Switchable quantized conductance in topological insulators revealed by the Shockley-Ramo theorem

TL;DR

This paper demonstrates a novel method using the Shockley-Ramo theorem to image and read out quantized conductance in topological insulators, revealing switchable, spin-polarized surface states without requiring coherent transport.

Contribution

It introduces an optoelectronic probe circuit based on the Shockley-Ramo theorem for detecting local conductance in topological insulators, enabling reliable, non-coherent read-out of surface states.

Findings

Observed conductance quantization at 1e2/h in topological insulators

Achieved switching of surface conductance via electrostatic field

Provided a global, non-coherent read-out scheme for surface states

Abstract

Crystals with symmetry-protected topological order, such as topological insulators, promise coherent spin and charge transport phenomena even in the presence of disorder at room temperature. Still, a major obstacle towards an application of topological surface states in integrated circuits is a clear, reliable, and straightforward read-out independent of a prevailing charge carrier density in the bulk. Here, we demonstrate how to image and read-out the local conductance of helical surface modes in the prototypical topological insulators Bi2Se3 and BiSbTe3. We apply the so-called Shockley-Ramo theorem to design an optoelectronic probe circuit for the gapless surface states, and surprisingly find a precise conductance quantization at 1e2/h. The unprecedented response is a clear signature of local spin-polarized transport, and it can be switched on and off via an electrostatic field effect. The macroscopic, global read-out scheme is based on the displacement current resistivity, and it does not require coherent transport between electrodes.6 It provides a generalizable platform for studying further non-trivial gapless systems such as Weyl-semimetals and quantum spin-Hall insulators.