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Register a new userAnomalous proximity effect in gold coated (110) $YBa_2Cu_3O_{7-delta}$ films: Penetration of the Andreev bound states
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Scanning tunneling spectroscopy of (110) $YBa_2Cu_3O_{7-delta}/Au$ bi-layers reveal a proximity effect markedly different from the conventional one. While proximity-induced mini-gaps rarely appear in the Au layer, the Andreev bound states clearly penetrate into the metal. Zero bias conductance peaks are measured on Au layers thinner than 7 nm with magnitude similar to those detected on the bare superconductor films. The peaks then decay abruptly with Au thickness and disappear above 10 nm. This length is shorter than the normal coherence length and corresponds to the (ballistic) mean free path.
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Scanning tunneling spectroscopy on gold layers over-coating textit{c}-axis $YBa_2Ca_3O_{7-delta}$ (YBCO) films reveals proximity induced gap structures. The gap size reduced exponentially with distance from textit{a}-axis facets, indicating that the proximity effect is primarily due to the (100) YBCO facets. The penetration depth of superconductivity into the gold is $sim 30$ nm, in good agreement with estimations for the dirty limit. The extrapolated gap at the interface is $sim 15$ meV, consistent with recent point-contact experiments. The proximity-induced order parameter appears to have predominant textit{s}-wave symmetry.
Scanning tunneling spectroscopy of thin epitaxial $SrRuO_{3}/(110)YBa_2Cu_3O_{7-delta}$ ferromagnet/superconductor bilayers, reveal a clear penetration of the Andreev bound states into the ferromagnetic layer. The penetration is manifested in the density of states of the ferromagnet as a split zero bias conductance peak with an imbalance between peak heights. Our data indicate that the splitting occurs at the superconductor side as a consequence of induced magnetization, confirming recent theoretical predictions. The imbalance is attributed to the spin polarization in the ferromagnet.
We study the penetration field $H_{rm P}$ for vortex nanocrystals nucleated in micron-sized samples with edges aligned along the nodal and anti-nodal directions of the d-wave superconducting parameter of Bi$_2$Sr$_2$CaCu$_2$O$_{8 - delta}$. Here we present evidence that the $H_{rm P}$ for nanocrystals nucleated in samples with edges parallel to the nodal direction is larger than for the antinodal case, $sim 72$,% at low temperatures. This finding supports the theoretical proposal that surface Andreev bound states appearing in a sample with edges parallel to the nodal direction would produce an anomalous Meissner current that increases the Bean-Livingston barrier for vortex penetration.This has been detected thanks to the nucleation of vortex nanocrystals with a significant surface-to-volume ratio.
We have measured Andreev reflections between an Au tip and Y_{1-x}Ca_{x}Ba_{2}Cu_{3}O_{7 - delta} thin films in the in-plane orientation. The conductance spectra are best fitted with a pair potential having the d_{x^{2}-y^{2}+is symmetry. We find that the amplitude of the is component is enhanced as the contact transparency is increased. This is an indication for an unusual proximity effect that modifies the pair potential in the superconductor near the surface with the normal metal.
We study dynamic fluctuation effects of $YBa_2Cu_3O_{7-delta}$ thin films in zero field around $T_c$ by doing frequency-dependent microwave conductivity measurements at different powers. The length scales probed in the experiments are varied systematically allowing us to analyze data which are not affected by the finite thickness of the films, and to observe single-parameter scaling. DC current-voltage characteristics have also been measured to independently probe fluctuations in the same samples. The combination of DC and microwave measurements allows us to precisely determine critical parameters. Our results give a dynamical scaling exponent $z=1.55pm0.15$, which is consistent with model E-dynamics.
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