Kavli Affiliate: Alessandra Lanzara
| First 5 Authors: Kefeng Wang, Ryo Mori, Zhijun Wang, Limin Wang, Jonathan Han Son Ma
| Summary:
The BaAl$_4$ 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 ThCr$_2$Si$_2$ family that hosts high-temperature
superconductivity and numerous magnetic and strongly correlated electron
systems. The MA$_4$ 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 uninteresting metals. Using electronic
structure calculations, symmetry analysis and topological quantum chemistry
techniques, we predict the exemplary compound BaAl$_4$ 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 $k_z$ direction and quadratic dispersion along the
$k_x/k_y$ direction resides on the rotational axis with $C_{4v}$ point group
symmetry. Electrical transport measurements reveal the presence of an extremely
large, unsaturating positive magnetoresistance in BaAl$_4$ despite an
uncompensated band structure, and quantum oscillations and angle-resolved
photoemission spectroscopy measurements confirm the predicted multiband
semimetal structure with pockets of Dirac holes and a Van Hove singularity
(VHS) remarkably consistent with the theoretical prediction. We thus present
BaAl$_4$ as a new topological semimetal, casting its prototype status into a
new role as building block for a vast array of new topological materials.
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