Evidence of double-gap superconductivity in noncentrosymmetric ${\mathrm{Nb}}_{0.18}{\mathrm{Re}}_{0.82}$ single crystals

We combine point contact spectroscopy with specific heat measurements to probe the superconducting state in noncentrosymmetric ${\mathrm{Nb}}_{0.18}{\mathrm{Re}}_{0.82}$ single crystals. The conductance spectra clearly exhibit a two-peak structure that is well reproduced within a two-band model with isotropic gaps in the spectrum. Such an observation is confirmed by distinct features of the specific heat both at low temperatures and in the range approaching the transition to the normal state. The analyses provide convincing evidence that the two-gap superconducting pairing is a robust feature of ${\mathrm{Nb}}_{0.18}{\mathrm{Re}}_{0.82}$.

Figure 1

X-ray Laue pattern of a ${\mathrm{Nb}}_{0.18}{\mathrm{Re}}_{0.82}$ crystal oriented with one of the principal axes perpendicular to the exposed surface.

Figure 2

Field emission scanning electron microscope image of (a) the ${\mathrm{Nb}}_{0.18}{\mathrm{Re}}_{0.82}$ single crystal ($ab$ plane) with Au wire and silver paste drop and (b) the Ag particles on the measured ${\mathrm{Nb}}_{0.18}{\mathrm{Re}}_{0.82}$ surface after removing the gold wire.

Figure 3

(a) Temperature dependence of the conductance spectra measured from 4.2 to 8.0 K. For clarity, the data are plotted with a constant offset along the linear vertical scale. The symbols are the experimental data and the solid lines are the fits to the data using the isotropic two-gap model as discussed in the main text. The arrows schematically indicate the positions of the dominant peaks in the spectrum at voltage biases $V_A$ and $V_B$. (b) Temperature dependence of the gaps ${\Delta }_A$ and ${\Delta }_B$ as extracted from the fits to the conductance spectra. (c) Conductance spectrum measured at $T=300$ mK at another position of the ${\mathrm{Nb}}_{0.18}{\mathrm{Re}}_{0.82}$ surface with respect to that in (a). This spectrum also exhibits the double gap structures at voltage biases $V_A$ and $V_B$.

Figure 4

Conductance spectra measured at $T=4.2$ K for different positions on sample 1 (i.e., A and B) and sample 2 of the ${\mathrm{Nb}}_{0.18}{\mathrm{Re}}_{0.82}$ surface. For clarity, the data are plotted with a constant offset along the linear vertical scale. The symbols are the experimental data and the arrows schematically indicate the positions of the two peaks in the spectrum at voltage biases $V_A$ and $V_B$.

Figure 5

Temperature dependence of the electronic contribution to the specific heat of a single crystal of ${\mathrm{Nb}}_{0.18}{\mathrm{Re}}_{0.82}$ in zero field. The solid line shows a fit to a two-band model, described in the text. The inset shows $C/T$ against $T^2$, in an applied field of 9 T. The dashed line shows a fit to the data above the transition as described in the text.

Figure 6

The top panel shows the derivative of the superconducting contribution of the specific heat [Eq. (3)] with respect to the reduced temperature $t=T/T_c$, for several different values of $\alpha =\Delta /k_{\mathrm{B}}{T}_c$. The bottom panel shows the derivative of the electronic contribution to the specific heat from Fig. 5. The solid line denotes the derivative of the two-band fit in Fig. 5, while the dashed and dotted lines show the derivative of the single band fit with $\alpha =1.98$ and 2.05, respectively.