Photocurrents in Bi2Se3: bulk versus surface, and injection versus shift currents

TL;DR

This paper investigates photocurrents in Bi2Se3, distinguishing between bulk and surface contributions, and injection versus shift currents, using optical methods that avoid invasive contacts, enabling detailed surface state analysis.

Contribution

The study introduces a method to isolate and analyze surface and bulk photocurrents in Bi2Se3 using one- and two-color excitation and terahertz detection, advancing surface state characterization.

Findings

Successfully separated surface and bulk photocurrent contributions.

Demonstrated optical excitation can probe topological surface states.

Established symmetry-based relations for current components.

Abstract

Optical injection and detection of charge currents can complement conventional transport and photoemission measurements without the necessity of invasive contact that may disturb the system being examined. This is a particular concern for the surface states of a topological insulator. In this work one- and two-color sources of photocurrents are examined in epitaxial, thin films of Bi2Se3. We demonstrate that optical excitation and terahertz detection simultaneously captures one- and two- color photocurrent contributions, as previously not required in other material systems. A method is devised to isolate the two components, and in doing so each can be related to surface or bulk excitations through symmetry. This strategy allows surface states to be examined in a model system, where they have independently been verified with angle-resolved photoemission spectroscopy.