Surface superconductivity in lead

A transition to the surface superconducting state is detected in lead single crystals at a temperature approximately 0.25 mK higher than the bulk superconducting transition temperature. The (H, T) phase diagram of this state is analyzed.     

Surface superconductivity and supercooling in lead

Observations are made of metastable supercooled normal bulk states in lead samples. It is demonstrated that such states are realized when the critical field of surface superconductivity H_c3 is lower than the critical field H_c and in the opposite case. Therefore, the surface superconductivity is not a nucleus with supercritical parameters for the bulk superconductivity.     

Surface and volume superconductivity of Pb embedded in nanopores

The heat capacity of lead embedded in glass nanopores (7 nm in diameter) and bulk lead was studied in the temperature range 2–40 K without a magnetic field and in magnetic fields of 1–8 T. The properties of lead nanoparticles and bulk lead were compared. The results obtained allowed us to separate the surface superconductivity from the volume superconductivity. The temperature dependence of the heat capacity of lead nanoparticles was shown to exhibit two superconducting transitions above and below the transition temperature for bulk lead (T _c = 7.2 K), which are associated with the surface and volume superconductivity. The upper critical fields H _c3 for the surface superconductivity and H _c2 for the volume superconductivity were determined. It turned out that these fields for Pb nanoparticles are two orders of magnitude higher than those for bulk lead. The “superconductor-normal metal” phase diagrams were constructed for lead nanoparticles. The study established an increase in the density of low-frequency excitations in Pb nanocrystals as compared to bulk Pb and a difference in the electronic heat capacity of Pb nanoparticles as compared to bulk Pb.     

Surface plasmons in layered structures and their role in superconductivity

The contribution of surface plasmons to electron-electron interaction is examined by calculating the effective Coulomb repulsion μ in the case of periodically spaced conducting planes separated by an insulator of dielectric constant ?_i. Substantial reduction in μ is obtained when $\text{?}{}_{\mathrm{i}}\text{》 1}$.     

Surface superconductivity in multilayered rhombohedral graphene: Supercurrent

The supercurrent for the surface superconductivity of a flat-band multilayered rhombohedral graphene is calculated. Despite the absence of dispersion of the excitation spectrum, the supercurrent is finite. The critical current is proportional to the zero-temperature superconducting gap, i.e., to the superconducting critical temperature and to the size of the flat band in the momentum space.     

Surface superconductivity induced by an external electric field

A theory of surface superconductivity induced by an external electric field in superconducting semimetals, metals and semiconductors is presented. It is shown that the inhomogeneous surface superconducting state deeply penetrates into the bulk of the sample. The dependence of the critical surface temperature and magnetic field on the external electric field is calculated.     

The effect of dielectric coating on the superconductivity of aluminum films

We report measurements on the changes of the superconducting transition temperatureT _c and the normal resistanceR _n of quench condensed aluminum films during dielectric coating. N₂ and noble gas coatings result in an exponential change of the superconducting properties due to the modification of the phonon spectrum. O₂ and Se coatings result in a nonmonotonic change ofT _c andR _n due to charge transfer processes and resonance scattering of the electrons into localized states within the coating layers.     

Surface tension of pure aluminum melt

The basic reason for the significant spread in the experimental data on the surface tension (σ) of aluminum is shown to be the adsorption of a residual gas in a vacuum chamber. The processing of the kinetic curves σ(θ) by a method proposed in this work yields for aluminum σ=1070 mJ/m².     

Surface structure and solidification morphology of aluminum nanoclusters

Classical molecular dynamics simulation with embedded atom method potential had been performed to investigate the surface structure and solidification morphology of aluminum nanoclusters Al_n (n=256, 604, 1220 and 2048). It is found that Al cluster surfaces are comprised of (1 1 1) and (0 0 1) crystal planes. (1 1 0) crystal plane is not found on Al cluster surfaces in our simulation. On the surfaces of smaller Al clusters (n=256 and 604), (1 1 1) crystal planes are dominant. On larger Al clusters (n=1220 and 2048), (1 1 1) planes are still dominant but (0 0 1) planes cannot be neglected. Atomic density on cluster (1 1 1)/(0 0 1) surface is smaller/larger than the corresponding value on bulk surface. Computational analysis on total surface area and surface energies indicates that the total surface energy of an ideal Al nanocluster has the minimum value when (0 0 1) planes occupy 25% of the total surface area. We predict that a melted Al cluster will be a truncated octahedron after equilibrium solidification.     

Surface superconductivity of dirty two-band superconductors: applications to MgB2

The minimal magnetic field H(c2) destroying superconductivity in the bulk of a superconductor is smaller than the magnetic field H(c3) needed to destroy surface superconductivity if the surface of a superconductor coincides with one of the crystallographic planes and is parallel to the external magnetic field. While for a dirty single-band superconductor the ratio of H(c3) to H(c2) is a universal temperature-independent constant 1.6946, for dirty two-band superconductors this is not the case. I show that in the latter case the interaction of the two bands leads to a novel scenario with the ratio H(c3)/H(c2) varying with temperature and taking values larger and smaller than 1.6946. The results are applied to MgB(2) and compared with recent experiments (A. Rydh, cond-mat/0307445).     

Surface guided modes in an aluminum oxide thin film

Energy losses due to the excitation of surface guided waves have been observed in inelastic electron scattering from thin films of aluminum oxide.     

Electronegativity and superconductivity

This article gives a new criterion for superconductivity. It can be applied to all the elements in the periodic table. The criterion shows that the values of electronegativity of all superconducting elements concentrate in a narrow range from 1.3–1.9 and that elements with values outside this region would be non-superconductive.Further study of the relation between electronegativity and superconductivity of elements will contribute to understanding the connection between the electrostatic action of the electron to the crystal lattice and superconductivity, which may further elucidate the mechanism of superconductivity.     

Stripes and superconductivity in cuprates

Holes doped into the CuO₂ planes of cuprate parent compounds frustrate the antiferromagnetic order. The development of spin and charge stripes provides a compromise between the competing magnetic and kinetic energies. Static stripe order has been observed only in certain particular compounds, but there are signatures which suggest that dynamic stripe correlations are common in the cuprates. Though stripe order is bad for superconducting phase coherence, stripes are compatible with strong pairing. Ironically, magnetic-field-induced stripe order appears to enhance the stability of superconducting order within the planes.     

Unconventional superconductivity in low density electron systems and conventional superconductivity in hydrogen metallic alloys

In this short review, we first discuss the results, which are mainly devoted to the generalizations of the famous Kohn–Luttinger mechanism of superconductivity in purely repulsive fermion systems at low electron densities. In the context of repulsive-U Hubbard model and Shubin–Vonsovsky model we consider briefly the superconducting phase diagrams and the symmetries of the order parameter in novel strongly correlated electron systems including idealized monolayer and bilayer graphene. We stress that purely repulsive fermion systems are mainly the subject of unconventional low-temperature superconductivity. To get the high temperature superconductivity in cuprates (with T_C of the order of 100 K) we should proceed to the t–J model with the van der Waals interaction potential and the competition between short-range repulsion and long-range attraction. Finally we note that to describe superconductivity in metallic hydrogen alloys under pressure (with T_C of the order of 200 K) it is reasonable to reexamine more conventional mechanisms connected with electron–phonon interaction. These mechanisms arise in the attractive-U Hubbard model with static onsite or intersite attractive potential or in more realistic theories (which include retardation effects) such as Migdal–Eliashberg strong coupling theory or even Fermi–Bose mixture theory of Ranninger et al. and its generalizations.     

Bipolaronic superconductivity in antiferromagnets

As a model of the cuprate superconductors, we have studied thep hole motion in a planar antiferromagnetic (AFM) background and ac-axis boson field. The indirect coupling between thed spins through thep holes is considered. In a range of the hole concentration, the indirect Cu–Cu interaction enhances the planar AFM coupling though it destroys the weakc-axis AFM order. At higher concentrations, the compensation of thed spins by thep holes occurs. For the strongp-d exchange coupling, thep holes can pair to form small magnetic bipolarons in the enhanced planar AFM background. The in-plane motion of the bipolarons is independent of thec-axis motion assisted by bosons. The superconducting properties of the cuprate superconductors are determined by a 2+1 dimensional bipolaron Hamiltonian. The results obtained from our model are consistent with the observations on the cuprate superconductors.     

Superfluidity and superconductivity in the universe

The paper aims to elucidate the current status of the problem concerning the existence and observation of superfluid and superconducting states in the universe, that is, under cosmic conditions. Following an introduction, the paper discusses Bose-Einstein condensation, superfluidity, and superconductivity; possibilities for the occurrence of superfluidity and superconductivity under cosmic conditions; superconductivity of dense, degenerate electron plasma (large planets, white dwarfs); superfluidity and superconductivity in neutron stars; and finally superfluidity in a cosmological neutrino “sea.”