Fixed-N Superconductivity: The Crossover from the Bulk to the Few-Electron Limit

Fabian Braun; Jan von Delft

doi:10.1103/PhysRevLett.81.4712

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Fixed-N Superconductivity: The Crossover from the Bulk to the Few-Electron Limit

Fabian Braun, Jan von Delft

DOI 10.1103/PhysRevLett.81.4712 · Physical Review Letters

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Abstract

We present a truly canonical theory of superconductivity in ultrasmall metallic grains by variationally optimizing fixed- N projected BCS wave functions, which yields the first full description of the entire crossover from the bulk BCS regime (mean level spacing d≪bulkgapΔ̃) to the “fluctuation-dominated” few-electron regime (d≫Δ̃). A wave-function analysis shows in detail how the BCS limit is recovered for d≪Δ̃, and how for d≫Δ̃ pairing correlations become delocalized in energy space. An earlier grand-canonical prediction for an observable parity effect in the spectral gaps is found to survive the fixed- N projection.

Source-reported materials — not catalogue approval

Formula	Reported Tc (K)	Pressure (GPa)	Type
Al Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula.	—	Pressure not reported	unknown