arXiv subjects

Y. Motome

Publications and source records attributed to Y. Motome.

18 recordsLinked to original sources

arXiv2025

High-field NMR study of field-induced states in Pb(TiO)Cu$_4$(PO$_4$)$_4$

The square cupola antiferromagnet Pb(TiO)Cu$_4$(PO$_4$)$_4$ exhibits the intriguing magnetoelectric responses arising from the consecutive change in the magnetic quadrupolar-type configuration of magnetic moments under external magnetic fields higher than 15 T. To clarify the high-field magnetic structures in Pb(TiO)Cu$_4$(PO$_4$)$_4$, an NMR measurement was performed in pulsed fields up to 32.2 T significantly extending the field range accessible by superconducting magnets. The double-peak structure of NMR spectra emerging above 29 T applied along the [001] direction evidences the successive magnetic transitions. The field dependence of NMR spectra was analyzed on the basis of cluster mean-field theory, which allows us to propose possible magnetic structures for the high-field magnetic states.

cond-mat.str-el↗

arXiv2023

Anomalous charge transport upon quantum melting of chiral spin order

A plethora of correlated and exotic metallic states have been identified on the border of itinerant magnetism, where the long-range spin texture is melted by tuning the magnetic transition temperature (T$_C$) towards zero, referred to as the quantum phase transition (QPT). So far, the study of QPT in itinerant magnets has mainly focused on low-T$_C$ materials (i.e., typically T$_C$ ~ 10 K) where the modification of electronic band structure is subtle, and only makes a small contribution to the QPT. Here we report a distinct example of a magnetic QPT accompanied by a gigantic modification of the electronic structure in FeGe, i.e., a well-studied itinerant chiral magnet hosting near-room-temperature (T$_C$ = 278 K) helical/skyrmion spin texture. The pressure-driven modification of the band structure (e.g., reduction of exchange splitting) is evidenced by magneto-transport study, suggesting a Fermi-surface reconstruction around the magnetic QPT (P ~19 GPa), in stark contrast to the case of typical metallic ferromagnets. Further application of pressure leads to a metal-to-insulator transition above P > 30 GPa, as also corroborated by our density-functional theory (DFT) calculation. Of particular interest is the occurrence of anomalous magneto-transport in the inhomogeneous short-range chiral-spin ground state (P = 20-30 GPa) above the QPT, with longitudinal fluctuations of magnetization. The unexpected observation of spontaneous anomalous Hall effect in this exotic quantum regime suggests macroscopic time-reversal symmetry (TRS) breaking, even in the absence of long-range magnetic order. Our findings mark the large body of unexplored high-T$_C$ itinerant magnets with broken inversion-symmetry as promising candidates of novel ground state formation near QPT.

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arXiv2022

Evidence for the first-order topological phase transition in a Kitaev spin liquid candidate $\alpha$-RuCl$ _3$

The Kitaev quantum spin liquid (QSL) on the two-dimensional honeycomb lattice epitomizes an entangled topological state, where the spins fractionalize into Majorana fermions. This state has aroused tremendous interest because it harbors non-Abelian anyon excitations. The half-integer quantized thermal Hall (HIQTH) conductance observed in $\alpha$-RuCl$_3$ is a key signature of these excitations. However, the fate of this topologically nontrivial state at intense fields remains largely elusive. Here, we report the thermal conductivity $\kappa$ and specific heat $C$ of $\alpha$-RuCl$_3$ with in-plane magnetic fields $H$. For the field direction perpendicular to the Ru-Ru bond, where the HIQTH effect is observed, we find a discontinuous jump in $\kappa(H)$ and a peak anomaly in $C(H)$ at $\mu_0H^*\approx11$\,T, evidencing a weak first-order phase transition. Remarkably, the HIQTH effect vanishes close to $H^*$. Furthermore, we find that the spin-fractionalization feature is retained well above $H^\ast$. These imply the emergence of the phase transition that separates two QSL phases with distinct topological properties.

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arXiv2022

Zoology of multiple-Q spin textures in a centrosymmetric tetragonal magnet with itinerant electrons

Magnetic skyrmion is a topologically stable particle-like swirling spin texture potentially suitable for high-density information bit, which was first observed in noncentrosymmetric magnets with Dzyaloshinskii-Moriya interaction. Recently, nanometric skyrmion has also been discovered in centrosymmetric rare-earth compounds, and the identification of their skyrmion formation mechanism and further search of nontrivial spin textures are highly demanded. Here, we have exhaustively studied magnetic structures in a prototypical skyrmion-hosting centrosymmetric tetragonal magnet GdRu2Si2, by performing the resonant X-ray scattering experiments. We identified a rich variety of double-Q magnetic structures, including the antiferroic order of meron(half-skyrmion)/anti-meronlike textures with fractional local topological charges. The observed intricate magnetic phase diagram has been successfully reproduced by the theoretical framework considering the four-spin interaction mediated by itinerant electrons and magnetic anisotropy. The present results will contribute to the better understanding of the novel skyrmion formation mechanism in this centrosymmetric rare-earth compound, and suggest that itinerant electrons can ubiquitously host a variety of unique multiple-Q spin orders in a simple crystal lattice system.

cond-mat.mtrl-sci↗

arXiv2020

Switching of band inversion and topological surface states by charge density wave

Topologically nontrivial materials host protected edge states associated with the bulk band inversion through the bulk-edge correspondence. Manipulating such edge states is highly desired for developing new functions and devices practically using their dissipation-less nature and spin-momentum locking. Here we introduce a transition-metal dichalcogenide VTe$_2$, that hosts a charge density wave (CDW) coupled with the band inversion involving V3$d$ and Te5$p$ orbitals. Spin- and angle-resolved photoemission spectroscopy with first-principles calculations reveal the huge anisotropic modification of the bulk electronic structure by the CDW formation, accompanying the selective disappearance of Dirac-type spin-polarized topological surface states that exist in the normal state. Thorough three dimensional investigation of bulk states indicates that the corresponding band inversion at the Brillouin zone boundary dissolves upon CDW formation, by transforming into anomalous flat bands. Our finding provides a new insight to the topological manipulation of matters by utilizing CDWs' flexible characters to external stimuli.

cond-mat.mtrl-sci↗

arXiv2020

Materials design of Kitaev spin liquids beyond the Jackeli-Khaliullin mechanism

The Kitaev spin liquid provides a rare example of well-established quantum spin liquids in more than one dimension. It is obtained as the exact ground state of the Kitaev spin model with bond-dependent anisotropic interactions. The peculiar interactions can be yielded by the synergy of spin-orbit coupling and electron correlations for specific electron configuration and lattice geometry, which is known as the Jackeli-Khaliullin mechanism. Based on this mechanism, there has been a fierce race for the materialization of the Kitaev spin liquid over the last decade, but the candidates have been still limited mostly to $4d$- and $5d$-electron compounds including cations with the low-spin $d^5$ electron configuration, such as Ir$^{4+}$ and Ru$^{3+}$. Here we discuss recent efforts to extend the material perspective beyond the Jackeli-Khaliullin mechanism, by carefully reexamining the two requisites, formation of the $j_{\rm eff}=1/2$ doublet and quantum interference between the exchange processes, for not only $d$- but also $f$-electron systems. We present three examples: the systems including Co$^{2+}$ and Ni$^{3+}$ with the high-spin $d^7$ electron configuration, Pr$^{4+}$ with the $f^1$-electron configuration, and polar asymmetry in the lattice structure. In particular, the latter two are intriguing since they may realize the antiferromagnetic Kitaev interactions, in contrast to the ferromagnetic ones in the existing candidates. This partial overview would stimulate further material exploration of the Kitaev spin liquids and its topological properties due to fractional excitations.

cond-mat.str-el↗

arXiv2020

Half-integer quantized anomalous thermal Hall effect in the Kitaev material $α$-RuCl$_3$

Heat transport mediated by Majorana edge modes in a magnetic insulator leads to a half-integer thermal quantum Hall conductance, which has recently been reported for the two-dimensional honeycomb material $α$-RuCl$_3$. While the conventional electronic Hall effect requires a perpendicular magnetic field, we find that this is not the case in $α$-RuCl$_3$. Strikingly, the thermal Hall plateau appears even for a magnetic field with no out-of-plane components. The field-angular variation of the quantized thermal Hall conductance has the same sign structure of the topological Chern number, which is either $\pm$1, as the Majorana band structure of the pure Kitaev spin liquid. This observation of a half-integer anomalous thermal Hall effect firmly establishes that the Kitaev interaction is primarily responsible and that the non-Abelian topological order associated with fractionalization of the local magnetic moments persists even in the presence of non-Kitaev interactions in $α$-RuCl$_3$.

cond-mat.str-el↗

arXiv2019

Negative thermal expansion in the plateau state of a magnetically-frustrated spinel

We report on negative thermal expansion (NTE) in the high-field, half-magnetization plateau phase of the frustrated magnetic insulator CdCr2O4. Using dilatometry, we precisely map the phase diagram at fields of up to 30T, and identify a strong NTE associated with the collinear half-magnetization plateau for B > 27T. The resulting phase diagram is compared with a microscopic theory for spin-lattice coupling, and the origin of the NTE is identified as a large negative change in magnetization with temperature, coming from a nearly-localised band of spin excitations in the plateau phase. These results provide useful guidelines for the discovery of new NTE materials.

cond-mat.str-el↗

arXiv2019

Thermodynamics of a gauge-frustrated Kitaev spin liquid

Two- and three-dimensional Kitaev magnets are prototypical frustrated quantum spin systems, in which the original spin degrees of freedom fractionalize into Majorana fermions and a $\mathbb{Z}_2$ gauge field -- a purely local phenomenon that reveals itself as a thermodynamic crossover at a temperature scale set by the strength of the bond-directional interactions. For conventional Kitaev magnets, the low-temperature thermodynamics reveals a second transition at which the $\mathbb{Z}_2$ gauge field orders and the system enters a spin liquid ground state. Here we discuss an explicit example that goes beyond this paradigmatic scenario -- the $\mathbb{Z}_2$ gauge field is found to be subject to geometric frustration, the thermal ordering transition is suppressed, and an extensive residual entropy arises. Deep in the quantum regime, at temperatures of the order of one per mil of the interaction strength, the degeneracy in the gauge sector is lifted by a subtle interplay between the gauge field and the Majorana fermions, resulting in the formation of a Majorana metal. We discuss the thermodynamic signatures of this physics obtained from large-scale, sign-free quantum Monte Carlo simulations.

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arXiv2018

Two-step gap opening across the quantum critical point in a Kitaev honeycomb magnet

Quantum spin liquid involves fractionalized quasipariticles such as spinons and visons. They are expressed as itinerant Majorana fermions and $Z_2$ fluxes in the Kitaev model with bond-dependent exchange interactions on a honeycomb spin lattice. The observation has recently attracted attention for a candidate material $α$-RuCl$_3$, showing spin liquid behaviour induced by a magnetic field. Since the observable spin excitation is inherently composed of the two quasiparticles, which further admix each other by setting in the magnetic field as well as non-Kitaev interactions, their individual identification remains challenging. Here we report an emergent low-lying spin excitation through nuclear magnetic and quadrupole resonance measurements down to $\sim 0.4$ K corresponding to $1/500$ of the exchange energy under the finely tuned magnetic field across the quantum critical point. We determined the critical behaviour of low-lying excitations and found evolution of two kinds of the spin gap at high fields. The two excitations exhibit repulsive magnetic field dependence, suggesting anti-crossing due to the hybridization between fractionalized quasiparticles.

cond-mat.str-el↗

arXiv2018

Majorana quantization and half-integer thermal quantum Hall effect in a Kitaev spin liquid

The quantum Hall effect (QHE) in two-dimensional (2D) electron gases, which is one of the most striking phenomena in condensed matter physics, involves the topologically protected dissipationless charge current flow along the edges of the sample. Integer or fractional electrical conductance are measured in units of $e^2/2π\hbar$, which is associated with edge currents of electrons or quasiparticles with fractional charges, respectively. Here we discover a novel type of quantization of the Hall effect in an insulating 2D quantum magnet. In $α$-RuCl$_3$ with dominant Kitaev interaction on 2D honeycomb lattice, the application of a parallel magnetic field destroys the long-range magnetic order, leading to a field-induced quantum spin liquid (QSL) ground state with massive entanglement of local spins. In the low-temperature regime of the QSL state, we report that the 2D thermal Hall conductance $κ_{xy}^{2D}$ reaches a quantum plateau as a function of applied magnetic field. $κ_{xy}^{2D}/T$ attains a quantization value of $(π/12)(k_B^2/\hbar)$, which is exactly half of $κ_{xy}^{2D}/T$ in the integer QHE. This half-integer thermal Hall conductance observed in a bulk material is a direct signature of topologically protected chiral edge currents of charge neutral Majorana fermions, particles that are their own antiparticles, which possess half degrees of freedom of conventional fermions. These signatures demonstrate the fractionalization of spins into itinerant Majorana fermions and $Z_2$ fluxes predicted in a Kitaev QSL. Above a critical magnetic field, the quantization disappears and $κ_{xy}^{2D}/T$ goes to zero rapidly, indicating a topological quantum phase transition between the states with and without chiral Majorana edge modes. Emergent Majorana fermions in a quantum magnet are expected to have a major impact on strongly correlated topological quantum matter.

cond-mat.str-el↗

arXiv2018

Unusual thermal Hall effect in a Kitaev spin liquid candidate $α$-RuCl$_3$

The Kitaev quantum spin liquid displays the fractionalization of quantum spins into Majorana fermions. The emergent Majorana edge current is predicted to manifest itself in the form of a finite thermal Hall effect, a feature commonly discussed in topological superconductors. Here we report on thermal Hall conductivity $κ_{xy}$ measurements in $α$-RuCl$_3$, a candidate Kitaev magnet with the two-dimensional honeycomb lattice. In a spin-liquid (Kitaev paramagnetic) state below the temperature characterized by the Kitaev interaction $J_K/k_B \sim 80$ K, positive $κ_{xy}$ develops gradually upon cooling, demonstrating the presence of highly unusual itinerant excitations. Although the zero-temperature property is masked by the magnetic ordering at $T_N=7$ K, the sign, magnitude, and $T$-dependence of $κ_{xy}/T$ at intermediate temperatures follows the predicted trend of the itinerant Majorana excitations.

cond-mat.str-el↗

arXiv2016

Fermionic response from fractionalization in an insulating two-dimensional magnet

Conventionally ordered magnets possess bosonic elementary excitations, called magnons. By contrast, no magnetic insulators in more than one dimension are known whose excitations are not bosons but fermions. Theoretically, some quantum spin liquids (QSLs) -- new topological phases which can occur when quantum fluctuations preclude an ordered state -- are known to exhibit Majorana fermions as quasiparticles arising from fractionalization of spins. Alas, despite much searching, their experimental observation remains elusive. Here, we show that fermionic excitations are remarkably directly evident in experimental Raman scattering data across a broad energy and temperature range in the two-dimensional material $α$-RuCl$_3$. This shows the importance of magnetic materials as hosts of Majorana fermions. In turn, this first systematic evaluation of the dynamics of a QSL at finite temperature emphasizes the role of excited states for detecting such exotic properties associated with otherwise hard-to-identify topological QSLs.

cond-mat.str-el↗

arXiv2014

Finite-Temperature Phase Transition to a Quantum Spin Liquid in a Three-Dimensional Kitaev Model on a Hyperhoneycomb Lattice

We present numerical evidence for the presence of a finite-temperature ($T$) phase transition separating paramagnet and quantum spin liquid in a three-dimensional variant of the Kitaev model defined on a hyperhoneycomb lattice in the limit of strong anisotropy; the model is mapped onto an effective Ising-type model, where elementary excitations consist of closed loops of flipped Ising-type variables. Analyzing this effective model by Monte Carlo simulation, we find a phase transition from quantum spin liquid to paramagnet at a finite critical temperature $T_c$. We also compute the magnetic properties in terms of the original quantum spins. We find that the magnetic susceptibility exhibits a broad hump above $T_c$, while it obeys the Curie law at high $T$ and approaches a nonzero Van Vleck-type constant at low $T$. Although the susceptibility changes continuously at $T_c$, its $T$ derivative shows critical divergence at $T_c$. We also clarify that the dynamical spin correlation function is momentum independent but shows quantized peaks corresponding to the discretized excitations. Although the phase transition accompanies no apparent symmetry breaking in terms of the Ising-type variables as well as the original quantum spins, we characterize it from a topological viewpoint. We find that, by defining the flux density for loops of the Ising-type variables, the transition is interpreted as the one occurring from the zero-flux quantum spin liquid to the nonzero-flux paramagnet; the latter has a Coulombic nature due to the local constraints. The role of global constraints on the Ising-type variables is examined in comparison with the results in the two-dimensional loop model. A correspondence of our model to the Ising model on a diamond lattice is also discussed. A possible relevance of our results to the recently-discovered hyperhoneycomb compound, $β$-Li$_2$IrO$_3$, is mentioned.

cond-mat.str-el↗

arXiv2013

Exploring the Dark Side of Cuprate Superconductors: s-wave Symmetry of the Pseudogap

We reveal the full energy-momentum structure of the pseudogap of underdoped high-Tc cuprate superconductors. Our combined theoretical and experimental analysis explains the spectral-weight suppression observed in the B2g Raman response at finite energies in terms of a pseudogap appearing in the single-electron excitation spectra above the Fermi level in the nodal direction of momentum space. This result suggests an s-wave pseudogap (which never closes in the energy-momentum space), distinct from the d-wave superconducting gap. Recent tunneling and photoemission experiments on underdoped cuprates also find a natural explanation within the s-wave-pseudogap scenario.

cond-mat.str-el↗

arXiv2011

Quantum Monte Carlo study of the transverse-field Ising model on a frustrated checkerboard lattice

We present the numerical results for low temperature behavior of the transverse-field Ising model on a frustrated checkerboard lattice, with focus on the effect of both quantum and thermal fluctuations. Applying the recently-developed continuous-time quantum Monte Carlo algorithm, we compute the magnetization and susceptibility down to extremely low temperatures while changing the magnitude of both transverse and longitudinal magnetic fields. Several characteristic behaviors are observed, which were not inferred from the previously studied quantum order from disorder at zero temperature, such as a horizontal-type stripe ordering at a substantial longitudinal field and a persistent critical behavior down to low temperature in a weak longitudinal field region.

cond-mat.str-el↗

arXiv2005

Study of the One- and Two-Band Models for Colossal Magnetoresistive Manganites Using the Truncated Polynomial Expansion Method

Considerable progress has been recently made in the theoretical understanding of the colossal magnetoresistance (CMR) effect in manganites. The analysis of simple models with two competing states and a resistor network approximation to calculate conductances has confirmed that CMR effects can be theoretically reproduced using non-uniform clustered states. In this paper, the recently proposed Truncated Polynomial Expansion method (TPEM) for spin-fermion systems is tested using the double-exchange one-band, with finite Hund coupling $J_{\rm H}$, and two-band, with infinite $J_{\rm H}$, models. Two dimensional lattices as large as 48$\times$48 are studied, far larger than those that can be handled with standard exact diagonalization (DIAG) techniques for the fermionic sector. The clean limit (i.e. without quenched disorder) is here analyzed in detail. Phase diagrams are obtained, showing first-order transitions separating ferromagnetic metallic from insulating states. A huge magnetoresistance is found at low temperatures by including small magnetic fields, in excellent agreement with experiments. However, at temperatures above the Curie transition the effect is much smaller confirming that the standard finite-temperature CMR phenomenon cannot be understood using homogeneous states. By comparing results between the two methods, TPEM and DIAG, on small lattices, and by analyzing the systematic behavior with increasing cluster sizes, it is concluded that the TPEM is accurate to handle realistic manganite models on large systems. Our results pave the way to a frontal computational attack of the colossal magnetoresistance phenomenon using double-exchange like models, on large clusters, and including quenched disorder.

cond-mat.mtrl-sci↗

arXiv2005

The Truncated Polynomial Expansion Monte Carlo Method for Fermion Systems Coupled to Classical Fields: A Model Independent Implementation

A software library is presented for the polynomial expansion method (PEM) of the density of states (DOS) developed by two of the authors (N.F. and Y. M.). The library provides all necessary functions for the use of the PEM and its truncated version (TPEM) in a model independent way. The PEM/TPEM replaces the exact diagonalization of the one electron sector in models for fermions coupled to classical fields. The computational cost of the algorithm is O(N) -- with N the number of lattice sites -- for the TPEM which should be contrasted with the computational cost of the diagonalization technique that scales as O(N^4). The method is applied for the first time to a double exchange model with finite Hund coupling and also to diluted spin-fermion models.

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