Kinetic Inductance and nonlinearity of MgB2 Films at 4K

TL;DR

This study fabricates and characterizes MgB₂ thin films at 4K, demonstrating their potential for non-linear kinetic inductance applications like quantum amplifiers, with promising current density and non-linearity metrics.

Contribution

It provides detailed measurements of MgB₂'s kinetic inductance and non-linearity at 4K, highlighting its suitability for quantum device applications and comparing favorably to other superconductors.

Findings

Measured kinetic inductance of ~5.5 pH/□ at 4.2 K

Achieved internal quality factors of ~3×10^4

Found high non-linear current density comparable to NbTiN and TiN

Abstract

We report on the fabrication and characterization of superconducting magnesium diboride (MgB$_2$) thin films intended for quantum-limited devices based non-linear kinetic inductance (NLKI) such as parametric amplifiers with either elevated operating temperatures or expanded frequency ranges. In order to characterize the MgB$_2$ material properties, we have fabricated coplanar waveguide (CPW) transmission lines and microwave resonators using $\approx$ 40 nm thick MgB$_2$ films with a measured kinetic inductance of $\sim$ 5.5 pH/$\square$ and internal quality factors, $Q_i \approx 3 \times 10^4$ at 4.2 K. We measure the NLKI in MgB$_2$ by applying a DC bias to a 6 cm long by 4 $\mu$m wide CPW transmission line, and measuring the resulting phase delay caused by the current dependent NLKI. We also measure the current dependent NLKI through CPW resonators that shift down in frequency with increased power applied through the CPW feedline. Using these measurements, we calculate the characteristic non-linear current parameter, $I_*$, for multiple CPW geometries. We find values for corresponding current density, $J_* = 12-22$~MA/cm$^2$ and a ratio of the critical current to the non-linear current parameter, $I_C/I_* = 0.14-0.26$, similar to or higher than values for other superconductors such as NbTiN and TiN.