, Volume 14, Issue 1, pp 103-108

An exact study of pairing fluctuations and phase diagrams in four-site Hubbard Nanoclusters

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Charge-spin separation and pairing fluctuations and pseudogaps are studied using the analytical eigenvalues of the four-site Hubbard Nanoclusters with the grand canonical and canonical ensemble approaches in a multidimensional parameter space of temperature (T), magnetic field (h), on-site interaction (U), chemical potential (μ), and number of electrons (N). The electron charge energy gap, with one hole off half filling, corresponds to an excitonic particle–hole pair binding energy Δ e-h > 0 at U > U c and vanishes at a critical parameter U c =  4.584. For UU c, particle–particle pair binding is found with a pairing energy Δp > 0. In addition, for UU c we find an electron pair binding instability at finite temperature near N ≈ 3, which manifests a possible pairing mechanism, a precursor to superconductivity, in small clusters. The resulting phase diagram, consisting of charge and spin pseudogaps, hole pairing near 1/8th filling with hole-rich and hole-poor regions in the ensemble of Nanoclusters, closely resembles the phase diagrams in the family of doped high-T c-cuprates.