Thermal critical points from competing singlet formations in fully frustrated bilayer antiferromagnets

TL;DR

This study explores the phase diagram and thermal transitions in a frustrated bilayer antiferromagnet, revealing a first-order quantum transition between two singlet states and a line of thermal critical points, with implications for understanding quantum disordered phases.

Contribution

It provides a comprehensive analysis combining quantum Monte Carlo, perturbation theory, and free-energy methods to map the phase diagram and critical behavior of a frustrated bilayer spin model.

Findings

Identification of a first-order quantum phase transition line.

Extension of this line into a wall of thermal first-order transitions.

Quadratic suppression of critical temperature in the strongly plaquettized regime.

Abstract

We examine the ground-state phase diagram and thermal phase transitions in a plaquettized fully frustrated bilayer spin-1/2 Heisenberg model. Based on a combined analysis from sign-problem free quantum Monte Carlo simulations, perturbation theory and free-energy arguments, we identify a first-order quantum phase transition line that separates two competing quantum-disordered ground states with dominant singlet formations on inter-layer dimers and plaquettes, respectively. At finite temperatures, this line extends to form a wall of first-order thermal transitions, which terminates in a line of thermal critical points. From a perturbative approach in terms of an effective Ising model description, we identify a quadratic suppression of the critical temperature scale in the strongly plaquettized region. Based on free-energy arguments we furthermore obtain the full phase boundary of the low-temperature dimer-singlet regime, which agrees well with the quantum Monte Carlo data.