Single-gap superconductivity in β-Bi2Pd
The β-Bi2Pd 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 β-Bi2Pd with the critical temperature Tc 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).].
Figure 1: Total heat capacity of the sample plus addenda in superconducting (0 T) and normal (1 T) states. The upper inset: the difference in entropy (purple line, right axis applies) and the thermodynamic critical field (black line, left axis). The lower inset: electronic specific heat from experimental data and theoretical curve from α model with the coupling ratio 2Δ/kBTc = 4.1.
Figure 2: Temperature dependence of the upper critical field for H in the c direction (closed symbols) and parallel to the ab planes (open symbols). Lines are fit to the WHH model. The lower inset: angular dependence of Tc2 measured at a fixed field of 0.1 T (symbols) and the theoretical curve from the Ginzburg-Landau model (line). The upper inset: deviation function of our measurements (red symbols) compared to those of Welp et al. (open circles) for a two gap superconductor,(Ba,K)Fe2As2.
Figure 3: The phase diagram of β-Bi2Pd showing Hcc1 (blue squares, the data are multiplied by a factor of 10 to be in scale) with the best fit (blue line) corresponding to 2Δ/kBTc = 4.1; Hc2 from heat capacity (circles) and resistivity (lozenges) for both H||c (filled symbols) and H||ab (open symbols) together with the WHH theoretical curves (gray full lines) as well as a Hc3 line (gray dashed line). (b) Local magnetic induction B as a function of applied field H measured by a Hall probe at selected temperatures with the field first increasing and then decreasing back to zero. (c) Superconducting transitions for H ⊥ ab revealed by the heat capacity (left axis) and resistivity measurements (right axis) at magnetic fields 0, 0.2, and 0.4 T, respectively.