A low energy muon spin rotation and point contact tunneling study of niobium films prepared for superconducting cavities

Abstract

Point contact tunneling (PCT) and low energy muon spin rotation (LE-µSR) are used to probe, on the same samples, the surface superconducting properties of micrometer thick niobium films deposited onto copper substrates using different sputtering techniques: diode, dc magnetron (dcMS) and HIPIMS. The combined results are compared to radio-frequency tests performances of RF cavities made with the same processes. Degraded surface superconducting properties are found to correlate to lower quality factors and stronger Q slope. In addition, both techniques find evidence for surface paramagnetism on all samples and particularly on Nb films prepared by HIPIMS. I. CURRENT LIMITATIONS OF NIOBIUM ON COPPER CAVITIESSuperconducting cavities prepared by coating a micrometer thick niobium film on a copper substrate enable a lower surface resistance compared to bulk niobium at 4.5 K the operation temperature of several accelerators using this technology, like the LHC or the HIE-Isolde at CERN. Additionally Nb/Cu cavities are more costeffective and do not require magnetic shielding. Thermal stability is enhanced avoiding quenches [1].Despite these advantages, this technology is currently not being considered for accelerators requiring highest accelerating gradient E acc or lowest surface resistance R S at temperatures of 2 K and below. The reason for this is that R S increases strongly with E acc . The origin of this field dependent surface resistance has been the subject of many past and recent studies [2][3][4][5], but is still far from being fully understood. The film thickness of about 1.5 µm is large compared to the London penetration depth λ L of about 32 nm. Differences in the performance (apart from thermal conductivity issues [6]) should therefore be correlated to the manufacturing procedure and the resulting surface structure. Since no single dominant source can be expected, several * Tobias.Junginger@helmholtz-berlin.de † TRIUMF Canada's National Laboratory for Particle and Nuclear Physics, Vancouver hypotheses need to be addressed individually to identify their origin and possibly mitigate their influence on the surface impedance and SRF cavity performances. A. Surface magnetism, a possible source of RF dissipationA possible source of dissipation under RF fields is surface magnetism in the oxide layer; magnetic impurities have a well known deleterious effect on superconductivity, causing inelastic scattering of the Cooper pairs [7] that increase the surface resistance R S and lower the quality factor Q ∝ 1/R S of superconducting RF cavities. The presence of localized magnetic moments on bulk niobium samples was revealed for the first time by Casalbuoni et al. in 2005 [8] by SQUID magnetometry. The AC magnetic susceptibility was measured in an external magnetic field of 0.7 T on Nb samples prepared with standard SRF cavity treatments, i.e. buffered chemical polishing (BCP), electropolishing (EP) and low temperature baking at about 120• C. In all cases, the samples displayed a Curie-Weiss behavior below 50 K indicativ...

High-Frequency Nonlinear Response of Superconducting Cavity-Grade Nb Surfaces

“…This sample is well characterized by pointcontact tunneling spectroscopy, low-energy muon spin rotation, and SEM. The point-contact tunneling measurements revealed that the sample has a distribution of superconducting gap values with a peak at ∆ = 1.48meV [41]. However, the tunneling data also show a few points with large zero-bias conductance peaks.…”

Determination of the Bardeen–Cooper–Schrieffer material parameters of the HIE-ISOLDE superconducting resonator

“…Muon spin rotation µSR [24] resulted in a consistent effective penetration depth calculated by [25] λ eff = λ L 1 + πξ 0 2l (15) when using (ξ 0 , λ L , l) obtained by the the BCS data fitting. The conventional DC 4-contact measurement for Residual-Resistivity Ratio (ρ(300K) /ρ(10K) by DC resistance ρ) was also done [4].…”

Influence of crystalline structure on rf dissipation in superconducting niobium

“…This remaining stress can be at the origin of local stress corrosion and the apparition of pits during subsequent etching. [25,31,32,83,140,[144][145][146].…”

“…Romanenko and collaborators worked on samples cut out of cavities (small grain and large grain, processed by BCP and EP) and compared cold and hot spots from the structural, morphological and chemical point of view [3,4,16,17]. Recently these results have been completed with magnetic characterization that highlights some aspects of vortice pinning [31,140].…”

“…It is interesting to note that Nb thin films show common properties with bulk niobium hotspots, in particular the existence of subgap states as measured with point contact tunneling [140], since one knows that thin films usually exhibit high crystalline defect density: strained lattice with higher T C , very small grains, high density of displaced and foreign atoms. Similar problems are also expected to be encountered for alternative SRF superconductors produced in the form of thin films like Nb 3 Sn, MgB 2 , multilayers.…”