Pressure-induced superconductivity in topological semimetal NbAs2

Yupeng Li,Zhu-An Xu,Ying Zhou,Zhaorong Yang,Chao An,Yi Zheng,Zhen Wang,Ranran Zhang,Xuliang Chen,Changyong Park,Yonghui Zhou,Chenqiang Hua,Yunhao Lu

doi:10.1038/s41535-018-0132-1

Abstract

Topological superconductivity with Majorana bound states, which are critical to implement nonabelian quantum computation, may be realized in three-dimensional semimetals with nontrivial topological feature, when superconducting transition occurs in the bulk. Here, we report pressure-induced superconductivity in a transition-metal dipnictide NbAs2. The emergence of superconductivity is not accompanied by any structural phase transition up to the maximum experimental pressure of 29.8 GPa, as supported by pressure-dependent synchrotron X-ray diffraction and Raman spectroscopy. Intriguingly, the Raman study reveals rapid phonon mode hardening and broadening above 10 GPa, in coincident with the superconducting transition. Using first-principle calculations, we determine Fermi surface change induced by pressure, which steadily increases the density of states without breaking the electron–hole compensation. Noticeably, the main hole pocket of NbAs2 encloses one time-reversal-invariant momenta of the monoclinic lattice, suggesting NbAs2 as a candidate of topological superconductors.

Highlights

There have been various proposals in search of quasiparticle excitations of Majorana fermions (MFs) in solids, which are the subject of both fundamental research and error-tolerant topological quantum computing.[1,2]
Chiral Majorana edge states are expected in two-dimensional (2D) chiral topological superconductors, consisting of a topological insulator in proximity to an s-wave superconductor.[9–12]
It is intriguing that superconducting phase transitions are widely observed in many topological materials when high pressure is applied, such as type II Weyl semimetals of MoTe213 and WTe2,14,15 Dirac semimetals of Cd3As216 and ZrTe5,17 and topological insulators of Bi2Se3,18 Bi2Te3,19 and Sb2Te3.20 Recently tip-induced superconductivity (SC) has attracted much attention because it could offer a new platform to study topological SC (TSC) in Dirac[21] and Weyl semimetals.[22]

Summary

INTRODUCTION

There have been various proposals in search of quasiparticle excitations of Majorana fermions (MFs) in solids, which are the subject of both fundamental research and error-tolerant topological quantum computing.[1,2] Superconductor–topological insulator (TI) heterostructures turn the surface Dirac fermions of topological insulators (TIs) into p-wave-like Cooper pairs.[3]. When spin–orbital coupling is included, the band anticrossing in MPn2 between the M-dxy and M À dx2Ày2 orbitals along the I–L–I′ direction are fully gapped,[34] resulting in weak topological insulator invariants of Z2 1⁄4 1⁄20; 111.28,31,35. Another interesting point in the MPn2 family is that magnetic field could induce Weyl points.[36]. The Cooper pair formation in NbAs2 may be correlated to the enhanced electron–phonon coupling under high pressure, since the Raman studies reveal significant phonon mode hardening and broadening above 10 GPa. It is noteworthy that, in the SC region, the main hole pocket of NbAs2 encloses the time-reversal-invariant (TRI) momenta M. These suggest NbAs2 as a candidate of TRI topological superconductors

RESULTS AND DISCUSSION

Findings

METHODS