Practical way to increase nonlinearity of kinetic inductance of superconductor

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 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 ρ S / ρ N ≫ 1 , it can be concluded that depositing a thin layer of a relatively low-resistivity metal onto a superconductor with high ρ is a practical method for achieving a large nonlinearity of the superconductor’s kinetic inductance.