Direction and symmetry transition of the vector order parameter in topological superconductors Cu$_x$Bi$_2$Se$_3$

TL;DR

This study investigates the vector order parameter and its pinning mechanism in Cu$_x$Bi$_2$Se$_3$ topological superconductors, revealing a transition from nematic to isotropic gap symmetry with increasing carrier density.

Contribution

It provides new insights into the directional behavior of the order parameter and identifies a symmetry transition driven by doping levels in Cu$_x$Bi$_2$Se$_3$.

Findings

Different $H_{c2}$ minima angles indicate varying $ extbf{d}$-vector directions.

High doping levels lead to disappearance of $H_{c2}$ anisotropy.

Transition from nematic to isotropic gap symmetry with increased carrier density.

Abstract

Topological superconductors have attracted wide-spreading interests for the bright application perspectives to quantum computing. Cu$_{0.3}$Bi$_2$Se$_3$ is a rare bulk topological superconductor with an odd-parity wave function, but the details of the vector order parameter $\textbf{{d}}$ and its pinning mechanism are still unclear. We have succeeded in growing Cu$_x$Bi$_2$Se$_3$ single crystals with unprecedented high doping levels. For samples with $x$ = 0.28, 0.36 and 0.37 with similar carrier density as evidenced by Knight shift, the in-plane upper critical field $H_{\rm c2}$ shows a two-fold symmetry. However, the angle at which the $H_{\rm c2}$ becomes minimal is different by 90$^\circ$ among them, which indicates that the $\textbf{{d}}$-vector direction is different for each crystal likely due to a different local environment. The carrier density for $x$ = 0.46 and 0.54 increases substantially compared to $x\leq$ 0.37. Surprisingly, the in-plane $H_{\rm c2}$ anisotropy disappears, indicating that the gap symmetry undergoes a transition from nematic to isotropic (possibly chiral) as carrier increases.