Muon spin rotation studies of niobium for superconducting rf applications

Beard, C.; Kolb, Philipp; Laxdal, Robert; Lockyer, N. S.; Longuevergne, David; Sonier, J. E.

doi:10.1103/physrevstab.16.062002

Cited by 20 publications

(23 citation statements)

References 26 publications

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“…The most apparent feature is the minimum observed at 50 K. In studies where the muon has been implanted in the bulk this feature has also been observed and was correlated to interstitial impurities [42]. A more recent study on cavity grade bulk niobium [43] has also found a minimum at 50 K. It has to be noted that in this study the variation of the hopping rate with temperature was much stronger than what is observed here. Also in [43] ν was obtained from zero field measurements only.…”

Section: Longitudinal Field Studies Of the Hipims Samplesupporting

confidence: 65%

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

Junginger

Calatroni

Sublet

et al. 2017

Supercond. Sci. Technol.

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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...

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Section: Longitudinal Field Studies Of the Hipims Samplesupporting

confidence: 65%

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

Junginger

Calatroni

Sublet

et al. 2017

Supercond. Sci. Technol.

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“…Muon hopping rate of ν ∼ 0.7 MHz was determined at 2.5 K by Grasselino et al from ZF µSR experiments. 31 In zero field the muon experiences a spin depolarization due to nuclear magnetic fields which for a static case is described with the Kubo-Toyabe function: 28…”

Section: A Effect Of Muon Diffusion On Nb µSr Spectramentioning

confidence: 99%

“…The strong collision model was used in Ref. 31 for estimate of the hopping rate ν. Muon spin polarization within this model is described as follows: 18…”

Section: A Effect Of Muon Diffusion On Nb µSr Spectramentioning

confidence: 99%

Evidence for Cooper pair diffraction on the vortex lattice of superconducting niobium

et al. 2013

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We investigated the Abrikosov vortex lattice (VL) of a pure Niobium single crystal with the muon spin rotation (µSR) technique. Analysis of the µSR data in the framework of the BCS-Gor'kov theory allowed us to determine microscopic parameters and the limitations of the theory. With decreasing temperature the field variation around the vortex cores deviates substantially from the predictions of the Ginzburg-Landau theory and adopts a pronounced conical shape. This is evidence of partial diffraction of Cooper pairs on the VL predicted by Delrieu for clean superconductors. PACS numbers: 74.25.Uv, 74.20.Fg, 76.75.+i The Ginzburg-Landau (GL) theory for superconductors is expressed in terms of an order parameter ∆(r). While the absolute value of ∆(r) determines the local superfluid density, its phase gradient is proportional to the local magnetic vector potential. This phase variation leads to the magnetic flux quantization and the formation of a periodic vortex lattice (VL) in type-II superconductors as was predicted by Abrikosov. 1 The VL field variation is uniquely characterized by two length scales, the magnetic penetration depth λ and the coherence length ξ GL . This simple model turned out to be quite successful in describing the behavior of a superconductor in a magnetic field 2,3 and serves as a basis for data analysis of experiments. 4-6 However, as was shown theoretically by Delrieu, 7 the GL model is unable to describe the magnetic response in clean superconductors at low temperatures and close to the upper critical field B c2 .Soon after the publication of the microscopic Bardeen-Cooper-Schrieffer (BCS) theory for conventional superconductors, 8 using a Green function formalism Gor'kov derived the GL equations from the BCS theory. 9 Based on Gor'kov's equation Delrieu analyzed the field variation for classical s−wave superconductors in the vicinity of B c2 . 7 He found that for clean superconductors the Cooper pairs (CPs) with balistic trajectories through the vortex cores diffract on the periodic potential of the VL. As a result, in the low temperature limit close to B c2 the spatial field variation around a vortex core has a conical shape, rather than the cosine-like GL behaviour, and the fields at the minimum and saddle points are interexchanged relative to the GL prediction. Nearly at the same time Brandt came to the same conclusion based on a nonlocal theory of superconductivity. 10 To observe the effect of diffraction of CPs, the carrier mean-free path ℓ mfp should exceed the intervortex distance, the measurements should be done in the vicinity of B c2 (T ) such that the ∆(r) gradient is negligible, and the temperature should be low to minimize thermal fluctuations.Although the theoretical study of the influence of the diffraction of CPs on the field variation was already per-formed in 1972, it was not investigated experimentally in detail so far. Early muon spin rotation (µSR) experiments revealed a linear high-field tail in the magnetic field distribution D exp c (B Z ), in agreement with the the...

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“…For this purpose samples of ellipsoidal geometry have been produced. In the experiments reported in [4], the magnetic field was applied perpendicular to the sample surface, unlike in accelerating cavities. To resemble the field geometry of SRF cavities, a spectrometer that allows the application of fields of up to 300 mT parallel to the sample surface was built.…”

Section: Introductionmentioning

confidence: 99%

Field of first magnetic flux entry and pinning strength of superconductors for rf application measured with muon spin rotation

Junginger

Abidi

Maffett

et al. 2018

Phys. Rev. Accel. Beams

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The performance of superconducting radiofrequency (SRF) cavities used for particle accelerators depends on two characteristic material parameters: field of first flux entry Hentry and pinning strength. The former sets the limit for the maximum achievable accelerating gradient, while the latter determines how efficiently flux can be expelled related to the maximum achievable quality factor. In this paper, a method based on muon spin rotation (µSR) is developed to probe these parameters on samples. It combines measurements from two different spectrometers, one being specifically built for these studies and samples of different geometries. It is found that annealing at 1400 • C virtually eliminates all pinning. Such an annealed substrate is ideally suited to measure Hentry of layered superconductors, which might enable accelerating gradients beyond bulk niobium technology.Recently, to reach high quality factors, a treatment procedure has been established baking cavities at 800 • C and injecting nitrogen gas at the end of this treatment. arXiv:1705.05480v3 [cond-mat.supr-con]

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Muon spin rotation studies of niobium for superconducting rf applications

Cited by 20 publications

References 26 publications

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

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

Evidence for Cooper pair diffraction on the vortex lattice of superconducting niobium

Field of first magnetic flux entry and pinning strength of superconductors for rf application measured with muon spin rotation

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