arXiv subjects

Eric M. Lechner

Publications and source records attributed to Eric M. Lechner.

4 recordsLinked to original sources

arXiv2026

Failure of Conventional Roughness Metrics in Assessing Field-Limiting Mesoscopic Topography in SRF Nb Films on Cu

A characteristic corrugated surface morphology of Nb films on Cu is identified, with mesoscopic features comparable to the London penetration depth and coherence length, and its impact on superconducting radio frequency cavity performance metrics is examined. Magnetic field enhancement factors and superheating field suppression factors are calculated within the London model for a representative corrugated geometry. These results demonstrate that roughness trends derived from buffered chemical polished and electropolished Nb cavities do not capture the impact of nanoscale surface morphology on SRF Nb thin film performance, as even surfaces that possess low average roughness can contain geometric features that generate substantial local field enhancement and significantly suppress the Bean Livingston barrier. The influence of surface roughness on impurity diffusion is also investigated, which highlights the roles of increased surface area and local geometric confinement in modifying near surface impurity distributions. Tracking the effect of impurity diffusion on the evolution of magnetic field enhancement, we show that geometrically confined impurity distributions can mitigate nanoscale magnetic field enhancement substantially.

cond-mat.supr-con↗

arXiv2026

Electropolishing-Induced Topographic Defects in Niobium: Insights and Implications for Superconducting Radio Frequency Applications

Electropolishing is the premier surface preparation method for high-Q, high-gradient superconducting RF cavities made of Nb. This leaves behind an apparently smooth surface, yet the achievable peak magnetic fields fall well below the superheating field of Nb, in most cases. In this work, the ultimate surface finish of electropolishing was investigated by studying its effect on highly polished Nb samples. Electropolishing introduces high slope angle sloped-steps at grain boundaries. The magnetic field enhancement and superheating field suppression factors associated with such a geometry are calculated in the London theory. Despite the by-eye smoothness of electropolished Nb, such defects compromise the stability of the low-loss Meissner state, likely limiting the achievable peak accelerating fields in superconducting RF cavities. Finally, the impact of surface roughness on impurity diffusion is investigated which can link surface roughness to the effectiveness of heat treatments like low-temperature baking or nitrogen infusion in the vortex nucleation or hydride hypotheses. Surface roughness tends to decrease the effective dose of impurities as a result of the expansion of impurities into regions with greater internal angle. The effective dose of impurities can be protected by minimizing slope angles and step heights, ensuring uniformity.

cond-mat.supr-con↗

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↗

arXiv2024

Analysis of Thermal Grooving Effects on Vortex Penetration in Vapor-Diffused Nb3Sn

While Nb3Sn theoretically offers better superconducting RF cavity performance (Q0 and Eacc) to Nb at any given temperature, peak RF magnetic fields consistently fall short of the 400 mT prediction. The relatively rough topography of vapor-diffused Nb3Sn is widely conjectured to be one of the factors that limit the attainable performance of Nb3Sn-coated Nb cavities prepared via Sn vapor diffusion. Here we investigate the effect of coating duration on the topography of vapor-diffused Nb3Sn on Nb and calculate the associated magnetic field enhancement and superheating field suppression factors using atomic force microscopy topographies. It is shown that the thermally grooved grain boundaries are major defects which may contribute to a substantial decrease in the achievable accelerating field. The severity of these grooves increases with total coating duration due to the deepening of thermal grooves during the coating process.

cond-mat.supr-con↗