Microscopic source of quasiparticle loss in tantalum resonators

Tantalum (Ta) based superconducting circuits have been demonstrated to enable world-record qubit coherence times (T₂) and quality factors [1,2], motivating a careful study of the microscopic origin of the remaining losses that limit their performance. We have recently shown [2] that temperature-dependent loss measurements reveal that some devices display behavior consistent with a lower superconducting critical temperature (T_c) than can be predicted by direct film characterization. Specifically, their constituent films have a single crystal structure associated with the high-T_c BCC (α) phase of Ta, and dc resistivity measurements show a T_c of over 4 K, while some resonators fabricated from these films show quasiparticle losses consistent with a T_c as low as 0.4 K. In addition, scanning SQUID measurements did not reveal any evidence of inhomogeneity in these films at the micrometer scale. Here, we present a comparative study of the structural and thermodynamic properties of Ta-based resonators with high- and low-T_c characteristics via X-ray diffraction and resistivity measurements. X-ray diffraction shows a clear correlation between the α-Ta (222) peak position and the apparent T_c obtained from microwave loss measurements. The α-Ta (222) peak associated with high-T_c resonators is consistently shifted to higher angles by ~0.6° compared to the low-apparent T_c films. Furthermore, resistivity as a function of temperature under applied magnetic field reveals that the magnetic field required to completely suppress the superconductivity is around twice as large in high-T_c films compared to low-apparent T_c ones. These results provide material proxies for optimizing Ta device quality, and demonstrate that detailed materials measurements can provide proxies for device performance.



[1] Nature Communications, 12(1), 1779 (2021)

[2] Physical Review X 13, 041005 (2023)