arXiv subjects

Yukitoshi Motome

Publications and source records attributed to Yukitoshi Motome.

At least 163 records · Page 9Linked to original sources

arXiv2015

Theory of Valence Transition in BiNiO$_3$

Motivated by the colossal negative thermal expansion recently found in BiNiO$_3$, the valence transition accompanied by the charge transfer between the Bi and Ni sites is theoretically studied. We introduce an effective model for Bi-$6s$ and Ni-$3d$ orbitals with taking into account the valence skipping of Bi cations, and investigate the ground-state and finite-temperature phase diagrams within the mean-field approximation. We find that the valence transition is caused by commensurate locking of the electron filling in each orbital associated with charge and magnetic orderings, and the critical temperature and the nature of the transitions are strongly affected by the relative energy between the Bi and Ni levels and the effective electron-electron interaction in the Bi sites. The obtained phase diagram well explains the temperature- and pressure-driven valence transitions in BiNiO$_3$ and the systematic variation of valence states for a series of Bi and Pb perovskite oxides.

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arXiv2015

Analysis of Charge-spin-orbital Fluctuations by Ab Initio Calculation and Random Phase Approximation: Application to Non-coplanar Antiferromagnet Cd2Os2O7

We present a systematic analysis on the basis of ab initio calculations and many-body perturbation theory for clarifying the dominant fluctuation in complex charge-spin-orbital coupled systems. For a tight-binding multiband model obtained from the maximally-localized Wannier function analysis of the band structure by the local density approximation, we take into account electron correlations at the level of random phase approximation. To identify the dominant fluctuation, we carry out the eigenmode analysis of the generalized susceptibility that includes all the multiple degrees of freedom: charge, spin, and orbital. We apply this method to the paramagnetic metallic phase of a pyrochlore oxide Cd$_2$Os$_2$O$_7$, which shows a metal-insulator transition accompanied by a peculiar noncoplanar antiferromagnetic order of all-in all-out type. We find that the corresponding spin fluctuation is dominantly enhanced by the on-site Coulomb repulsions in the presence of strong spin-orbit coupling and trigonal crystal field splitting. Our results indicate that the combined method offers an effective tool for the systematic analysis of potential instabilities in strongly correlated electron materials.

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arXiv2015

Phase Diagram of the Kitaev-type Model on a Decorated Honeycomb Lattice in the Isolated Dimer Limit

An effective model in the isolated dimer limit of the Kitaev-type model on a decorated honeycomb lattice is investigated at finite temperature. The ground state of this model is exactly shown to be a chiral spin liquid with spontaneous breaking of time reversal symmetry. We elaborate the finite-temperature phase diagram by using the mean-field approximation and Monte Carlo simulation. We find that the phase transition between the high-temperature paramagnetic phase and the low-temperature chiral spin liquid phase is always of second order in the Monte Carlo results, although a tricritical point appears in the mean-field phase diagram. The finite-size scaling analysis of the Monte Carlo data indicates that the phase transition belongs to the two-dimensional Ising universality class.

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arXiv2015

Thermodynamics of Chiral Spin Liquids with Abelian and Non-Abelian Anyons

Thermodynamic properties of chiral spin liquids are investigated for a variant of the Kitaev model defined on a decorated honeycomb lattice. Using the quantum Monte Carlo simulation, we find that the model exhibits a finite-temperature phase transition associated with the time reversal symmetry breaking, in both topologically trivial and nontrivial regions. While changing the exchange constants, the phase transition changes from continuous to discontinuous one, apparently correlated with the change in the excitations from Abelian to non-Abelian anyons. We show this coincidence by computing the topological quantities: the Chern number and the thermal Hall conductivity. In addition, we find, as a diagnostic of the chiral spin liquids, successive crossovers with multi-stage entropy release above the critical temperature, which indicates that the hierarchical fractionalization of a quantum spin occurs differently between the two regions.

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arXiv2015

Spontaneous formation of kagome network and Dirac half-semimetal on a triangular lattice

In spin-charge coupled systems, geometrical frustration of underlying lattice structures can give rise to nontrivial magnetic orders and electronic states. Here we explore such a possibility in the Kondo lattice model with classical localized spins on a triangular lattice by using a variational calculation and simulated annealing. We find that the system exhibits a four-sublattice collinear ferrimagnetic phase at 5/8 filling for a large Hund's-rule coupling. In this state, the system spontaneously differentiates into the up-spin kagome network and the isolated down-spin sites, which we call the kagome network formation. In the kagome network state, the system becomes Dirac half-semimetallic: The electronic structure shows a massless Dirac node at the Fermi level, and the Dirac electrons are almost fully spin polarized due to the large Hund's-rule coupling. We also study the effect of off-site Coulomb repulsion in the kagome network phase where the system is effectively regarded as a 1/3-filling spinless fermion system on the kagome lattice. We find that, at the level of the mean-field approximation, a $\sqrt{3} \times \sqrt{3}$-type charge order occurs in the kagome network state, implying the possibility of fractional charge excitations in this triangular lattice system. Moreover, we demonstrate that the kagome network formation with fully-polarized Dirac electrons are controllable by an external magnetic field.

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arXiv2015

A strong coupling theory for electron-mediated interactions in double-exchange models

We present a theoretical framework for evaluating effective interactions between localized spins mediated by itinerant electrons in double-exchange models. Performing the expansion with respect to the spin-dependent part of the electron hopping terms, we show a systematic way of constructing the effective spin model in the large Hund's coupling limit. As a benchmark, we examine the accuracy of this method by comparing the results with the numerical solutions for the spin-ice type model on a pyrochlore lattice. We also discuss an extension of the method to the double-exchange models with Heisenberg and $XY$ localized spins.

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arXiv2015

Spontaneous multipole ordering by local parity mixing

Broken spatial inversion symmetry in spin-orbital coupled systems leads to a mixing between orbitals with different parity, which results in unusual electronic structures and transport properties. We theoretically investigate the possibility of multipole ordering induced by a parity mixing. In particular, we focus on the system in which the parity mixing appears in a sublattice-dependent form. Starting from the periodic Anderson model with such a local parity mixing, we derive an extended Kondo lattice model with sublattice-dependent antisymmetric exchange couplings between itinerant electrons and localized spins. By the variational calculation, simulated annealing, and Monte Carlo simulation, we show that the model on a quasi-one-dimensional zig-zag lattice exhibits an odd-parity multipole order composed of magnetic toroidal and quadrupole components at and near half filling. The multipole order causes a band deformation with the band bottom shift and a magnetoelectric response. The results suggest that unusual odd-parity multipole orders will be widely observed in multi-orbital systems with local parity mixing.

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arXiv2015

Exotic magnetic phases in an Ising-spin Kondo lattice model on a kagome lattice

Magnetic and electronic states of an Ising-spin Kondo lattice model on a kagome lattice are investigated by a Monte Carlo simulation. In addition to the conventional ferromagnetic and ferrimagnetic orders, we show that this model exhibits several thermally-induced phases, such as partially disordered, Kosteritz-Thouless-like, and loop-liquid states. In the partially disordered state, we show that the magnetic transition is associated with the charge-gap formation. We find that the density of state shows characteristic peaks reflecting the underlying spin texture. In the loop-liquid state, we also find a peak in a different manner in the density of states as well as in the optical conductivity. This is a consequence of the formation of closed loops of the same spin sites in the loop-liquid state. Our results elucidates the peculiar cooperation between thermal fluctuations and the spin-charge interplay in this frustrated itinerant electron system.

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arXiv2014

Topological semimetal-to-insulator phase transition between noncollinear and noncoplanar multiple-Q states on a square-to-triangular lattice

Noncollinear and noncoplanar magnetic orders lead to unusual electronic structures and transport properties. We here investigate two types of multiple-Q magnetically ordered states and a topological phase transition between them in two dimensions. One is a coplanar but noncollinear double-Q state on a square lattice, which is a semimetal accommodating massless Dirac electrons. The other is a noncoplanar triple-Q state on a triangular lattice, which is a Chern insulator showing the quantum anomalous Hall effect. We discuss the peculiar electronic structures in these two multiple-Q states in a unified way on the basis of the Kondo lattice model, which suggests a quantum phase transition between the two states in a continuous change of lattice geometry between the square and triangular lattices. We systematically examine the possibility of such a transition by using the mean-field approximation for the ground state of the periodic Anderson model. After clarifying the parameter region in which the double-Q (triple-Q) state is stabilized on the square (triangular) lattice, we show that a continuous topological phase transition indeed takes place between the double-Q Dirac semimetal and the triple-Q Chern insulator on the square-to-triangular lattice. The nature of the transition is discussed by the topologically-protected edge states as well as the bulk magnetic and electronic properties. The results indicate that unusual critical phenomena may occur at finite temperature related with multiple-Q chiral spin-liquid states.

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arXiv2014

Vaporization of Kitaev spin liquids

Quantum spin liquid is an exotic quantum state of matter in magnets. This state is a spin analogue of the liquid helium which does not solidify down to the lowest temperature due to strong quantum fluctuations. In conventional fluids, liquid and gas possess the same symmetry and adiabatically connect to each other by bypassing the critical end point. We find that the situation is qualitatively different in quantum spin liquids realizing in a three-dimensional Kitaev model; both gapless and gapped quantum spin liquid phases at low temperatures are always distinguished from the high-temperature paramagnet (spin gas) by a phase transition. The results challenge common belief that the absence of thermodynamic singularity down to the lowest temperature is a symptom of a quantum spin liquid.

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arXiv2014

Surface and interface effects on a magnetic Chern insulator

Effects of an open surface on a magnetic Chern insulator are investigated in comparison with those of an interface to a capping magnetic layer. In magnets, an open surface often perturbs the magnetic order by a reconstruction of the magnetic moment directions near the surface. On the other hand, in topological insulators, it leads to the formation of topologically protected surface states. These two contrasting effects may coexist in magnetic Chern insulators, which give rise to nontrivial surface reconstruction. For instance, the chiral edge current is largely enhanced by the edge reconstruction in a two-dimensional magnetic Chern insulator realized in a quarter-filled Kondo lattice model on a triangular lattice. We here show that the edge reconstruction can be described semiquantitatively by a simple junction model between the bulk topological magnetic state and a ferromagnetic capping layer. We further clarify how the chiral edge current is affected by the magnetic structure in the capping layer. Our results indicate that the topological edge state can be controlled magnetically through the junctions.

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arXiv2014

Low-energy Majorana states in spin-liquid transitions in a three-dimensional Kitaev model

A three-dimensional Kitaev model on a hyperhoneycomb lattice is investigated numerically at finite temperature. The Kitaev model is one of the solvable quantum spin models, where the ground state is given by gapped and gapless spin liquids, depending on the anisotropy of the interactions. This model can be rewritten as a free Majorana fermion system coupled with $Z_2$ variables. The density of states of Majorana fermions shows an excitation gap in the gapped region, while it is semimetallic in the gapless region reflecting the Dirac node. Performing the Monte Carlo simulation, we calculate the temperature dependence of the Majorana spectra. We find that the semimetallic dip is filled up as temperature increases in the gapless region, but surprisingly, the spectrum develops an excitation gap in the region near the gapless-gapped boundary. Such changes of the low-energy spectrum appear sharply at the transition temperature from the spin liquid to the paramagnetic state. The results indicate that thermal fluctuations of the $Z_2$ fields significantly influence the low-energy state of Majorana fermions, especially in the spin liquid formation.

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arXiv2014

Toroidal order in a partially disordered state on a layered triangular lattice: implication to UNi4B

A partial disorder on a layered triangular lattice is theoretically investigated from a viewpoint of toroidal ordering and magnetoelectric effects. We consider an extended periodic Anderson model including a site-dependent antisymmetric spin-orbit coupling between conduction and localized electrons. We show that, by the mean-field approximation, the model exhibits a coplanar vortex-lattice-type magnetic order as observed in a hexagonal uranium compound UNi4B, in the parameter region with intermediate hybridization and electron correlation. This peculiar state accommodates a toroidal order, which leads to the linear magnetoelectric effect. We discuss the implications of our results to UNi4B, focusing on the possible source of the site-dependent antisymmetric spin-orbit coupling.

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arXiv2014

Quantum spin Hall effect in a two-orbital model on a honeycomb lattice

The spin Hall effect is investigated in a two-orbital tight-binding model on a honeycomb lattice. We show that the model exhibits three topologically-different insulating phases at half filling, which are distinguished by different quantized values of the spin Hall conductivity. We analytically determine the phase boundaries, where the valence and conduction bands touch with each other with forming the Dirac nodes at the Fermi level. The results are discussed in terms of the effective antisymmetric spin-orbit coupling. The relation to the Kane-Mele model and implications for a magnetoelectric effect are also discussed.

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arXiv2014

Magnetic Field Effect in One-Dimensional Charge Ordering Systems

We study effects of an external magnetic field on charge ordering in the one-dimensional extended Hubbard model at quarter filling by means of the quantum Monte Carlo method. We find that the Zeeman coupling enhances the charge order correlation, which is prominent when the system is located in the critical regime near the charge ordering transition at zero magnetic field. This behavior is interpreted in terms of the crossover to the fully spin-polarized limit where the model is exactly solvable. Furthermore, by incorporating the interchain Coulomb repulsion, we show that the charge-ordering transition temperature is raised by the magnetic field. We also discuss the relevance of our results to magnetoresistance effects observed in molecular conductors.

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arXiv2014

Effect of magnetoelastic coupling on spin-glass behavior in Heisenberg pyrochlore antiferromagnets with bond disorder

Motivated by puzzling aspects of spin-glass behavior reported in frustrated magnetic materials, we theoretically investigate effects of magnetoelastic coupling in geometrically frustrated classical spin models. In particular, we consider bond-disordered Heisenberg antiferromagnets on a pyrochlore lattice coupled to local lattice distortions. By integrating out the lattice degree of freedom, we derive an effective spin-only model, the bilinear-biquadratic model with bond disorder, which is analyzed by classical Monte Carlo simulations. First, we discuss the phase diagrams as well as thermodynamic and magnetic properties. We show that the spin-glass transition temperature is largely enhanced by the spin-lattice coupling $b$ in the weakly disordered regime. This enhancement is ascribed to the suppression of thermal fluctuations in semidiscrete degenerate manifold formed in the presence of the spin-lattice coupling. We also find that, as increasing the strength of disorder, the system shows a concomitant transition of the nematic order and spin glass at a temperature determined by $b$, being almost independent of bond disorder. Although further-neighbor exchange interactions originating in the cooperative lattice distortions result in the spin-lattice order in the weakly disordered regime, the concomitant transition remains robust. We investigate the nature of the concomitant transition by analyzing the hysteresis of the magnetic susceptibility, the nonlinear susceptibility, and the specific heat. Furthermore, we discuss linear and nonlinear magnetic susceptibilities in the high-temperature paramagnetic phase as well as single-spin flip dynamics in the nematic phase. All these results are discussed in comparison with experiments for typical pyrochlore magnets, such as Y2Mo2O7 and ZnCr2O4.

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arXiv2014

Entanglement Spectrum in Cluster Dynamical Mean-Field Theory

We study the entanglement spectrum of the Hubbard model at half filling on a kagome lattice. The entanglement spectrum is defined by the set of eigenvalues of reduced thermal density matrix, which is naturally obtained in the framework of the dynamical mean-field theory. Adopting the cluster dynamical mean-field theory combined with continuous-time auxiliary-field Monte Carlo method, we calculate the entanglement spectrum for a three-site triangular cluster in the kagome Hubbard model. We find that the results at the three-particle sector well captures the qualitative nature of the system. In particular, the eigenvalue of the reduced density matrix, corresponding to the chiral degrees of freedom, exhibits characteristic temperature scale T_{\rm chiral}, below which a metallic state with large quasiparticle mass is stabilized. The entanglement spectra at different particle number sectors also exhibit characteristic changes around T_{\rm chiral}, implying the development of inter-triangular ferromagnetic correlations in the correlated metallic regime.

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arXiv2014

Multiple-Q Instability by (d-2)-dimensional Connections of Fermi Surfaces

We propose a general mechanism of multiple-Q ordering with noncollinear and noncoplanar spin textures in itinerant magnets. By analyzing the fourth-order perturbation with respect to the spin-charge coupling, we find that the instability toward multiple-Q ordering is caused by (d-2)-dimensional connections of the Fermi surfaces in the extended Brillouin zone by the multiple-Q wave vectors: zero(one)-dimensional point (line) connections in the two(three)-dimensional systems. The instabilities are obtained as the "fixed points" in the two-parameter flow diagram, irrespective of lattice structures. The hidden Fermi surface instability provides a universal origin of noncollinear and noncoplanar instabilities common to frustrated and unfrustrated lattice systems.

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