Optimization of Nb Superconducting Films for Quantum Devices by H-T Phase Boundary Analysis*

A critical area for the improvement of quantum devices is the materials optimization of superconducting films used in the construction of qubit elements and resonators, where careful considerations of purity and morphology must be made to reduce the effects of TLSs. In materials such as Nb, this involves simultaneously refining both the interfacial properties of the film as well as the superconductivity of the bulk. 

In this talk I discuss our current approach to assessing materials properties of superconducting films through careful transport studies of the superconducting H-T phase boundary. Our method utilizes a resistance feedback technique to continuously map T_c under applied field and allows for a rapid mapping of the phase diagram and extraction of the coherence length ξ₀ in the low field G.L. limit, which in certain cases can be a better indicator of film quality than T_c alone. In addition, non-linearity of the low field limit and transition width broadening are readily observed with high fidelity, opening up additional metrics for evaluation. 

Through comparison of Nb films produced under various growth conditions, we demonstrate the value of our approach in providing detailed assessment of the properties of Nb films of various purities and morphologies.