Direction and symmetry transition of the vector order parameter in topological superconductors CuxBi2Se3

T Kawai,K Matano,Guo-Qing Zheng,T Kambe,Y Kandori,Y Honoki,C G Wang

doi:10.1038/s41467-019-14126-w

Abstract

Topological superconductors have attracted wide-spreading interests for the bright application perspectives to quantum computing. Cu0.3Bi2Se3 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 CuxBi2Se3 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 Hc2 shows a two-fold symmetry. However, the angle at which the Hc2 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 Hc2 anisotropy disappears, indicating that the gap symmetry undergoes a transition from nematic to isotropic (possibly chiral) as carrier increases.

Highlights

Topological superconductors have attracted wide-spreading interests for the bright application perspectives to quantum computing
For samples with x = 0.28, 0.36, and 0.37 which have the same size of the Knight shift, we find a two-fold symmetry in the in-plane Hc2 by ac susceptibility and magnetoresistance measurements, in agreement with previous reports[16,24,25,26,27,28]
The obtained Tc and shielding fraction (SF) for most samples are close to the values reported by Kriener et al.[34]

Summary

Introduction

Topological superconductors have attracted wide-spreading interests for the bright application perspectives to quantum computing. Cu0.3Bi2Se3 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. The in-plane Hc2 anisotropy disappears, indicating that the gap symmetry undergoes a transition from nematic to isotropic (possibly chiral) as carrier increases. Majorana fermions (excitations) are expected to appear on edges or in the vortex cores[1,2]. Such novel edge states can potentially be applied to fault tolerant non-Abelian quantum computing[3,4]. Clear evidence for odd-parity superconductivity had not been found until very recently[16]. Cu-doped topological insulator CuxBi2Se317 had been proposed as a candidate[14], experiments had been controversial[18,19,20]

Methods

Results

Conclusion