Superconducting nanofilms: molecule-like pairing induced by quantum confinement (original) (raw)
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Superconducting nanofilms: Andreev-type states induced by quantum confinement
Physical Review B, 2008
Quantum confinement of the transverse electron motion is the major effect governing the superconducting properties of high-quality metallic nanofilms, leading to a nonuniform transverse distribution of the superconducting condensate. In this case the order parameter can exhibit significant local enhancements due to these quantum-size effects and, consequently, quasiparticles have lower energies when they avoid the local enhancements of the pair condensate. Such excitations can be considered as new Andreev-type quasiparticles but now induced by quantum confinement. By numerically solving the Bogoliubov-de Gennes equations and using Anderson's approximate solution to these equations, we: ͑a͒ formulate a criterion for such new Andreev-type states ͑NATS͒ and ͑b͒ study their effect on the superconducting characteristics in metallic nanofilms. We also argue that nanofilms made of low-carrier-density materials, e.g., of superconducting semiconductors, can be a more optimal choice for the observations of NATS and other quantum-size superconducting effects.
Physical Review B, 2012
To date, several experimental groups reported measurements of the thickness dependence of Tc of atomically uniform single-crystalline Pb nanofilms. The reported amplitude of the Tc-oscillations varies significantly from one experiment to another. Here we propose that the reason for this unresolved issue is an interplay of the quantum-size variations in the single-electron density of states with thickness-dependent oscillations in the phonon mediated electron-electron coupling. Such oscillations in the coupling depend on the substrate material, the quality of the interface, the protection cover and other details of the fabrication process, changing from one experiment to another. This explains why the available data do not exhibit one-voice consistency about the amplitude of the Tc-oscillations. Our analyses are based on a numerical solution of the Bogoliubovde Gennes equations for a superconducting slab.
Nanoscale superconductivity: Nanowires and nanofilms
Physica C: Superconductivity, 2008
Quantum confinement of electrons in highly crystalline nanowires and nanofilms results in the formation of a series of subbands that move in energy with changing wire/film thickness. When the bottom of such a subband moves through the Fermi surface, the density of states changes and a size-dependent superconducting resonance appears, leading to quantum-size oscillations in the critical temperature T c (the order parameter and energy gap) and the critical magnetic field H c as function of the thickness. Our theoretical formulation is based on a numerical solution of the Bogoliubov-de Gennes equations in the clean limit. A quantitative description is given of recent experimental data on the thickness dependence of T c in Al and Sn nanowires, and on the film thickness dependence of T c in Al and Pb nanofilms. In the presence of quantum confinement the spatial distribution of the pair condensate is very inhomogeneous, which leads to the formation of new Andreev-type states induced by quantum confinement. Our investigation suggests that these states can play an important role in superconducting nanowires, decreasing the ratio of the energy gap to the critical temperature. We also show that for cylindrical nanowires with diameters K 10-15 nm, the superconducting-to-normal phase transition driven by a parallel magnetic field becomes of first order. The critical field is strongly enhanced, and exhibits pronounced quantum-size oscillations.
Nanowires and nanofilms: Superconductivity in quantum-size regime
Physica C: Superconductivity, 2008
Quantum confinement is the major mechanism governing superconductivity in highly crystalline metallic specimens with nanoscale dimensions. Changes in the single-electron spectrum due to size quantization result in quantum-size variations of the superconducting properties (e.g., critical temperature and critical magnetic field) with profound enhancements at the points of the superconducting resonances. Our investigation is based on a self-consistent numerical solution of the Bogoliubov-de Gennes equations for clean metallic nanofilms and nanowires.
Physical Review Letters, 2011
The electron-phonon coupling (EPC) strength for each phonon mode in superconducting Pb films is measured by inelastic helium atom scattering (IHAS). This surprising ability of IHAS relies on two facts: (a) In ultrathin metal films, the EPC range exceeds the film thickness, thus enabling IHAS to detect most film phonons, even 1 nm below the surface; (b) IHAS scattering amplitudes from single phonons are shown, by first-principle arguments, to be proportional to the respective EPC strengths. Thus IHAS is the first experiment providing mode-selected EPC strengths (mode-lambda spectroscopy).
Atomically flat superconducting nanofilms: multiband properties and mean-field theory
Superconductor Science and Technology, 2015
Recent progress in materials synthesis enabled fabrication of superconducting atomically flat single-crystalline metallic nanofilms with thicknesses down to a few monolayers. Interest in such nano-thin systems is attracted by the dimensional 3D-2D crossover in their coherent properties which occurs with decreasing the film thickness. The first fundamental aspect of this crossover is dictated by the Mermin-Wagner-Hohenberg theorem and concerns frustration of the long-range order due to superconductive fluctuations and the possibility to track its impact with an unprecedented level of control. The second important aspect is related to the Fabri-Pérot modes of the electronic motion strongly bound in the direction perpendicular to the nanofilm. The formation of such modes results in a pronounced multiband structure that changes with the nanofilm thickness and affects both the mean-field behavior and superconductive fluctuations. Though the subject is very rich in physics, it is scarcely investigated to date. The main obstacle is that there are no manageable models to study a complex magnetic response in this case. Full microscopic consideration is rather time consuming, if practicable at all, while the standard Ginzburg-Landau theory is not applicable. In the present work we review the main achievements in the subject to date, and construct and justify an efficient multiband mean-field formalism which allows for numerical and even analytical treatment of nano-thin superconductors in applied magnetic fields.
Shape-Resonant Superconductivity in Nanofilms: from Weak to Strong Coupling
Journal of Superconductivity and Novel Magnetism, 2016
Ultrathin superconductors of different materials are becoming a powerful platform to find mechanisms for enhancement of superconductivity, exploiting shape resonances in different superconducting properties. Here we evaluate the superconducting gap and its spatial profile, the multiple gap components, and the chemical potential, of generic superconducting nanofilms, considering the pairing attraction and its energy scale as tunable parameters, from weak to strong coupling, at