arXiv subjects

Yukitoshi Motome

Publications and source records attributed to Yukitoshi Motome.

At least 181 records · Page 10Linked to original sources

arXiv2014

Spontaneous Parity Breaking in Spin-Orbital Coupled Systems

Effects of spontaneous parity breaking by charge, spin, and orbital orders are investigated in a two-band Hubbard model on a honeycomb lattice. This is a minimal model in which the inter-orbital hopping, atomic spin-orbit coupling, and strong electron correlation give rise to fascinating properties, such as the magnetoelectric effects, quantum spin Hall effect, and spin or valley splitting in the band structure. We perform the symmetry analysis of possible broken-parity states and the mean-field analysis of their competition. We find that the model at 1/4 filling exhibits a spin-orbital composite ordered state and a charge ordered state, in addition to a paramagnetic quantum spin-Hall insulator. We show that the composite ordered phase exhibits two types of magnetoelectric responses. The charge ordered state shows spin splitting in the band structure, while the topological nature varies depending on electron correlations.

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arXiv2014

Spin-orbit coupling in octamers in a spinel sulfide CuIr$_2$S$_4$: Competition between spin-singlet and quadrupolar states, and its relevance to remnant paramagnetism

We theoretically investigate magnetic properties in the low-temperature phase with the formation of eight-site clusters, octamers, in the spinel compound CuIr$_2$S$_4$. The octamer state was considered to be a spin-singlet state induced by a Peieirls instability through the strong anisotropy of $d$ orbitals, the so-called orbital Peierls state. We reexamine this picture by taking into account the spin-orbit coupling which was ignored in the previous study. We derive a low-energy effective model between $j_{\rm eff}=1/2$ quasispins on Ir$^{4+}$ cations in an octamer from the multiorbital Hubbard model with the strong spin-orbit coupling by performing the perturbation expansion from the strong correlation limit. The effective Hamiltonian is in the form of the Kitaev-Heisenberg model but with an additional interaction, a symmetric off-diagonal exchange interaction originating from the perturbation process including both d-d and d-p-d hopping. Analyzing the effective Hamiltonian on two sites and the octamer by the exact diagonalization, we find that there is a competition between a spin-singlet state and a quadrupolar state. The former singlet state is a conventional one, adiabatically connected to the orbital-Peierls state. On the other hand, the latter quadrupolar state is stabilized by the additional interaction, which consists of a linear combination of different total spin momenta along the spin quantization axis. In the competing region, the model exhibits paramagnetic behavior with a renormalized small effective moment at low temperature. This peculiar remnant paramagnetism is not obtained in the Kitaev-Heisenberg model without the additional interaction. Our results renew the picture of the octamer state and provide a scenario for the intrinsic paramagnetic behavior recently observed in a muon spin rotation experiment [K. M. Kojima et al., Phys. Rev. Lett. 112, 087203 (2014)].

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arXiv2014

Reconstruction of chiral edge states in a magnetic Chern insulator

We study reconstruction of the chiral edge states in a magnetic Chern insulator in the Kondo lattice model on a triangular lattice at 1/4 filling. In this state, the spin scalar chirality associated with a four-sublattice noncoplanar magnetic order assures the topological nature characterized by a nonzero Chern number. Performing a Langevin-based numerical simulation for finite-size clusters with the open boundary condition in one direction, we clarify how the magnetic and electronic properties are modulated near the edges of the system. As a result, we find that the magnetic state near the edges is reconstructed to develop ferromagnetic spin correlations. At the same time, the electronic state is also modified and the chiral edge current is enhanced. We discuss this enhancement from the viewpoint of the electronic band structure for the gapless edge states.

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arXiv2014

Toroidal order in metals without local inversion symmetry

Toroidal order, given by a composite of electric and magnetic orders, manifests itself not only in the peculiar magnetism but also in anomalous transport and magnetoelectric effect. We report our theoretical results on the influence and stability of a toroidal order in metals on the basis of a microscopic model. We consider an effective single-band Hubbard-type model with a site-dependent antisymmetric spin-orbit coupling, which is derived from a four-band tight-binding model including the atomic spin-orbit coupling, off-site hybridizations between orbitals with different parities, and odd-parity crystalline electric field. For this single-band model on a layered honeycomb lattice, we investigate the electronic structure, magnetotransport, and magnetoelectric effect in the toroidal ordered state with a vortex-like magnetic structure. The ferroic order of the microscopic toroidal moments acts as an effective gauge field for electrons, which modulates the electronic band structure with a shift of the band bottom in the momentum space. In addition, the site-dependent antisymmetric spin-orbit coupling gives rise to highly anisotropic Hall responses. The most salient feature is two different types of magnetoelectric responses: one is a magnetic order with net toroidal magnetization induced by an electric current perpendicular to the planes, and the other is a uniform transverse magnetization induced by an electric current within the planes. We examine the ground state of the effective model by the mean-field approximation, and show that the toroidal order is stabilized by strong electron correlations at low electron density. We also discuss the temperature dependence of the magnetoelectric effects associated with spontaneous toroidal ordering. Implications to experiments are also presented.

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arXiv2014

3D Dirac Electrons on a Cubic Lattice with Noncoplanar Multiple-Q Order

Noncollinear and noncoplanar spin textures in solids manifest themselves not only in their peculiar magnetism but also in unusual electronic and transport properties. We here report our theoretical studies of a noncoplanar order on a simple cubic lattice and its influence on the electronic structure. We show that a four-sublattice triple-Q order induces three-dimensional massless Dirac electrons at commensurate electron fillings. The Dirac state is doubly degenerate, while it splits into a pair of Weyl nodes by lifting the degeneracy by an external magnetic field; the system is turned into a Weyl semimetal in applied field. In addition, we point out the triple-Q Hamiltonian in the strong coupling limit is equivalent to the 3D π-flux model relevant to an AIII topological insulator. We examine the stability of such a triple-Q order in two fundamental models for correlated electron systems: a Kondo lattice model with classical localized spins and a periodic Anderson model. For the Kondo lattice model, performing a variational calculation and Monte Carlo simulation, we show that the triple-Q order is widely stabilized around 1/4 filling. For the periodic Anderson model, we also show the stability of the same triple-Q state by using the mean-field approximation. For both models, the triple-Q order is widely stabilized via the couplings between conduction electrons and localized electrons even without any explicit competing magnetic interactions and geometrical frustration. We also show that the Dirac electrons induce peculiar surface states: Fermi "arcs" connecting the projected Dirac points, similarly to Weyl semimetals.

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arXiv2013

Spin-orbital frustration in pyrochlores A2Mo2O7

Electronic and magnetic properties of molybdenum pyrochlores A2Mo2O7 are studied by the fully relativistic density-functional theory plus on-site repulsion (U) method, with focusing on the spin-glass insulating material Y2Mo2O7. We find that the system exhibits peculiar competition in energy between different magnetic states in the large-U insulating region. The magnetic competition cannot be explained by the conventional picture based on the geometrical frustration of isotropic Heisenberg antiferromagnetic exchange interactions. Through an analysis by using a generalized spin model, we find that the effective spin interactions are distinct from the simple Heisenberg form and strongly anisotropic in spin space. We also reveal that they give rise to keen competition between antiferromagnetic and ferromagnetic states. The complex form of the magnetic interactions indicates a crucial role of the orbital degree of freedom. Analyzing a three-orbital Hubbard model, we clarify that the magnetic competition is tightly connected with orbital frustration in the 4d2 electronic configuration through the spin-orbital interplay. The results challenge the conventional picture of the spin-glass behavior that attributes the origin to the geometrical frustration of purely antiferromagnetic exchange interactions.

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arXiv2013

Effect of Quantum Spin Fluctuation on Scalar Chiral Ordering in the Kondo Lattice Model on a Triangular Lattice

Spin scalar chiral ordering gives rise to nontrivial topological characters and peculiar transport properties. We here examine how quantum spin fluctuations affect the spin scalar chiral ordering in itinerant electron systems. We take the Kondo lattice model on a triangular lattice, and perform the linear spin wave analysis in the Chern insulator phases with spin scalar chiral ordering obtained in the case that the localized spins are classical. We find that, although the quantum fluctuation destabilizes the spin scalar chiral phase at 3/4 filling that originates from the perfect nesting of Fermi surface, it retains the phase at 1/4 filling that is induced by an effective positive biquadratic interaction. The reduction of the ordered magnetic moment by the zero-point quantum fluctuation is considerably small, compared with those in spin-only systems. The results suggest that the Chern insulator at 1/4 filling remains robust under quantum fluctuations.

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arXiv2013

Invariant Energy Levels and Flat Band Engineering in a Kondo Lattice Model on Geometrically Frustrated Lattices

We show the existence of invariant energy levels in a Kondo lattice model on an isolated complete graph, such as a triangle and a tetrahedron. These energy levels always have fixed eigenenergies $t \pm J/2$, irrespective of the configuration of localized moments ($t$ is the transfer integral of conduction electrons and $J$ is the spin-charge coupling constant). We also extend the analysis to geometrically frustrated lattices by using the complete graphs as basic building blocks. We show that the construction rule for the invariant energy levels leads to the existence condition of localized states, if the model is defined on the triangle-based line graphs, such as a kagome lattice. We further propose a procedure of engineering isolated flat bands with broken time-reversal symmetry, which are separated from other dispersive bands with finite energy gaps.

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arXiv2013

Charge Order with a Noncoplanar Triple-Q Magnetic Order on a Cubic Lattice

Ground state of the periodic Anderson model on a cubic lattice is investigated by mean-field calculations, with focusing on the possibility of charge ordering. We show that three different charge-ordered phases appear at 3/2 filling, which are accompanied by ferromagnetic, noncoplanar triple-Q, and ferrimagnetic order, respectively. The triple-Q magnetic order is likely stabilized by emergent geometrical frustration in the charge-ordered phase with the ordering wave vector (pi,pi,pi). We show that the nearest-neighbor hopping between localized electrons stabilizes the triple-Q phase relative to the other two phases. We also discuss the electronic structure in the triple-Q charge-ordered phase.

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arXiv2013

Linear Spin Wave Analysis for General Magnetic Orders in the Kondo Lattice Model

We extend the formulation of the spin wave theory for the Kondo lattice model, which was mainly used for the ferromagnetic metallic state, to general magnetic orders including complex noncollinear and noncoplanar orders. The 1/S expansion is reformulated in the matrix form depending on the size of the magnetic unit cell. The noncollinearity and noncoplanarity of the localized moments are properly taken into account by the matrix elements of the para-unitary matrix used in the diagonalization of the Bogoliubov-de Gennes type Hamiltonian for magnons. We apply the formulation within the linear spin wave approximation to a typical noncollinear case, the $120^{\circ}$ N{é}el order on a triangular lattice at half filling. We calculate the magnon excitation spectrum and the quantum correction to the magnitude of ordered moments as functions of the strength of the Hund's-rule coupling, $J_{\rm H}/t$. We find that the magnon excitation shows softening at $J_{\rm H}/t \simeq 2.9$, which indicates that the $120^{\circ}$ order is destabilized for smaller $J_{\rm H}/t$. On the other hand, we show that the $120^{\circ}$ order is stable in the entire range of $J_{\rm H}/t \gtrsim 2.9$, and, in the limit of $J_{\rm H}/t \to \infty$, the form of the spin wave spectrum approaches that for the antiferromagnetic Heisenberg model, while the bandwidth is proportional to $t^2/J_{\rm H}$. The reduction of the ordered moment is smaller than that for the spin-only model, except in the vicinity of the softening.

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arXiv2013

Monte Carlo Study of an Effective Ising Model for the Spin-ice type Kondo Lattice Model

An effective Ising model for the spin-ice type Kondo lattice model is investigated by the classical Monte Carlo simulation. We clarify the magnetic phase diagram with four phases: ice-ferro, ice-(0,0,2π), 32-sublattice, and all-in/all-out ordered states. The result well reproduces the phase diagram of the Kondo lattice model studied previously [H. Ishizuka et al.: J. Phys. Soc. Jpn. 81 (2012) 113706], which suggests that the RKKY interactions up to third neighbors are sufficient to describe the magnetic properties of the itinerant electron model. We discuss the peculiar nature of phase transitions: the suppression of the critical temperatures down to zero temperature between two ice phases and the presence of the tricritical points on the phase boundary of the 32-sublattice ordered state.

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arXiv2013

On the Stability of Quantum Hall Kagome-ice Insulator

The stability of the kagome-ice insulator against the three dimensionality and the canting angle of the spin anisotropy axes is studied numerically using the exact diagonalization method. The kagome-ice insulator, which was recently found at 2/3 electron filling in the double-exchange model on a kagome lattice, is a Chern insulator showing a quantized Hall response despite the absence of magnetic long-range order. We find that this peculiar state remains stable when the lattice is extended to three dimensions in an anisotropic pyrochlore lattice in the weak interlayer coupling region. On the other hand, we show that the kagome-ice insulator is also stable against the change of the canting angle. Furthermore, we find another kagome-ice insulator at 1/3 filling in a different range of the canting angle. Our results indicate that the quantum anomalous Hall response is widely observed in the variants of the kagome-ice double-exchange systems.

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arXiv2013

Spontaneous Spatial Inversion Symmetry Breaking and Spin Hall Effect in a Spin-ice Double-exchange Model on a Pyrochlore Lattice

A formation of tetrahedral spin clusters is discovered by Monte Carlo simulation for a spin-ice type double-exchange model on a pyrochlore lattice. The spin-cluster phase is magnetically disordered, but breaks spatial inversion symmetry spontaneously by developing noncoplanar four-spin molecules periodically on the pyrochlore lattice. We find that the system exhibits a nonzero spin Hall conductivity in the spin-cluster phase. The result suggests that an intersite-multipole order induces the unconventional spin Hall state without the spin-orbit interaction.

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arXiv2013

Loop Liquid in an Ising-Spin Kondo Lattice Model on a Kagome Lattice

Phase diagram of an Ising-spin Kondo lattice model on a kagome lattice is investigated by a Monte Carlo simulation. We find that the system exhibits a peculiar ferrimagnetic state at a finite temperature, in which each triangle is in a two-up one-down spin configuration but the spin correlation does not develop any superstructure. We call this state the loop liquid, as it is characterized by the emergent degree of freedom, self-avoiding up-spin loops. We elucidate that the system shows phase transitions from the loop liquid to ferrimagnetically ordered states and a crossover to a partially ferromagnetic state by changing the electron density and temperature. These can be viewed as crystallization and cohesion of the loops, respectively. We demonstrate that the loop formation is observed in the optical conductivity as a characteristic resonant peak.

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arXiv2013

Polynomial Expansion Monte Carlo Study of Frustrated Itinerant Electron Systems: Application to a Spin-ice type Kondo Lattice Model on a Pyrochlore Lattice

We present the benchmark of the polynomial expansion Monte Carlo method to a Kondo lattice model with classical localized spins on a geometrically frustrated lattice. The method enables to reduce the calculation amount by using the Chebyshev polynomial expansion of the density of states compared to a conventional Monte Carlo technique based on the exact diagonalization of the fermion Hamiltonian matrix. Further reduction is brought by a real-space truncation of the vector-matrix operations. We apply the method to the model with spin-ice type Ising spins on a three-dimensional pyrochlore lattice, and carefully examine the convergence in terms of the order of polynomials and the truncation distance. We find that, in a wide range of electron density at a relatively weak Kondo coupling compared to the noninteracting bandwidth, the results by the polynomial expansion method show good convergence to those by the conventional method within reasonable numbers of polynomials. This enables us to study the systems up to 4x8^3 = 2048 sites, while the previous study by the conventional method was limited to 4x4^3 = 256 sites. On the other hand, the real-space truncation is not helpful in reducing the calculation amount for the system sizes that we reached, as the sufficient convergence is obtained when most of the sites are involved within the truncation distance. The necessary truncation distance, however, appears not to show significant system size dependence, suggesting that the truncation method becomes efficient for larger system sizes.

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arXiv2013

Thermally-induced Phases in an Ising Kondo Lattice Model on a Triangular Lattice: Partial Disorder and Kosterlitz-Thouless State

Magnetic and electronic properties of a Kondo lattice model with Ising localized spins are studied on an isotropic triangular lattice. By using Monte Carlo simulation, we present that the model shows a rich phase diagram with four dominant states: two-sublattice stripe, three-sublattice ferrimganetic, partially disordered, and Kosterlitz-Thouless like quasi-long-range ordered states. Among them, the partially disordered state and Kosterlitz-Thouless like state are intermediate phases induced by thermal fluctuations in the phase competing regime; they are present only at finite temperatures and eventually taken over by another phases as the temperature is further lowered. Although the Kosterlitz-Thouless like state was found also in triangular Ising antiferromagnets with further-neighbor interactions, the partially disordered state has not been reported in the localized spin only models in two dimensions. Interestingly, the partially disordered phase is also peculiar in the charge degree of freedom of itinerant electrons; it is insulating and accompanied by charge disproportionation. From a combined analysis of a mean-field calculation of the band structure and Monte Carlo simulation, we conclude that the partial disorder in the present model is stabilized by the Slater mechanism.

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arXiv2013

Charge Order in a Two-Dimensional Kondo Lattice Model

The possibility of charge order is theoretically examined for the Kondo lattice model in two dimensions, which does not include bare repulsive interactions. Using two complementary numerical methods, we find that charge order appears at quarter filling in an intermediate Kondo coupling region. The charge ordered ground state is an insulator exhibiting an antiferromagnetic order at charge-poor sites, while the paramagnetic charge-ordered state at finite temperatures is metallic with pseudogap behavior. We confirm that the stability of charge order is closely related with the local Kondo-singlet formation at charge-rich sites. Our results settle the controversy on charge order in the Kondo lattice model in realistic spatial dimensions.

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arXiv2012

Quantum Anomalous Hall Effect in Kagome Ice

An anomalous Hall insulator without magnetic long-range ordering is theoretically reported in the absence of the relativistic spin-orbit coupling. It is realized in itinerant electrons coupled with the Ising spins on a <111> kagome plane of pyrochlore spin ice in applied magnetic field. We find that the kagome-ice type local spin correlation in the magnetization plateau state opens a charge gap without magnetic ordering, which results in quantization of the Hall conductivity. By Monte Carlo simulation, we identify the anomalous Hall insulating region in the magnetic phase diagram, in addition to another anomalous Hall insulator in a fully-saturated state.

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