Filling-enforced Magnetic Dirac Semimetals in Two Dimensions

TL;DR

This paper introduces a new class of two-dimensional magnetic Dirac semimetals protected by combined crystal and antiferromagnetic symmetries, capable of hosting a single Dirac point at the Fermi level.

Contribution

The study identifies and characterizes a novel magnetic Dirac semimetal phase stabilized by symmorphic crystal symmetries and antiferromagnetic time-reversal symmetry, distinct from conventional time-reversal symmetric phases.

Findings

Presence of a single, stable Dirac point in FeSe monolayers.

Dirac point acts as a quantum critical point between different topological phases.

Density functional calculations confirm the existence of this phase in specific materials.

Abstract

Filling-enforced Dirac semimetals, or those required at specific fillings by the combination of crystalline and time-reversal symmetries, have been proposed and discovered in numerous materials. However, Dirac points in these materials are not generally robust against breaking or modifying time-reversal symmetry. We present a new class of two-dimensional Dirac semimetal protected by the combination of crystal symmetries and a special, antiferromagnetic time-reversal symmetry. Systems in this class of magnetic layer groups, while having broken time-reversal symmetry, still respect the operation of time-reversal followed by a half-lattice translation. In contrast to 2D time-reversal-symmetric Dirac semimetal phases, this magnetic Dirac phase is capable of hosting just a single isolated Dirac point at the Fermi level, and that Dirac point can be stabilized solely by symmorphic crystal symmetries. We find that this Dirac point represents a new quantum critical point, and lives at the boundary between Chern insulating, antiferromagnetic topological crystalline insulating, and trivial insulating phases. We present density functional theoretic calculations which demonstrate the presence of this 2D magnetic Dirac semimetallic phase in FeSe monolayers and discuss the implications for engineering quantum phase transitions in these materials.