Non-linear transport without spin-orbit coupling or warping in two-dimensional Dirac semimetals

TL;DR

This paper demonstrates that two-dimensional Dirac semimetals without tilt or warping can exhibit non-linear transport phenomena driven by Berry curvature dipoles, leading to novel helicity-dependent photocurrents and thermal responses.

Contribution

It reveals that Dirac quasiparticles from merging Dirac points can produce a non-zero Berry curvature dipole without tilt or warping, expanding understanding of non-linear effects in 2D Dirac materials.

Findings

Non-zero Berry curvature dipole in untitled, unwarped 2D Dirac semimetals.

Independence of BCD from Dirac velocity in these systems.

Potential for helicity-dependent photocurrent and nonlinear Nernst effect.

Abstract

It has recently been realized that the first-order moment of the Berry curvature, namely the Berry curvature dipole (BCD) can give rise to non-linear current in a wide variety of time-reversal invariant and non-centrosymmetric materials. While the BCD in two-dimensional Dirac systems is known to be finite only in the presence of either substantial spin-orbit coupling where low-energy Dirac quasiparticles form tilted cones or higher order warping of the Fermi surface, we argue that the low-energy Dirac quasiparticles arising from the merging of a pair of Dirac points without any tilt or warping of the Fermi surface can lead to a non-zero BCD. Remarkably, in such systems, the BCD is found to be independent of Dirac velocity as opposed to the Dirac dispersion with a tilt or warping effects. We further show that the proposed systems can naturally host helicity-dependent photocurrent due to their linear momentum-dependent Berry curvatures. Finally, we discuss an important byproduct of this work, i.e., nonlinear anomalous Nernst effect as a second-order thermal response.