고경춘 교수

We theoretically investigate magnon-phonon hybrid excitations in two-dimensional ferromagnets. The bulk bands of hybrid excitations, which are referred to as magnon polarons, are analytically shown to be topologically nontrivial, possessing finite Chern numbers. We also show that the Chern numbers of magnon-polaron bands and the number of band-crossing lines can be manipulated by an effective magnetic field. For experiments, we propose to use the thermal Hall conductivity as a probe of the finit

Recently, topological responses of magnons have emerged as a central theme in magnetism and spintronics. However, resulting Hall responses are typically weak and infrequent, since, according to present understanding, they arise from effective spin-orbit couplings, which are weaker compared to the exchange energy. Here, by investigating transport properties of magnon orbital moments, we predict that the magnon orbital Nernst effect is an intrinsic characteristic of the honeycomb antiferromagnet a

We theoretically investigate coupled gyration modes of magnetic solitons whose distances to the nearest neighbors are staggered. In a one-dimensional bipartite lattice, we analytically and numerically find that there is a midgap gyration mode bounded at the domain wall connecting topologically distinct two phases which is analogous to the Su-Schrieffer-Heeger model. As a technological application, we show that a one-dimensional domain-wall string in a two-dimensional soliton lattice can serve as

We theoretically investigate topological spin transport of the magnon polarons in a bilayer magnet with two-dimensional square lattices. Our theory is motivated by recent reports on the van der Waals magnets which show the reversible electrical switching of the interlayer magnetic order between antiferromagnetic and ferromagnetic orders. The magnetoelastic interaction opens band gaps and allows the interband transition between different excitation states. In the layered antiferromagnet, due to t

The topological properties of a certain class of spinless three-band Hamiltonians are shown to be summed up by the skyrmion number in momentum space, analogous to the case of a two-band Hamiltonian. A topological tight-binding Hamiltonian on a kagome lattice is analyzed from this viewpoint. When such a Hamiltonian is ``folded,'' the two bands with opposite Chern numbers merge into a degenerate band exhibiting a non-Abelian gauge connection. A conserved pseudospin current operator can be construc

Current-induced magnetization switching by spin-orbit torques has become of technological interest for spintronics, because it provides high-speed operation while preserving device stability. However, conventional spin-orbit-torque switching requires a rather large switching current, and an external in-plane magnetic field for deterministic switching. In this study an alternative switching scheme using a circularly polarized alternating current is investigated, which allows not only a low switch

We theoretically investigate the thermal Hall transport of magnon-polarons in a two-dimensional honeycomb antiferromagnetic insulator under the influence of a perpendicular magnetic field, varying in strength. The application of a perpendicular magnetic field induces a magnetic phase transition from the collinear antiferromagnetic phase to the spin-flop phase, leading to a significant alteration in Hall transport across the transition point. In this paper, our focus is on the intrinsic contribut

It is known that in chiral magnets with intrinsic inversion symmetry breaking, two spin waves moving in opposite directions can propagate at different velocities, exhibiting a phenomenon called magnetochiral nonreciprocity, which allows for realizations of certain spin logic devices such as a spin-wave diode. Here, we theoretically demonstrate that the spin-wave nonreciprocity can occur without intrinsic bulk chirality in easy-cone ferromagnets and easy-cone antiferromagnets. Specifically, we sh

Defect formation in the one-dimensional topological three-band model, spanned by the generators of SU(2) and containing a central flat band, is examined within both lattice and continuum theories. Classic results of Jackiw-Rebbi and Rice-Mele for the soliton charge are generalized. The charge is twice the value obtained for the two-band case with the corresponding parameters. A sudden jump in the soliton charge from zero to one as the soliton state passes through the central flat band is predict

In optics, the Fabry–Pérot interferometer is a basic building block, enabling a selection of light with a specific wavelength. We theoretically propose a magnonic version of the Fabry–Pérot interferometer based on two magnetic domain walls. By exploiting the interaction of a domain wall and spin waves, in particular, the phenomenon that a sufficiently narrow domain wall serves as an effective mirror for spin waves as it reflects spin wave almost completely, a magnonic interferometer is contrived

Autoresonance is a self-sustained resonance mechanism due to a driving force whose frequency monotonically varies with time. We theoretically show that the autoresonance mechanism allows an efficient switching of perpendicular magnetization by spin-orbit spin-transfer torques. We find that a threshold current for the autoresonant switching can be much smaller than that of conventional spin-orbit torque switching driven by a DC current. Moreover, the suggested scheme allows fully deterministic sw

Recently, the switching of a perpendicularly magnetized ferromagnet (FM) by injecting an in-plane current into an attached non-magnet (NM) has become of emerging technological interest. This magnetization switching is attributed to the spin-orbit torque (SOT) originating from the strong spin-orbit coupling of the NM layer. However, the switching efficiency of the NM/FM structure itself may be insufficient for practical use, as for example, in spin transfer torque (STT)-based magnetic random acce

A BPS limit is systematically derived for straight multi- D- and DF-strings from the D3D¯3 system in the context of boundary superstring field theory. The BPS limit is obtained in the limit of thin D(F)-strings, where the Bogomolny equation supports singular static multi-D(F)-string solutions. For the BPS multi-string configurations with arbitrary separations, BPS sum rule is fulfilled under a Gaussian type tachyon potential and reproduces exactly the descent relation. For the DF-strings ((p,q)-

The scalar spin chirality, which characterizes the fundamental unit of noncoplanar spin structures, plays an important role in rich chiral physics of magnetic materials. In particular, the intensive research efforts over the past two decades have demonstrated that the scalar spin chirality is the source of various novel Hall transports in solid-state systems, offering a primary route to bring about chiral phenomena in condensed matter physics. However, in all of the previous studies, the scalar

We obtain a new type of $\mathcal{N}=3$ Yang-Mills Chern-Simons theory from the Mukhi-Papageorgakis Higgs mechanism of the $\mathcal{N}=3$ Gaiotto-Tomasiello theory. This theory has $\mathcal{N}=1$ BPS fuzzy funnel solution, which is expressed in terms of the seven generators of SU(3), excluding ${T}_{8}$. We propose that this is an effective theory of multiple D2-branes with D6- and D8-branes background in massive IIA string theory.