A cascading nonlinear magneto-optical effect in topological insulators

TL;DR

This paper demonstrates a nonlinear magneto-optical technique to detect the crossover from 3D to 2D topological insulators by observing changes in Kerr rotation periodicity, enabling Dirac cone prediction without surface-sensitive measurements.

Contribution

It introduces a cascading nonlinear magneto-optical effect as a new method to identify topological phase transitions in TIs through optical measurements.

Findings

Change in Kerr rotation periodicity at critical thickness.

Prediction of Dirac cone formation in TI films.

Method applicable to various TIs and 2D materials.

Abstract

Topological insulators (TIs) are characterized by possessing metallic (gapless) surface states and a finite band-gap state in the bulk. As the thickness of a TI layer decreases down to a few nanometer, hybridization between the top and bottom surfaces takes place due to quantum tunneling, consequently at a critical thickness a crossover from a 3D-TI to a 2D insulator occurs. Although such a crossover is generally accessible by scanning tunneling microscopy, or by angle-resolved photoemission spectroscopy, such measurements require clean surfaces. Here, we demonstrate that a cascading nonlinear magneto-optical effect induced via strong spin-orbit coupling can examine such crossovers. The helicity dependence of the time-resolved Kerr rotation exhibits a robust change in periodicity at a critical thickness, from which it is possible to predict the formation of a Dirac cone in a film several quintuple layers thick. This method enables prediction of a Dirac cone using a fundamental nonlinear optical effect that can be applied to a wide range of TIs and related 2D materials.