  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 nanoohms
surface resistances is a key metric for the wider adoption of SRF technology.
Here we report ZrNb(CO) RF superconducting films with high critical
temperatures (Tc) achieved for the first time under ambient pressure. 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. Our 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, we avoid hexagonal Zr phases that are equilibrium-stable
but degrade Tc. X-ray and electron diffraction combined with photoelectron
spectroscopy reveal a system containing cubic ZrNb mixed with rocksalt NbC and
low-dielectric-loss ZrO2. We demonstrate proof-of-concept RF performance of
ZrNb(CO) on an SRF sample test system. BCS resistance trends lower than
reference Nb, while quench fields occur at approximately 35 mT. Our results
demonstrate the potential of ZrNb(CO) thin films for particle accelerator and
other SRF applications.
