Rinsuke Yamada

We report the first observation of a topological thermal Hall effect (tTHE) of electrons moving through a magnetic skyrmion lattice (SkL) of short period, where the skyrmion diameter is only a few nanometers. In the hexagonal intermetallic Gd2PdSi3, we observe a characteristic anomaly in the magnetic field dependence of the thermal Hall conductivity κxy, which scales well with the anomaly in the electrical Hall conductivity σxy and is closely related to the topological winding number of the SkL ground state. The relative magnitude of entropy and charge currents, defined as the Lorenz ratio ∼κxy/(σxyT), is consistent with a nondissipative, or intrinsic, mechanism for the topological Hall effect. We stress that the Berry phase in momentum space (k-space) causes such a nondissipative Hall transport, independent of the carrier relaxation time.

A fundamental question in condensed matter physics concerns how topological electronic states are influenced by many-body correlations, and magnetic Weyl semimetals represent an important material platform to address this problem. However, the magnetic structures realized in these materials are limited, and in particular, no clear example of an undistorted helimagnetic state has been definitively identified. Here, we report clear evidence of a harmonic helimagnetic cycloid with an incommensurate magnetic propagation vector Q in the Weyl semimetal GdAlSi via resonant elastic X-ray scattering, including rigorous polarization analysis. This cycloidal structure is consistent with the Dzyaloshinskii–Moriya interaction prescribed by the polar crystal structure of GdAlSi. Upon applying a magnetic field, the cycloid undergoes a transition to a novel multi-Q state. This field-induced, noncoplanar texture is consistent with our numerical spin model, which incorporates the Dzyaloshinskii–Moriya interaction and, crucially, anisotropic exchange interactions. The perfectly harmonic Weyl helimagnet GdAlSi serves as a prototypical platform to study electronic correlation effects in periodically modulated Weyl semimetals.

The dynamic motion of topological defects in magnets induces an emergent electric field, as exemplified by the continuous flow of skyrmion vortices. However, the electrodynamics underlying this emergent field remains poorly understood. In this context, magnetic domain walls - one dimensional topological defects with two collective modes, sliding and spin tilt - offer a promising platform for exploration. Here, we demonstrate that the dissipative motion of domain walls under oscillatory current excitation generates an emergent electric field. We image domain patterns and quantify domain wall length under applied magnetic fields in mesoscopic devices based on the magnetic Weyl semimetal NdAlSi. These devices exhibit exceptionally strong domain wall scattering and a pronounced emergent electric field, observed in the imaginary component of the complex impedance. Spin dynamics simulations reveal that domain wall sliding dominates over spin tilting, where the phase delay of the domain wall motion with respect to the driving force impacts the emergent electric field. Our findings establish domain-wall dynamics as a platform for studying emergent electromagnetic fields and motivate further investigations on the coupled motion of magnetic solitons and conduction electrons.

Antiferromagnetic states with a spin-split electronic structure give rise to spintronic, magnonic and electronic phenomena despite (near-)zero net magnetization^{1,2,3,4,5,6,7}. The simplest odd-parity spin splitting—p wave—was originally proposed to emerge from a collective instability in interacting electron systems^8,9,10,11,12. Recent theory has identified a distinct route to realize p-wave spin-split electronic bands without strong correlations^13,14, termed p-wave magnetism. Here we demonstrate an experimental realization of a metallic p-wave magnet. The odd-parity spin splitting of delocalized conduction electrons arises from their coupling to an antiferromagnetic texture of localized magnetic moments: a coplanar spin helix whose magnetic period is an even multiple of the chemical unit cell, as revealed by X-ray scattering experiments. This texture breaks space-inversion symmetry but approximately preserves time-reversal symmetry up to a half-unit-cell translation—thereby fulfilling the symmetry conditions for p-wave magnetism. Consistent with theoretical predictions, our p-wave magnet shows a characteristic anisotropy in the electronic conductivity^13,14,15. Relativistic spin–orbit coupling and a tiny spontaneous net magnetization further break time-reversal symmetry, resulting in a giant anomalous Hall effect (Hall conductivity >600 S cm⁻¹, Hall angle >3%), for an antiferromagnet. Our model calculations show that the spin-nodal planes found in the electronic structure of p-wave magnets are readily gapped by a small perturbation to induce the anomalous Hall effect. We establish metallic p-wave magnets as an ideal platform to explore the functionality of spin-split electronic states in magnets, superconductors, and in spintronic devices.

In kagome metals, flat electronic bands induced by frustrated hopping are a platform for strong electron correlations. Here, we investigate the superconductivity in the kagome system LaRu 3 ( Si 1 − x Ge x ) 2 by chemical pressure tuning while preserving the Ru- 4 d states that constitute the kagome flat bands. We observe a sizable enhancement in the density of states up to x = 0.07 , as determined by the specific heat, with a concomitant increase in the superconducting transition temperature T c . Ge dopants induce a uniaxial lattice expansion along the c axis. Our first-principles calculations suggest that this mitigates the detrimental effect of hybridization between kagome layers and reduces the dispersion of the Ru- d x 2 − y 2 flat band. The calculated chemical potential moves closer to the maximum in the energy-dependent density of states. Our result is consistent with a theoretical prediction of tunable flat-band superconductivity in LaRu 3 Si 2 by mode-selective coupling between specific kagome phonons and the Ru- d x 2 − y 2 orbitals.