ZrNb(CO) RF Superconducting Thin Film with High Critical Temperature in the Theoretical Limit

Abstract

Superconducting radio‐frequency (SRF) resonators are critical components for particle accelerator applications, such as free‐electron lasers, and for emerging technologies in quantum computing. Developing advanced materials and their deposition processes to produce RF superconductors that yield nΩ surface resistances is a key metric for the wider adoption of SRF technology. Here, ZrNb(CO) RF superconducting films with high critical temperatures (Tc) achieved for the first time under ambient pressure are reported. The attainment of a Tc near the theoretical limit for this material without applied pressure is promising for its use in practical applications. A range of Tc, likely arising from Zr doping variation, may allow a tunable superconducting coherence length that lowers the sensitivity to material defects when an ultra‐low surface resistance is required. The ZrNb(CO) films are synthesized using a low‐temperature (100 – 200 °C) electrochemical recipe combined with thermal annealing. The phase transformation as a function of annealing temperature and time is optimized by the evaporated Zr‐Nb diffusion couples. Through phase control, one avoids hexagonal Zr phases that are equilibrium‐stable but degrade Tc. X‐ray and electron diffraction combined with photoelectron spectroscopy reveal a system containing cubic β‐ZrNb mixed with rocksalt NbC and low‐dielectric‐loss ZrO2. Proof‐of‐concept RF performance of ZrNb(CO) on an SRF sample test system is demonstrated. BCS resistance trends lower than reference Nb, while quench fields occur at approximately 35 mT. The results demonstrate the potential of ZrNb(CO) thin films for particle accelerators and other SRF applications.

Smooth, homogeneous, high-purity Nb3Sn superconducting RF resonant cavity by seed-free electrochemical synthesis

“…This results in an abrupt change in the magnetic fields in the vicinity of the cavity. The cryostat is magnetically shielded such that the dominant component of the magnetic fields in the cryostat aligns along the cavity's central axis with a magnitude <10 mG. We recorded either an increase or a decrease in the magnetic fields depending on the orientation to indicate the T c (for more information about this measurement setup, see [47,48]).…”

“…Other hypotheses involve Nb 3 Sn's small coherence length (∼3 nm), making it sensitive to defects like grain boundaries [71], as well as the impact of surface oxides [72][73][74][75]. We are exploring alternative materials like ZrNb(CO) with a suitable coherence length and low-dielectric-loss ZrO 2 [48]. To understand the refinement of Nb 3 Sn achieved, we probed impurities by SIMS (figures 8(e) and S20) and XPS (figure 8(f)) depth profiling.…”