Possible structural origin of superconductivity in Sr-doped Bi2Se3

TL;DR

This study investigates the structural origins of superconductivity in Sr-doped Bi2Se3, revealing that interstitial Sr atoms, rather than intercalated ones, are likely responsible for superconductivity, supported by experimental and computational evidence.

Contribution

The paper provides new insights into the doping structure of Sr-doped Bi2Se3, showing that interstitial Sr atoms are key to superconductivity, challenging previous assumptions about intercalation in van der Waals gaps.

Findings

Superconductivity correlates with n-type Sr dopants.

Interstitial Sr atoms are more likely responsible for superconductivity.

Doping site stability depends on Sr concentration and processing conditions.

Abstract

Doping bismuth selenide (Bi2Se3) with elements such as copper and strontium (Sr) can induce superconductivity, making the doped materials interesting candidates to explore potential topological superconducting behaviors. It was thought that the superconductivity of doped Bi2Se3 was induced by dopant atoms intercalated in van der Waals gaps. However, several experiments have shown that the intercalation of dopant atoms may not necessarily make doped Bi2Se3 superconducting. Thus, the structural origin of superconductivity in doped Bi2Se3 remains an open question. Herein, we combined material synthesis and characterization, high-resolution transmission electron microscopy, and first-principles calculations to study the doping structure of Sr-doped Bi2Se3. We found that the emergence of superconductivity is strongly related with n-type dopant atoms. Atomic-level energy-dispersive X-ray mapping revealed various n-type Sr dopants that occupy intercalated and interstitial positions. First-principles calculations showed that the formation energy of a specific interstitial Sr doping position depends strongly on Sr doping level. This site changes from a metastable position at low Sr doping level to a stable position at high Sr doping level. The calculation results explain why quenching is necessary to obtain superconducting samples when the Sr doping level is low and also why slow furnace cooling can yield superconducting samples when the Sr doping level is high. Our findings suggest that Sr atoms doped at interstitial locations, instead of those intercalated in van der Waals gaps, are most likely to be responsible for the emergence of superconductivity in Sr-doped Bi2Se3.