  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.

Strongly Correlated Electrons