Kinetic inductance in superconducting CoSi₂ coplanar microwave transmission lines

We have looked into cobalt disilicide (CoSi₂) as a potential building block for superconducting quantum circuits. In order to achieve this, we annealed a thin layer of Co to create microwave cavities with thickness of d = 10-105 nm from CoSi₂ embedded in the silicon substrate. The cavity properties were measured as a function of temperature and power. In the films measuring 10 and 25 nm, we find a significant kinetic inductance L_K with a non-BCS power-law variation δL_K ∝ T^4.3±0.2 at low temperatures. The quality factor of the studied microwave resonances varied from 3 × 10³ (d = 10 nm) to ∼5 × 10⁴ (d = 105 nm) and increased as d(A − log d) with thickness, with two-level systems having very little effect. The power dependence of kinetic inductance was analyzed in terms of heat flow due to electron-phonon coupling, which was found to be stronger than estimated for heat relaxation by regular quasiparticles.