how to bring them together, stabilizing a skyrmion fluid in between; a region where the density of well-defined skyrmions can be tuned before obtaining an ordered phase. We investigate in particular the melting of the skyrmion solid. Our analysis also brings to light a long-range ordered phase with Z$_3$ symmetry. At last, we initiate the study of this rich magnetic background on conduction electrons that are coupled to the local spins. We study how the different chiral magnetic textures stabilized in this model (skyrmion solid, liquid and gas and chiral spin liquid) induce a topological Quantum Hall effect. We observe in the ordered skyrmion phase the appearance of Landau levels which persist even in the skyrmion-liquid regime and gradually disappear as the skyrmion density decreases to form a gas.

cond-mat.stat-mech↗

arXiv2023

Schwinger boson theory of the J1,J2=J3 kagome antiferromagnet

We study the kagome antiferromagnet for quantum spin-1/2 with first J1, second J2 and third J3 neighbour exchanges, along the J2 = J3 = J line. We use Schwinger-boson mean-field theory for the precise determination of the phase diagram, and two different rewritings of the Hamiltonian to build an intuition about the origin of the transitions. The spin liquid obtained at J = 0 remains essentially stable over a large window, up to J = 1/3, because it is only weakly frustrated by the J term. Then at J = 1/2, the intermediate Z2 spin liquid condenses into a long-range chiral order because of the change of nature of local magnetic fluctuations. As a side benefit, our Hamiltonian rewriting offers an exact solution for the ground state of our model on a Husimi cactus.

cond-mat.str-el↗

arXiv2023

Non-local spin correlation as a signature of Ising anyons trapped in vacancies of the Kitaev spin liquid

In the Kitaev chiral spin liquid, Ising anyons are realized as $Z_2$ fluxes binding Majorana zero modes, which, however, are thermal excitations with finite decay rates. On the other hand, a lattice vacancy traps a $Z_2$ flux even in the ground state, resulting in the stable realization of a Majorana zero mode in a vacancy. We demonstrate that spin-spin correlation functions between two vacancy sites exhibit long-range correlation arising from the fractionalized character of Majorana zero modes, in spite of the strong decay of bulk spin correlations. Remarkably, this non-local spin correlation does not decrease as the distance between two vacancy sites increases, signaling Majorana teleportation. Furthermore, we clarify that the non-local correlation can be detected electrically via the measurement of non-local conductance between two vacancy sites, which is straightforwardly utilized for the readout of Majorana qubits. These findings pave the way to the measurement-based quantum computation with Ising anyons trapped in vacancies of the Kitaev spin liquid.

cond-mat.str-el↗

arXiv2022

Matrix Product Renormalization Group: Potential Universal Quantum Many-Body Solver

The density matrix renormalization group (DMRG) is a celebrated tensor network algorithm, which computes the ground states of one-dimensional quantum many-body systems very efficiently. Here we propose an improved formulation of continuous tensor network algorithms, which we name a matrix product renormalization group (MPRG). MPRG is a universal quantum many-body solver, which potentially works at both zero and finite temperatures, in two and higher dimensions, and is even applicable to open quantum systems. Furthermore, MPRG does not rely on any variational principles and thus supports any kind of non-Hermitian systems in any dimension. As a demonstration, we present critical properties of the Yang-Lee edge singularity in one dimension as a representative non-Hermitian system.

cond-mat.str-el↗

arXiv2022

Flux roughening in spin ice with mixed $\pm J$ interactions

Spin ice presents a typical example of classical spin liquid, where conserved magnetic fluxes emerge from microscopic spin degrees of freedom. In this letter, we investigate the effect of perturbation by magnetic charge disorder in two-dimensional spin ice. To this aim, we develop a novel cluster update algorithm, which enables fast relaxation of magnetic charges. The efficient Monte Carlo calculation reveals a drastic change of spin structure factor as doping magnetic charges: the pinch point, characterizing the spin ice, is gradually replaced by a diffusive peak. We derive an analytical relation connecting the flux fluctuation and the spin structure factor, and explain the evolution of diffusive peak in terms of the roughening of magnetic fluxes.

cond-mat.str-el↗

arXiv2022

Phonon thermal Hall effect in a metallic spin ice

It has become common knowledge that phonons can generate thermal Hall effect in a wide variety of materials, although the underlying mechanism is still controversial. We study longitudinal $κ_{xx}$ and transverse $κ_{xy}$ thermal conductivity in Pr$_2$Ir$_2$O$_7$, which is a metallic analogue of spin ice. Despite the presence of mobile charge carriers, we find that both $κ_{xx}$ and $κ_{xy}$ are dominated by phonons. A $T/H$ scaling of $κ_{xx}$ unambiguously reveals that longitudinal heat current is substantially impeded by resonant scattering of phonons on paramagnetic spins. Upon cooling, the resonant scattering is strongly affected by a development of spin ice correlation and $κ_{xx}$ deviates from the scaling in an anisotropic way with respect to field directions. Strikingly, a set of the $κ_{xx}$ and $κ_{xy}$ data clearly shows that $κ_{xy}$ correlates with $κ_{xx}$ in its response to magnetic field including a success of the $T/H$ scaling and its failure at low temperature. This remarkable correlation provides solid evidence that an indispensable role is played by spin-phonon scattering not only for hindering the longitudinal heat conduction, but also for generating the transverse response.

cond-mat.str-el↗

arXiv2021

Magnetic Switching by Oxygen Adsorption in Metal-Organic Framework Systems

In this letter, we address magnetization switching by oxygen adsorption in porous metal-organic framework systems. To this end, we construct a simple localized spin model combined with a Langmuir-type formula for oxygen adsorption and study its finite-temperature properties using Monte Carlo simulation. We successfully explain the main features of this phenomenon, such as the discontinuous changes in magnetic states, sensitivity of the magnetic transition temperatures to oxygen pressure, and absence of singularities in adsorbed oxygen. Based on this model, we also reproduce the observed magnetic transition temperatures for a typical value of oxygen adsorption energy.

cond-mat.mtrl-sci↗

arXiv2020

STM as a single Majorana detector of Kitaev's chiral spin liquid

In this letter, we propose a local detection scheme for the Majorana zero mode (MZM) carried by a vison in Kitaev's chiral spin liquid (CSL) using scanning tunneling microscopy (STM). The STM introduces a single Majorana into the system through hole/charge injection and the Majorana interacts with the MZM to form a stable composite object. We derive the exact analytical expression of single-hole Green's function in the Mott insulating limit of Kitaev's model and show that the differential conductance has split peaks, as a consequence of resonant tunneling through the vison-hole composite. The peak splitting scales with the binding energy of vison-hole composite, which is comparable to the Majorana gap in CSL, well within the reach of experimental observation.

cond-mat.str-el↗

arXiv2020

Field-selective classical spin liquid and magnetization plateaus on kagome lattice

We obtain a classical spin liquid (CSL) phase by applying a magnetic field in $J_1$-$J_2$-$J_3$ Ising model on a kagome lattice. As we proved in the previous study [Phys. Rev. Lett. {\bf 119}, 077207 (2017)], this model realizes one species of CSL, the hexamer CSL, at zero magnetic field, which consists of macroscopically degenerate spin configurations with mixed total magnetization, $M$. The magnetic field selects its subset, which can be mapped to a trimer covering of the dual lattice, and forms a magnetization plateau of $M=1/9$. In addition to this CSL, we find two other magnetization plateaus at $M=5/9$ and $17/27$, which are ascribed to the "multimer" superstructures on a dual lattice.

cond-mat.str-el↗

arXiv2019

Spectroscopy of Majorana modes of non-Abelian vortices in Kitaev's chiral spin liquid

We study the temperature (T) dependence of the dynamical structure factor of the chiral spin liquid phase of Kitaev's honeycomb model. We find, using a recently developed analytical approach, direct signatures of the Majorana modes associated with the non-Abelian Z_2 vortices (visons). In particular, a characteristic multiplet of discrete peaks at low but nonzero-T and its field dependence reflect the existence of thermally activated visons whose varying separation yields different splittings of the Majorana `zero' modes. The resonant processes involved are specific to the Majorana modes, and will persist even in the presence of weak non-integrable interactions.

cond-mat.str-el↗