Topological Hall and Nernst effects in spin-chiral magnets*

We are interested in the effect of spin-chirality on the electronic band structure of solids, where topological electronic states may be generated without explicitly invoking spin-orbit coupling. To study the impact of spin-chirality on the Bloch wave functions, it is ideal to choose materials where the length scale of a spin-twist is comparable to the size of a crystallographic unit cell. Highly symmetric intermetallics with tiny skyrmion vortices, representing quantized units of spin chirality, are a perfect playground in this respect [1,2]. In Gd2PdSi3 with triangular lattice of magnetic moments and with skyrmions smaller than 2.8 nanometers in diameter, our meticulous transport experiments revealed giant Hall and Nernst responses [3]. These indicate the importance of Berry-phase theory in a momentum-space picture, beyond the conventional real-space picture suitable to larger-sized skyrmion lattice materials. At temperatures significantly above the transition to long-range order, Dzyaloshinskii-Moriya interactions can imprint themselves as a chiral habit of dynamical spin fluctuations [4]. We demonstrate the modification of Bloch wavepackets and their band dispersion due to thermal fluctuations in a trimer-based ferromagnet with Dzyaloshinskii-Moriya interactions. Intriguing features, such as the comparable size of Hall and thermoelectric Nernst conductivities when measured in fundamental units, are discussed in light of recent advances in ab-initio theoretical modeling.