BCS-BEC Crossover of Triplet Exciton Condensation in Bilayer Systems

TL;DR

This paper investigates the BCS-BEC crossover of triplet exciton condensation in a bilayer Hubbard model, revealing how exciton properties and quantum criticality evolve across different coupling regimes.

Contribution

It provides a detailed analysis of exciton formation and condensation across the BCS-BEC crossover using dynamical spin structure factors and wave functions.

Findings

In the BCS regime, excitons form and condense simultaneously at the phase transition.

In the BEC regime, bound states are well-defined across the Brillouin zone with small particle-hole pairs.

Quantum criticality may emerge from intermediate coupling materials in the crossover regime.

Abstract

We study the BCS-BEC crossover phenomenon of triplet exciton condensation using a half-filled bilayer Hubbard model. We calculate the dynamical spin structure factor and the exciton wave function on the phase boundary between the antiferromagnetically ordered and disordered phases. In the BCS regime, the formation and condensation of particle-hole bound states, namely excitons, occur simultaneously at the phase transition point, and the exciton wave function is extended in real space. In the BEC regime, on the other hand, bound states are well defined over the whole Brillouin zone also in the disordered phase, and the size of particle-hole pairs is smaller than a lattice space. Quantum criticality of charge-spin-orbital entangled states can emerge from intermediate coupling materials in the crossover regime.