Phonon softening in nanostructured phonon-mediated superconductors (review).

Various aspects of phonon spectrum changes in nanostructured phonon-mediated superconductors are considered. It is shown how, with the development of experimental techniques and, accordingly, obtaining new results, the understanding of the influence of the surface and nanoscale on the magnitude of the electron-phonon interaction and the critical temperatureTcchanged and deepened. The review is organized as follows. After theIntroduction, in thesecondpart we give the quick theoretical background for the description of superconductivity within the framework of various formalisms. In thethirdpart we describe the properties of nanostructured (granular) thin films paying attention to the impact of grain sizes and methods of deposition on theTcvalue. The role of material parameters is underlined and different aspects of the behavior of granular thin films are discussed. In thefourthsection the impact of external sources of modification of the phonon spectra like noble gases and organic molecules are considered. Problems and progress in this area are discussed. Thefifthpart is dedicated to the phonon modification and related quantum size effects in nanostructured superconductors. In thesixthpart we review the results of direct evidence of phonon softening in nanostructured superconductors and in theseventhsection we discuss a possible alternative description of the superconducting properties of nanostructured superconductors related to the concept of metamaterials. In theeighthandninthparts we review the impact of substrates with lattice mismatched parameters and graphene sheets, respectively, on the modification of the phonon spectrum and enhancement of superconductivity in various superconducting thin films. Finally, in the lasttenthsection we consider the nonequilibrium superconductivity driven by femtosecond pulses of light, which leads to generation of coherent phonons and to a significant increase in the critical temperature in a number of superconducting materials.

Correlation analysis of vibration modes in physical vapour deposited Bi2Se3 thin films probed by the Raman mapping technique.

In this work, the Raman spectroscopy mapping technique is used for the analysis of mechanical strain in Bi2Se3 thin films of various (3-400 nm) thicknesses synthesized by physical vapour deposition on amorphous quartz and single-layer graphene substrates. The evaluation of strain effects is based on the correlation analysis of in-plane (E2 g) and out-of-plane (A2 1g) Raman mode positions. For Bi2Se3 films deposited on quartz, experimental datapoints are scattered along the line with a slope of ∼0.85, related to the distribution of hydrostatic strain. In contrast to quartz/Bi2Se3 samples, for graphene/Bi2Se3 heterostructures with the same thicknesses, an additional negative slope of ∼-0.85, which can be associated with the distribution of the in-plane (a-b) biaxial tensile strain due to the film-substrate lattice mismatch, is observed. The algorithm of phonon deformation potential (PDP) calculation based on the proposed strain analysis for the 3 nm thick Bi2Se3 film deposited on the graphene substrate, where the strain is considered to be coherent across the thickness, is demonstrated. The PDPs for biaxial in-plane strain of the Bi2Se3 3 nm film in in-plane and out-of-plane modes are equal to -7.64 cm-1/% and -6.97 cm-1/%, respectively.

Low temperature injected-caused charge carrier instability in n-type silicon below insulator-to-metal transition.

We report on the electric transport properties of Si heavily doped with Sb at concentration just below the insulator-to-metal transition in the temperature range 1.9-3.0 K for current density J < 0.2 A cm-2. The change in the sign of the temperature dependence of the differential resistivity [Formula: see text] was observed: the d[Formula: see text]/dT is positive if J < 0.045 A cm-2 whereas it becomes negative at J > 0.045 A cm-2. The effect is explained assuming the exchange by electrons between the upper Hubbard band (UHB) and the conduction band. The obtained J dependencies of the activation energy, nonequilibrium concentration, mobility and scattering time of the conduction electrons correspond well to this hypothesis. The reason for charge instability is the Coulomb repulsion between electrons occupying states both in the UHB and conduction band. The estimated J dependencies of the conduction electrons lifetime and concentration of the D- states in the UHB strongly supports this assumption.

NbN superconducting nanonetwork fabricated using porous silicon templates and high-resolution electron beam lithography.

Superconducting NbN nanonetworks with a very small number of interconnected nanowires, with diameter of the order of 4 nm, are fabricated combining a bottom-up (use of porous silicon nanotemplates) with a top-down technique (high-resolution electron beam lithography). The method is easy to control and allows the fabrication of devices, on a robust support, with electrical properties close to a one-dimensional superconductor that can be used fruitfully for novel applications.

Superconducting nanowire quantum interference device based on Nb ultrathin films deposited on self-assembled porous Si templates.

Magnetoresistance oscillations were observed on networks of superconducting ultrathin Nb nanowires presenting evidence of either thermal or quantum activated phase slips. The magnetic transport data, discussed in the framework of different scenarios, reveal that the system behaves coherently in the temperature range where the contribution of the fluctuations is important.

Upper critical magnetic fields in superconductor/ferromagnet hybrids.

We measured parallel upper critical magnetic fields in Nb/PdNi and Nb/CuNi bilayers and Nb/PdNi/Nb and Nb/CuNi/Nb trilayers. In the bilayers case the measurements reveal a dependence of the reduced two-dimensional-three-dimensional crossover temperature, t(cr) = T(cr)/T(c), on the different values of the superconductor (S)/ferromagnet (F) interface transparencies. In the case of the trilayers we observe that the reduced crossover temperature as a function of the ferromagnetic layer thickness, d(F), reaches a value equal to one only when the π-phase occurs in these systems. Also in this case the influence of the interface transparency on the observed results is discussed.