Rotational Symmetry Breaking in a Trigonal superconductor Nb-doped Bi2Se3

TL;DR

This study provides evidence that Nb-doped Bi2Se3 exhibits rotational symmetry breaking in its superconducting state, indicating nematic order and topological odd-parity superconductivity, with magnetic response strongly coupled to crystal symmetry.

Contribution

It demonstrates the coupling between superconducting magnetic response and crystal symmetry, revealing nematic order and symmetry breaking in Nb-doped Bi2Se3.

Findings

Magnetic response couples strongly to 3-fold crystal symmetry.

Superconducting hysteresis shows 2-fold and 4-fold symmetry.

Heat capacity indicates no line node in the gap.

Abstract

The search for unconventional superconductivity has been focused on materials with strong spin-orbit coupling and unique crystal lattices. Doped bismuth selenide (Bi$_2$Se$_3$) is a strong candidate given the topological insulator nature of the parent compound and its triangular lattice. The coupling between the physical properties in the superconducting state and its underlying crystal symmetry is a crucial test for unconventional superconductivity. In this paper, we report direct evidence that the superconducting magnetic response couples strongly to the underlying 3-fold crystal symmetry in the recently discovered superconductor with trigonal crystal structure, niobium (Nb)-doped bismuth selenide (Bi$_2$Se$_3$). More importantly, we observed that the magnetic response is greatly enhanced along one preferred direction spontaneously breaking the rotational symmetry. Instead of a simple 3-fold crystalline symmetry, the superconducting hysteresis loop shows dominating 2-fold and 4-fold symmetry. This observation confirms the breaking of the rotational symmetry and indicates the presence of nematic order in the superconducting ground state of Nb-doped Bi$_2$Se$_3$. Further, heat capacity measurements display an exponential decay in superconducting state and suggest that there is no line node in the superconducting gap. These observations provide strong evidence of odd-parity topological superconductivity.