Absence of Andreev bound states in $\beta \text{−}{\mathrm{PdBi}}_2$ probed by point-contact Andreev reflection spectroscopy

We report point-contact spectroscopic studies on high quality single crystalline $\beta \text{−}{\mathrm{PdBi}}_2$ with a superconducting $T_c=5.2\mathrm{K}$. Our measurements show a typical double-peak structure in the voltage-biased junction conductance at low temperatures, probably excluding the existence of topological Andreev bound states. The conductance curves can be well fitted with the standard Blonder-Tinkham-Klapwijk model for $s$-wave superconductors, yielding a superconducting gap ${\mathrm{\Delta }}_0\simeq 0.92\mathrm{meV}$ with $2{\mathrm{\Delta }}_0/k_B{T}_c\sim 4.1$. Our results strongly suggest that $\beta \text{−}{\mathrm{PdBi}}_2$ is a conventional intermediate-coupling $s$-wave superconductor.

Figure 1

(a) The electrical resistivity of a $\beta \text{−}{\mathrm{PdBi}}_2$ single crystal, showing a superconducting transition temperature of 5.2 K, while the single crystal x-ray diffraction pattern at room temperature is shown in the inset. (b) The temperature dependence of zero-bias conductance of a point contact on a single crystal of $\beta \text{−}{\mathrm{PdBi}}_2,G_0\left(T\right)$, showing a superconducting transition temperature consistent with resistivity measurements. (c) The magnetic susceptibility of a $\beta \text{−}{\mathrm{PdBi}}_2$ single crystal. Squares and solid circles represent zero-field cooling (ZFC) and field cooling (FC), respectively, in a magnetic field of 10 Oe.

Figure 2

(a) Point-contact spectra of $\beta \text{−}{\mathrm{PdBi}}_2$ for different contacts at 1.8 K with the typical double-peak structure. The conductance curves are shifted for clarity. (b) The temperature evolution of $G\left(V\right)$ for one set of point-contact spectra; (c) zero-bias conductance peaks observed at 1.8 K, which are probably due to the contact heating effect.

Figure 3

(a) Temperature dependence of the normalized conductance curves for one set of Ag-paint/$\beta \text{−}{\mathrm{PdBi}}_2$ point-contact spectra (dots) with BTK optimal fittings included for comparison (solid lines). (b) Two sets of BTK-fitted superconducting gaps $\mathrm{\Delta }$ in $\beta \text{−}{\mathrm{PdBi}}_2$ as a function of temperature, which follow the standard BCS curve (solid line), yielding $2{\mathrm{\Delta }}_0/k_B{T}_c\sim 4.10$.

Figure 4

(a) Point-contact spectra of $\beta \text{−}{\mathrm{PdBi}}_2$ at 1.8 K in different magnetic fields, where the conductance curves are shifted for clarity and the superconductivity is completely suppressed in a field of 0.6 T. (b) BTK-fitted superconducting gap of $\beta \text{−}{\mathrm{PdBi}}_2$ at 1.8 K under different magnetic fields in comparison with the theoretical predictions of a BCS superconductor in an applied field (solid line).