Superconducting proximity effect and the Fermi velocity in the surface-state of SmB$_{\mathrm{6}}$ thin films

SmB$_{\mathrm{6}}$ recently has been predicted to be topological Kondo insulator. Here, we investigate the Fermi velocity ($v_{F})$ of SmB$_{\mathrm{6}}$ using transport measurements and a study on the superconducting proximity effect, independently. In the transport measurement, SmB$_{\mathrm{6}}$ thin films show thickness-independent transport characteristics at low temperatures, which is a strong evidence for the presence of the surface conducting channel as well as the insulting bulk state as the nature of Kondo insulator. We estimate the thickness of the surface-state to be $\approx $ 7 nm and the $v_{F}$ to be \textasciitilde 10$^{\mathrm{5}}$ m/s. In order to carry out the proximity effect investigation, we fabricated superconducting Nb/SmB$_{\mathrm{6}}$ bilayers \textit{in-situ. }We performed Usadel fitting to the variation of critical temperatures of the Nb layers due to the proximity effect. Interestingly, only the fitting regarding a 2D surface model yielded the consistent value of the $v_{F}$ with the value obtained from the transport measurement as well as the reported value from the quantum oscillation measurement. These results indicate that SmB$_{\mathrm{6}}$ has a true 2D surface-channel responsible for the observed proximity effect.