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We report measurements of the Meissner screening profile in a Nb (300 nm)/ Al 2 O 3 thin film using 8 Li β -detected nuclear magnetic resonance (β -NMR). The NMR probe 8 Li was ion-implanted into the Nb film at energies ≤ 20 keV, corresponding to mean stopping depths comparable to Nb 's magnetic penetration depth λ. 8 Li 's strong dipole–dipole coupling with the host 93 Nb nuclei provided a "cross-relaxation" channel that dominated in low magnetic fields, which conferred indirect sensitivity to the local magnetic field via the spin-lattice relaxation (SLR) rate 1 / T 1. From a fit of the 1 / T 1 data to a model accounting for its dependence on temperature, magnetic field, and 8 Li + implantation energy, we obtained a magnetic penetration depth λ 0 = 51.5(22) nm, consistent with a relatively short carrier mean-free-path ℓ = 18.7(29) nm typical of similarly prepared Nb films. The results presented here constitute an important step toward using 8 Li β -NMR to characterize bulk Nb samples with engineered surfaces, which are often used in the fabrication of particle accelerators. [ABSTRACT FROM AUTHOR]

This document is a comment on a previous study that examined the Meissner screening properties of niobium (Nb) in a superconducting radio frequency (SRF) cavity. The authors of the comment re-analyzed the data from the original study and found no significant changes to the Meissner screening profile of Nb after mild baking. They also discovered a non-superconducting "dead layer" at the surface of the Nb sample, which may indicate spatial inhomogeneities in the screening properties. The authors recommend further high-resolution measurements to investigate this further. [Extracted from the article]

A new high field spectrometer has been built to extend the capabilities of the β-detected nuclear magnetic resonance (β-NMR) facility at TRIUMF. This new beamline extension allows β-NMR spectroscopy to be performed with fields up to 200 mT parallel to a sample's surface (perpendicular to the ion beam), allowing depth-resolved studies of local electromagnetic fields with spin polarized probes at a much higher applied magnetic field than previously available in this configuration. The primary motivation and application is to allow studies of superconducting radio frequency (SRF) materials close to the critical fields of Nb metal, which is extensively used to fabricate SRF cavities. The details of the design considerations and implementation of the ultra-high vacuum (UHV) system, ion optics, and beam diagnostics are presented here. Commissioning of the beamline and spectrometer with radioactive ions are also reported here. Future capabilities and applications in other areas are also described. [ABSTRACT FROM AUTHOR]

We investigated the depth, temperature, and molecular-weight (MW) dependence of the γ-relaxation in polystyrene glasses using implanted 8Li+ and β-detected nuclear magnetic resonance. Measurements were performed on thin films with MW ranging from 1.1 to 641 kg/mol. The temperature dependence of the average 8Li spin–lattice relaxation time ( T 1 avg ) was measured near the free surface and in the bulk. Spin–lattice relaxation is caused by phenyl ring flips, which involve transitions between local minima over free-energy barriers with enthalpic and entropic contributions. We used transition state theory to model the temperature dependence of the γ-relaxation, and hence T 1 avg . There is no clear correlation of the average entropy of activation ( Δ ‡ S ̄ ) and enthalpy of activation ( Δ ‡ H ̄ ) with MW, but there is a clear correlation between Δ ‡ S ̄ and Δ ‡ H ̄ , i.e., entropy–enthalpy compensation. This results in the average Gibbs energy of activation, Δ ‡ G ̄ , being approximately independent of MW. Measurements of the temperature dependence of T 1 avg as a function of depth below the free surface indicate the inherent entropic barrier, i.e., the entropy of activation corresponding to Δ ‡ H ̄ = 0, has an exponential dependence on the distance from the free surface before reaching the bulk value. This results in Δ ‡ G ̄ near the free surface being lower than the bulk. Combining these observations results in a model where the average fluctuation rate of the γ-relaxation has a "double-exponential" depth dependence. This model can explain the depth dependence of 1 / T 1 avg in polystyrene films. The characteristic length of enhanced dynamics is ∼6 nm and approximately independent of MW near room temperature. [ABSTRACT FROM AUTHOR]

β-detected NMR (β-NMR) has been used to study the molecular-scale dynamics of lithium ions in thin films of poly(ethylene oxide) (PEO) containing either lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) or lithium trifluoroacetate (LiTFA) salts at monomer-to-salt ratios (EO/Li) of 8.3. The results are compared with previous β-NMRmeasurements on purePEOandPEOwith lithium triflate (LiOTf) at the same loading [McKenzie et al., J. Am. Chem. Soc. 136, 7833 (2014)]. Activated hopping of 8Li+ was observed in all of the films above ∼250 K, with the hopping parameters strongly correlated with the ionicity of the lithium salt rather than the polymer glass transition temperature. The preexponential factor increases exponentially with ionicity, while the activation energy for hopping increases approximately linearly, going from 6.3±0.2 kJ mol-1 in PEO:LiTFA to 17.8±0.2 kJ mol-1 in PEO:LiTFSI. The more rapid increase in the pre-exponential factor outweighs the effect of the larger activation energy and results in 8Li+ hopping being fastest in PEO followed by PEO:LiTFSI, PEO:LiOTf and PEO:LiTFA. [ABSTRACT FROM AUTHOR]