Superconductivity in topologically nontrivial material Au2Pb

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Superconductivity in topologically nontrivial material Au 2Pb

Ying Xing 1,2,5,He Wang 1,2,5,Chao-Kai Li 1,2,5,Xiao Zhang 1,2,Jun Liu 3,Yangwei Zhang 1,2,Jiawei Luo 1,2,Ziqiao Wang 1,2,Yong Wang 3,Langsheng Ling 4,Mingliang Tian 4,Shuang Jia 1,2,Ji Feng 1,2,Xiong-Jun Liu 1,2,Jian Wei 1,2and Jian Wang 1,2

INTRODUCTION

The recent discovery of three-dimensional (3D)Dirac semimetals,1–12as an intermediate state between a trivial insulator and a topological insulator,has stimulated extensive research on these materials.In 3D Dirac semimetals,the conduction and valence bands contact only at Dirac points in the Brillouin zone,and gap formation is forbidden by crystalline symmetry.Much recent excitement has surrounded the materials like Na 3Bi 2and Cd 3As 2,3in which the Dirac cone semimetal states were predicted theoretically and soon identi ?ed by Angle-resolved photoemission spectroscopy (ARPES)experiments.4,6,11Surprisingly,the superconductivity induced by hard point contact (PC)on Cd 3As 2was recently reported and unconventional superconducting order parameter was suggested by the zero-bias conductance peak (ZBCP)and double conductance peaks (DCPs)symmetric around zero bias.13,14In addition,pressure-induced superconductivity in Cd 3As 2was further con ?rmed by hydrostatic pressure experiment.15

Another promising material is the cubic Laves phase Au 2Pb,which reveals the signature for the symmetry-protected Dirac semimetal state at temperature 4100K.However,after the structural phase transition,the Dirac cone is gapped o 100K.16Here we report systematic studies of low-temperature transport and diamagnetic properties,PC measurements and ?rst-principles calculations of Au 2Pb single crystal.The superconductivity of Au 2Pb is characterised,and the anomalous quasi-linear H c2(T)in Au 2Pb suggests the presence of an unconventional super-conducting state.The T c

onset and H c2revealed by the PC measurement with a gold tip is consistent with that by the bulk transport and magnetisation measurements,but with a hard W tip,the gap is largely enhanced as shown by the increases of both T c

onset and H c2.By comparing the computed Fermi surfaces of Au 2Pb of relaxed lattice structure and those under 1%uniaxial compression,we ?nd that the Fermi surfaces are enlarged and the

density of states near the Fermi energy increases under the applied external pressure,which agrees with the observed increase of T c

onset in the PC measurements.More importantly,the possibility of topological superconductivity is suggested by ?rst-principles calculations,which show nontrivial topological properties of the projected pseudospin texture corresponding to the p -and d -orbitals near the Fermi energy,as well as the experimental observations of anomalous quasi-linear H c2(T)behaviour and unusual PCS feature detected by W tip.These characteristics make Au 2Pb a potential candidate material for topological superconductor.

RESULTS

Sample structure

High-quality single crystal of Au 2Pb samples were synthesised by self-?ux method in the evacuated quartz ampoule (see Materials and Methods).The powder X-ray diffraction (XRD)pattern is given in Figure 1a,indicating a crystal structure of Au 2Pb and exhibiting a cubic Laves phase with a 1=a 2=a 3=7.9603Åat room temperature.The single crystal XRD pattern only shows (111)re ?ections,suggesting the sample surface of the crystal is (111)plane (inset of Figure 1a).The atomic high-resolution transmission electron microscopy (HRTEM)image (Figure 1b),together with the selected area electron diffraction pattern (Figure 1c),further demonstrate the high-quality single crystal nature of Au 2Pb samples.

Superconductivity

The superconductivity in Au 2Pb has been con ?rmed by our systematic low-temperature transport and diamagnetic measurements.Figure 2a displays the sample resistivity (ρ)as a function of temperature at zero magnetic ?eld.The behaviour is

1

International Center for Quantum Materials,School of Physics,Peking University,Beijing,China;2Collaborative Innovation Center of Quantum Matter,Beijing,China;3Center of Electron Microscopy,State Key Laboratory of Silicon Materials,School of Materials Science and Engineering,Zhejiang University,Hangzhou,China and 4High Magnetic Field Laboratory,Chinese Academy of Sciences,Anhui,China.

Correspondence:J Feng (jfeng11@)or J Wei (weijian6791@)or J Wang (jianwangphysics@)5

These authors contributed equally to this work.

Received 26May 2016;revised 21June 2016;accepted 21June 2016

/npjquantmats

Published in partnership with Nanjing University

metallic with an anomaly of ρ(T )at 100K due to a structural phase transition (cubic Laves phase 4100K).16A steep step at 50K and hysteresis at 40K are observed with cooling and heating measurements,owing to structural transition to orthorhombic phase o 40K (the inset of Figure 2a).On cooling down to lower temperature,bulk T c

onset ~1.3K and zero resistance transition temperature T c zero

~1.18K are observed,comparable to the

previous reports.16,17

The superconductivity in Au 2Pb is further con ?rmed by Meissner effect measurements.In Figure 2b,the susceptibility (χ)versus temperature curves show quite sharp drops at 1.15K,in good agreement with T c

zero from ρ(T )curve.Magnetic ?eld dependence of the magnetisation (M (H ))curves at various temperatures (inset of Figure 2b)exhibit the expected quasi-linear behaviour at low ?elds but deviate from linearity above the lower critical ?eld H c1.Figure 2c reveals the suppression of the superconducting state of Au 2Pb by the magnetic ?eld perpendicular to (111)plane (H ?).The superconducting transition becomes broader and shifts to lower temperature with increasing ?elds.Magnetotransport measurements were carried out at various temperatures from 0.18to 1.4K (Figure 2d).The onse …… 此处隐藏：6165字，全部文档内容请下载后查看。喜欢就下载吧 ……