Single-gap superconductivity in β − B i 2 Pd

The $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{B}{\mathrm{i}}_{2}\mathrm{Pd}$ compound has been proposed as another example of a multigap superconductor [Imai et al., J. Phys. Soc. Jpn. 81, 113708 (2012)]. Here, we report on measurements of several important physical quantities capable of showing a presence of multiple energy gaps on our superconducting single crystals of $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{B}{\mathrm{i}}_{2}\mathrm{Pd}$ with the critical temperature ${T}_{c}$ close to 5 K. The calorimetric study via a sensitive ac technique shows a sharp anomaly at the superconducting transition, however only a single energy gap is detected. Also other characteristics inferred from calorimetric measurements as the field dependence of the Sommerfeld coefficient and the temperature and angular dependence of the upper critical magnetic field point unequivocally to standard single $s$-wave gap superconductivity. The Hall-probe magnetometry provides the same result from the analysis of the temperature dependence of the lower critical field. A single-gapped BCS density of states is detected by the scanning tunneling spectroscopy measurements. Then, the bulk as well as the surface sensitive probes evidence a standard conventional superconductivity in this system where the topologically protected surface states have been recently detected by angle-resolved photoemission spectroscopy [Sakano et al., Nat. Commun. 6, 8595 (2015).].