Development of NbTi and Nb3Sn thin films for innovative superconducting axion haloscopes

The axion is a hypothetical particle introduced as solution to the strong CP problem; its mass is predicted to be in the range of 10 - 10^3 ueV. Due to its weak interaction it is a candidate cold dark matter particle. This particle can be detected by resonant cavities, or haloscopes, immersed in an intense static magnetic field and tuned to the axion frequency - in the microwave range - exploiting the axion-photon coupling described by the inverse Primakoff effect. Superconducting (SC) materials offer the possibility to enhance the sensitivity of the resonator because of their higher quality factor in a magnetic field compared to copper. The ones eligible for this application are the type-II superconductors, which can sustain high magnetic fields - up to several teslas - without breaking the SC state. The objective of this work is the development of NbTi and Nb3Sn films for SC haloscopes coated via magnetron sputtering. The Nb3Sn critical magnetic field of 30 T makes it a suitable replacement for NbTi, whose critical field is 12 T. For NbTi, several tests on planar samples have been made to investigate some deposition parameters. The obtained composition is around 50 % in weight of Ti, which is in the desired range to have the maximum upper critical magnetic field. Copper interdiffusion in the film has been addressed with a niobium diffusion barrier. Four copper haloscopes (a total of eight half cells) have been coated with a 2.5 um NbTi film, one of which also with the Nb diffusion barrier of 1 um. The cavity of nominal frequency 7GHz have been tested in a cryogenic environment and immersed in a magnetic field up to 11 T. The figure of merit relevant to the axion search resulted, at 8 T and 4.2 K, one order of magnitude higher than a copper cavity. The quality factor of the resonator results limited by the uncoated part of the cavity. XRD and third harmonic AC magnetic susceptibility revealed the presence of a single SC phase. An ultra high vacuum deposition chamber dedicated to Nb3Sn has been set up to operate at the high temperature required for the formation of the Nb3Sn phase. XRD and EDS analysis confirmed the presence of the SC phase. The composition of the films is sub-stoichiometric: about 22 atomic % of Sn. A thermal treatment of the sample at different temperatures and a higher deposition pressure demonstrated to increase the critical temperature. The Nb diffusion barrier has been implemented also for Nb3Sn to reduce the Cu content in the film. It showed to increase the composition, by blocking the Sn diffusion towards the substrate, and the critical temperature. Moreover, it allows to obtain higher Tc at a lower deposition temperature compared to the copper substrate. This is a noticeable result in the optics of SRF cavities, which cannot sustain high temperature treatments.