Light control of surface\u2013bulk coupling by terahertz vibrational coherence in a topological insulator

Xu Yang,Zhaoyu Liu,Chirag Vaswani,Jacek K Furdyna,Xin Zhao,Caizhuang Wang,Di Cheng,Jigang Wang,Yongxin Yao,Malgorzata Dobrowolska-Furdyna,Kaiming Ho,Liang Luo,Richard H J Kim,Xinyu Liu,Ilias E Perakis

doi:10.1038/s41535-020-0215-7

Xu Yang, Zhaoyu Liu + Show 13 more

Open Access

https://doi.org/10.1038/s41535-020-0215-7

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Abstract

The demand for disorder-tolerant quantum logic and spin electronics can be met by generating and controlling dissipationless spin currents protected by topology. Dirac fermions with helical spin-locking surface transport offer a way of achieving such a goal. Yet, surface-bulk coupling can lead to strong Dirac electron scattering with bulk carriers and phonons as well as impurities, assisted by such dissipative channel, which results in “topological breakdown”. Here, we demonstrate that coherent lattice vibrations periodically driven by a single-cycle terahertz (THz) pulse can significantly suppress such dissipative channel in topological insulators. This is achieved by reducing the phase space in the bulk available for Dirac fermion scattering into during coherent lattice oscillations in Bi2Se3. This light-induced suppression manifests as a remarkable transition exclusively in surface transport, absent for bulk, above the THz electric fields for driving coherent phonons, which prolongs the surface transport lifetime. These results, together with simulations, identify the critical role of spin–orbit coupling for the “phase space contraction” mechanism that suppresses the surface-bulk coupling. Imposing vibrational quantum coherence into topological states of matter may become a universal light control principle for reinforcing the symmetry-protected helical transport.

Highlights

There is increasing evidence that coherently driven states can protect and enhance the stability of quantum systems
Our results reveal a “phase space contraction” mechanism assisted by spin–orbit coupling that is responsible for coherent phonon-enabled suppression of the surfacebulk coupling in dressed topological states out of equilibrium
This lineshape has been established as arising from joint carrier conductivity responses from both bulk and THz esluercfatrcoedystnaatmesicpslutusdaieEs1uopf hBoi2nSoen3.2m3 FoodreT, Hcoznesxisctietendt with prior states, i.e., 1Department of Physics and Astronomy and Ames Laboratory-U.S DOE, Iowa State University, Ames, IA 50011, USA. 2Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA. 3Department of Physics, University of Alabama at Birmingham, Birmingham, AL 35294-1170, USA. 4These authors contributed : Xu Yang, Liang Luo. ✉email: jgwang@ameslab.gov

Summary

Introduction

There is increasing evidence that coherently driven states can protect and enhance the stability of quantum systems. This effect of coherent phonons is in stark contrast to that of incoherent phonons from thermal excitations, which diminishes topological enhancement of surface transport via random lattice scattering, heating, surface-bulk charge transfer and coupling. Experimental realizations of coherent phonon generation have been explored in TIs,[7,13,14] the influence of such vibrational coherence on the surface helical transport has not been observed until now

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