Thermodynamic evidence for nematic superconductivity in Cu$_x$Bi$_2$Se$_3$

TL;DR

This study provides thermodynamic evidence for nematic superconductivity in Cu$_x$Bi$_2$Se$_3$, revealing spontaneous rotational symmetry breaking and confirming its topologically non-trivial nature through specific-heat measurements.

Contribution

The paper reports the first thermodynamic evidence of nematic superconductivity in Cu$_x$Bi$_2$Se$_3$, demonstrating spontaneous rotational symmetry breaking in bulk properties.

Findings

Two-fold symmetric behavior in specific heat under magnetic field

Confirmation of topologically non-trivial superconductivity

Evidence for an odd-parity nematic superconducting state

Abstract

Unconventional superconductivity is characterized by the spontaneous symmetry breaking of the macroscopic superconducting wavefunction in addition to the gauge symmetry breaking, such as rotational-symmetry breaking with respect to the underlying crystal-lattice symmetry. Particularly, superconductivity with spontaneous rotational-symmetry breaking in the wavefunction amplitude and thus in bulk properties, not yet reported previously, is intriguing and can be termed "nematic" superconductivity in analogy to nematic liquid-crystal phases. Here, based on specific-heat measurements of the single-crystalline Cu$_x$Bi$_2$Se$_3$ under accurate magnetic-field-direction control, we report thermodynamic evidence for nematic superconductivity, namely, clear two-fold-symmetric behavior in a trigonal lattice. The results indicate realization of an "odd-parity nematic" state, feasible only by macroscopic quantum condensates and distinct from nematic states in liquid crystals. The results also confirm topologically non-trivial superconductivity in Cu$_x$Bi$_2$Se$_3$.