Correlating Electron–Phonon Coupling and In Situ High-Temperature Atomic-Scale Surface Structure at the Metallic Nb(100) Surface by Helium Atom Scattering and Density Functional Theory

Abstract

Helium (He) atom scattering (HAS) simultaneously measured the surface electron−phonon coupling (EPC, SEPC) constant (λ, λ S ) and in situ high-temperature atomicscale surface structure of the unreconstructed, metallic Nb(100) surface. The Nb(100) surface λ S is 0.50 ± 0.08, and its atomic-scale surface structure is confirmed. The λ S measured for the Nb(100) surface is ∼1/2 the reported bulk Nb λ values. The significance of Nb(100)'s diminished EPC was elucidated by estimating relevant superconducting properties from the measured λ S , surface Debye temperature, known material parameters, and well-established equations. Density functional theory (DFT) with local averaging agrees well with the HAS data. A critical temperature (T C ) of 1.4−3.6 K, a superheating field (H sh at 2 K) ≤ 0.16 T, and a superconducting gap (at 2 K) ≤ 1.0 meV were estimated from these measurements. These results indicate that the Nb(100) surface has decreased superconducting properties relative to the bulk. This study shows that these effects may also be due to the interface itself even without oxygen. These results contain the first λ measured for the metallic Nb(100) and any Nb surface. These measurements begin a fundamental understanding of the atomic-scale surface structure's effect on EPC and superconductivity in Nb.

Distinguishing the Roles of Atomic-Scale Surface Structure and Chemical Composition in Electron Phonon Coupling of the Nb(100) Surface Oxide Reconstruction

“…The work presented herein provides the first measurement of λ of the (3 × 1)-O/Nb(100) surface oxide reconstruction. These results distinguish the effect of the surface oxide reconstruction from that of interstitial oxygen on Nb(100)’s surface superconductivity . These findings begin to fill a critical gap in the foundational knowledge of the effect of atomic scale surface structure and composition on SRF cavity performance.…”

“…Conventional superconducting states form due to electron–phonon interactions that condense electrons into Cooper pairs. ,− The electron–phonon coupling (EPC) constant (λ) is a dimensionless constant that quantifies the effective strength of electron–phonon interactions in a material. , Furthermore, this constant determines many superconducting properties such as the critical temperature ( T C ) and superconducting gap. ,, We have brought into effective action helium (He) atom scattering’s (HAS) sensitivity to both in situ high-temperature surface structure and surface EPC (SEPC) constant (λ S ) to connect atomic-scale surface structure with its effect on λ S and thus surface superconductivity. Previously, we have demonstrated this capability by measuring λ S and atomic-scale surface structure simultaneously on the unreconstructed Nb(100) surface and now set forth in this work the results of a similar study on the Nb(100) surface oxide reconstruction …”

“…Following a similar approach to that used for the metallic, unreconstructed Nb(100) surface, we considered the range of interaction of the electron density of the He atom compared to the electron density of the atoms on the surface, given the distance between them. The upper bound averages over single atoms in the unit cell, considering a shorter interaction range than expected.…”

“…Such an understanding of the role of surface structure and chemical composition, as well as their resulting effects on superconductivity at the surface, remains elusive. This relationship is not well understood, and we have only just begun its elucidation with simultaneous in situ atomic scale surface structure and surface EPC measurements on the bare metallic Nb(100) surface …”

“…While we have recently made the first measurement of surface EPC on any Nb surface, specifically the metallic, unreconstructed Nb(100) surface, EPC in bulk Nb has been studied thoroughly. − The λ of bulk Nb has been measured by electronic Raman scattering (1.15), proximity electron tunneling spectroscopy (1.04 ± 0.06), , femtosecond pump–probe measurements (1.16 ± 0.11), and calculated with McMillan’s expression for T C from accompanying measurements of appropriate normal-state parameters (0.92) . Recently, we measured the EPC of the bare metal surface to be 0.50 and used it to estimate benchmark superconducting properties, correlating atomic scale structure to speculative SRF cavity performance . The diminished EPC at the surface of the unreconstructed metallic Nb(100) surface implies that even without O or reconstruction, the presence of an interface approximately halves the λ.…”