Crystalline symmetry-protected non-trivial topology in prototype compound BaAl4

Abstract

The BaAl4 prototype crystal structure is the most populous of all structure types, and is the building block for a diverse set of sub-structures including the famous ThCr2Si2 family that hosts high-temperature superconductivity and numerous magnetic and strongly correlated electron sys- tems. The MA4 family of materials (M=Sr, Ba, Eu; A=Al, Ga, In) themselves present an intriguing set of ground states including charge and spin orders, but have largely been considered as uninter- esting metals. Using electronic structure calculations, symmetry analysis and topological quantum chemistry techniques, we predict the exemplary compound BaAl4 to harbor a three-dimensional Dirac spectrum with non-trivial topology and possible nodal lines crossing the Brillouin zone, wherein one pair of semi-Dirac points with linear dispersion along the kz direction and quadratic dispersion along the kx/ky direction resides on the rotational axis with C4v point group symme- try. Electrical transport measurements reveal the presence of an extremely large, unsaturating positive magnetoresistance in BaAl4 despite an uncompensated band structure, and quantum oscil- lations and angle-resolved photoemission spectroscopy measurements confirm the predicted multi- band semimetal structure with pockets of Dirac holes and a Van Hove singularity (VHS) remarkably consistent with the theoretical prediction. We thus present BaAl4 as a new topological semimetal, casting its prototype status into a new role as building block for a vast array of new topological materials.