Manifestation of a Second Dirac Surface State and Bulk Bands in THz Radiation from Topological Insulators.

Chien-Ming Tu,Chih-Wei Luo,Tien-Tien Yeh,Helmuth Berger,Ku-Ding Tsuei,Jiunn-Yuan Lin,Shin-An Ku,Yi-Ru Chen,Wen-Yen Tzeng,Raman Sankar,Kaung-Hsiung Wu,Fang-Cheng Chou,Cheng-Maw Cheng,Takayoshi Kobayashi,Hsueh-Ju Chen,Jenh-Yih Juang

doi:10.1038/srep14128

Abstract

Topological insulators (TIs) are interesting quantum matters that have a narrow bandgap for bulk and a Dirac-cone-like conducting surface state (SS). The recent discovered second Dirac surface state (SS) and bulk bands (BBs) located ~1.5 eV above the first SS are important for optical coupling in TIs. Here, we report on the time-domain measurements of THz radiation generated from TIs n-type Cu0.02Bi2Se3 and p-type Bi2Te3 single crystals by ultrafast optical pulse excitation. The observed polarity-reversal of the THz pulse originated from transient current is unusual, and cannot be reconciled with the photo-Dember effect. The second SS and BBs are found to be indispensable for the explanation of the unusual phenomenon. Thanks to the existence of the second SS and BBs, TIs manifest an effective wide band gap in THz generation. The present study demonstrates that time-domain THz spectroscopy provide rich information of the optical coupling and the electronic structure of TIs.

Highlights

A new phase of quantum matters called topological insulators (TIs) are theoretically predicted and experimentally observed recently, and their band structures are composed of insulating bulk bands and a conducting surface state (SS)
This study reports the time domain measurements of THz radiation generated from the surface and bulk of n-type Cu0.02Bi2Se3 and p-type Bi2Te3 single crystals
The P-polarized THz radiation from both Cu0.02Bi2Se3 and Bi2Te3 do not show a significant three-fold symmetry that is related to the crystalline structure, and this excludes optical rectification (OR) as the mechanism for P-polarized THz radiation from TIs

Summary

Introduction

A new phase of quantum matters called topological insulators (TIs) are theoretically predicted and experimentally observed recently, and their band structures are composed of insulating bulk bands and a conducting surface state (SS). The SS results from a strong spin-orbital interaction and band inversion[1,2] and exhibits novel properties, such as the protection by time-reversal symmetry against backscattering from nonmagnetic impurities[3,4,5] and spin-polarized current[6,7,8]. These properties hint at potential applications in spintronic devices and quantum computations. With this time-domain technique, the 2nd SS and BBs manifest themselves in the spectrum, and prove vital in generating the THz radiation through a transient drift current

Methods

Results

Conclusion