Drive the Dirac Electrons into Cooper Pairs in SrxBi2Se3

TL;DR

This study demonstrates that surface Dirac electrons in SrxBi2Se3 can form Cooper pairs, providing evidence for topological superconductivity by showing their condensation into the superconducting state.

Contribution

It provides the first systematic experimental evidence that Dirac electrons on the surface of an intrinsic topological superconductor can participate in Cooper pairing.

Findings

Superconductivity is induced only by intercalated Sr atoms.

Full superconducting gaps are observed without in-gap states.

Surface Dirac electrons condense into Cooper pairs below the bulk gap.

Abstract

Topological superconductor is a very interesting and frontier topic in condensed matter physics1. Despite the tremendous efforts in exploring the topological superconductivity, its presence is however still under heavy debates. The Dirac electrons are supposed to exist in a thin layer of the surface of a topological insulator. Due to the finite spin-orbital coupling, these electrons will have a spin-momentum locking effect. In this case, the superfluid with the spin singlet Cooper pairing is not completely comforted by the Dirac electrons. It thus remains unclear whether and how the Dirac electrons fall into Cooper pairing in an intrinsic superconductor with the topological surface states. In this work, we show the systematic study of scanning tunneling microscope/spectroscopy on the possible topological superconductor SrxBi2Se3. We first show that only the intercalated (or inserted), not the substituted Sr atoms can induce superconductivity. Then we show the full superconducting gaps without any abnormal in-gap density of states as expected theoretically for the bulk topological superconductivity. However, we find that the surface Dirac electrons will simultaneously condense into the superconducting state when the energy is smaller than the bulk superconducting gap. This vividly demonstrates how the surface Dirac electrons are driven into Cooper pairs.