Vortex Motion Induced Losses in Tantalum Resonators

Tantalum (Ta) based superconducting circuits have been demonstrated to enable record qubit coherence times (T₂) and quality factors (Q) [1, 2], motivating a careful study of the microscopic origin of the remaining losses that limit their performance. We have recently shown [2] that the losses in Ta-based resonators are dominated by several types of surface and bulk saturable two-level systems (TLSs). We also observe that some devices exhibit loss that is exponentially activated at low temperature, even though they are fabricated from films that have a single crystal structure associated with the high-T_𝑐 BCC (𝛼) phase of Ta. Specifically, dc resistivity measurements show a superconducting critical temperature (T_𝑐) of over 4 K, while resonators fabricated from these films show losses that increase exponentially with temperature with an activation energy as low as 0.3 K. Here, we present a comparative study of the structural and thermodynamic properties of Ta-based resonators and identify free vortex motion loss as the source of thermally activated microwave loss. Through careful magnetoresistance and X-ray diffraction measurements, we observe that the increased loss occurs for films that are in the clean limit, where the superconducting coherence length (𝜉) is shorter than the mean free path (𝑙). Vortex-motion-induced losses are suppressed for films in the dirty limit, which show evidence of structural defects that can pin vortices. We verify this hypothesis by explicitly pinning vortices via patterning and find that we can suppress the loss by microfabrication.



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

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