arXiv subjects

Masaru Kato

Publications and source records attributed to Masaru Kato.

14 recordsLinked to original sources

arXiv2019

Dependences of local density of state on temperature, size, and shape in two-dimensional nano-structured superconductors

In this paper, we investigate a local density of state (LDOS) in two-dimensional nano-structured superconductors. We solve the Bogoliubov-de Gennes equations self-consistently with the two-dimensional finite element method. In nano-structured superconductors, the LDOS as a function of the energy has many discrete peaks. A discretization of the LDOS comes from a discretization of energy levels due to the quantum confinement effect in nano-structured systems. When the temperature increases, a width of a peak in the LDOS is spread to a large energy range and neighbor peaks are superposed due to the thermal effect. On the other hand, for the fixed temperature, the behavior of the LDOS is different between nano-scaled rectangular and square systems. In the nano-scaled rectangular system, when only a lateral length increases, a contribution of the quantum confinement effect from the lateral side is suppressed, while the contribution from a longitudinal side remains large. Then, some peaks are left in the LDOS even when the lateral length is very large. These peaks form a periodic structure and can be regarded as gaps in a multi gap structure due to the quantum confinement effect. On the other hand, energy levels in the square system tend to arrange equally. Then, in the square system, peaks in the LDOS which exist in the rectangular system are small. Also, the period between peaks in the LDOS in the square system is smaller than the period in the rectangular system.

cond-mat.supr-con↗

arXiv2019

Critical states in superconducting plate: Structure dependence

We study vortex penetration into two-layer structures of superconducting plates under a perpendicular magnetic field. We solve the heat transport equation and the Maxwell equations with the current-voltage relation for superconductor, simultaneously, and obtain magnetic flux and current densities. We show how magnetic flux structure depends on the structure, especially distance of two-layer of superconductors.

cond-mat.supr-con↗

arXiv2018

Novel vortex structures in the three-dimensional superconductor under the helical magnetic field from the chiral helimagnet

We have investigated vortex structures in three-dimensional superconductors under a helical magnetic field from a chiral helimagnet numerically. In order to obtain vortex structures, we solve three-dimensional Ginzburg-Landau equations with the finite element method. The distribution of the helical magnetic field is assumed to be proportional to the distribution of the magnetic moments in the chiral helimagnet. Then, the magnetic field is the same direction in the yz-plane and helical rotation along the helical axis. Under this helical magnetic field, vortices appear to be perpendicular to the surface of the superconductor. But we have found that there are tilted vortices toward the helical axis, although there is no component of the magnetic field along the helical axis. This vortex structure depends on the chirality of the distribution of the helical magnetic field.

cond-mat.supr-con↗

arXiv2018

Phase transition of vortex states in two-dimensional superconductors under a oscillating magnetic field from the chiral helimagnet

We have investigated vortex states in two-dimensional superconductors under a oscillating magnetic field from a chiral helimagnet. We have solved the two-dimensional Ginzburg-Landau equations with finite element method. We have found that when the magnetic field from the chiral helimagnet increases, vortices appear all at once in all periodic regions. This transition is different from that under the uniform magnetic field. Under the composite magnetic field with the oscillating and uniform fields (down-vortices), vortices antiparallel to the uniform magnetic field disappear. Then, the small uniform magnetic field easily remove down-vortices.

cond-mat.supr-con↗

arXiv2016

Dependence of Vortex States in Superconductors on a Chiral Helimagnet and an Applied Magnetic Field

We study effects of a chiral helimagnet (CHM) on vortex states in a superconductor, solving the Ginzburg-Landau equations in a chiral helimagnet/superconductor bilayer system. We found that vortices form a periodically modulated triangular lattice, because the magnetic field from the chiral helimagnet $H_{CHM}$ oscillates spatially. An increase of a critical current is expected, because vortices are pinned by the $H_{CHM}$.

cond-mat.supr-con↗

arXiv2016

Effects of chiral helimagnets on vortex states in a superconductor

We have investigated vortex states in chiral helimagnet/superconductor bilayer systems under an applied external magnetic field Happl, using the Ginzburg-Landau equations. Effect of the chiral helimagnet on the superconductor is taken as a magnetic field HCHM, which is perpendicular to the superconductor and oscillates spatially. For Happl = 0 and weak HCHM, there appear pairs of up-and down-vortices. Increasing Happl, down-vortices gradually disappear, and number of up-vortices increases in the large magnetic field region. Then, up-vortices form parallel, triangular, or square structures.

cond-mat.supr-con↗

arXiv2016

Excitation spectra and wave functions of quasiparticle bound states in bilayer Rashba superconductors

We study the excitation spectra and the wave functions of quasiparticle bound states at a vortex and an edge in bilayer Rashba superconductors under a magnetic field. In particular, we focus on the quasiparticle states at the zero energy in the pair-density wave state in a topologically non-trivial phase. We numerically demonstrate that the quasiparticle wave functions with zero energy are localized at both the edge and the vortex core if the magnetic field exceed the critical value.

cond-mat.supr-con↗

arXiv2015

Inhomogeneous Electronic Distribution in High-Tc Cuprates

We theoretically investigate the doping evolution of the electronic state of high-Tc cuprate on both sides of the half-filling on the basis of the three-dimensional three-band Hubbard model with a layered structure using the Hartree-Fock approximation. Once a small amount of holes or electrons are doped into the half-filled state, our model exhibits the charge-transfer insulator-to-metal transition along with a chemical potential jump. At the same time, the doped holes or electrons are inhomogeneously distributed, and they tend to form clusters in the vicinity of the half-filling. This suggests the possibility of microscopic phase separation with the separation between the metallic and the insulating regions.

cond-mat.str-el↗

arXiv2008

Metallic State of the Three-band Hubbard Model with Super-lattice Structure

We investigate the dynamical superlattice correlation in the two-dimensional three-band Hubbard model on the basis of the unrestricted fluctuation exchange approximation. We calculate the one-particle spectral function, the spin correlation function and the charge correlation function at finite temperature. We find that some experimental results can be reproduced consistenly by taking inhomogenous distribution of Cu 3d electrons into account. The correlation functions suggest that several kinds of instabilities with spatial inhomogenieties exist in some regions, where these instabilities significantly affect the one-particle spectral fuctions.

cond-mat.str-el↗

arXiv2003

Ginzburg-Landau calculations of d-wave superconducting dot in s-wave superconducting matrix

We have developed a numerical method that calculated superconducting states and magnetic field distributions for the composite structures of the High-Tc superconductor and the conventional superconductor in arbitrary geometries. We show spontaneous magnetic flux appears at the corner of the boundary of these two superconductors. Also we propose High-Tc superconducting dot embedded in conventional superconductors, which is named as d-dot and show the spatial distribution of superconducting order parameters and the magnetic field.

cond-mat.supr-con↗

arXiv2000

Bound states and extended states around a single vortex in the d-wave superconductors

Making use of the Bogoliubov-de Gennes equation for the d-wave superconductors, we investigate the quasi-particle spectrum around a single vortex. Taking $p_Fξ=10$, we found that there are bound states which are localized around the vortex core, and extended states which are rather uniform, for $|E|<Δ$ where $E$ is the quasi-particle energy and $Δ$ is the asymptotic value of the order parameter for away from the vortex.

cond-mat.supr-con↗

arXiv2000

Quasi-Particle Spectrum around a Single Vortex in Superconductors - s-Wave Case -

Making use of the Bogoliubov-de Gennes equation, we study the quasi-particle spectrum and the vortex core structure of a single vortex in quasi 2D s-wave superconductors for small p_F xi_0, where p_F is the Fermi momentum and xi_0=v_F/Delta_0 is the coherence length (hbar=1). During our numerical calculation the particle number is conserved for each p_F xi_0. In particular, we find that there are only 1 or 2 bound states for p_F xi_0=1. Also, for p_F xi_0=1, the Kramer-Pesch effect ceases to exist at around T/T_c= 0.3.

cond-mat.supr-con↗

arXiv2000

Stripe orders in the extended Hubbard model

We study stripe orders of charge and spin density waves in the extended Hubbard model with the nearest-neighbor Coulomb repulsion V within the mean field approximation. We obtain V vs. T(temperature) phase diagram for the on-site Coulomb interaction U/t=8.0 and the filling n=0.8, here t is a nearest-neighbor transfer energy. Our result shows that the diagonal stripe spin density wave state (SDW) is stable for small V, but for large V the most stable state changes to a charge density wave-antiferromagnetic (CDW-AF) state. Especially we find at low temperature and for a certain range of value of V, a vertical stripe CDW-AF state becomes stable.

cond-mat.str-el↗

arXiv1998

Quasi-particle spectrum around a single vortex in s-wave superconductors

Making use of the Bogoliubov-de Gennes equation, we study the quasi-particle spectrum and the vortex core structure of a single vortex in quasi 2D s-wave superconductors for small $p_Fξ_0$, where $p_F$ is the Fermi momentum and $ξ_0=v_F/Δ_0$ is the coherence length($\hbar=1$). In particular we find that the number of bound states decreases rapidly for decreasing $p_Fξ_0$. Also for $p_Fξ_0\sim 1$, the Kramer-Pesch effect stops around $T/T_c\simeq 0.3$.

cond-mat.supr-con↗