Predicted novel type of photoinduced topological phase transition accompanied by collision and collapse of Dirac-cone pair in organic salt $\alpha$-(BEDT-TTF)$_2$I$_3$

TL;DR

This paper predicts a new type of photoinduced topological phase transition in an organic salt, involving collision and collapse of Dirac points, expanding the understanding of light-induced topological phenomena beyond conventional gap-opening mechanisms.

Contribution

It introduces a novel photoinduced topological phase transition mechanism involving Dirac point collision and collapse in an organic salt, using Floquet theory.

Findings

Collision and collapse of Dirac points under elliptically polarized light.

Transition from topological semimetal to normal insulator.

Potential experimental detection via Hall conductivity measurements.

Abstract

Photoinduced topological phase transitions in the Dirac-electron systems have attracted intensive research interest since its theoretical prediction in graphene, where the application of circularly polarized light opens a gap at the Dirac points and renders the system a topologically nontrivial Chern insulator phase through breaking the time-reversal symmetry. However, most of the previously studied phenomena in two-dimensional Dirac systems are basically based on the same physicical mechanism, i.e., the gap opening in the Dirac electron bands with circularly polarized light, and it is lacking in variety of the physics. In this paper, we theoretically predict a novel type of photoinduced topological phase transition accompanied by collision and collapse of gapped Dirac points in the organic salt $\alpha$-(BEDT-TTF)$_2$I$_3$. By constructing the Floquet theory for this compound, we demonstrate that the irradiation of elliptically polarized light causes collision of the Dirac points through the photoinduced band deformation and their collapse, which eventually results in the topological phase transition from a topological semimetal with gapped Dirac cones to a normal insulator when the elliptical axis is oriented at a specfic angle with respect to the crystallographic axes. We argue that this novel photoinduced phase transition can be experimentally detected by the measurement of Hall conductivity. The present work enriches the fundamental physics of photoinduced topological phase transitions and thus contribute to development of this rapidly growing research field.