Nematic topological superconducting phase in Nb-doped Bi2Se3

Junying Shen,Zengle Huang,Chang-Woo Cho,Kam Tuen Law,Noah Fan Qi Yuan,Rolf Lortz,Seng Huat Lee,Wen-Yu He,Yew San Hor

doi:10.1038/s41535-017-0064-1

Abstract

A nematic topological superconductor has an order parameter symmetry, which spontaneously breaks the crystalline symmetry in its superconducting state. This state can be observed, for example, by thermodynamic or upper critical field experiments in which a magnetic field is rotated with respect to the crystalline axes. The corresponding physical quantity then directly reflects the symmetry of the order parameter. We present a study on the superconducting upper critical field of the Nb-doped topological insulator NbxBi2Se3 for various magnetic field orientations parallel and perpendicular to the basal plane of the Bi2Se3 layers. The data were obtained by two complementary experimental techniques, magnetoresistance and DC magnetization, on three different single crystalline samples of the same batch. Both methods and all samples show with perfect agreement that the in-plane upper critical fields clearly demonstrate a two-fold symmetry that breaks the three-fold crystal symmetry. The two-fold symmetry is also found in the absolute value of the magnetization of the initial zero-field-cooled branch of the hysteresis loop and in the value of the thermodynamic contribution above the irreversibility field, but also in the irreversible properties such as the value of the characteristic irreversibility field and in the width of the hysteresis loop. This provides strong experimental evidence that Nb-doped Bi2Se3 is a nematic topological superconductor similar to the Cu- and Sr-doped Bi2Se3.

Highlights

Published in partnership with Nanjing University the irreversible properties, such as the width of the hysteresis loop In Fig. 3 we show a selection of magnetization hysteresis loop at a fixed magnetic field value
The magneto-resistive and the magnetic upper critical field transitions of NbxBi2Se3 show a clear two-fold symmetry instead of the expected three-fold symmetry, which suggests that the superconducting state of NbxBi2Se3 is nematic and spontaneously breaks the three-fold crystalline symmetry, in perfect agreement with what has been observed for CuxBi2Se328,29 and SrxBi2Se3.30,31 Since the metal dopants are intercalated in the van der Waals gap between the Bi2Se3 layers, it has been argued that 1D clustering in the form of stripe like ion patters could be responsible for the twofold anisotropy.[30]
While we cannot rule this out from our experiments, the fact that the normal state resistance does not show any dependence on the in-plane field direction does make this scenario unlikely, and rather suggests a true nematic superconducting state

Summary

Introduction

As shown by many experimental groups recently, topological insulators such as Bi2Se3 can become superconducting when doped by metals such as Cu, Sr and Nb.[19,20,21,22,23,24,25] Due to the nontrivial topology in the normal state band structures, the nature of pairing in these superconducting states has attracted much interest. It was proposed that Cu-doped Bi2Se3 could be a topological superconductor with odd parity pairing, which belongs to the Au representation of the D3d point group.[26] There are Majorana surface states associated with these superconducting phases that should cause zero-bias conductance peaks in tunneling experiments.[21] the zero-bias conductance peak found in an early experiment on CuxBi2Se3 is missing in more surface sensitive scanning tunneling experiments.[27]

Methods

Results

Conclusion