arXiv subjects

Jun-ichiro Inoue

Publications and source records attributed to Jun-ichiro Inoue.

15 recordsLinked to original sources

arXiv2024

Anisotropy in magnetoelastic effects of YCo5

We develop the phenomenological theory for uniaxial ferromagnets, and derive explicit formulae for their magnetostriction (MS) and magnetic anisotropy energy (MAE) generated by the magnetoelastic (ME) effects. An analysis of the experimental results for the MS in hexagonal YCo$_{5}$ using the formulae obtained reveals that 1) an elastic anomaly should appear in the elastic constant $c_{33}$, and 2) the MAE caused by the ME effects could be non-negligible. The calculated results of electronic structure under lattice deformation indicate that the particular arrangement of Co atoms in the lattice is related to the anisotropy of MS in YCo$_{5}$. It is suggested that the anisotropic MS observed for various Y-compounds could be interpreted in a similar manner.

cond-mat.mtrl-sci↗

arXiv2021

Strain Engineering of Magnetic Anisotropy in Epitaxial Films of Cobalt Ferrite

Perpendicular magnetic anisotropy (PMA) energy up to $K_{\mathrm{u}}=6.1\pm0.8$ MJ m$^{-3}$ is demonstrated in this study by inducing large lattice-distortion exceeding 3% at room temperature in epitaxially distorted cobalt ferrite Co$ _{x} $Fe$ _{3-x} $O$ _{4} $ (x = 0.72) (001) thin films. Although the thin film materials include no rare-earth elements or noble metals, the observed $ K_{u} $ is larger than that of the neodymium-iron-boron compounds for high-performance permanent magnets. The large PMA is attributed to the significantly enhanced magneto-elastic effects, which are pronounced in distorted films with epitaxial lattice structures upon introducing a distortion control layer of composition Mg$ _{2-x} $Sn$_{1+x}$O$ _{4} $. Surprisingly, the induced $ K_{u} $ can be quantitatively explained in terms of the agreement between the local crystal field of Co$ ^{2+} $ and the phenomenological magneto-elastic model, indicating that the linear response of induced $K_u$ is sufficiently valid even under lattice distortions as large as 3.2%. Controlling tetragonal lattice deformation using a non-magnetic spinel layer for ferrites could be a promising protocol for developing materials with large magnetic anisotropies.

cond-mat.mtrl-sci↗

arXiv2021

Spin reorientation in tetragonally distorted spinel oxide NiCo$_2$O$_4$ epitaxial films

We experimentally investigated the magnetic properties of NiCo$_2$O$_4$ epitaxial films known to be conductive oxides with perpendicular magnetic anisotropy (PMA) at room temperature. Both magneto-torque and magnetization measurements at various temperatures provide clear experimental evidence of the spin reorientation transition at which the MA changes from PMA to easy-cone magnetic anisotropy (ECMA) at a certain temperature ($T_{\rm{SR}}$). ECMA was commonly observed in films grown by pulsed laser deposition and reactive radio frequency magnetron sputtering, although $T_{\mathrm{SR}}$ is dependent on the growth method as well as the conditions. The cone angles measured from the $c$-axis increased successively at $T_{\mathrm{SR}}$ and approached a maximum of 45-50 degrees at the lowest measurement temperature of 5 K. Calculation with the cluster model suggests that the Ni$^{3+}$ ions occupying the $T_d$ site could be the origin of the ECMA. Both the magnetic properties and the results of the calculation based on the cluster model indicate that the ECMA is attributable to the cation anti-site distribution of Ni$^{3+}$, which is possibly formed during the growth process of the thin films.

cond-mat.mtrl-sci↗

arXiv2009

Electrically controlled superconducting states at the heterointerface SrTiO$_3$/LaAlO$_3$

We study the symmetry of Cooper pair in a two-dimensional Hubbard model with the Rashba-type spin-orbit interaction as a minimal model of electron gas generated at a heterointerface of SrTiO$_3$/LaAlO$_3$. Solving the Eliashberg equation based on the third-order perturbation theory, we find that the gap function consists of the mixing of the spin-singlet $d_{xy}$-wave component and the spin-triplet $(p_x\pm ip_y)$-wave one due to the broken inversion symmetry originating from the Rashba-type spin-orbit interaction. The ratio of the d-wave and the p-wave component continuously changes with the carrier concentration. We propose that the pairing symmetry is controlled by tuning the gate voltage.

cond-mat.supr-con↗

arXiv2008

Intrinsic spin Hall effect in graphene: Numerical calculations in a multi-orbital mode

We study the spin Hall effect (SHE) in graphene using a realistic multi-orbital tight-binding model that includes the atomic spin-orbit interaction. The SHE is found to be induced by the spin-dependent Aharonov-Bohm phase. In the metallic case, the calculated values for the spin Hall conductivity (SHC) are much smaller than the quantized Hall conductivity for realistic parameter values of metallic graphene. In the insulating case, quantization of the SHC is violated due to the multi-orbital effect. The present study suggests that the SHE in a honeycomb lattice is enhanced by chemical doping, such as the substitution of carbon atoms with boron atoms.

cond-mat.mes-hall↗

arXiv2008

Predominant Magnetic States in Hubbard Model on Anisotropic Triangular Lattices

Using an optimization variational Monte Carlo method, we study the half-filled-band Hubbard model on anisotropic triangular lattices, as a continuation of the preceding study [J. Phys. Soc. Jpn 75, 074707 (2006)]. We introduce two new trial states: (i) A coexisting state of (π,π)-antiferromagnetic (AF) and a d-wave singlet gaps, in which we allow for a band renormalization effect, and (ii) a state with an AF order of 120^\circ spin structure. In both states, a first-order metal-to-insulator transition occurs at smaller U/t than that of the pure d-wave state. In insulating regimes, magnetic orders always exist; an ordinary (π,π)-AF order survives up to t'/t\sim 0.9 (U/t=12), and a 120^\circ-AF order becomes dominant for t'/t \gsim 0.9. The regimes of the robust superconductor and of the nonmagnetic insulator the preceding study proposed give way to these magnetic domains.

cond-mat.supr-con↗

arXiv2007

Anomalous Hall effect in a two-dimensional electron gas

The anomalous Hall effect in a magnetic two-dimensional electron gas with Rashba spin-orbit coupling is studied within the Kubo-Streda formalism in the presence of pointlike potential impurities. We find that all contributions to the anomalous Hall conductivity vanish to leading order in disorder strength when both chiral subbands are occupied. In the situation that only the majority subband is occupied, all terms are finite in the weak scattering limit and the total anomalous Hall conductivity is dominated by skew scattering. We compare our results to previous treatments and resolve some of the discrepancies present in the literature.

cond-mat.mtrl-sci↗

arXiv2006

Superconductivity and a Mott Transition in a Hubbard Model on an Anisotropic Triangular Lattice

A half-filled-band Hubbard model on an anisotropic triangular lattice (t in two bond directions and t' in the other) is studied using an optimization variational Monte Carlo method, to consider the Mott transition and superconductivity arising in κ-BEDT-TTF_2X. Adopting wave functions with doublon-holon binding factors, we reveal that a first-order Mott (conductor-to-nonmagnetic insulator) transition takes place at U=U_c approximately of the band width, for a wide range of t'/t. This transition is not directly connected to magnetism. Robust d-wave superconductivity appears in a restricted parameter range: immediately below U_c and moderate strength of frustration (0.4\lsim t'/t\lsim 0.7), where short-range antiferromagnetic correlation sufficiently develops but does not come to a long-range order. The relevance to experiments is also discussed.

cond-mat.supr-con↗

arXiv2006

Vertex Corrections on the Anomalous Hall Effect in Spin-polarized Two-dimensional Electron Gases with Rashba Spin-orbit Interaction

We study the effect of disorder on the intrinsic anomalous Hall (AH) conductivity in a spin-polarized two-dimensional electron gas with a Rashba-type spin-orbit interaction. We find that AH conductivity vanishes unless the lifetime is spin-dependent, similar to the spin Hall (SH) conductivity in the non-magnetic system. In addition, we find that the SH conductivity does not vanish in the presence of magnetic scatterers. We show that the SH conductivity can be controlled by changing the amount of the magnetic impurities.

cond-mat.mes-hall↗

arXiv2005

Nonmonotonic d_{x^2-y^2}-Wave Superconductivity in Electron-Doped Cuprates Viewing from the Strong-Coupling Side

Applying a variational Monte Carlo method to a two-dimensional t-J model, we study the nonmonotonic d_{x^2-y^2}-wave superconductivity, observed by Raman scattering and ARPES experiments in the electron-doped cuprates. As a gap function in the trial state, we extend the d-wave form (ext.d) so as to have its maxima located near the hot spots of the system. It is found that, in contrast to the hole-doped case, the ext.d wave is always more stable than the simple d wave in the electron-doped case, and the magnetic correlation of the wave vector (π,π) as well as the pair correlation is enhanced. These results corroborate spin-correlation-mediated superconductivity in cuprates, recently argued from a FLEX calculation. In addition, we confirm that s- and p-wave symmetries are never stabilized even in the over-doped regime.

cond-mat.supr-con↗

arXiv2004

Variational Monte Carlo studies of a t-J model on an anisotropic triangular lattice

With the insulating phase of κ-(BEDT-TTF)_2X in mind, we study a t-J model on an anisotropic triangular lattice, where the hopping integral is t' in one of the three directions, using a variational Monte Carlo method. By changing the value of t'/t, we study the stability of superconducting (SC) states with d- and d+id-wave symmetries and of an antiferromagnetic (AF) state. As t'/t decreases from 1, the stable state immediately switches from the d+id wave to the d wave. The AF state is stabilized from the normal spin liquid state for t'/t\lsim 0.7 at half filling. We also take account of Nagaoka ferromagnetism and a phase separation.

cond-mat.supr-con↗

arXiv2004

Variational Monte Carlo Studies of Pairing Symmetry for the t-J Model on a Triangular Lattice

As a model of a novel superconductor Na_xCoO_2\cdotyH_2O, a single-band t-J model on a triangular lattice is studied, using a variational Monte Carlo method. We calculate the energies of various superconducting (SC) states, changing the doping rate δand sign of t for small J/|t|. Symmetries of s, d, and d+id (p+ip and f) waves are taken up as candidates for singlet (triplet) pairing. In addition, the possibility of Nagaoka ferromagnetism and inhomogeneous phases is considered. It is revealed that, among the SC states, the d+id wave always has the lowest energy, which result supports previous mean-field studies. There is no possibility of triplet pairing, although the f-wave state becomes stable against a normal state in a special case (δ=0.5 and t<0). For t<0, the complete ferromagnetic state is dominant in a wide range of δand J/|t|, which covers the realistic parameter region of superconductivity.

cond-mat.supr-con↗

arXiv2004

Suppression of the Persistent Spin Hall Current by Defect Scattering

We study the linear response spin Hall conductivity of a two-dimensional electron gas (2DEG) in the presence of the Rashba spin orbit interaction in the diffusive transport regime. When defect scattering is modeled by isotropic short-range potential scatterers the spin Hall conductivity vanishes due to the vertex correction. A non-vanishing spin Hall effect may be recovered for dominantly forward defect scattering.

cond-mat.mes-hall↗

arXiv2002

Diffuse transport and spin accumulation in a Rashba two-dimensional electron gas

The Rashba Hamiltonian describes the splitting of the conduction band as a result of spin-orbit coupling in the presence of an asymmetric confinement potential and is commonly used to model the electronic structure of confined narrow-gap semiconductors. Due to the mixing of spin states some care has to be exercised in the calculation of transport properties. We derive the diffusive conductance tensor for a disordered two-dimensional electron gas with spin-orbit interaction and show that the applied bias induces a spin accumulation, but that the electric current is not spin-polarized.

cond-mat.mes-hall↗

arXiv1999

Intrinsic Josephson Effect in the Layered Two-dimensional t-J Model

The intrinsic Josephson effect in the high-Tc superconductors is studied using the layered two-dimensional t-J model. The d.c.Josephson current which flows perpendicular to the t-J planes is obtained within the mean-field approximation and the Gutzwiller approximation. We find that the Josephson current has its maximum near the optimum doping region as a function of the doping rate.

cond-mat.supr-con↗