Point-contact Andreev reflection spectroscopy on the misfit phase superconductors ${\left(\mathrm{PbSe}\right)}_{1.12}\left({\mathrm{TaSe}}_2\right)$ doped with Cl or Br

We report point-contact Andreev reflection spectroscopy studies on possible topological superconductor candidates Cl- (Br-) doped (${\mathrm{PbSe})}_{1.12}\left({\mathrm{TaSe}}_2\right)$ with superconducting transition temperature $T_c\sim$ 1.2 K. A common double-peak feature is observed for the conductance curves at low temperature in the absence of a zero-bias conductance peak. We analyze conductance curves with the Blonder-Tinkham-Klapwijk model and the extracted superconducting gap follows a typical Bardeen-Cooper-Schrieffer temperature- and field-dependent behavior, with $2{\mathrm{\Delta }}_0/k_B{T}_c\approx 3.7$ and 3.8 for Cl and Br doping, respectively. Our results strongly suggest that both Cl- and Br-doped $({\mathrm{PbSe})}_{1.12}\left({\mathrm{TaSe}}_2\right)$ are fully gapped superconductors in an intermediate-coupling regime.

Figure 1

(a) Schematic illustration of the crystal structure for the misfit phase compound $\mathrm{(}{\mathrm{PbSe})}_{1.12}{\mathrm{TaSe}}_2$ adapted from Ref. [41]; (b) Single crystal x-ray diffraction of PSTS-Cl and PSTS-Br in the [001] direction, suggesting an effective doping of Cl and Br into $\mathrm{(}{\mathrm{PbSe})}_{1.12}{\mathrm{TaSe}}_2$; [(c) and (e)] the temperature dependence of point-contact zero-bias conductance $G_0(T$) in comparison with the electrical resistivity for PSTS-Cl and PSTS-Br, respectively. [(d) and (f)] Representative point-contact conductance curves $G\left(V\right)$ at 0.3 K for PSTS-Cl and PSTS-Br, respectively. The curves are shifted vertically for clarity and solid lines are the optimal BTK fitting curves.

Figure 2

(a) Temperature evolution of soft point-contact conductance curves $G\left(V\right)$ of PSTS-Cl (open symbols) in comparison with their optimal BTK fitting curves with $Z=1.3$ (solid lines). (b) The superconducting gap as a function of temperature $\mathrm{\Delta }\left(T\right)$ (half-filled circles) follows the BCS behavior (solid line) while the ratio between the smearing parameter $\mathrm{\Gamma }$ and superconducting gap $\mathrm{\Delta }$, $\mathrm{\Gamma }/\mathrm{\Delta }$ increases dramatically close to $T_c$ (blue squares).

Figure 3

(a) Magnetic field evolution of soft point-contact conductance $G\left(V\right)$ of PSTS-Cl (open symbols) at 0.3 K with the field in the [001] direction. The curves are shifted for clarity and compared with their optimal BTK fitting curves (solid lines). (b) The extracted superconducting gap as a function of field $\mathrm{\Delta }\left(H\right)$ (half-filled circles) is consistent with the theoretical predicted behavior (solid line), while the smearing parameter $\mathrm{\Gamma }$ (blue stars) increases as a function of magnetic field.

Figure 4

[(a) and (b)] Temperature and magnetic field evolution of one representative set of PCARS data on PSTS-Br (open symbols) in comparison with their optimal BTK fitting curves with $Z=0.43$ (solid lines). The conductance curves are shifted for clarity and the magnetic filed is applied in the [001] direction at 0.3 K. [(c) and (d)] The extracted superconducting gap $\mathrm{\Delta }$ from BTK model as a function of temperature and magnetic field, respectively. The gap $\mathrm{\Delta }$ follows the theoretical predicted behaviors in both cases (solid lines).