Topological Magnetic Heuslers : Role of Symmetry and the Berry Phase

Topological materials are a new class of quantum materials where gapless electronic excitations are observed, which are protected by the crystal symmetry and topology of the bulk band structure. It was recently realized that the Berry phase is deeply connected with these topological states of matter and can be easily tuned via the crystal symmetry, band structure, and by engineering the number of valance electrons in topological Heuslers. We synthesized single crystals of these various topological Heuslers and investigate their magnetic, electrical, and thermal transport properties, as well as the electronic band structure with angle-resolved photoemission spectroscopy experiments. We performed topological band theory calculations in order to gain a better understanding of these materials. We found that Co2MnGa, the first three-dimensional topological magnet, shows one of the highest anomalous Hall conductivities (AHC) ~ (1600 Ωcm) at 2 K and the highest known anomalous Hall angle (AHA) (up to 12%) of any magnetic Heuslers at room temperature. We observed a remarkably high anomalous Nernst thermopower Syx A of ~6.0 μV K at 1 T magnetic field and at room temperature, which is beyond the magnetizationscaling relation in any compound reported so far in literature. We also show how the AHC in topological Heuslers can be tuned from zero to a colossal value that is independent of the sample’s magnetization. We also found that the metallic ferrimagnetic compound Mn2CoGa compensates the AHC to zero due to topological reasons, regardless of its high magnetic moment (~2 μB). We illustrate how a metallic magnet can be converted to a topologically trivial semiconductor by symmetry engineering and extend this understanding to a series of compounds like spin-gapless semiconductors.