arXiv subjects

Toru Kikuchi

Publications and source records attributed to Toru Kikuchi.

9 recordsLinked to original sources

arXiv2019

Current-induced modulation of interfacial Dzyaloshinskii-Moriya interaction

The Dzyaloshinskii-Moriya (DM) interaction is an antisymmetric exchange interaction that is responsible for the emergence of chiral magnetism. The origin of the DM interaction, however, remains to be identified albeit the large number of studies reported on related effects. It has been recently suggested that the DM interaction is equivalent to an equilibrium spin current density originating from spin-orbit coupling, an effect referred to as the spin Doppler effect. The model predicts that the DM interaction can be controlled by spin current injected externally. Here we show that the DM exchange constant ($D$) in W/CoFeB based heterostructures can be modulated with external current passed along the film plane. At higher current, $D$ decreases with increasing current, which we infer is partly due to the adiabatic spin transfer torque. At lower current, $D$ increases linearly with current regardless of the polarity of current flow. The rate of increase in $D$ with the current density agrees with that predicted by the model based on the spin Doppler effect. These results imply that the DM interaction at the HM/FM interface partly originates from an equilibrium interface spin (polarized) current which can be modulated externally.

cond-mat.mes-hall↗

arXiv2018

Spin Connections for Nonrelativistic Electrons on Curves and Surfaces

We propose a basic theory of nonrelativistic spinful electrons on curves and surfaces. In particular, we discuss the presence and effects of spin connections, which describe how spinors and vectors couple to the geometry of curves and surfaces. We derive explicit expressions of spin connections by performing simple dimensional reduction from the three-dimensional flat space. The spin connections act on electrons as spin-dependent magnetic fields, which have been known as `pseudomagnetic fields' in the context of, e.g., graphenes and Dirac/Weyl semimetals. We propose that these spin-dependent magnetic fields are present universally on curves and surfaces, acting on electrons regardless of the kinds of their spinorial degrees of freedom and their dispersion relations. We discuss that the curvature effects via spin connections will induce the spin Hall effect and induce the Dzyaloshinskii--Moriya interactions between magnetic moments on curved surfaces, without relying on the relativistic spin-orbit couplings. We also mention the importance of spin connections on orbital physics of electrons on curved geometry.

cond-mat.mes-hall↗

arXiv2018

First-Principles Evaluation of the Dzyaloshinskii--Moriya Interaction

We review recent developments of formulations to calculate the Dzyaloshinskii--Moriya (DM) interaction from first principles. In particular, we focus on three approaches. The first one evaluates the energy change due to the spin twisting by directly calculating the helical spin structure. The second one employs the spin gauge field technique to perform the derivative expansion with respect to the magnetic moment. This gives a clear picture that the DM interaction can be represented as the spin current in the equilibrium within the first order of the spin-orbit couplings. The third one is the perturbation expansion with respect to the exchange couplings and can be understood as the extension of the Ruderman--Kittel--Kasuya--Yosida (RKKY) interaction to the noncentrosymmetric spin-orbit systems. By calculating the DM interaction for the typical chiral ferromagnets Mn$_{1-x}$Fe$_x$Ge and Fe$_{1-x}$Co$_x$Ge, we discuss how these approaches work in actual systems.

cond-mat.mes-hall↗

arXiv2016

Doppler shift picture of the Dzyaloshinskii--Moriya interaction

We present a physical picture for the emergence of the Dzyaloshinskii--Moriya (DM) interaction based on the idea of the Doppler shift by an intrinsic spin current induced by spin--orbit interaction under broken inversion symmetry. The picture is confirmed by a rigorous effective Hamiltonian theory, which reveals that the DM coefficient is given by the magnitude of the intrinsic spin current. The expression is directly applicable to first principles calculations and clarifies the relation between the interaction and the electronic band structures. Quantitative agreement with experimental results is obtained for the skyrmion compounds Mn$_{1-x}$Fe$_x$Ge and Fe$_{1-x}$Co$_x$Ge.

cond-mat.mes-hall↗

arXiv2015

Spin Dynamics with Inertia in Metallic Ferromagnets

Non-adiabatic contribution of environmental degrees of freedom yields effective inertia of spin in effective spin dynamics. In this paper, we study several aspects of the inertia of spin in metallic ferromagnets. (i) a concrete expression of the spin inertia $m_s$: $m_s=\hbar S_c/(2g_{\rm sd})$, where $S_c$ is the spin polarization of conduction electrons and $g_{\rm sd}$ is the $sd$ coupling constant. (ii) dynamical behavior of spin with inertia, discussed from viewpoints of a spinning top and of a particle on a sphere. (iii) behavior of spin waves and domain walls in the presence of inertia, and behavior of spin with inertia in the case of a time-dependent magnetic field.

cond-mat.mes-hall↗

arXiv2012

Rotating string in doubled geometry with generalized isometries

In this paper, we first construct a globally well-defined non-geometric background which contains several branes in type II string theory compactified on a 7-torus. One of these branes is called 5^2_2, which is a codimension-2 object and has a non-trivial monodromy given by a T-duality transformation. The geometry near the 5^2_2-brane is shown to approach the non-geometric background constructed in arXiv:1004.2521. We then construct the solution of a fundamental string rotating along a non-trivial cycle in the 5^2_2 background. Although the background is not axisymmetric in the usual sense, we show that it is actually axisymmetric as a doubled geometry by explicitly finding a generalized Killing vector. We perform a generalized coordinate transformation into a system where the generalized isometry is manifest, and show that the winding and momentum charges of the string solution is explicitly conserved in that system.

hep-th↗

arXiv2010

Relativistic Collective Coordinate Quantization of Solitons: Spinning Skyrmion

We develop a consistent relativistic generalization of collective coordinate quantization of field theory solitons. Our principle of introducing collective coordinates is that the equations of motion of the collective coordinates ensure those of the original field theory. We illustrate this principle with the quantization of spinning degrees of freedom of Skyrmion representing baryons. We calculate the leading relativistic corrections to the static properties of nucleons, and find that the corrections are non-negligible ones of 10% to 20%.

hep-th↗

arXiv2010

Relativistic Collective Coordinate System of Solitons and Spinning Skyrmion

We consider constructing the relativistic system of collective coordinates of a field theory soliton on the basis of a simple principle: The collective coordinates must be introduced into the static solution in such a way that the equation of motion of the collective coordinates ensures that of the original field theory. As an illustration, we apply this principle to the quantization of spinning motion of the Skyrmion by incorporating the leading relativistic correction to the rigid body approximation. We calculate the decay constant and various static properties of nucleons, and find that the relativistic corrections are in the range of 5% -- 20%. We also examine how the baryons deform due to the spinning motion.

hep-th↗

arXiv2009

Melting Spectral Functions of the Scalar and Vector Mesons in a Holographic QCD Model

We investigate the finite-temperature spectral functions of heavy quarkonia by using the soft-wall AdS/QCD model. We discuss the scalar, the pseudo-scalar, the vector, and the axial-vector mesons and compare their qualitative features of the melting temperature and growing width. We find that the axial-vector meson melts earlier than the vector meson, while there appears only a slight difference between the scalar and pseudo-scalar mesons which also melt earlier than the vector meson.

hep-ph↗