Investigation of the two-gap superconductivity in a few-layer ${\mathrm{NbSe}}_2$-graphene heterojunction

We investigated the superconductivity in a few-layer ${\mathrm{NbSe}}_2$-graphene heterojunction by differential conductance spectroscopy. Because of the gate-tunable Fermi level of the few-layer graphene, used here as a tunneling electrode in a nano-point-contact spectroscopy setup, the differential conductance of the heterojunction showed highly sensitive dependence on the gate voltage, which allowed us to probe the nature of the superconducting gap functions with unprecedented detail by continuously tuning the transparency of the junction between the spectroscopic tunneling and the Andreev reflection limits. Characteristic features associated with a two-gap superconductivity in ${\mathrm{NbSe}}_2$ were reproducibly observed in both limits and between, e.g., in the form of a central conductance dip with two sets of coherence peaks when the Fermi level was close to the charge neutrality point of graphene. From fits with the Blonder-Tinkham-Klapwijk model, two gaps with their temperature dependence were extracted. The two gaps associated with the two-band superconductivity in ${\mathrm{NbSe}}_2$ followed the expected temperature behavior in the limit of weak interband scattering, with a gap to $T_c$ ratio suggesting a weak to moderately strong coupling in few-layer systems.

Figure 1

Structure of the ${\mathrm{NbSe}}_2$-graphene heterojunction. (a) Optical images of the heterostructure before top BN capping. (b) Sketch of the device structure and the measurement setup for differential conductance measurement. Scale bar: 3 $\mu \mathrm{m}$.

Figure 2

Gate dependency of the differential conductance of the ${\mathrm{NbSe}}_2$-graphene junction. (a) Differential conductance of the junction at various gate voltages at 2 K. (b) Mapping of the normalized differential conductance versus gate voltage and bias voltage.

Figure 3

Temperature dependence and BTK fitting of the normalized differential conductance of the graphene-${\mathrm{NbSe}}_2$ junction. Gate voltage: (a) $-4.0\text{V}$, (b) $-4.2\text{V}$. The lines are the fitting curves, and the dots are the experimental raw data at various temperatures normalized by the data at 5 K.

Figure 4

Temperature dependence of the superconducting gap values extracted from BTK fits. Open symbols correspond to the larger gap, and solid symbols correspond to the smaller gap. The triangles and squares denote different gate voltages, respectively, as indicated in the legend. The two solid lines are fits of the two gaps using two-band BCS theory with small interband scattering. The ratio of interband scattering to intraband interaction is $7\times {10}^{-3}$.