Quasiparticle spectroscopy in tantalum films with different Ta/sapphire interfaces

Abstract

One of the crucial aspects of current research in quantum information science is the identification and control of loss mechanisms in superconducting circuits. Although microwave measurements directly quantify device performance, additional techniques that probe quasiparticle excitations in superconducting films are needed to understand the microscopic mechanisms underlying dissipation and decoherence. Here, we present results from quasiparticle spectroscopy of Ta/sapphire films by measuring the Meissner-state magnetic susceptibility using a precision frequency-domain resonator specifically designed for thin films. We find direct evidence for additional low-energy excitations in samples with lower internal quality factors. These excitations are consistent with deep subgap states due to two-level systems, Yu-Shiba-Rusinov states near the gap edge, and perhaps other pair-breaking mechanisms. The developed non-destructive frequency-domain quasiparticle spectroscopy is a valuable addition to the quantum materials toolbox.

Figures (5)

• Figure 1: (a) RRR and $Q_{i}$ for three different types of samples. (b) Corresponding surface oxide thickness and $T_{c}$. The data are taken from Table II of Ref. lwn1-fznb.
• Figure 2: Cross-sectional scanning transmission electron microscopy (STEM) images of Samples A(a-c), B(d-f), and C(g-i). (a) Low mag HAADF-STEM images showing overall thin film on substrate structure and (b-c) atomic resolution HAADF-STEM images of surface and interface regions of Sample A. (d) Low mag HAADF-STEM images showing overall thin film on substrate structure and (e-f) atomic resolution HAADF-STEM images of surface and interface regions of Sample B. (g) Low mag HAADF-STEM images showing overall thin film on substrate structure and (h-i) atomic resolution HAADF-STEM images of surface and interface regions of Sample C.
• Figure 3: (a) Change in the resonant frequency over the full temperature range in two tantalum samples each of A, B and C types. Superconducting transition temperatures are similar and consistent with transport measurements, where sample B has the highest and sample C has the lowest $T_{c}$. (b) Magnetic susceptibility obtained from the data in panel (a) as a function of normalized temperature, $t=T/T_{c}$.
• Figure 4: (a) Low-temperature magnetic susceptibility in a restricted vertical scale highlighting deviations from a clean activated response. (b) Further zoom emphasizing the low-$T$ behavior used for power-law fitting. Compare the vertical scale with Fig. \ref{['fig:chi']}.
• Figure 5: Correlation between low-temperature susceptibility behavior and device performance. Left axis: power-law exponent $n$ from low-$T$ ($t\le0.33$) fits. Right axis: inverse susceptibility change $\Delta\chi^{-1}$ in the range $t\le 0.3$. The horizontal axis shows the internal quality factor $Q_i$.