Exact Thermodynamics of Pairing and Charge-spin Separation Crossovers in Small Hubbard Nanoclusters

TL;DR

This paper provides an exact analysis of thermodynamics in small Hubbard nanoclusters, revealing charge and spin pairing, phase transitions, and similarities to high-temperature superconductor phase diagrams.

Contribution

It offers the first exact numerical study of pairing and magnetic crossovers in small Hubbard clusters, including criteria for quantum critical points and finite-temperature crossovers.

Findings

Charge and spin pairings observed in small clusters.

Transitions from pairing to ferromagnetic insulators driven by electron repulsion.

Phase diagrams resemble those of doped high-Tc cuprates.

Abstract

The exact numerical diagonalization and thermodynamics in an ensemble of small Hubbard clusters in the ground state and finite temperatures reveal intriguing insights into the nascent charge and spin pairings, Bose condensation and ferromagnetism in nanoclusters. The phase diagram off half filling strongly suggests the existence of subsequent transitions from electron pairing into unsaturated and saturated ferromagnetic Mott-Hubbard like insulators, driven by electron repulsion. Rigorous criteria for the existence of quantum critical points in the ground state and corresponding crossovers at finite temperatures are formulated. The phase diagram for 2x4-site clusters illustrates how these features are scaled with cluster size. The phase separation and electron pairing, monitored by a magnetic field and electron doping, surprisingly resemble phase diagrams in the family of doped high Tc cuprates.