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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>L</mml:mi> <mml:mi>k</mml:mi> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> . Specifically, the change in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>L</mml:mi> <mml:mi>k</mml:mi> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> with increasing current reached 70% in the NbN/Mo bilayer at liquid helium temperature, whereas in the NbN strip, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>L</mml:mi> <mml:mi>k</mml:mi> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>L</mml:mi> <mml:mi>k</mml:mi> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>ρ</mml:mi> <mml:mi>S</mml:mi> </mml:msub> </mml:mrow> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mrow> <mml:msub> <mml:mi>ρ</mml:mi> <mml:mi>N</mml:mi> </mml:msub> </mml:mrow> <mml:mo>≫</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:math> , it can be concluded that depositing a thin layer of a relatively low-resistivity metal onto a superconductor with high <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>ρ</mml:mi> </mml:mrow> </mml:math> is a practical method for achieving a large nonlinearity of the superconductor’s kinetic inductance.