Optical Signatures of the Tunable Band Gap and Valley-Spin Coupling in Silicene

TL;DR

This paper explores how the optical properties of silicene and similar materials change with an electrically tunable band gap, revealing signatures of different phases and enabling control over spin-valley coupling using circularly polarized light.

Contribution

It demonstrates how dynamical conductivity measurements can identify various phases and control spin-valley polarization in silicene through electric field tuning.

Findings

Optical signatures distinguish topological and trivial phases.

Electric field can switch the Drude response on and off.

Circularly polarized light selectively probes spin and valley indices.

Abstract

We investigate the optical response of the silicene and similar materials, such as germanene, in the presence of an electrically tunable band gap for variable doping. The interplay of spin orbit coupling, due to the buckled structure of these materials, and a perpendicular electric field gives rise to a rich variety of phases: a topological or quantum spin Hall insulator, a valley-spin-polarized metal and a band insulator. We show that the dynamical conductivity should reveal signatures of these different phases which would allow for their identification along with the determination of parameters such as the spin orbit energy gap. We find an interesting feature where the electric field tuning of the band gap might be used to switch on and off the Drude intraband response. Furthermore, in the presence of spin-valley coupling, the response to circularly polarized light as a function of frequency and electric field tuning of the band gap is examined. Using right- and left-handed circular polarization it is possible to select a particular combination of spin and valley index. The frequency for this effect can be varied by tuning the band gap.