Diverging shift current responses in the gapless limit of two-dimensional systems

TL;DR

This paper investigates the behavior of shift current responses in two-dimensional systems as they approach the gapless limit, revealing a divergence proportional to the inverse of the gap size in Weyl semimetals.

Contribution

It demonstrates the divergence of shift current responses in 2D Weyl semimetals as the energy gap closes, highlighting a fundamental difference from electric polarization behavior.

Findings

Shift current diverges as the gap approaches zero.

Behavior differs from electric polarization, which exhibits a jump.

Quantitative relationship between polarization and shift current breaks down.

Abstract

The shift current responses of two-dimensional systems in the gapless limit are investigated. As the energy gap becomes smaller, the interband transition probability becomes larger at the band edges. We found the divergence of the shift current response in a manner proportional to the inverse of the gap size when the system becomes a two-dimensional Weyl semimetal in the gapless limit. This behavior is different from that for electric polarization, which has a jump across the gapless case. It means that the quantitative relationship between electric polarizations and shift currents is broken in this gapless limit.