arXiv subjects

Ryan M. L. McFadden

Publications and source records attributed to Ryan M. L. McFadden.

At least 19 recordsLinked to original sources

arXiv2026

Modernization and Statistical Validation of a Multilayer TRIM.SP Code for Low-Energy Muon and Ion Implantation

We describe the staged modernization of a local multilayer TRIM.SP implementation, denoted TRIM.SP-NL. The work included behavior-preserving refactoring, correction of localized bookkeeping and initialization defects, support for more than five elements per layer, reproducible build modes, and a runtime-selectable random-number-generator (RNG) interface. The RANLUX RNG remains the default reference backend, while PCG32 and xoshiro256** provide faster alternatives without changing the input-deck format. The code validation combined exact regression testing with statistical comparisons over 13 representative target multilayer configurations, comprising 143 energy-configuration points per backend and 429 simulations in total. Across the test suite, PCG32 and xoshiro256** reduced runtime by factors of 1.98 and 2.01, respectively. The implanted, backscattered, and transmitted fractions, as well as the mean implantation depths, range straggling, and representative depth profiles showed no systematic RNG dependence. Together with a Node.js-based graphical interface for setup, scans, and result collection, the Fortran engine forms the open-source TRIM.SP-NL Workbench.

physics.acc-ph↗

arXiv2026

Insight into SRF cavity performance from simulations of Nb's surface oxide dissolution and diffusion

We report simulations of the dissolution and diffusion of Nb's surface oxide layer in vacuum. While this chemical doping process is important for the surface preparation of Nb superconducting radio frequency (SRF) cavities - common components of particle accelerators - quantitatively linking the resulting oxygen distributions to superconducting performance remains challenging. In this work, we simulate the reaction-diffusion process numerically for treatment temperatures $T = 50^{\circ}$C to $200^{\circ}$C and times $t = 0.5$ h to $120$ h, and calculate the effect of the spatially inhomogeneous oxygen doping on Nb's superconducting properties. We find that oxygen doping redistributes the Meissner screening current, reducing its value at the surface and shifting its maximum several nanometres into the material. These results provide a microscopic link between oxygen diffusion profiles and the electromagnetic response of Nb relevant for SRF cavity operation. This work provides a quantitative framework linking oxygen diffusion profiles to superconducting performance and establishes a foundation for future studies involving time-dependent and multi-step heat treatment protocols.

cond-mat.supr-con↗

arXiv2026

Contrasting impurity-induced magnetism and dynamics in 2H-MoTe2

We investigate the behavior of interstitial $^8$Li$^+$ implanted near the surface of 2H-MoTe$_2$ using $β$-detected NMR. We find that, unlike the muon, $^8$Li$^+$ does not show any signature of induced magnetism. This result is consistent with density functional theory, which identifies the Li stopping site at the 2a Wyckoff position in the van der Waals gap and confirms the absence of detectable Li-induced electronic spin polarization. Both the spin-lattice relaxation and the resonance lines show evidence of strong spin dynamics above $\sim 200$ K, reminiscent of local stochastic $^8$Li$^+$ motion within a cage. The resonance line shape consists of quadrupolar satellites on top of a broad central peak. To better understand the interaction of $^8$Li$^+$ with the host material, we employ a frequency-comb measurement, by simultaneously exciting four frequencies corresponding to the first-order quadrupolar satellite transitions, $ν_0 \pm 3ν_{\mathrm{comb}}$ and $ν_0 \pmν_{\mathrm{comb}}$ around the Larmor frequency $ν_0$ as a function of $ν_{\mathrm{comb}}$. This offers an enhanced sensitivity to the quadrupolar split portion of the line. Using this method, we find a small decrease of the quadrupolar frequency with increasing temperature, showing the typical behavior associated with thermally excited phonons and the absence of any magnetic response which was observed with other defects in 2H-MoTe$_2$.

cond-mat.mtrl-sci↗

arXiv2026

Niobium's intrinsic coherence length and penetration depth revisited using low-energy muon spin spectroscopy and secondary-ion mass spectrometry

We report direct, simultaneous measurements of the London penetration depth ($λ_L$) and Bardeen-Cooper-Schrieffer (BCS) coherence length ($ξ_0$) in oxygen-doped niobium, with impurity concentrations spanning the "clean" to "dirty" limits. Two depth-resolved techniques - low-energy muon spin spectroscopy (LE-$μ$SR) and secondary-ion mass spectrometry (SIMS) - were used to quantify the element's Meissner screening profiles, analyzed within a framework that accounts for nonlocal electrodynamics. The analysis indicates intrinsic length scales of $λ_L = 29.1(10)$ nm and $ξ_0 = 39.9(25)$ nm, corresponding to a Ginzburg-Landau (GL) parameter of $κ= 0.70(5)$. The obtained $λ_L$ and $κ$ values, accurately quantified at the nanoscale, are smaller than values commonly used in applications and modeling, and indicate that clean niobium lies at the boundary between type-I and type-II superconductivity, supporting the contemporary view that its intrinsic state may be type-I.

cond-mat.supr-con↗

arXiv2025

Implantation studies of low-energy positive muons in niobium thin films

Here we study the range of keV positive muons $μ^+$ implanted in Nb$_2$O$_5$($x$ nm)/Nb($y$ nm)/SiO$_2$(300 nm)/Si [$x$ = 3.6 nm, 3.3 nm; $y$ = 42.0 nm, 60.1 nm] thin films using low-energy muon spin spectroscopy (LE-$μ$SR). At implantation energies 1.3 keV $\leq E \leq$ 23.3 keV, we compare the measured diamagnetic $μ^+$ signal fraction $f_{\mathrm{dia.}}$ against predictions derived from implantation profile simulations using the TRIM.SP Monte Carlo code. Treating the implanted $μ^+$ as light protons, we find that simulations making use of updated stopping cross section data are in good agreement with the LE-$μ$SR measurements, in contrast to parameterizations found in earlier tabulations. Implications for other studies relying on accurate $μ^+$ stopping information are discussed.

cond-mat.mtrl-sci↗

arXiv2025

Superconducting properties of thin film $\mathrm{Nb_{1-x}Ti_xN}$ studied via the NMR of implanted $^8$Li

We report measurements of the normal-state and superconducting properties of thin-film $\mathrm{Nb_{1-x}Ti_xN}$ using $^{8}$Li $β$-detected nuclear magnetic resonance ($β$-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 $β$-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 $λ(0 \; \mathrm{K})= 180.57 \pm 0.30$ 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 $Δ(0 \; \mathrm{K}) = 2.60 \pm 0.12$ meV and modest Dynes-like broadening. Our measurements suggest a gap ratio $2Δ(0 \; \mathrm{K})/k_\mathrm{B}T_\mathrm{c}(0 \; \mathrm{T}) = 3.92 \pm 0.25$, consistent with strong-coupling behavior. Sources for the dynamic high-$T$ relaxation are suggested.

cond-mat.supr-con↗

arXiv2025

Lithium-ion dynamics in synthetic quartz studied via the NMR of implanted $^{8}$Li$^{+}$

We report $β$-detected nuclear magnetic resonance ($β$-NMR) measurements of implanted $^{8}$Li$^{+}$ in a synthetic single crystal of $α$-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.

cond-mat.mtrl-sci↗

arXiv2024

Search for inhomogeneous Meissner screening in Nb induced by low-temperature surface treatments

Empirical surface treatments, such as low-temperature baking (LTB) in a gaseous atmosphere or in vacuum, are important for the surface preparation of Nb superconducting radio frequency (SRF) cavities. These treatments inhomogeneously dope the first $\sim$50 nm of Nb's subsurface and are expected to impart depth-dependent characteristics to its Meissner response; however, direct evidence supporting this remains elusive, suggesting the effect is subtle. In this work, we revisit the Meissner profile data for several LTB treatments obtained from low-energy muon spin rotation (LE-$μ$SR) experiments [A. Romanenko et al., Appl. Phys. Lett. 104, 072601 (2014) and R. M. L. McFadden et al., Phys. Rev. Appl. 19, 044018 (2023)], and search for signatures of inhomogeneous field screening. Using a generalized London expression with a recently proposed empirical model for a depth-dependent magnetic penetration depth $λ(z)$, we obtain improved fits to the Meissner data, revealing that the presence of a non-superconducting surface "dead layer" $d \geq 25$ nm is a strong indicator of a reduced supercurrent density at shallow subsurface depths. Our analysis supports the notion that vacuum annealing at 120 $^{\circ}$C for 48 h induces a depth-dependent Meissner response, which has consequences for Nb's ability to maintain a magnetic-flux-free state. Evidence of similar behavior from a "nitrogen infusion" treatment is less compelling. Suggestions for further investigation into the matter are provided.

cond-mat.supr-con↗

arXiv2024

Measurements of the first-flux-penetration field in surface-treated and coated Nb: Distinguishing between surface pinning and an interface energy barrier

We report measurements of the first-flux-penetration field in surface-treated and coated Nb samples using muon spin rotation ($μ$SR). Using thin Ag foils as energy moderators for the implanted muon spin-probes, we "profile" the vortex penetration field $μ_{0} H_{\mathrm{vp}}$ at sub-surface depths on the order of $\sim 10$ $\mathrmμ$m to $\sim 100$ $\mathrmμ$m. In a coated sample [Nb$_3$Sn(2 $\mathrmμ$m)/Nb], we find that $μ_{0} H_{\mathrm{vp}}$ is depth-independent with a value of 234.5(35) mT, consistent with Nb's metastable superheating field and suggestive of surface energy barrier for flux penetration. Conversely, in a surface-treated sample [Nb baked in vacuum at 120 $^{\circ}$C for 48 h], vortex penetration onsets close to pure Nb's lower critical field $μ_{0}H_\mathrm{c1} \approx 170$ mT, but increases with increasing implantation depth, consistent with flux-pinning localized at the surface. The implication of these results for technical applications of superconducting Nb, such as superconducting radio frequency (SRF) cavities, is discussed.

cond-mat.supr-con↗

arXiv2024

Depth-resolved Characterization of Meissner Screening Breakdown in Surface Treated Niobium

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 $β$-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 (RF) fields near the Meissner breakdown limits of Nb.

cond-mat.supr-con↗

arXiv2023

Ion-Implanted $^8$Li Nuclear Magnetic Resonance in Highly Oriented Pyrolytic Graphite

We report $β$-detected nuclear magnetic resonance of ultra-dilute $^{8}$Li$^{+}$ implanted in highly oriented pyrolytic graphite (HOPG). The absence of motional narrowing and diffusional spin-lattice relaxation implies Li$^+$ is not appreciably mobile up to 400 K, in sharp contrast to the highly lithiated stage compounds. However, the relaxation is remarkably fast and persists down to cryogenic temperatures. Ruling out extrinsic paramagnetic impurities and intrinsic ferromagnetism, we conclude the relaxation is due to paramagnetic centers correlated with implantation. While the resulting effects are not consistent with a Kondo impurity, they also differ from free paramagnetic centers, and we suggest that a resonant scattering approach may account for much of the observed phenomenology.

cond-mat.mtrl-sci↗

arXiv2023

Depth-resolved measurement of the Meissner screening profile in a niobium thin film from spin-lattice relaxation of the implanted $β$-emitter $^{8}$Li

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 $\leq$ 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 $\ell$ = 18.7(29) nm typical of similarly prepared Nb films. The results presented here constitute an important step towards using $^{8}$Li $β$-NMR to characterize bulk Nb samples with engineered surfaces, which are often used in the fabrication of particle accelerators.

cond-mat.mtrl-sci↗

arXiv2023

Comment on "Strong Meissner screening change in superconducting radio frequency cavities due to mild baking" [Appl. Phys. Lett. 104, 072601 (2014)]

In a recent Letter by Romanenko et al., the authors used low-energy muon spin rotation (LE-$μ$SR) to measure the Meissner screening profile in cutouts from Nb superconducting radio frequency (SRF) cavities, systematically comparing how different surface treatments affect the screening properties of the elemental type-II superconductor. They reported a "strong" modification to the character of the screening profile upon mild baking at 120 $^{\circ}$C for 48 h, which was interpreted as a depth-dependent carrier mean-free-path resulting from a "gradient in vacancy concentration" near the surface. While this observation led to speculation that this surface treatment yields an "effective" superconducting bilayer, we suggest that its likeness to such is accidental and that the behavior is an artifact from the analysis.

cond-mat.supr-con↗

arXiv2023

Evidence for current suppression in superconductor-superconductor bilayers

Superconducting radio frequency (SRF) cavities, which are critical components in many particle accelerators, need to be operated in the Meissner state to avoid strong dissipation from magnetic vortices. For a defect-free superconductor, the maximum attainable magnetic field for operation is set by the superheating field, $B_{\mathrm{sh}}$, which directly depends on the surface current. In heterostructures composed of different superconductors, the current in each layer depends not only on the properties of the individual material, but also on the electromagnetic response of the adjacent layers through boundary conditions at the interfaces. Three prototypical bilayers [$\mathrm{Nb_{1-x}Ti_xN}$(50 nm)/Nb, $\mathrm{Nb_{1-x}Ti_xN}$(80 nm)/Nb, and $\mathrm{Nb_{1-x}Ti_xN}$(160 nm)/Nb] are investigated here by depth-resolved measurements of their Meissner screening profiles using low-energy muon spin rotation (LE-$μ$SR). From fits to a model based on London theory (with appropriate boundary and continuity conditions), a magnetic penetration depth for the thin $\mathrm{Nb_{1-x}Ti_xN}$ layers of $λ_\mathrm{Nb_{1-x}Ti_xN} =$ 182.5(31) nm is found, in good agreement with literature values for the bulk alloy. Using the measured $λ_\mathrm{Nb_{1-x}Ti_xN}$, the maximum vortex-free field, $B_{\mathrm{max}}$, of the superconductor-superconductor (SS) bilayer structure was estimated to be 610(40) mT. The strong suppression of the surface current in the $\mathrm{Nb_{1-x}Ti_xN}$ layer suggests an optimal thickness of $\sim 1.4 λ_{\mathrm{Nb_{1-x}Ti_xN}} =$ 261(14) nm.

cond-mat.supr-con↗

arXiv2022

Depth-resolved measurements of the Meissner screening profile in surface-treated Nb

We report depth-resolved measurements of the Meissner screening profile in several surface-treated Nb samples using low-energy muon spin rotation (LE-$μ$SR). In these experiments, implanted positive muons, whose stopping depths below Nb's surface were adjusted between ~10 nm to ~150 nm, reveal the field distribution inside the superconducting element via their spin-precession (communicated through their radioactive decay products). We compare how the field screening is modified by different surface treatments commonly employed to prepare superconducting radio frequency (SRF) cavities used in accelerator beamlines. In contrast to an earlier report [A. Romanenko et al., Appl. Phys. Lett. 104 072601 (2014)], we find no evidence for any "anomalous" modifications to the Meissner profiles, with all data being well-described by a London model. Differences in screening properties between surface treatments can be explained by changes to the carrier mean-free-paths resulting from dopant profiles near the material's surface.

cond-mat.supr-con↗

arXiv2022

A New High Parallel-Field Spectrometer at TRIUMF's $β$-NMR Facility

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, 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.

cond-mat.mtrl-sci↗

arXiv2021

Nuclear magnetic resonance of ion implanted $^8$Li in ZnO

We report on the stability and magnetic state of ion implanted $^8$Li in single crystals of the semiconductor ZnO using $β$-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.

cond-mat.mtrl-sci↗

arXiv2020

Local electronic and magnetic properties of the doped topological insulators Bi$_{2}$Se$_{3}$:Ca and Bi$_{2}$Te$_{3}$:Mn investigated using ion-implanted $^{8}$Li $β$-NMR

We report $β$-detected nuclear magnetic resonance ($β$-NMR) measurements in Bi$_{2}$Se$_{3}$:Ca (BSC) and Bi$_{2}$Te$_{3}$:Mn (BTM) single crystals using $^{8}$Li$^{+}$ implanted to depths on the order of 100 nm. Above $\sim 200$ K, spin-lattice relaxation (SLR) reveals diffusion of $^{8}$Li$^{+}$, with activation energies of $\sim 0.4$ eV ($\sim 0.2$ eV) in BSC (BTM). At lower temperatures, the nuclear magnetic resonance (NMR) properties are those of a heavily doped semiconductor in the metallic limit, with Korringa relaxation and a small, negative, temperature-dependent Knight shift in BSC. From this, we make a detailed comparison with the isostructural tetradymite Bi$_{2}$Te$_{2}$Se (BTS) [McFadden et al., Phys Rev. B 99, 125201 (2019)]. In the magnetic BTM, the effects of the dilute Mn moments predominate, but remarkably the $^{8}$Li signal is not wiped out through the magnetic transition at 13 K, with a prominent critical peak in the SLR that is suppressed in a high applied field. This detailed characterization of the $^{8}$Li NMR response is an important step towards using depth-resolved $β$-NMR to study the low-energy properties of the chiral topological surface state (TSS). With the bulk NMR response now established in several Bi$_{2}$Ch$_{3}$ tetradymite topological insulators (TIs), the prospect of directly probing their chiral TSS using the depth resolution afforded by $β$-NMR remains strong.

cond-mat.mtrl-sci↗