Nanostructured surfaces

Conductance dips in point-contact Andreev reflection spectra
Volkov S.1, Zhitlukhina E.2, Gregor M.1, Plecenik T.1, Plecenik A.1, Belogolovskii M.2
1 Faculty of Mathematics, Physics and Informatics, Comenius University in Bratislava, Mlynská dolina F2, 842 48 Bratislava, Slovakia.
2 Faculty of Physics and Technology, Donetsk National University, 600-richchia Str., 21, Vinnytsia-21021, Ukraine

Actually, point-contact Andreev reflection (PCAR) spectroscopy has become a powerful tool for investigating superconducting materials. In the most popular version of this technique, a sharp nano-sized tip forms a ballistic contact with a surface of the superconductor. Under ideal conditions, its differential conductance G(V) = dI(V)/dV versus voltage V is dominated by Andreev reflection processes and well-developed Blonder-Tinkham-Klapwijk (BTK) model provides valuable insight on the nature of superconductivity in the samples. Despite significant progress in the development of this approach, some unexpected features, which cannot be appropriately explained by the BTK theory, are frequently arising in the G(V) curves. Understanding the origin of these features would provide an objective interpretation of the PCAR spectra unsuitable for fitting with the BTK theory.
In contrast to the BTK results, the differential conductance does not plateau at high biases but frequently exhibits symmetrical over voltage dips usually ignored when the fitting the measured curves to theoretical ones. In this work, we show that the full G(V)  characteristic can be well described by a unified model that assumes the presence of a degraded or proximity-induced superconducting nanoscale sheath at the surface of a superconductor studied. The calculations are based on a scattering formalism taking into account Andreev electron-into-hole (and inverse) reflections at normal metal-superconductor interfaces as well as transmission and backscattering events at the interfaces between metallic layers. Our analysis shows that the PCAR spectra with G(V)  dips at finite biases probe superconducting correlations in various depths of the sample, depending on the bias voltage.
To support the interpretation of previously unexplained conductance dips, we have reconsidered our PCAR data for MgB2/Al/interface/Pt heterostructures [1] and performed new experiments on NbN/NiCu/Al/interface/Pt multilayers. In analyzing the data, we allow for a lower pairing potential at the superconductor surface and obtain a good agreement with the experiment.

1.	Zhitlukhina E., Dvoranová M.,·Plecenik T.,·Gregor M., Belogolovskii M., Plecenik A. Electron–boson coupling in superconductors studied by a self‑formed nanofilament device // Appl Nanosci. - 2019. - https://doi.org/10.1007/s13204-019-01082-6