Shun Okumura 교수

The magnetic hedgehog lattice (HL) is a noncoplanar magnetic texture with a periodic array of magnetic monopoles and antimonopoles. Despite phenomenological and numerical studies thus far, there remain open issues on the microscopic origin, especially with respect to the recent experimental findings of two different types of HLs even at zero magnetic field. Here, we study the stability of the HLs for an effective spin model with long-range interactions arising from the itinerant nature of electr

Photoluminescent (PL) properties related to Te(4+) species in zinc borate glasses are examined. Broad emission was observed by the excitation of the PL excitation peak of Te(4+) present at the optical absorption edge. The emission intensity of Te(4+) in 5TeO(2)-50ZnO-45B(2)O(3) glass was thermally quenched in a temperature region over 100 K, suggesting that concentration quenching preferentially occurred. The lifetime of the emission was approximately 2.5 μs, which is characteristic of relaxatio

Chiral magnets often show intriguing magnetic and transport properties associated with their peculiar spin textures. A typical example is a chiral soliton lattice, which is found in monoaxial chiral magnets, such as CrNb$_3$S$_6$ and Yb(Ni$_{1-x}$Cu$_x$)$_3$Al$_9$ in an external magnetic field perpendicular to the chiral axis. Here, we theoretically investigate the electronic and magnetic properties in the chiral soliton lattice by a minimal itinerant electron model. Using variational calculatio

It is well established that spin-transfer torques exerted by in-plane spin currents give rise to a motion of magnetic skyrmions resulting in a skyrmion Hall effect. In films of finite thickness or in three-dimensional bulk samples the skyrmions extend in the third direction forming a string. We demonstrate that a spin current flowing longitudinally along the skyrmion string instead induces a Goldstone spin wave instability. Our analytical results are confirmed by micromagnetic simulations of bot

Received 1 December 2023DOI:https://doi.org/10.1103/PhysRevB.108.219902©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasHelicoidal magnetic textureOptical conductivityPhotovoltaic effectPhysical SystemsChiral magnetsTechniquesKondo lattice modelMethods in transportOptical second-harmonic generationCondensed Matter, Materials & Applied Physics

Abstract We theoretically study the electrical conductivity in a one-dimensional helimagnet whose spin texture changes from helimagnetic to conical magnetic, and to forced ferromagnetic state while increasing the magnetic field along the helical axis. We find that the conductivity in the helimagnetic state at zero field depends on the electron filling and the coefficient of the spin-charge coupling. We also find that the conductivity in the conical magnetic state changes nonlinearly to the appli

The hedgehog lattice (HL) is a three-dimensional topological spin texture hosting a periodic array of magnetic monopoles and antimonopoles. It has been studied theoretically for noncentrosymmetric systems with the Dzyaloshinskii-Moriya interaction, but the stability, as well as the magnetic and topological properties, remains elusive in the centrosymmetric case. We here investigate the ground state of an effective spin model with long-range bilinear and biquadratic interactions for a centrosymme

Despite proposing a quantum generative model for time series that successfully learns correlated series with multiple Brownian motions, the model has not been adapted and evaluated for financial problems. In this study, a time-series generative model was applied as a quantum generative model to actual financial data. Future data for two correlated time series were generated and compared with classical methods such as long short-term memory and vector autoregression. Furthermore, numerical experi

We show the relationship between the Fourier coefficients and the barren plateau problem emerging in parameterized quantum circuits. In particular, the sum of squares of the Fourier coefficients is exponentially restricted concerning the qubits under the barren plateau condition. Throughout theory and numerical experiments, we introduce that this property leads to the vanishing of a probability and an expectation formed by parameterized quantum circuits. The traditional barren plateau problem re

Chiral magnets, which break both spatial inversion and time reversal symmetries, carry a potential for quadratic optical responses. Despite the possibility of enhanced and controlled responses through the magnetic degree of freedom, the systematic understanding remains yet to be developed. We here study nonlinear optical responses in a prototypical chiral magnetic state with a one-dimensional conical order by using the second-order response theory. We show that the photovoltaic effect and the se