Resistance and current-voltage characteristics of individual superconducting $\mathrm{Nb}{\mathrm{Se}}_2$ nanowires

Resistance and current-voltage characteristics of individual superconducting $\mathrm{Nb}{\mathrm{Se}}_2$ nanowires are investigated. In the current-voltage curves, a stairlike structure is observed, indicating the possible formation of phase-slip centers. A close examination of the current-voltage characteristic in a selected high quality $\mathrm{Nb}{\mathrm{Se}}_2$ nanowire with a diameter of $75\mathrm{nm}$ reveals that the characteristic voltages in the stairlike structure follow the BCS-like temperature dependence of superconducting gaps vanishing at $T_C$. While the phase-slip center mechanism remains to be a plausible explanation of the observed features, an alternative model involving multigap Josephson tunneling is proposed to account for the BCS-like temperature dependence. From the BCS fits, two distinct superconducting gaps are extracted. Moreover, the critical current of the $75\mathrm{nm}$ nanowire at low temperatures as well as near $T_C$ can also be described by the Ambegaokar-Baratoff relation for multigap Josephson junctions. Our data suggest the possible observation of multiband superconductivity in $\mathrm{Nb}{\mathrm{Se}}_2$ and are in good agreement with the predictions of recent band structure and Fermi surface calculations on $\mathrm{Nb}{\mathrm{Se}}_2$.

Figure 1

(a) Resistance versus temperature for a superconducting $\mathrm{Nb}{\mathrm{Se}}_2$ NW with a diameter of $\sim 75\mathrm{nm}$, applying a current of $200\mathrm{nA}$. (b) $R$ and $dR∕dT$ versus $T$ of the $\mathrm{Nb}{\mathrm{Se}}_2$ NW device for a broader temperature range. Inset: A SEM image of the $\mathrm{Nb}{\mathrm{Se}}_2$ NW device.

Figure 2

(a) Voltage-current $\left(V\text{−}I\right)$ curves of the 4P $\mathrm{Nb}{\mathrm{Se}}_2$ NW device measured in zero applied magnetic field for $T<T_C$. (b) Temperature dependence of the reduced critical current.

Figure 3

(a) Resistance versus temperature for an underconverted superconducting $\mathrm{Nb}{\mathrm{Se}}_2$ NW with a diameter of $\sim 40\mathrm{nm}$, applying a current of $10\mathrm{nA}$. (b) Voltage-current $\left(V\text{−}I\right)$ curves and $dV∕dI$ of the $\mathrm{Nb}{\mathrm{Se}}_2$ NW device in (a) measured in zero applied magnetic field at $T=1.9\mathrm{K}$. (c) $R$ versus $T$ for two high quality $\mathrm{Nb}{\mathrm{Se}}_2$ NWs with diameters of 250 and $300\mathrm{nm}$, respectively. (d) Voltage-current $\left(V\text{−}I\right)$ curves of the two $\mathrm{Nb}{\mathrm{Se}}_2$ NW devices in (c); the 250 and $300\mathrm{nm}$ NWs are measured in zero applied magnetic field and at 1.9 and $2\mathrm{K}$, respectively.

Figure 4

(a) Current-voltage $\left(I\text{−}V\right)$ of the $\mathrm{Nb}{\mathrm{Se}}_2$ NW device. The arrows on each curve indicate, from left to right, voltages corresponding to twice the lower energy gaps $\left(2{\Delta }_1\right)$, the sum of the two different gaps $({\Delta }_1+{\Delta }_2)$, and twice the high energy gap $\left(2{\Delta }_2\right)$. (b) Temperature variations of $2{\Delta }_2$, and ${\Delta }_1+{\Delta }_2$ Solid lines are the fits to the BCS theory.