arXiv subjects

Eric M. Lechner

Publications and source records attributed to Eric M. Lechner.

3 recordsLinked to original sources

arXiv2025

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 ($\lambda_L$) and Bardeen-Cooper-Schrieffer (BCS) coherence length ($\xi_0$) in oxygen-doped niobium, with impurity concentrations spanning the "clean" to "dirty" limits. Two depth-resolved techniques - low-energy muon spin spectroscopy (LE-$\mu$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 $\lambda_L = 29.1(10)$ nm and $\xi_0 = 39.9(25)$ nm, corresponding to a Ginzburg-Landau (GL) parameter of $\kappa = 0.70(5)$. The obtained $\lambda_L$ and $\kappa$ 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

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↗

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↗