Spin-polarized Weyl cones and giant anomalous Nernst effect in ferromagnetic Heusler films

Kazuki Sumida,Yuya Sakuraba,Takashi Kono,Koji Miyamoto,Taichi Okuda,Kazuki Goto,Masaaki Kakoki,Akio Kimura,Yoshio Miura,Keisuke Masuda,Weinan Zhou

doi:10.1038/s43246-020-00088-w

Abstract

Weyl semimetals are characterized by the presence of massless band dispersion in momentum space. When a Weyl semimetal meets magnetism, large anomalous transport properties emerge as a consequence of its topological nature. Here, using in−situ spin- and angle-resolved photoelectron spectroscopy combined with ab initio calculations, we visualize the spin-polarized Weyl cone and flat-band surface states of ferromagnetic Co2MnGa films with full remanent magnetization. We demonstrate that the anomalous Hall and Nernst conductivities systematically grow when the magnetization-induced massive Weyl cone at a Lifshitz quantum critical point approaches the Fermi energy, until a high anomalous Nernst thermopower of ~6.2 μVK−1 is realized at room temperature. Given this topological quantum state and full remanent magnetization, Co2MnGa films are promising for realizing high efficiency heat flux and magnetic field sensing devices operable at room temperature and zero-field.

Highlights

Weyl semimetals are characterized by the presence of massless band dispersion in momentum space
The experimental evidence for broken mirror symmetry was not provided by the recent angle-resolved photoelectron spectroscopy (ARPES) measurement on bulk Co2MnGa crystal because the remanent magnetization was negligible as applying external magnetic fields is not permitted in this measurement
By in situ spin-resolved ARPES (SARPES), we provided a direct visualization of the spin-polarized and massive Weyl cones and the peculiar surface state under mirror-symmetry breaking

Summary

Introduction

Weyl semimetals are characterized by the presence of massless band dispersion in momentum space. The observed ANE thermopower of single crystalline bulk Co2MnGa at room temperature, ~6.0 μV K−1 is an order of magnitude larger than that of other ferromagnets with similar magnetizations[7,9] These transverse properties are postulated to arise from a Berry curvature emerging within band structures near the Fermi energy (EF)[11,12]. A Co2MnGa Heusler alloy has been theoretically predicted to be a ferromagnetic Weyl semimetal with a high Curie temperature and has been experimentally demonstrated in the bulk form to exhibit large anomalous transport properties under an external magnetic field[7,9,27] The nature of this highly symmetric crystal (Fig. 1a) creates mirror-symmetry-protected Weyl nodal lines in the band structure as encountered by theory and experiments[28,29]. The ANE reaches thermopower of ~6.2 μV K−1 at room temperature, which is the highest amongst magnetic films to the best of our knowledge

Methods

Results

Conclusion