The College of William and Mary, Williamsburg, USA
JLAB, Newport News, Virginia, USA
Superconducting thin films and multilayers have attracted the attention of the scientific community due to the promise of overcoming the maximum field gradients that SRF cavities can withstand, pushing them above 100 MeV/m *. Nevertheless, in order to achieve the desired properties, special attention needs to be devoted to the epitaxy and growth mode of such thin films, taking into account multiple aspects such as crystalline quality, lattice strain, grain size, etc. We present a complete correlation between morphology, structure and superconducting properties such as critical field, critical temperature and complex susceptibility for single crystal Nb(110) thin films sputter deposited on a-plane sapphire substrates. The influence of strain and grain boundaries in the superconducting transition is analyzed in detail, since the lattice mismatch between Nb and sapphire induces strain in the first atomic layers and may affect the superconducting properties of the thin films. AC susceptibility techniques allow us to identify the dissipative effects in the lattice associated with the presence of defects, thus allowing us to tune the growth conditions to minimize their effect.
*A. Gurevich, Applied Physics Letters 88 (1), 012511 (2006).
The College of William and Mary, Williamsburg, USA
JLAB, Newport News, Virginia, USA
SRF technology used in linear accelerators is based on bulk Nb cavities that have high cost and are approaching the maximum field gradients they can withstand*. Thus, development of a suitable alternative to bulk Nb is needed. Attempts have been made to implement Nb-coated Cu cavities since the thermal conductivity of Cu is better than bulk Nb**. Our studies show that the transport properties of Nb, in particular the residual resistance ratio (RRR), are better when epitaxially grown on crystalline ceramics (i.e. MgO and Al2O3) compared to Cu templates. Since grain boundaries are one of the main obstacles to superconducting transport, we show how the increased number of crystallographic domains that can occur during epitaxial Nb growth onto Cu surfaces leading to higher density of grain boundaries can explain our results. We propose a route to improved performance while maintaining thermal efficiency by using seed-layers on Cu templates that can decrease grain boundary density. We will show our correlated studies of microstructure and surface morphology and the resulting transport/susceptibility properties illustrating possible mechanisms to improve cavity performance of such films.
* P. Kneisel et al., Proceedings of 2005 Particle Accelerator Conference, Knoxville, TN, TPPT076 (2005).
** S. Calatroni, Physica C 441, 95 (2006).
The College of William and Mary, Williamsburg, USA
JLAB, Newport News, Virginia, USA
Surface and interface roughness are critical factors in determining the technological viability of many systems, in particular the development of next-generation superconducting radio frequency (SRF) cavities. Thus, we have undertaken a systematic effort to investigate the surface evolution of epitaxially grown Nb thin films under specific deposition conditions. This is important since ongoing efforts to improve cavity’s performance have considered the possibility of multi-layered thin film coatings* as an alternative to the current bulk Nb technology. We examined the surface morphology of epitaxial Nb films grown on MgO at different stages during growth and applied dynamical scaling analysis to the surface features. Our thin film nucleation and growth kinetics studies are relevant since thin films may differ from bulk systems due to limited material supply as well as stress contributions from lattice mismatch with the substrate. This can induce significant surface roughness which can in turn lead to undesirable effects for SRF applications. Our studies may offer a venue to minimize these drawbacks by suitable choice of thin film growth parameters and substrates.
* A. Gurevich, Appl. Phys. Lett. 88, 012511 (2006).