Practical way to increase nonlinearity of kinetic inductance of superconductor

Abstract: This work demonstrates that depositing a thin layer of Mo (5-15 nm) onto a 10 nm thick NbN strip leads to a significant increase in the nonlinearity of the kinetic inductance $L_k$. Specifically, the change in $L_k$ with increasing current reached 70% in the NbN/Mo bilayer at liquid helium temperature, whereas in the NbN strip, $L_k$ changed by only 10% in the superconducting state. In addition to altering the nonlinear properties, the Mo layer caused a significant increase in the critical current at low temperatures (up to 2 times in the case of a 5 nm thick Mo layer). The increased nonlinearity of $L_k$ can be explained by two factors: i) a reduction of the critical supervelocity at which the superconducting state becomes unstable with respect to vortex formation when a Mo layer is deposited on NbN, and ii) a higher sensitivity of the induced superconductivity in Mo to supervelocity/supercurrent. Considering the results on the transport properties of SN bilayers with a high ratio of layer resistivities $\rho_S/\rho_N >> 1$, it can be concluded that depositing a thin layer of a relatively low-resistivity metal onto a superconductor with high $\rho$ is a practical method for achieving a large nonlinearity of the superconductor's kinetic inductance.