Ultrafast Terahertz Conductivity Probes of Topologically Enhanced Surface Transport Driven by Mid-Infrared Laser Pulses in Bi$_2$Se$_3$

TL;DR

This study demonstrates that mid-infrared femtosecond pulses can selectively enhance and probe surface transport in topological insulator Bi$_2$Se$_3$, revealing faster surface conduction and a new method for characterizing topological materials.

Contribution

It introduces a novel mid-infrared excitation technique to isolate and analyze surface transport in topological insulators without interference from bulk conduction.

Findings

Surface transport in Bi$_2$Se$_3$ is enhanced by mid-infrared pulses.

Surface conduction is faster than bulk, with a ratio of approximately 3.80.

The method distinguishes surface from bulk contributions at low temperatures.

Abstract

The recent discovery of topology-protected charge transport of ultimate thinness on surfaces of three-dimensional topological insulators (TIs) are breaking new ground in fundamental quantum science and transformative technology. Yet a challenge remains on how to isolate and disentangle helical spin transport on the surface from bulk conduction. Here we show that selective midinfrared femtosecond photoexcitation of exclusive intraband electronic transitions at low temperature underpins topological enhancement of terahertz (THz) surface transport in doped Bi2Se3, with no complication from interband excitations or need for controlled doping. The unique, hot electron state is characterized by conserved populations of surface/bulk bands and by frequency-dependent hot carrier cooling times that directly distinguish the faster surface channel than the bulk. We determine the topological enhancement ratio between bulk and surface scattering rates, i.e., $\gamma_\text{BS}/\gamma_\text{SS}\sim$3.80 in equilibrium. These behaviors are absent at elevated lattice temperatures and for high pumpphoton frequencies and uences. The selective, mid-infrared-induced THz conductivity provides a new paradigm to characterize TIs and may apply to emerging topological semimetals in order to separate the transport connected with the Weyl nodes from other bulk bands.