Direction and symmetry transition of the vector order parameter in topological superconductors CuxBi2Se3
T. Kawai, C. G. Wang, Y. Kandori, Y. Honoki, K. Matano, T. Kambe, Guo-qing Zheng
Abstract
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 d and its pinning mechanism are still unclear. Here, we succeed 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 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.