Phonon softening in nanostructures phonon-mediated superconductors (review)

Abstract

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 infl uence of the surface and nanoscale on the magnitude of the electron–phonon interaction and the critical temperature T c changed and deepened. The review is organized as follows. After the Introduction, in the second part we give the quick theoretical background for the description of superconductivity within the framework of various formalisms. In the third part we describe the properties of nanostructured (granular) thin fi lms paying attention to the impact of grain sizes and methods of deposition on the T c value. The role of material parameters is underlined and different aspects of the behavior of granular thin fi lms are discussed. In the fourth section the impact of external sources of modifi cation of the phonon spectra like noble gases and organic molecules are considered. Problems and progress in this area are discussed. The fi fth part is dedicated to the phonon modifi cation and related quantum size effects in nanostructured superconductors. In the sixth part we review the results of direct evidence of phonon softening in nanostructured superconductors and in the seventh section we discuss a possible alternative description of the superconducting properties of nanostructured superconductors related to the concept of metamaterials. In the eighth and ninth parts we review the impact of substrates with lattice mismatched parameters and graphene sheets, respectively, on the modifi cation of the phonon spectrum and enhancement of superconductivity in various superconducting thin fi lms. Finally, in the last tenth section we consider the nonequilibrium superconductivity driven by femtosecond pulses of light, which leads to generation of coherent phonons and to a signifi cant increase in the critical temperature in a number of superconducting materials.