Semi-Dirac and Weyl Fermions in Transition Metal Oxides

TL;DR

This paper predicts the existence of three-dimensional semi-Dirac fermions in certain transition metal oxides with specific crystalline symmetries, and explores their potential for tunable electronic properties and novel device applications.

Contribution

It identifies a new class of 3D semi-Dirac fermions protected by nonsymmorphic symmetry in transition metal oxides, and demonstrates how strain and symmetry breaking can induce Weyl points and anomalous Hall effects.

Findings

3D semi-Dirac fermions are protected by nonsymmorphic symmetry.

Broken time-reversal symmetry leads to Weyl points.

Strain tuning affects Berry phase and electronic properties.

Abstract

We show that a class of compounds with $I$4/$mcm$ crystalline symmetry hosts three-dimensional semi-Dirac fermions. Unlike the known two-dimensional semi-Dirac points, the degeneracy of these three-dimensional semi-Dirac points is not lifted by spin-orbit coupling due to the protection by a nonsymmorphic symmetry -- screw rotation in the $a-b$ plane and a translation along the $c$ axis. This crystalline symmetry is found in tetragonal perovskite oxides, realizable in thin films by epitaxial strain that results in a$^0$a$^0$c$^-$-type octahedral rotation. Interestingly, with broken time-reversal symmetry, two pairs of Weyl points emerge from the semi-Dirac points within the Brillouin zone, and an additional lattice distortion leads to enhanced intrinsic anomalous Hall effect. The ability to tune the Berry phase by epitaxial strain can be useful in novel oxide-based electronic devices.