$\require{mhchem}$Misfit phase $\ce{(BiSe)_{$1.10$}NbSe2}$ as the origin of superconductivity in nobium-doped bismuth selenide

TL;DR

This study reveals that superconductivity in Nb-doped Bi2Se3 originates from a misfit phase, (BiSe)_{1.10}NbSe2, rather than Nb intercalation, challenging previous assumptions about its structure.

Contribution

The paper demonstrates that the superconductivity in Nb-doped Bi2Se3 is due to a misfit phase impurity, not Nb intercalation, providing a new understanding of its structural origin.

Findings

Superconductivity arises from the (BiSe)_{1.10}NbSe2} misfit phase.

Nb does not intercalate into Bi2Se3 as previously thought.

The misfit phase is present as an impurity at low Nb concentrations and as the main phase at higher concentrations.

Abstract

$\require{mhchem}$Topological superconductivity is of great contemporary interest and has been proposed in doped $\ce{Bi2Se3}$ in which electron-donating atoms such as Cu, Sr or Nb have been intercalated into the $\ce{Bi2Se3}$ structure. For $\ce{Nb_{x}Bi2Se3}$, with $\text{T}_\text{c} \sim 3 \ \text{K}$, it is assumed in the literature that Nb is inserted in the van der Waals gap. However, in this work an alternative origin for the superconductivity in Nb-doped $\ce{Bi2Se3}$ is established. In contrast to previous reports, it is deduced that Nb intercalation in $\ce{Bi2Se3}$ does not take place. Instead, the superconducting behaviour in samples of nominal composition $\ce{Nb_{x}Bi2Se3}$ results from the $\ce{(BiSe)_{$1.10$}NbSe2}$ misfit phase that is present in the sample as an impurity phase for small $x$ ($0.01 \leq x \leq 0.10$) and as a main phase for large $x$ ($x = 0.50$). The structure of this misfit phase is studied in detail using a combination of X-ray diffraction and transmission electron microscopy techniques.