Thickness-dependent evolution between Dirac and Weyl semimetal states of Bi_0.96Sb_0.04 films revealed by optical techniques

Phase transition in solids can be induced by various methods such as temperature change, application of voltage and pressure, chemical doping, and so on. Especially, it is known that Sb doping to semimetal Bi induces the shift at L-point bands (L_s, L_a) in Bi and makes the Dirac semimetal state at 4% Sb doping. Recently, it was reported that the Dirac-to-Weyl semimetal transition can be achieved at room temperature by adjusting the thickness of Bi_0.96Sb_0.04 films. In this work, we employed various optical measurement techniques to trace the Dirac-to-Weyl semimetal states of Bi_0.96Sb_0.04 films with a variation of the film thickness t from 3 nm to 300 nm. Using the optical second harmonic generation measurement, we revealed that the inversion symmetry breaking occurs only when t<10 nm. Correspondingly, we found a clear signature of the Weyl state from the helicity-dependent emission of the terahertz wave. We further discuss the change in the electronic structure between Dirac and Weyl semimetal states based on other optical results of photo-excited charge carriers as well as the free carriers in the static state.