Dirac Surface States and Nature of Superconductivity in Noncentrosymmetric BiPd

TL;DR

This study investigates the electronic and superconducting properties of noncentrosymmetric BiPd, revealing Dirac surface states with helical spin polarization and characterizing its superconductivity as topologically trivial with an isotropic s-wave gap.

Contribution

It combines macroscopic, atomic-scale, and computational methods to analyze the surface states and superconducting nature of BiPd, providing new insights into noncentrosymmetric superconductors.

Findings

Dirac-cone surface states with helical spin polarization in BiPd

Superconductivity in BiPd is topologically trivial with an isotropic s-wave gap

First-principles calculations support experimental observations

Abstract

In non-magnetic bulk materials, inversion symmetry protects the spin degeneracy. If the bulk crystal structure lacks a centre of inversion, however, spin-orbit interactions lift the spin degeneracy, leading to a Rashba metal whose Fermi surfaces exhibit an intricate spin texture. In superconducting Rashba metals a pairing wavefunction constructed from these complex spin structures will generally contain both singlet and triplet character. Here we examine the possible triplet components of the order parameter in noncentrosymmetric BiPd, combining for the first time in a noncentrosymmetric superconductor macroscopic characterization, atomic-scale ultra-low-temperature scanning tunnelling spectroscopy, and relativistic first-principles calculations. While the superconducting state of BiPd appears topologically trivial, consistent with Bardeen-Cooper-Schrieffer theory with an order parameter governed by a single isotropic s-wave gap, we show that the material exhibits Dirac-cone surface states with a helical spin polarization.