Superconducting gap of the noncentrosymmetric superconductor α-BiPd

In most known superconductors, inversion symmetry and Pauli exclusion ensure that the Cooper pair wave function can be separated into an orbital component that has either even or odd parity, and a spin component which is then either singlet or triplet, but this does not hold in general. In the absence of an inversion center in the crystal structure, parity is not a good quantum number, and the Cooper pairs will be some mixture of singlet and triplet. We report ultra-low temperature scanning tunneling microscopy/spectroscopy (ULT-STM/STS) measurements at temperatures down to 15 mK on the recently rediscovered non-centrosymmetric superconductor α-BiPd. Tunneling spectra show only a single superconducting gap with Δ(0)= 0.6 meV, the temperature and magnetic field dependence of which are found to be well described by BCS theory and a single s-wave gap. Our results provide an upper limit for a possible triplet component in α-BiPd of 10 μ eV.