Direction and symmetry transition of the vector order parameter in topological superconductors Cu_xBi₂Se₃

Topological superconductors have attracted wide-spreading interests for the bright application perspectives to quantum computing. Cu_0.3Bi₂Se₃ is a rare bulk topological superconductor with an odd-parity wave function, but the details of the vector order parameter d and its pinning mechanism are still unclear. Here, we succeed in growing Cu_xBi₂Se₃ 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 the Knight shift, the in-plane upper critical field H_c2 shows a two-fold symmetry. However, the angle at which the H_c2 becomes minimal is different by 90° among them, which indicates that the 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 ≤ 0.37. Surprisingly, the in-plane H_c2 anisotropy disappears, indicating that the gap symmetry undergoes a transition from nematic to isotropic (possibly chiral) as carrier increases.