A cascading nonlinear magneto-optical effect in topological insulators

Richarj Mondal,Paul Fons,Yuta Saito,Muneaki Hase,Shuichi Murakami,Junji Tominaga,Alexander V Kolobov,Yuki Aihara

doi:10.1038/s41598-018-22196-x

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 nanometers, 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.

Highlights

Higher-order nonlinear optical processes in solids are sensitive to the crystal lattice symmetry through the nonlinear susceptibilities χ(n)(n = 2, 3, ...)[1]
We demonstrate that nonlinear magnetic excitation and the subsequent probing of the cascaded second-order susceptibility can explicitly detect the presence of a surface Dirac cone and revisit the 2D insulator to 3D-Topological insulators (TIs) crossover in X2Te3 (X = Bi, Sb) thin films, whose thickness are less than 10 nm
We have focused on p-type Sb2Te3 since it is generally difficult to investigate its topological properties by conventional techniques such as ARPES

Summary

OPEN A cascading nonlinear magnetooptical effect in topological insulators

Richarj Mondal1,Yuta Saito 2,Yuki Aihara[1], Paul Fons 2, Alexander V. 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. The periodicity (i.e. π) of the helicity dependence of the Δθk signal is the same with that observed in crystalline GaAs20, which belongs to normal insulators as well as for the 2 QL Sb2Te3 sample (Fig. 3a) This is in accordance with the fact that topological surface states are absent in the surface of the 2 QL thick Sb2Te3 sample, and it shows similar behavior to the normal insulator GeTe

The measured ratic nature of the

Conclusion

Methods

Additional Information