Observability of superconductivity in Sr-doped Bi2Se3 at the surface using scanning tunneling microscope

TL;DR

This study investigates the surface superconductivity in Sr-doped Bi2Se3 using STM, finding it absent on clean surfaces but observable in micro-flakes where the topological surface state is disrupted, suggesting surface conditions influence superconductivity.

Contribution

The paper reveals that surface superconductivity in Sr-doped Bi2Se3 depends on the integrity of the topological surface state and is observable only in micro-flakes where strain destroys the TSS.

Findings

No superconducting gap observed on clean surfaces with STM.

Superconducting gap detected in micron-sized flakes transferred to STM tips.

Surface superconductivity is suppressed by the topological surface state and local electric fields.

Abstract

The superconducting materials family of doped Bi2Se3 remains intensively studied in the field of condensed matter physics due to strong experimental evidence for topologically non-trivial superconductivity in the bulk. However, at the surface of these materials, even the observation of superconductivity itself is still controversial. We use scanning tunneling microscopy (STM) down to 0.4 K to show that on the surface of bulk superconducting SrxBi2Se3, no gap in the density of states is observed around the Fermi energy as long as clean metallic probe tips are used. Nevertheless, using scanning electron microscopy and energy-dispersive X-ray analysis, we find that micron-sized flakes of SrxBi2Se3 are easily transferred from the sample onto the STM probe tip and that such flakes consistently show a superconducting gap in the density of states. We argue that the superconductivity in SrxBi2Se3 crystals does not extend to the surface when the topological surface state (TSS) is intact, but in micro-flakes the TSS has been destroyed due to strain and allows the superconductivity to extend to the surface. To understand this phenomenon, we propose that the local electric field, always found in electron doped Bi2Se3 in the presence of the TSS due to an intrinsic upward band bending, works against superconductivity at the surface.