Distortions to the penetration depth and coherence length of superconductor/normal-metal superlattices

Abstract

Superconducting (S) thin film superlattices composed of Nb and a normal metal spacer (N) have been extensively utilized in Josephson junctions given their favorable surface roughness compared to Nb films of comparable thickness. In this work, we characterize the London penetration depth and Ginzburg-Landau coherence lengths of S /N superlattices using polarized neutron reflectometry and electrical transport. Despite the normal metal spacer layers being only approximately 8% of the total superlattice thickness, we surprisingly find that the introduction of these thin N spacers between S layers leads to a dramatic increase in the measured London penetration depth compared to that of a single Nb film of comparable thickness. Using the measured values for the effective inand out-of-plane coherence lengths, we quantify the induced anisotropy of the superlattice samples and compare to a single Nb film sample. From these results, we find that that the superlattices behave similarly to layered 2D superconductors.

Enhancement of supercurrent through ferromagnetic materials by interface engineering

“…Our samples are in the intermediate case, with each electrode thickness within a factor of two of its λ value. We discovered recently [29,30] that λ for our bottom [Nb/Al] 3 /Nb multilayer is of order 185 nm -much longer than the value of 85 nm we expect for our sputtered Nb films [31]. If we use λ…”

Absence of magnetic interactions in Ni–Nb ferromagnet–superconductor bilayers

“…The key observable of Bergeret et al is that the induced ferromagnetism decays over the coherence length (ξ), which for thin film Nb is much shorter than λ L [21][22][23]. Additionally, spin-triplet pairs can modify the magnetic response of F-S systems, for example, by introducing a paramagnetic Meissner component [24][25][26][27][28][29], observed experimentally by the muon technique [30][31][32].…”

“…In the first experiment, we use polarized neutron reflectometry (PNR) as a sensitive probe of screening and buried interfaces in our Ni-Nb bilayer. Previously, we have used PNR to measure λ L = 96 ± 9 nm (uncertainty represents one standard error) inside a 200 nm thick Nb single layer film in the Meissner state [23]. However, unlike our previous study, here we add the Ni (2.8 nm) layer below the Nb (200 nm) layer.…”

Supercurrent diode effect in thin film Nb tracks

“…The increased is expected for the epitaxial Nb due to the decrease in crystallographic defects such as grain boundaries. The properties of our polycrystalline Nb thin films are reported elsewhere 25 .…”

“…We next study the track width dependence of the supercurrent diode effect in the polycrystalline and epitaxial Nb samples. For polycrystalline Nb, the London penetration depth λ L = 96 nm 25 , providing an estimate of the Pearl penetration depth λ P = 2λ 2 L /t ≈ 300 nm. For comparable epitaxial Nb, λ L tends towards bulk 26 providing an estimate λ P ≈ 50 nm.…”