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.

A magnetic order can be completely suppressed at zero temperature (T ), by doping carriers or applying pressure, at a quantum critical point (QCP) , around which physical properties change drastically. However, the situation is unclear for an electronic nematic order that breaks rotation symmetry. Here we report nuclear magnetic resonance (NMR) studies on NaFe 1−x Co x As where magnetic and nematic transitions are well separated. The NMR spectrum is sensitive to inhomogeneous magnetic fields in the vortex state, which is related to London penetration depth λ L that measures the electron mass m * . We discovered two peaks in the doping dependence of λ 2 L (T ∼0); one at x M =0.027 where the spin-lattice relaxation rate shows quantum critical behavior, and another at x c =0.032 around which the nematic transition temperature extrapolates to zero and the electrical resistivity shows a T -linear variation. Our results indicate that a nematic QCP lies beneath the superconducting dome at x c where m * is enhanced. The impact of the nematic fluctuations on superconductivity is discussed.