Superconducting Gap Engineering in Tantalum-Alloy-Based Resonators

Utilizing tantalum (Ta) in superconducting circuits has led to significant improvements, such as high qubit lifetime (T1) and quality factors in both qubits and 

resonators, suggesting that material optimization plays an important role in the development of superconducting circuits. Thus we here explore 

superconducting gap engineering in Ta-based devices as a powerful strategy for expanding the range of suitable host materials. By alloying 20 atomic 

percent (at.%) Hf into Ta thin films, we achieve a superconducting transition temperature (Tc) of 6.09 K as observed in DC transport measurements, 

reflecting an increase in the superconducting gap. We systematically vary deposition conditions to control film orientation and transport properties of 

Ta-Hf alloy thin films. We then confirm the enhancement in Tc via microwave measurements at millikelvin temperatures. We verify the ∼40% increase in 

Tc relative to bare Ta devices, while the loss contributions from two-level systems (TLSs) and quasi-particles (QPs) remain unchanged in the low temperature 

regime. These findings emphasize the promise of material engineering in superconducting circuits and point to many potential material candidates for 

further exploration.

Link to the article is here.