High-temperature superconductivity using a model of hydrogen bonds.

Recently, there has been much interest in high-temperature superconductors and more recently in hydrogen-based superconductors. This work offers a simple model that explains the behavior of the superconducting gap based on naive BCS (Bardeen-Cooper-Schrieffer) theory and reproduces most effects seen in experiments, including the isotope effect and [Formula: see text] enhancement as a function of pressure. We show that this is due to a combination of the factors appearing in the gap equation: the matrix element between the proton states and the level splitting of the proton.

Reduction of Electron Repulsion and Enhancement of T_{c} in Small Diffusive Superconducting Grains.

The superconducting properties of small metallic grains has been a topic of active research for half a century now. Early experiments demonstrated a remarkable rise in the critical temperature, T_{c}, with reducing grain size in a variety of materials. In two-dimensional diffusive superconductors, T_{c} is decreased due to enhanced Coulomb repulsion.

Efficiency and dissipation in a two-terminal thermoelectric junction, emphasizing small dissipation.

The efficiency and cooling power of a two-terminal thermoelectric refrigerator are analyzed near the limit of vanishing dissipation (ideal system), where the optimal efficiency is the Carnot one, but the cooling power vanishes. This limit, where transport occurs only via a single sharp electronic energy, has been referred to as "strong coupling" or "the best thermoelectric." Confining the discussion to the linear-response regime, it is found that "parasitic" effects that make the system deviate from the ideal limit, and reduce the efficiency from the Carnot limit, are crucial for the usefulness of the device.

Scale-dependent competing interactions: sign reversal of the average persistent current.

The interaction-induced orbital magnetic response of a nanoscale system, modeled by the persistent current in a ring geometry, is evaluated for a system which is a superconductor in the bulk. The interplay of the renormalized Coulomb and Fröhlich interactions is crucial. The diamagnetic response of the large superconductor may become paramagnetic when the finite-size-determined Thouless energy is larger than or on the order of the Debye energy.