Spin-Valley Optical Selection Rule and Strong Circular Dichroism in Silicene

TL;DR

This paper explores silicene's unique spin-valley optical selection rules and strong circular dichroism, revealing how these properties differ between topological and band insulator phases and enabling spin-polarized electron excitation.

Contribution

It demonstrates the drastic change in optical selection rules across the phase transition, providing a method to identify phases and enabling spin-valley selective optical excitation.

Findings

Silicene exhibits strong circular dichroism linked to spin-valley selection rules.

The selection rule changes drastically at the topological phase transition.

Optical excitation can generate spin-polarized currents with phase-dependent spin directions.

Abstract

Silicene (a monolayer of silicon atoms) is a topological insulator, which undergoes a topological phase transition to a band insulator under an external electric field. The spin polarization is unique and opposite at the K and K' points due to the spin-orbit coupling. Accordingly, silicene exhibits a strong circular dichroism with respect to optical absorption, obeying a certain spin-valley selection rule. It is remarkable that this selection rule is drastically different between these two types of insulators owing to a band inversion taking place at the phase transition point. Hence we can tell experimentally whether silicene is in the topological or band insulator phase by circular dichroism. Furthermore the selection rule enables us to excite electrons with definite spin and valley indices by optical absorption. Photo-induced current is spin polarized, where the spin direction is different between the topological and band insulators. It is useful for future spintronics applications.