arXiv subjects

Akihisa Koga

Publications and source records attributed to Akihisa Koga.

At least 55 records · Page 3Linked to original sources

arXiv2020

Antiferromagnetically ordered state in the half-filled Hubbard model on the Socolar dodecagonal tiling

We investigate the antiferromagnetically ordered state in the half-filled Hubbard model on the Socolar dodecagonal tiling. When the interaction is introduced, the staggered magnetizations suddenly appear, which results from the existence of the macroscopically degenerate states in the tightbinding model. The increase of the interaction strength monotonically increases the magnetizations although its magnitude depends on the local environments. Magnetization profile is discussed in the perpendicular space. The similarity and difference are also addressed in magnetic properties in the Hubbard model on the Penrose, Ammann-Beenker, and Socolar dodecagonal tilings.

cond-mat.str-el↗

arXiv2020

Effect of Electron-Electron Interactions on Metallic State in Quasicrystals

We theoretically study the effect of electron-electron interactions on the metallic state of quasicrystals. To address the problem, we introduce the extended Hubbard model on the Ammann-Beenker tiling as a simple theoretical model. The model is numerically solved within an inhomogeneous mean-field theory. Because of the lack of periodicity, the metallic state is nonuniform in the electron density even in the noninteracting limit. We clarify how this charge distribution pattern changes with electron-electron interactions. We find that the intersite interactions change the distribution substantially and in an electron-hole asymmetric way. We clarify the origin of these changes through the analyses in the real and perpendicular spaces. Our results offer a fundamental basis to understand the electronic states in quasicrystalline metals.

cond-mat.str-el↗

arXiv2020

Superlattice structure in the antiferromagnetically ordered state in the Hubbard model on the Ammann-Beenker tiling

We study magnetic properties in the half-filled Hubbard model on the Ammann-Beenker tiling. First, we focus on the domain structure with locally eightfold rotational symmetry to examine the strictly localized confined states for the tightbinding model. We count the number of vertices and confined states in the larger domains generated by the deflation operations systematically. Then, the fraction of the confined states, which plays an important role for magnetic properties in the weak coupling limit, is obtained as $p=1/2τ^2$, where $τ(=1+\sqrt{2})$ is the silver ratio. It is also found that the wave functions for confined states are densely distributed in the system and thereby the introduction of the Coulomb interactions immediately induces the finite staggered magnetizations. Increasing the Coulomb interactions, the spatial distribution of the magnetizations continuously changes to those of the Heisenberg model. We discuss crossover behavior in the perpendicular space representation and reveal the superlattice structure in the spatial distribution of the staggered magnetizations.

cond-mat.str-el↗

arXiv2020

Collective Modes in Excitonic Insulators: Effects of Electron-Phonon Coupling and Signatures in Optical Response

We consider a two-band spinless model describing an excitonic insulator (EI) on the two-dimensional square lattice with anisotropic hopping parameters and electron-phonon (el-ph) coupling, inspired by the EI candidate Ta$_2$NiSe$_5$. We systematically study the nature of the collective excitations in the ordered phase which originates from the interband Coulomb interaction and the el-ph coupling. When the ordered phase is stabilized only by the Coulomb interaction (pure EI phase), its collective response exhibits a massless phase mode in addition to the amplitude mode. We show that in the BEC regime, the signal of the amplitude mode becomes less prominent and that the anisotropy in the phase mode velocities is relaxed compared to the model bandstructure. Through coupling to the lattice, the phase mode acquires a mass and the signal of the amplitude mode becomes less prominent. Importantly, character of the softening mode at the boundary between the normal semiconductor phase and the ordered phase depends on the parameter condition. In particular, we point out that even for el-ph coupling smaller than the Coulomb interaction the mode that softens to zero at the boundary can have a phonon character. We also discuss how the collective modes can be observed in the optical conductivity. Furthermore, we study the effects of nonlocal interactions on the collective modes and show the possibility of realizing a coexistence of an in-gap mode and an above-gap mode split off from the single amplitude mode in the system with the local interaction only.

cond-mat.str-el↗

arXiv2020

Excitonic condensation reflecting electronic states in two-band Penrose-Hubbard model

We study the excitonic insulating (EI) phase in the two-band Hubbard models on the Penrose tiling. Performing the real-space mean-field calculations systematically, we obtain the ground state phase diagrams for the vertex and center models. We find that, in some regimes, the stable EI phase is induced by small interband interactions. We argue that this originates from the electron-hole pairing for the completely or nearly degenerate states, which are characteristic of the Penrose tiling. We also study spatial distribution of the order parameter, mapping it to the perpendicular space.

cond-mat.str-el↗

arXiv2020

Spin transport in the Quantum Spin Liquid State in the $S=1$ Kitaev model: role of the fractionalized quasiparticles

We investigate the real-time spin response of the $S=1$ Kitaev model upon stimuli of a pulsed magnetic field in one of the edges using the exact diagonalization method. It is found that the pulsed magnetic field has no effect on the appearance of the spin moments in the quantum spin liquid region, but induces the spin oscillations in the other edge region with a small magnetic field. This is understood by the existence of the itinerant quasiparticles, which carry the spin excitations without the spin polarization in the quantum spin liquid state. This suggests that the spin fractionalizations occur in the $S=1$ Kitaev model as well as the exactly solvable $S=1/2$ Kitaev one and the fractionalized quasiparticles play an essential role in the spin transport.

cond-mat.str-el↗

arXiv2019

Majorana-mediated spin transport without spin polarization in Kitaev quantum spin liquids

We study the spin transport through the quantum spin liquid (QSL) by investigating the real-time and real-space dynamics of the Kitaev spin system with a zigzag structure in terms of the time-dependent Majorana mean-field theory. After the magnetic field pulse is introduced to one of the edges, the spin moments are excited in the opposite edge region although no spin moments are induced in the Kitaev QSL region. This unusual spin transport originates from the fact that the $S=1/2$ spins are fractionalized into the itinerant and localized Majorana fermions in the Kitaev system. Although both Majorana fermions are excited by the magnetic pulse, only the itinerant Majorana fermions flow through the bulk regime without the spin excitation, resulting in the spin transport in the Kitaev system. We also demonstrate that this phenomenon can be observed even in the system with the Heisenberg interactions using the exact diagonalization.

cond-mat.str-el↗

arXiv2019

Ferromagnetically ordered metal in the single-band Hubbard model

We study a ferromagnetic instability in a single-band Hubbard model on the hypercubic lattice away from half filling. Using dynamical mean-field theory with the continuous-time quantum Monte Carlo simulations based on the segment algorithm, we calculate the magnetic susceptibility in the weak and strong coupling regions systematically. We then find how ferromagnetic fluctuations are enhanced when the interaction strength and density of holes are varied. The efficiency of the double flip updates in the Monte Carlo simulations is also addressed.

cond-mat.str-el↗

arXiv2019

Magnetic properties of the S = 1 Kitaev model with anisotropic interactions

We investigate magnetic properties in the $S=1$ Kitaev model in the anisotropic limit. Performing the fourth-order perturbation expansion with respect to the $x$-bonds, $y$-bonds, and magnetic field, we derive the effective Hamiltonian, where the low-energy physics should be described by the free spins with an effective magnetic field. Making use of the exact diagonalization method for small clusters, we discuss ground-state properties in the system complementary.

cond-mat.str-el↗

arXiv2019

Residual Entropy and Spin Fractionalizations in the Mixed-Spin Kitaev Model

We investigate ground-state and finite temperature properties of the mixed-spin $(s, S)$ Kitaev model. When one of spins is half-integer and the other is integer, we introduce two kinds of local symmetries, which results in a macroscopic degeneracy in each energy level. Applying the exact diagonalization to several clusters with $(s, S)=(1/2, 1)$, we confirm the presence of this large degeneracy in the ground states, in contrast to the conventional Kitaev models. By means of the thermal pure quantum state technique, we calculate the specific heat, entropy, and spin-spin correlations in the system. We find that in the mixed-spin Kitaev model with $(s, S)=(1/2, 1)$, at least, the double peak structure appears in the specific heat and the plateau in the entropy at intermediate temperatures, indicating the existence of the spin fractionalization. Deducing the entropy in the mixed-spin system with $s, S\le 2$ systematically, we clarify that the smaller spin-$s$ is responsible for the thermodynamic properties at higher temperatures.

cond-mat.str-el↗

arXiv2019

Ferromagnetic Instability for single-band Hubbard model in the strong-coupling regime

We study a ferromagnetic instability in a doped single-band Hubbard model by means of dynamical mean-field theory with the continuous-time quantum Monte Carlo simulations. Examining the effect of the strong correlations in the system on the hypercubic and Bethe lattice, we find that the ferromagnetically ordered state appears in the former, while it does not in the latter. We also reveal that the ferromagnetic order is more stable in the case that the noninteracting DOS exhibits a slower decay in the high-energy region. The present results suggest that, in the strong-coupling regime, the high-energy part of DOS plays an essential role for the emergence of the ferromagnetically ordered state, in contrast to the Stoner criterion justified in the weak interaction limit.

cond-mat.str-el↗

arXiv2019

Low temperature properties in the Bilayer Kitaev model

The ground state of the bilayer Kitaev model with the Heisenberg-type interlayer exchange interaction is investigated by means of the exact diagonalization. Calculating the ground-state energy, local quantity defined on each plaquette, and dynamical spin structure factor, we obtain results suggesting the existence of a quantum phase transition between the Kitaev quantum spin liquid (QSL) and dimer singlet states when the interlayer coupling is antiferromagnetic. On the other hand, increasing the ferromagnetic interlayer coupling, there exists no singularity in the physical quantities, suggesting that the $S=1/2$ Kitaev QSL state realized in each layer adiabatically connects to another QSL state realized in the $S=1$ Kitaev model. Thermodynamic properties are also studied by means of the thermal pure quantum state method.

cond-mat.str-el↗

arXiv2018

Quasiperiodicity and valence fluctuation in the spin-1/2 Falicov-Kimball model

We study the spin-1/2 Falicov-Kimball model with conduction and localized $f$ electrons on the Penrose lattice using the real-space dynamical mean-field theory. By changing the $f$ electron level, the $f$ electron density at each site changes continuously, in contrast to periodic systems with first-order valence transitions. In the intermediate valence regime, the local $f$ electron number strongly depends on a wider range of the Penrose structure surrounding its lattice site, in spite of the local interaction between the conduction and $f$ electrons. The temperature dependence of the magnetic response is also discussed.

cond-mat.str-el↗

arXiv2018

Staggered ordered phases in the three-orbital Hubbard model

We study ordered phases with broken translational symmetry in the half-filled three-orbital Hubbard model with antiferromagnetic Hund coupling by means of dynamical mean-field theory (DMFT) and continuous-time quantum Monte Carlo simulations. The stability regions of the antiferro-orbital (AFO), antiferro-magnetic (AFM), and charge density wave (CDW) states are determined by measuring the corresponding order parameters. We introduce two symmetrically distinct AFO order parameters and show that these are the primary order parameters in the phase diagram. The CDW and AFM states appear simultaneously with these two types of AFO orders in the weak and strong coupling region, respectively. The DMFT phase diagram is consistent with the results obtained by the Hartree approximation and strong-coupling perturbation theory. In the weak coupling regime, a nontrivial exponent $β=3/2$ is found for the CDW order parameter, which is related to the coupling between the CDW and AFO orders in the Landau theory characteristic for the three-orbital model. We also demonstrate the existence of a metallic AFO state without any charge disproportions and magnetic orders, which appears only at finite temperatures.

cond-mat.str-el↗

arXiv2018

Quantum and classical behavior of spin-$S$ Kitaev models in an anisotropic limit

We study low-energy properties of spin-$S$ Kitaev models in an anisotropic limit. The effective form of a local conserved quantity is derived in the low-energy subspace. We find this is the same as that of $S=1/2$ case for the half-integer spins but shows a different form for the integer spins. Applying the perturbation theory to the anisotropic Kitaev model, we obtain the effective Hamiltonian. In the integer spin case, the effective model is equivalent to a free spin model under an uniform magnetic field, where quantum fluctuations are quenched. On the other hand, in the half-integer case, the system is described by the toric code Hamiltonian, where quantum fluctuations play a crucial role in the ground state. The boundary effect in the anisotropic Kitaev system is also discussed.

cond-mat.str-el↗

arXiv2018

Symmetry breaking states in the half-filled two-orbital Hubbard model with crystalline electric field

We investigate the half-filled two-orbital Hubbard model with the crystalline electric field using dynamical mean-field theory combined with the continuous-time quantum Monte Carlo simulations. We systematically study how the interplay of the intra- and interorbital Coulomb interations together with the Hund coupling realizes the diagonal and off-diagonal ordered states. It is found that the antiferroorbital ordered state is realized in the Hubbard model, in addition to the antiferromagnetically ordered and excitonic states. The competition between the antiferroorbital ordered and excitonic states close to the band insulating state is addressed.

cond-mat.str-el↗

arXiv2018

Ground-state properties for bilayer Kitaev model: dimer expansion study

We study ground state properties in the bilayer Kitaev model by means of the dimer expansion. The existence of parity symmetries in the system reduces the computational cost significantly. This allows us to expand the ground state energy and interlayer spin-spin correlation up to 30th order in the interdimer Kitaev coupling. The numerical calculations clarify that the dimer singlet state is indeed realized in the wide parameter region.

cond-mat.str-el↗

arXiv2018

Spontaneously orbital-selective superconductivity in a three-orbital Hubbard model

We study a three-orbital Hubbard model with negative Hund coupling in infinite dimensions, combining dynamical mean-field theory with continuous time quantum Monte Carlo simulations. This model, which is relevant for the description of alkali-doped fullerides, has previously been shown to exhibit a spontaneous orbital selective Mott phase in the vicinity of the superconducting phase. Calculating the pair potential and double occupancy in each orbital, we study the competition between different homogeneous ordered states and determine the corresponding finite temperature phase diagram of the model. We identify two distinct types of spontaneous orbital-selective Mott states and show that an orbital-selective $s$-wave superconducting state with one superconducting and two metallic orbitals is spontaneously realized between the conventional $s$-wave superconducting phase and these two kinds of spontaneously orbital-selective Mott states.

cond-mat.supr-con↗