Kazushi Aoyama

Stable pairing states of superfluid $^{3}\mathrm{He}$ in aerogel are examined in the case with a global uniaxial anisotropy which may be created by applying a uniaxial stress to the aerogel. Due to such a global anisotropy, the stability region of an Anderson-Brinkman-Morel (ABM) pairing state becomes wider. In a uniaxially stretched aerogel, the pure polar pairing state with a horizontal line node is predicted to occur, as a three-dimensional superfluid phase, over a measurable width just below

The hedgehog lattice, a three-dimensional periodic array of magnetic monopoles and antimonopoles, is known to be realized in the presence of the Dzyaloshinskii-Moriya interaction (DMI). Here, the authors report a new mechanism other than the conventional DMI. They demonstrate that the hedgehog lattice can be induced by frustration in DMI-free antiferromagnets on the breathing pyrochlore lattice and that an applied magnetic field leads to a nonzero net spin chirality, which, in a metallic system,

The effect of spatial inhomogeneity on the properties of a two-dimensional noncentrosymmetric superconductor in an in-plane magnetic field is investigated, as it can be realized in LaAlO(3)-SrTiO(3) interfaces. We demonstrate that the spatial variation of Rashba spin-orbit coupling yields a local magnetic flux pattern due to the field-induced inhomogeneous helical phase. For sufficiently strong fields, vortices can nucleate at inhomogeneities of the Rashba spin-orbit coupling.

We theoretically investigate effects of lattice distortions on the spin ordering of classical Heisenberg antiferromagnets on the breathing pyrochlore lattice. In the model, local lattice distortions originating from the site vibration are taken into account to yield effective spin interactions via the spin-lattice coupling (SLC). The breathing lattice alternation is characterized by the ratio of the nearest-neighbor interaction for large tetrahedra to that for small ones, ${J}^{\ensuremath{'}}/J

Effects of local lattice distortions on the spin ordering are investigated for the antiferromagnetic classical Heisenberg model on the pyrochlore lattice. It is found by Monte Carlo simulations that the spin-lattice coupling (SLC) originating from site phonons induces a first-order transition into two different types of collinear magnetic ordered states. The state realized at the stronger SLC is cubic symmetric characterized by the magnetic (1/2,1/2,1/2) Bragg peaks, while that at the weaker SLC

Transport properties of the classical antiferromagnetic XXZ model on the square lattice have been theoretically investigated, putting emphasis on how the occurrence of a phase transition is reflected in spin and thermal transports. As is well known, the anisotropy of the exchange interaction $\mathrm{\ensuremath{\Delta}}\ensuremath{\equiv}{J}_{z}/{J}_{x}$ plays a role to control the universality class of the transition of the model, i.e., either a second-order transition at ${T}_{N}$ into a magn

We theoretically investigate the appearance of spatially modulated superconducting states in mesoscopic superconducting thin-wall cylinders in a magnetic field at low temperatures. Quantization of the electron motion around the circumference of the cylinder leads to a discontinuous evolution of the spatial modulation of the superconducting order parameter along the transition line T(c)(H). We show that this discontinuity leads to the nonmonotonic behavior of the specific heat jump at the onset o

We have theoretically investigated transport properties of the classical Heisenberg antiferromagnet on the triangular lattice, in which a binding-unbinding topological transition of Z_{2} vortices is predicted to occur at a finite temperature T_{v}. It is shown by means of the hybrid Monte Carlo and spin-dynamics simulations that the longitudinal spin-current conductivity exhibits a divergence at T_{v}, while the thermal conductivity only shows a monotonic temperature dependence with no clear an

Domains in noncentrosymmetric materials represent regions of different crystal structure and spin-orbit coupling. Twin boundaries separating such domains display unusual properties in noncentrosymmetric superconductors (NCSs), where magnetoelectric effects influence the local lower and upper critical magnetic fields. As a model system, we investigate NCSs with tetragonal crystal structure and Rashba spin-orbit coupling (RSOC), and with twin boundaries parallel to their basal planes. There, we re

We show that classical Heisenberg antiferromagnets on the breathing kagome lattice can be a platform to realize a zero-field topological order of the scalar spin chirality which can be viewed as a miniature skyrmion crystal (SkX) of discrete form with a small number of spins in its magnetic unit cell. In the model, a third nearest-neighbor (NN) antiferromagnetic interaction along the bond direction ${J}_{3}$ and the breathing bond alternation characterized by the ratio of the NN interaction for

The equal-spin pairing state, the so-called A-like phase, of superfluid $^{3}\mathrm{He}$ in aerogels is studied theoretically in the Ginzburg-Landau region by examining thermodynamics, and the resulting equilibrium phase diagram is mapped out. We find that the ABM pairing state with presumably quasi-long-ranged superfluid order has a lower free energy than the planar and ``robust'' states and is the best candidate of the A-like phase with a strange lowering of the polycritical point observed ex

We theoretically investigate pairing states of the spin-triplet $p$-wave superfluid ${}^{3}$He confined in narrow cylinders. The surface-induced distortion and the multiple internal degrees of freedom of the order parameter lead to the occurrence of a stripe structure along the cylinder axis in the superfluid ${}^{3}$He-B phase. We show that in sufficiently small cylinders with an anisotropic surface scattering, the stripe order with broken translational symmetry may be stabilized as the lowest

Effects of impurity scatterings on the strong-coupling (SC) contribution, stabilizing the ABM (axial) pairing state, to the quartic term of the Ginzburg-Landau free energy of superfluid $^{3}\mathrm{He}$ are theoretically studied to examine recent observations suggestive of an anomalously small SC effect in superfluid $^{3}\mathrm{He}$ in aerogels. To study the SC corrections, two approaches are used. One is based on a perturbation in the short-range repulsive interaction, and the other is a phe

In two-dimensional superconductors with a Rashba-type spin-orbit coupling, it is known that an in-plane magnetic field can induce a helical superconducting (SC) state with a phase modulation ${e}^{i\mathbf{q}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{r}}$. Here, we theoretically investigate the stability of a stripe order, a weight-biased superposition state composed of $+\mathbf{q}$ and $\ensuremath{-}\mathbf{q}$ modes taking the form of ${\mathrm{\ensuremath{\Delta}}}_{+}{e}^{i\mathbf{q

We theoretically investigate a ${J}_{1}\text{\ensuremath{-}}{J}_{3}$ classical Heisenberg model on the breathing pyrochlore lattice, where the nearest-neighbor (NN) exchange interactions for small and large tetrahedra, ${J}_{1}$ and ${J}_{1}^{\ensuremath{'}}$, take different values due to the breathing bond alternation and ${J}_{3}$ is the third NN antiferromagnetic interaction along the bond direction. It is found by means of Monte Carlo simulations that for large ${J}_{3}$, a hedgehog lattice,