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We report direct measurements of the magnetic field screening at the limits of the Meissner phase for two superconducting niobium (Nb) samples. The samples are processed with two different surface treatments that have been developed for superconducting radio-frequency (SRF) cavity applications—a “baseline” treatment and an oxygen-doping (“O-doping”) treatment. The measurements show: (1) that the screening length is significantly longer in the “O-doping” sample compared to the “baseline” sample; (2) that the screening length near the limits of the Meissner phase increases with applied field; (3) the evolution of the screening profile as the material transitions from the Meissner phase to the mixed phase; and (4) a demonstration of the absence of any screening profile for the highest applied field, indicative of the full flux entering the sample. Measurements are performed utilizing the β -detected nuclear magnetic resonance ( β -NMR) technique that allows depth resolved studies of the local magnetic field within the first 100 nm of the surface. The study takes advantage of the β -SRF beamline, a new facility at TRIUMF, Canada, where field levels up to 200 mT are available parallel to the sample surface to replicate radio frequency fields near the Meissner breakdown limits of Nb.

In most superconductors electrons form Cooper pairs in a spin-singlet state mediated by either phonons or by long-range interactions such as spin fluctuations. The superconductor UTe 2 is a rare material wherein electrons are believed to form pairs in a unique spin-triplet state with potential topological properties. While spin-triplet pairing may be mediated by ferromagnetic or antiferromagnetic fluctuations, experimentally, the magnetic properties of UTe 2 are unclear. By way of muon spin rotation/relaxation (μSR) measurements on independently grown UTe 2 single crystals we demonstrate the existence of magnetic clusters that gradually freeze into a disordered spin frozen state at low temperatures. Our findings suggest that inhomogeneous freezing of magnetic clusters is linked to the ubiquitous residual linear term in the temperature dependence of the specific heat ( C ) and the low-temperature upturn in C / T versus T . The omnipresent magnetic inhomogeneity has potential implications for experiments aimed at establishing the intrinsic low-temperature properties of UTe 2 . UTe 2 receives significant attention as it may be an example of a spin-triplet superconductor but many features of this material are still to be fully understood. Here, the authors use muon spin rotation to investigate the existence of low-temperature magnetic clusters in single crystals of UTe 2 and discuss the potential relationship with the temperature dependent behaviour of the specific heat.

The Kondo insulator SmB 6 is purported to develop into a robust topological insulator at low temperatures. Yet there are several puzzling and unexplained physical properties of the insulating bulk. It has been proposed that bulk spin excitons may be the source of these anomalies and may also adversely affect the topologically protected metallic surface states. Here, we report muon spin rotation measurements of SmB 6 that show thermally activated behavior for the temperature dependence of the transverse-field relaxation rate below 20 K and a decreasing contact hyperfine field contribution to the positive muon Knight shift below 5–6 K. Our data are consistent with the freezing out of a bulk low-energy (~1 meV) spin exciton concurrent with the appearance of metallic surface conductivity. Furthermore, our results support the idea that spin excitons play some role in the anomalous low-temperature bulk properties of SmB 6 . Kondo insulators: the importance of spin excitons The role of spin excitons in the bulk properties of the Kondo insulator SmB6 is a matter of debate; now, Jeff E. Sonier at the Simon Fraser University in Canada and colleagues use muon spin spectroscopy to shed new light on the question. SmB6 is a good candidate 3D topological insulator with a strong insulating gap, but the physics of this material is still not fully elucidated. In particular, SmB-6 exhibits low-temperature thermodynamic and transport anomalies that have been attributed to spin excitons, which might negatively impact the topological surface states. The authors present evidence for a low-energy bulk spin exciton that freezes below the temperature at which the surface conductivity arises and seems to be related to the anomalous low-temperature bulk properties of SmB6, contributing to the understanding of this material.

We report direct measurements of the magnetic field screening at the limits of the Meissner phase for two superconducting Nb samples. The samples are processed with two different surface treatments that have been developed for superconducting radio-frequency cavity applications -- a \"baseline\" treatment and an oxygen-doping (\"O-doping\") treatment. The measurements show: 1) that the screening length is significantly longer in the \"O-doping\" sample compared to the \"baseline\" sample; 2) that the screening length near the limits of the Meissner phase increases with applied field; 3) the evolution of the screening profile as the material transitions from the Meissner phase to the mixed phase; and 4) a demonstration of the absence of any screening profile for the highest applied field, indicative of the full flux entering the sample. Measurements are performed utilizing the \\(\\beta\\)-detected nuclear magnetic resonance (\\(\\beta\\)-NMR) technique that allows depth resolved studies of the local magnetic field within the first 100 nm of the surface. The study takes advantage of the \\(\\beta\\)-SRF beamline, a new facility at TRIUMF, Canada, where field levels up to 200 mT are available parallel to the sample surface to replicate radio frequency (RF) fields near the Meissner breakdown limits of Nb.

Star lattice, which can be visualized as a honeycomb network with each vertex replaced by a triangle, provides a rare platform for realizing exotic quantum states such as quantum spin liquids and disorder-driven random-singlet (RS) states. Herein, we investigate the ground-state properties of the three-dimensional (3D) stuffed hyper-star lattice Li\\(_2\\)Cu\\(_2\\)(MoO\\(_4\\))\\(_3\\), which exhibits a crossover from short-range spin correlations to a disorder-driven RS-like state below \\(T^{*}\\sim\\)15.8 K. Thermodynamic and microscopic measurements capture this crossover through a change in the power-law behavior of various observables, from \\(\\sim T^{0.25}\\) for \\(T > T^{*}\\) to \\(\\sim T^{-0.50}\\) for \\(T < T^{*}\\). Upon further cooling, a quasi-frozen state emerges near \\(T_{\\rm f} = 0.32\\) K, likely associated with weakly coupled spin chains within the hyper-star spin network. Our results underscore the crucial role of orphan spins and weak residual interactions in stabilizing a disorder-driven quantum-disordered state in 3D.

We report measurements of the normal-state and superconducting properties of thin-film \\(\\mathrm{Nb_{1-x}Ti_xN}\\) using \\(^{8}\\)Li \\(\\beta\\)-detected nuclear magnetic resonance (\\(\\beta\\)-NMR). In these experiments, radioactive \\(^{8}\\)Li\\(^{+}\\) probes were implanted \\(\\sim21\\) nm below the surface of a \\(\\mathrm{Nb_{1-x}Ti_xN}\\)(91 nm) film in \\(\\mathrm{Nb_{0.75}Ti_{0.25}N}\\)(91 nm)/AlN(4 nm)/Nb and its NMR response recorded (via \\(^{8}\\)Li's \\(\\beta\\)-emissions) between 4.6 K and 270 K in a 4.1 T field applied normal to its surface. Resonance measurements reveal wide, symmetric lineshapes at all temperatures, with significant additional broadening below the film's superconducting transition temperature \\(T_\\mathrm{c}(0 \\; \\mathrm{T}) = 15.4 \\pm 0.7\\) K due to vortex lattice formation. Fits to a broadening model find a magnetic penetration depth \\(\\lambda(0 \\; \\mathrm{K})= 180.57 \\pm 0.30\\) nm and upper critical field \\(B_\\mathrm{c2}(0 \\; \\mathrm{K})= 18 \\pm 4\\) T, consistent with literature estimates. Spin-lattice relaxation (SLR) measurements find a Korringa response at low temperatures, with dynamic (i.e., thermally activated) contributions dominating above \\(\\sim100\\) K. Below \\(T_\\mathrm{c}\\), we observe a small Hebel-Slichter coherence peak characterized by a superconducting energy gap \\(\\Delta(0 \\; \\mathrm{K}) = 2.60 \\pm 0.12\\) meV and modest Dynes-like broadening. Our measurements suggest a gap ratio \\(2\\Delta(0 \\; \\mathrm{K})/k_\\mathrm{B}T_\\mathrm{c}(0 \\; \\mathrm{T}) = 3.92 \\pm 0.25\\), consistent with strong-coupling behavior. Sources for the dynamic high-\\(T\\) relaxation are suggested.

We report \\(\\)-detected nuclear magnetic resonance (\\(\\)-NMR) measurements of implanted \\(^8\\)Li\\(^+\\) in a synthetic single crystal of \\(\\)-SiO\\(_2\\) (quartz). At 6.55 Tesla, the spectrum is comprised of a large amplitude broad resonance and a quadrupolar multiplet that is only revealed by an RF comb excitation. The quadrupole splitting is surprisingly small, increases with temperature, and provides information on the implantation site. Supercell density functional theory calculations show that the small EFG is consistent with an in-channel interstitial site (Wyckoff 3\\(a\\)). The spin-lattice relaxation is unexpectedly fast and strongly temperature dependent with a diffusive peak above 200 K and a second more prominent relaxation peak at lower temperature. Analysis of the diffusive relaxation yields an activation barrier 178(43) meV for the isolated Li\\(^+\\), in the range of other measurements and calculations. To account for many of the other features of the data, it is suggested that some of the implanted ions trap an electron forming the neutral Li\\(^0\\), which is stable over a narrow range of temperatures.

We report measurements of the Meissner screening profile in a Nb(300 nm)/Al\\(_{2}\\)O\\(_{3}\\) thin film using \\(^{8}\\)Li \\(\\beta\\)-detected nuclear magnetic resonance (\\(\\beta\\)-NMR). The NMR probe \\(^{8}\\)Li was ion-implanted into the Nb film at energies \\(\\leq\\) 20 keV, corresponding to mean stopping depths comparable to Nb's magnetic penetration depth \\(\\lambda\\). \\(^{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 \\(\\lambda_{0}\\) = 51.5(22) nm, consistent with a relatively short carrier mean-free-path \\(\\ell\\) = 18.7(29) nm typical of similarly prepared Nb films. The results presented here constitute an important step towards using \\(^{8}\\)Li \\(\\beta\\)-NMR to characterize bulk Nb samples with engineered surfaces, which are often used in the fabrication of particle accelerators.

A new high field spectrometer has been built to extend the capabilities of the \\(\\beta\\)-detected nuclear magnetic resonance (\\(\\beta\\)-NMR) facility at TRIUMF. This new beamline extension allows \\(\\beta\\)-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, 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.

We report on the stability and magnetic state of ion implanted \\(^8\\)Li in single crystals of the semiconductor ZnO using \\(\\beta\\)-detected nuclear magnetic resonance. At ultradilute concentrations, the spectra reveal distinct Li sites from 7.6 to 400 K. Ionized shallow donor interstitial Li is stable across the entire temperature range, confirming its ability to self-compensate the acceptor character of its (Zn) substitutional counterpart. Above 300 K, spin-lattice relaxation indicates the onset of correlated local motion of interacting defects, and the spectra show a site change transition from disordered configurations to substitutional. Like the interstitial, the substitutional shows no resolved hyperfine splitting, indicating it is also fully ionized above 210 K. The electric field gradient at the interstitial \\(^8\\)Li exhibits substantial temperature dependence with a power law typical of non-cubic metals.