Proximity effect between two superconductors spatially resolved by scanning tunneling spectroscopy

We present a combined experimental and theoretical study of the proximity effect in an atomic-scale controlled junction between two different superconductors. Elaborated on a Si(111) surface, the junction comprises a Pb nanocrystal with an energy gap of 1.2 meV, connected to a crystalline atomic monolayer of lead with a gap of 0.23 meV. Using in situ scanning tunneling spectroscopy we probe the local density of states of this hybrid system both in space and in energy, at temperatures below and above the critical temperature of the superconducting monolayer. Direct and inverse proximity effects are revealed with high resolution. Our observations are precisely explained with the help of a self-consistent solution of the Usadel equations. In particular, our results demonstrate that in the vicinity of the Pb islands, the Pb monolayer locally develops a finite proximity-induced superconducting order parameter, well above its own bulk critical temperature. This leads to a giant proximity effect where the superconducting correlations penetrate inside the monolayer a distance much larger than in a non-superconducting metal.Comment: 13 pages, 14 figures, accepted for publication in Physical Review

Confinement of superconducting fluctuations due to emergent electronic inhomogeneities

The microscopic nature of an insulating state in the vicinity of a superconducting state, in the presence of disorder, is a hotly debated question. While the simplest scenario proposes that Coulomb interactions destroy the Cooper pairs at the transition, leading to localization of single electrons, an alternate possibility supported by experimental observations suggests that Cooper pairs instead directly localize. The question of the homogeneity, granularity, or possibly glassiness of the material on the verge of this transition is intimately related to this fundamental issue. Here, by combining macroscopic and nano-scale studies of superconducting ultrathin NbN films, we reveal nanoscopic electronic inhomogeneities that emerge when the film thickness is reduced. In addition, while thicker films display a purely two-dimensional behaviour in the superconducting fluctuations, we demonstrate a zero-dimensional regime for the thinner samples precisely on the scale of the inhomogeneities. Such behavior is somehow intermediate between the Fermi and Bose insulator paradigms and calls for further investigation to understand the way Cooper pairs continuously evolve from a bound state of fermionic objects into localized bosonic entities.Comment: 29 pages 9 figure

Spectroscopic evidence for strong correlations between local superconducting gap and local Altshuler-Aronov density-of-states suppression in ultrathin NbN films

Disorder has different profound effects on superconducting thin films. For a large variety of materials, increasing disorder reduces electronic screening which enhances electron-electron repulsion. These fermionic effects lead to a mechanism described by Finkelstein: when disorder combined to electron-electron interactions increases, there is a global decrease of the superconducting energy gap $\Delta$ and of the critical temperature $T_c$, the ratio $\Delta$/$k_BT_c$ remaining roughly constant. In addition, in most films an emergent granularity develops with increasing disorder and results in the formation of inhomogeneous superconducting puddles. These gap inhomogeneities are usually accompanied by the development of bosonic features: a pseudogap develops above the critical temperature $T_c$ and the energy gap $\Delta$ starts decoupling from $T_c$. Thus the mechanism(s) driving the appearance of these gap inhomogeneities could result from a complicated interplay between fermionic and bosonic effects. By studying the local electronic properties of a NbN film with scanning tunneling spectroscopy (STS) we show that the inhomogeneous spatial distribution of $\Delta$ is locally strongly correlated to a large depletion in the local density of states (LDOS) around the Fermi level, associated to the Altshuler-Aronov effect induced by strong electronic interactions. By modelling quantitatively the measured LDOS suppression, we show that the latter can be interpreted as local variations of the film resistivity. This local change in resistivity leads to a local variation of $\Delta$ through a local Finkelstein mechanism. Our analysis furnishes a purely fermionic scenario explaining quantitatively the emergent superconducting inhomogeneities, while the precise origin of the latter remained unclear up to now.Comment: 11 pages, 4 figure

Scanning tunneling spectroscopy study of the proximity effect in a disordered two-dimensional metal

The proximity effect between a superconductor and a highly diffusive two-dimensional metal is revealed in a scanning tunneling spectroscopy experiment. The in situ elaborated samples consist of superconducting single crystalline Pb islands interconnected by a nonsuperconducting atomically thin disordered Pb wetting layer. In the vicinity of each superconducting island the wetting layer acquires specific tunneling characteristics which reflect the interplay between the proximity-induced superconductivity and the inherent electron correlations of this ultimate diffusive two-dimensional metal. The observed spatial evolution of the tunneling spectra is accounted for theoretically by combining the Usadel equations with the theory of dynamical Coulomb blockade; the relevant length and energy scales are extracted and found in agreement with available experimental dataWe thank Hermann Grabert for useful discussions. This work was supported by grants from the University Pierre et Marie Curie (UPMC) ‘‘Emergence’’ and by CNRS Ph.D. Grant (L. S.-G.). J. C. C. and F. S. B. acknowledge financial support from the Spanish MICINN (Contracts No. FIS2011-28851-C02-01 and No. FIS2011-28851- C02-02

Электронный рост нанообъектов Pb на поверхностях Si

Выполнены исследования особенностей формирования металлических наноостровков Pb на поверхности кремния методом сканирующей туннельной микроскопии. Показано, что рост наноостровков Pb на поверхности Si происходит в рамках модели Странски—Крастанова; вместе с тем формирование островков сопровождается их расслоением с характерным масштабом 2 нм (7 монослоёв Pb). Обнаруженное явление рассматривается в связи с минимизацией энергии в квантовых ямах, образующихся вследствие эффекта квантовой локализации, и объясняется в рамках модели электронного роста.Виконано дослідження особливостей формування металевих наноострівців Pb на поверхні кремнію методою сканівної тунельної мікроскопії. Показано, що ріст наноострівців Pb на поверхні Si відбувається в рамках моделю Странскі—Крастанова; разом з тим формування острівців супроводжується їх розшаруванням з характерним масштабом 2 нм (7 моношарів Pb). Виявлене явище розглядається в зв’язку з мінімізацією енергії в квантових ямах, які утворюються внаслідок ефекту квантової локалізації, та пояснюються в рамках моделю електронного росту.We report on Pb-islands growth on a surface of silicon. Using the scanning tunnelling microscopy, we show that, while in general the growth follows the Stranski—Krastanov scenario, the formation of Pb islands is accompanied by their lamination with a characteristic scale of two nanometers (7 monolayers of Pb). Such an effect manifests the energy minimum in quantum wells due to the quantum confinement, and it can be explained within the scope of the electronic-growth model

Ex situ elaborated proximity mesoscopic structures for ultrahigh vacuum scanning tunneling spectroscopy

We apply ultrahigh vacuum Scanning Tunneling Spectroscopy (STS) at ultra-low temperature to study proximity phenomena in metallic Cu in contact with superconducting Nb. In order to solve the problem of Cu-surface contamination, Cu(50nm)/Nb(100nm) structures are grown by respecting the inverted order of layers on SiO2/Si substrate. Once transferred into vacuum, the samples are cleaved at the structure-substrate interface. As a result, a contamination-free Cu-surface is exposed in vacuum. It enables high-resolution STS of superconducting correlations induced by proximity from the underlying superconducting Nb layer. By applying magnetic field, we generate unusual proximity-induced superconducting vortices and map them with a high spatial and energy resolution. The suggested method opens a way to access local electronic properties of complex electronic mesoscopic devices by performing ex situ STS under ultrahigh vacuum. © 2014 AIP Publishing LLC

On the importance of measuring accurately LDOS maps using scanning tunneling spectroscopy in materials presenting atom-dependent charge order: the case of the correlated Pb/Si(111) single atomic layer

We show how to properly extract the local charge order in two-dimensional materials from scanning tunneling microscopy/spectroscopy (STM/STS) measurements. When the charge order presents spatial variations at the atomic scale inside the unit cell and is energy dependent, particular care should be taken. In such cases the use of the lock-in technique, while acquiring an STM topography in closed feedback loop, leads to systematically incorrect dI/dV measurements giving a false local charge order. A correct method is either to perform a constant height measurement or to perform a full grid of dI/dV(V) spectroscopies, using a bias voltage setpoint outside the material bandwidth where the local density-of-states (LDOS) is spatially homogeneous. We take as a paradigmatic example of two-dimensional material the 1/3 single-layer Pb/Si(111). As large areas of this phase cannot be grown, charge ordering in this system is not accessible to angular resolved photoemission or grazing x-ray diffraction. Previous investigations by STM/STS supplemented by {\it ab initio} Density Functional Theory (DFT) calculations concluded that this material undergoes a phase transition to a low-temperature $3\times 3$ reconstruction where one Pb atom moves up, the two remaining Pb atoms shifting down. A third STM/STS study by Adler {\it et al.} [PRL 123, 086401 (2019)] came to the opposite conclusion, i.e. that two Pb atoms move up, while one Pb atom shifts down. This latter erroneous conclusion comes from a misuse of the lock-in technique. In contrast, using a full grid of dI/dV(V) spectroscopy measurements, we show that the energy-dependent LDOS maps agree very well with state-of-the-art DFT calculations confirming the one-up two-down charge ordering. This structural and charge re-ordering in the $3\times 3$ unit cell is equally driven by electron-electron interactions and the coupling to the substrate.Comment: 11 pages, 3 figure