Attraction from frustration in ladder systems

TL;DR

This paper investigates multi-particle bound states in frustrated ladder systems of bosons and fermions, revealing how kinetic frustration leads to effective attraction and complex many-body states, with potential experimental implications.

Contribution

It uncovers the formation of multi-particle bound states driven by kinetic frustration, demonstrating their properties and existence in both bosonic and fermionic ladder systems, including novel many-body phases.

Findings

Bound states between holes and magnons with energies up to 0.6t.

Formation of clusters of holes and magnons with antiferromagnetic correlations.

Existence of bound and antibound states in various ladder geometries.

Abstract

We analyze the formation of multi-particle bound states in ladders with frustrated kinetic energy in two component bosonic and two component fermionic systems. We focus on the regime of light doping relative to insulating states at half-filling, spin polarization close to 100 percent, and strong repulsive interactions. A special feature of these systems is that the binding energy scales with single particle tunneling $t$ rather than exchange interactions, since effective attraction arises from alleviating kinetic frustration. For two component Fermi systems on a zigzag ladder we find a bound state between a hole and a flipped spin (magnon) with a binding energy that can be as large as $0.6t$. We demonstrate that magnon-hole attraction leads to formation of clusters comprised of several holes and magnons and expound on antiferromagentic correlations for the transverse spin components inside the clusters. We identify several many-body states that result from self-organization of multi-particle bound states, including a Luttinger liquid of hole-magnon pairs and a density wave state of two hole - three magnon composites. We establish a symmetry between the spectra of Bose and Fermi systems and use it to establish the existence of antibound states in two component Bose mixtures with SU(2) symmetric repulsion on a zigzag ladder. We also consider Bose and Fermi systems on a square ladder with flux and demonstrate that both systems support bound states. We discuss experimental signatures of multi-particle bound states in both equilibrium and dynamical experiments. We point out intriguing connections between these systems and the quark bag model in QCD.