Superconducting fluctuations in small grains - the Universal Hamiltonian and the reduced BCS model

TL;DR

This paper investigates how electron fluctuations in small superconducting grains, modeled by the reduced BCS and Universal Hamiltonian, influence their superconducting properties and related experimental phenomena.

Contribution

It demonstrates that electron fluctuations far from the Fermi level dominate superconductivity in small grains, providing insights into experimental observables.

Findings

Electron fluctuations far from the Fermi energy are dominant.

Superconducting properties are significantly affected by these fluctuations.

Implications for spin susceptibility and persistent currents are discussed.

Abstract

Small superconducting grains are discussed in the frameworks of both the reduced BCS Hamiltonian and the Universal Hamiltonian. It is shown that fluctuations of electrons in levels far from the Fermi energy dominate superconducting properties in small and ultrasmall grains. Experimental consequences related to the spin susceptibility and persistent currents of grains and rings with weak electron-electron interactions are discussed.