Magnetic flux distribution, quasiparticle spectroscopy, and quality factors in Nb films for superconducting qubits

Abstract

Niobium is a practical material platform for superconducting microwave circuits; however, device-level performance can vary significantly depending on film growth and processing conditions. We compare three epitaxial Nb films grown on $c-$plane sapphire substrates under nominally identical conditions, except for the deposition temperature. To correlate internal quality factors, $Q_{\mathrm {i}}$, with material properties, we combine magneto-optical imaging of magnetic flux distribution with quasiparticle spectroscopy via measurements of the London penetration depth, $λ(T)$. In the low-$Q_{\mathrm i}$ film, there is a lesser ability to screen the magnetic field and an irregular temperature variation of $λ(T)$, implying the existence of localized in-gap states. High $Q_{\mathrm i}$ films show the opposite trend. We conclude that our measurements provide an efficient method for characterizing and optimizing superconducting films for quantum informatics applications.

Figures (5)

• Figure 1: Nb films studied in this work. Top panel shows superconducting transition temperature, $T_c$; bottom panel shows corresponding $RRR=R(300\,\text{K})/R(T_c)$. Data from Ref. mcfadden2025.
• Figure 2: Magneto-optical imaging of penetrating magnetic flux after cooling in zero-field (ZFC) to 4 K. Brighter contrast corresponds to higher $B_{z}$. The columns show three types of films, and the rows show flux penetration after applying 40 Oe (top) and then increasing it to 80 Oe (bottom). The low-$Q_{\mathrm {i}}$ film is clearly distinct, exhibiting a granular structure, whereas the high-$Q_{\mathrm {i}}$ film displays the lowest flux penetration; see the text for discussion.
• Figure 3: Flux penetration depth as a function of resonator quality factor. The depth of the flux front decreases with increasing $Q_i$, indicating that high-$Q$ films possess superior screening capacity and higher critical current densities.
• Figure 4: Magneto-optical Faraday images of the remanent state after cooling in a 320 Oe to 4 K and turning the field off. The low-quality factor film shows prominent granular structure and mesoscopic inhomogeneity. However, at the macro scale, all films exhibit a roof-like shape of trapped flux, as expected from the Bean model.
• Figure 5: Main frame: raw data - resonant frequency shift as a function of temperature. Inset: low-temperature variation of the magnetic susceptibility at low temperatures. Note the extremely small range shown, from -1.000 to -0.994.