Tuning the Parity Mixing of Singlet-Septet Pairing in a Half-Heusler Superconductor

K Ishihara,Y Miao,R Masuki,T Shibauchi,R Arita,T Nomoto,O Pavlosiuk,M Hirayama,T Takenaka,Y Mizukami,M Konczykowski,P Wiśniewski,D Kaczorowski,M Yamashita,K Hashimoto

doi:10.1103/physrevx.11.041048

Abstract

In superconductors, electrons with spin ${s=1/2}$ form Cooper pairs whose spin structure is usually singlet (${S=0}$) or triplet (${S=1}$). When the electronic structure near the Fermi level is characterized by fermions with angular momentum ${j=3/2}$ due to strong spin-orbit interactions, novel pairing states such as even-parity quintet (${J=2}$) and odd-parity septet (${J=3}$) states become allowed. Prime candidates for such exotic states are half-Heusler superconductors, which exhibit unconventional superconducting properties, but their pairing nature remains unsettled. Here we show that the superconductivity in the noncentrosymmetric half-Heusler LuPdBi can be consistently described by the admixture of isotropic even-parity singlet and anisotropic odd-parity septet pairing, whose ratio can be tuned by electron irradiation. From magnetotransport and penetration depth measurements, we find that carrier concentrations and impurity scattering both increase with irradiation, resulting in a nonmonotonic change of the superconducting gap structure. Our findings shed new light on our fundamental understanding of unconventional superconducting states in topological materials.

Highlights

Half-Heusler materials, RPtBi and RPdBi, where R is a rare-earth element, are of notable interest as they can provide a new platform for topological phenomena [1,2,3]
We find from magnetic penetration depth measurements that the gap structure in LuPdBi changes from nodeless to nodal, and to nodeless again, by increasing the irradiation dose, which can be semiquantitatively explained by the singlet-septet pairing state, taking into account the above two effects
Our ab initio calculations of the electronic structure reveal that the Γ8 band formed by j 1⁄4 3=2 fermions dominates near the Fermi energy EF [Fig. 1(b)], which confirms that LuPdBi gives a platform of the j 1⁄4 3=2 superconductivity

Summary

Introduction

Half-Heusler materials, RPtBi and RPdBi, where R is a rare-earth element, are of notable interest as they can provide a new platform for topological phenomena [1,2,3]. Since the crystal structure of half-Heusler materials has no inversion center [Fig. 1(a)], they are classified as noncentrosymmetric superconductors having spin-split Fermi surfaces (FSs) In such superconductors with no inversion center, even-parity and odd-parity pairing states are allowed to admix [13,14,15], which has been experimentally shown by the contrasting superconducting gap structures in Li2Pd3B and Li2Pt3B with different SOI [16,17]. While experimental studies on YPtBi have reported spin-split FSs and a superconducting gap structure with line nodes [12,20], 2160-3308=21=11(4)=041048(12)

Methods

Results

Conclusion