arXiv subjects

A. Koga

Publications and source records attributed to A. Koga.

At least 19 recordsLinked to original sources

arXiv2026

Impact of Stealthy Hyperuniform Magnetic Impurity Configurations on Bulk Magnetism in a Two-dimensional Heisenberg Model

We investigate an antiferromagnetic quantum Heisenberg model on a square lattice with high-spin magnetic impurities to clarify how random and stealthy hyperuniform impurity configurations influence the bulk magnetic properties. Stealthy hyperuniform configurations are generated using generalized cost functions that interpolate between square-lattice-like and triangular-lattice-like arrangements. Using linear spin-wave theory for the mixed-spin model, we demonstrate that triangular-lattice-like arrangements yield a larger average staggered magnetization than both random and square-lattice-like cases. This enhancement originates from sublattice effects: while the square-lattice-like configuration enforces nearest-neighbor impurities to occupy opposite sublattices due to its bipartite structure, the triangular-lattice-like arrangement allows same-sublattice nearest-neighbor pairs, thereby strengthening cooperative magnetic enhancement.

cond-mat.dis-nn↗

arXiv2023

Hyperuniformity in two-dimensional periodic and quasiperiodic point patterns

We study hyperuniform properties in various two-dimensional periodic and quasiperiodic point patterns. Using the histogram of the two-point distances, we develop an efficient method to calculate the hyperuniformity order metric, which quantifies the regularity of the hyperuniform point patterns. The results are compared with those calculated with the conventional running average method. To discuss how the lattice symmetry affects the order metric, we treat the trellis and Shastry-Sutherland lattices with the same point density as examples of periodic lattices, and Stampfli hexagonal and dodecagonal quasiperiodic tilings with the same point density as examples of quasiperiodic tilings. It is found that the order metric for the Shastry-Sutherland lattice (Stampfli dodecagonal tilings) is smaller than the other in the periodic (quasiperiodic) tiling, meaning that the order metric is deeply related to the lattice symmetry. Namely, the point pattern with higher symmetry is characterized by the smaller order metric when their point densities are identical. Order metrics for several other quasiperiodic tilings are also calculated.

cond-mat.stat-mech↗

arXiv2022

Efficient Scheme for Time-dependent Thermal Pure Quantum State: Application to the Kitaev Model with Armchair Edges

We consider the time-dependent thermal pure quantum state method and introduce the efficient scheme to evaluate the change in physical quantities induced by the time-dependent perturbations, which has been proposed in our previous paper [H. Taguchi et al., Phys. Rev. B 105, 125137 (2022)]. Here, we treat the Kitaev model to consider the Majorana-mediated spin transport, as an example. We demonstrate how efficient our scheme is to evaluate spin oscillations induced by the magnetic field pulse.

cond-mat.str-el↗

arXiv2022

Field-direction Dependence of Majorana-mediated Spin Transport

We study the field-direction dependence of the Majorana-mediated spin transport in the Kitaev clusters with zigzag and armchair edges, applying a static magnetic field to one of the edges and a magnetic pulsed field to the other edges. By means of the exact diagonalization method, we calculate the time-evolution of the spin moments in both edge regions to clarify how the directions of two fields and shape of the edges affect the Majorana-mediated spin transport.

cond-mat.str-el↗

arXiv2018

Incipient and well-developed entropy plateaus in spin-S Kitaev models

We present results on entropy and heat-capacity of the spin-S honeycomb-lattice Kitaev models using high-temperature series expansions and thermal pure quantum (TPQ) state methods. We study models with anisotropic couplings $J_z=1\ge J_x=J_y$ for spin values 1/2, 1, 3/2, and 2. We show that for $S>1/2$, any anisotropy leads to well developed plateaus in the entropy function at an entropy value of $\frac{1}{2}\ln{2}$, independent of $S$. However, in the absence of anisotropy, there is an incipient entropy plateau at $S_{max}/2$, where $S_{max}$ is the infinite temperature entropy of the system. We discuss possible underlying microscopic reasons for the origin and implications of these entropy plateaus.

cond-mat.str-el↗

arXiv2018

Magnetic properties in the metallic magnets with large anisotropy

We study low temperature properties in the metallic magnets, considering the itinerant electron mediated ferromagnetism. Applying the Monte Carlo simulations to the extended double exchange model, we discuss reorientation phase transition and anisotropy field for the metallic magnets.

cond-mat.str-el↗

arXiv2018

Charge Kondo Effect and Superconductivity in the Falikov-Kimball model with the Pair Hopping

We study the Falikov-Kimball model with the pair hopping between the conduction and localized bands to discuss how the charge Kondo effect is realized. By combining dynamical mean-field theory with the continuous time quantum Monte Carlo method, we clarify that the charge Kondo state survives even at zero temperature and this competes with the charge ordered and s-wave superconducting states. The role of the interorbital repulsion for the superconducting state is also addressed.

cond-mat.str-el↗

arXiv2012

A study on correlation effects in two dimensional topological insulators

We investigate correlation effects in two dimensional topological insulators (TI). In the first part, we discuss finite size effects for interacting systems of different sizes in a ribbon geometry. For large systems, there are two pairs of well separated massless modes on both edges. For these systems, we analyze the finite size effects using a standard bosonization approach. For small systems, where the edge states are massive Dirac fermions, we use the inhomogeneous dynamical mean field theory (DMFT) combined with iterative perturbation theory as an impurity solver to study interaction effects. We show that the finite size gap in the edge states is renormalized for weak interactions, which is consistent with a Fermi-liquid picture for small size TIs. In the second part, we investigate phase transitions in finite size TIs at zero temperature focusing on the effects of possible inter-edge Umklapp scattering for the edge states within the inhomogeneous DMFT using the numerical renormalization group. We show that correlation effects are effectively stronger near the edge sites because the coordination number is smaller than in the bulk. Therefore, the localization of the edge states around the edge sites, which is a fundamental property in TIs, is weakened for strong coupling strengths. However, we find no signs for "edge Mott insulating states" and the system stays in the topological insulating state, which is adiabatically connected to the non-interacting state, for all interaction strengths smaller than the critical value. Increasing the interaction further, a nearly homogeneous Mott insulating state is stabilized.

cond-mat.str-el↗

arXiv2010

Polarized superfluid state in a three-dimensional fermionic optical lattice

We study ultracold fermionic atoms trapped in a three dimensional optical lattice by combining the real-space dynamical mean-field approach with continuous-time quantum Monte Carlo simulations. For a spin-unpolarized system we show results the density and pair potential profile in the trap for a range of temperatures. We discuss how a polarized superfluid state is spatially realized in the spin-polarized system with harmonic confinement at low temperatures and present the local particle density, local magnetization, and pair potential.

cond-mat.quant-gas↗

arXiv2008

Mott insulating state in a quarter-filled two-orbital Hubbard chain with different bandwidths

We investigate the ground-state properties of the one-dimensional two-band Hubbard model with different bandwidths. The density-matrix renormalization group method is applied to calculate the averaged electron occupancies $n$ as a function of the chemical potential $μ$. Both at quarter and half fillings, "charge plateaux" appear in the $n$-$μ$ plot, where $dμ/dn$ diverges and the Mott insulating states are realized. To see how the orbital polarization in the one-quarter charge plateau develops, we apply the second-order perturbation theory from the strong-coupling limit at quarter filling. The resultant Kugel-Khomskii spin-orbital model includes a $magnetic$ field coupled to orbital pseudo-spins. This field originates from the discrepancy between the two bandwidths and leads to a finite orbital pseudo-spin magnetization.

cond-mat.str-el↗

arXiv2008

Zero-temperature Phase Diagram of Two Dimensional Hubbard Model

We investigate the two-dimensional Hubbard model on the triangular lattice with anisotropic hopping integrals at half filling. By means of a self-energy functional approach, we discuss how stable the non-magnetic state is against magnetically ordered states in the system. We present the zero-temperature phase diagram, where the normal metallic state competes with magnetically ordered states with $(π, π)$ and $(2π/3, 2π/3)$ structures. It is shown that a non-magnetic Mott insulating state is not realized as the ground state, in the present framework, but as a meta-stable state near the magnetically ordered phase with $(2π/3, 2π/3)$ structure.

cond-mat.str-el↗

arXiv2005

Mott transitions in two-orbital Hubbard systems

We investigate the Mott transitions in two-orbital Hubbard systems. Applying the dynamical mean field theory and the self-energy functional approach, we discuss the stability of itinerant quasi-particle states in each band. It is shown that separate Mott transitions occur at different Coulomb interaction strengths in general. On the other hand, if some special conditions are satisfied for the interactions, spin and orbital fluctuations are equally enhanced at low temperatures, resulting in a single Mott transition. The phase diagrams are obtained at zero and finite temperatures. We also address the effect of the hybridization between two orbitals, which induces the Kondo-like heavy fermion states in the intermediate orbital-selective Mott phase.

cond-mat.str-el↗

arXiv2004

Superconductivty without inversion symmetry: MnSi versus CePt_3Si

Superconductivity in materials without spatial inversion symmetry is studied. We show that in contrast to common believe, spin-triplet pairing is not entirely excluded in such systems. Moreover, paramagnetic limiting is analyzed for both spin-singlet and triplet pairing. The lack of inversion symmetry reduces the effect of the paramagnetic limiting for spin-singlet pairing. These results are applied to MnSi and CePt_3Si.

cond-mat.str-el↗

arXiv2003

Hole Dynamics in the Orthogonal-Dimer Spin System

The dynamics of a doped hole in the orthogonal-dimer spin system is investigated systematically in one, two and three dimensions. By combining the bond-operator method with the self-consistent Born approximation, we argue that a dispersive quasi-particle state in the dimer phase is well defined even for quasi-two-dimensional systems. On the other hand, a doped hole in the plaquette-singlet phase hardly itinerates, forming an almost localized mode. We further clarify that although the quasi-particle weight in the dimer phase is decreased in the presence of the interchain coupling, it is not suppressed but even enhanced upon the introduction of the interlayer coupling.

cond-mat.str-el↗

arXiv2003

Hole-Doping Effects on a Two-dimensional Kondo Insulator

We study the effects of hole doping on the two-dimensional Heisenberg-Kondo model around the quantum critical point, where the spin liquid phase (Kondo insulator) and the magnetically ordered phase are separated via a second-order phase transition. By means of the self-consistent Born approximation within the bond operator formalism as well as the standard spin wave theory, we discuss dynamical properties of a doped hole. It is clarified that a quasi-particle state stabilized in the spin liquid phase is gradually obscured as the system approaches the quantum critical point. This is also the case for the magnetically ordered phase. We argue the similarity and the difference between these two cases.

cond-mat.str-el↗

arXiv2000

Magnetic Double Structure for S=1, 1/2 Mixed-Spin Systems

We investigate the zero-temperature and the finite-temperature properties of the two-dimensional antiferromagnetic quantum spin system composed of the s=1/2 and s=1 spins. The spin excitation spectrum as well as the thermodynamic quantities are computed by means of the Schwinger-boson mean-field theory. We discuss how the magnetic double structure with the gapful and the gapless excitations is generated, and apply the results systematically to the Haldane gap system in a staggered magnetic field and also to the weakly coupled ferrimagnetic mixed-spin chains. It is confirmed that the results obtained are consistent with the experiments for the quasi-one-dimensional Haldane compounds $R_2 \rm BaNiO_5$.

cond-mat.str-el↗

arXiv1999

Quantum Phase Transitions in Two-Dimensional Spin Systems with Ladder, Plaquette and Mixed-Spin Structures

Quantum phase transitions between the magnetically ordered and disordered states are studied for the two-dimensional antiferromagnetic quantum spin systems with ladder, plaquette, and mixed-spin structures. Starting with properly chosen singlet-cluster configurations, we perform the series expansion for the staggered magnetic susceptibility. The phase boundary is determined by applying the Dlog and biased Padé approximants to the staggered susceptibility thus obtained. The resulting phase diagram allows us to discuss the quantum phase transitions quantitatively, which agrees fairly well with the quantum Monte Carlo results for several cases previously studied.

cond-mat.str-el↗

arXiv1998

Two-Dimensional Quantum Spin Systems with Ladder and Plaquette Structure

We investigate low-energy properties of two-dimensional quantum spin systems with the ladder and plaquette structures, which are described by a generalized antiferromagnetic Heisenberg model with both of the bond and spin alternations. By exploiting a non-linear $σ$ model technique and a modified spin wave approach, we evaluate the spin gap and the spontaneous magnetization to discuss the quantum phase transition between the ordered and disordered states. We argue how the spin-gapped phase is driven to the antiferromagnetic phase in the phase diagram.

cond-mat.str-el↗