Hierarchical mean-field approach to the $J_1$-$J_2$ Heisenberg model on a square lattice

TL;DR

This paper introduces a hierarchical mean-field approach to analyze the $J_1$-$J_2$ Heisenberg model on a square lattice, revealing the nature of quantum phases and phase transitions with a focus on symmetry and relevant degrees of freedom.

Contribution

It develops a novel hierarchical mean-field method that preserves symmetry and identifies relevant degrees of freedom, providing new insights into the phase diagram of the $J_1$-$J_2$ model.

Findings

The symmetric plaquette covering reproduces the known phase diagram.

The intermediate phase is a plaquette crystal with a finite gap.

The Ne9el to paramagnetic transition is continuous and fits the Ginzburg-Landau-Wilson paradigm.

Abstract

We study the quantum phase diagram and excitation spectrum of the frustrated $J_1$-$J_2$ spin-1/2 Heisenberg Hamiltonian. A hierarchical mean-field approach, at the heart of which lies the idea of identifying {\it relevant} degrees of freedom, is developed. Thus, by performing educated, manifestly symmetry preserving mean-field approximations, we unveil fundamental properties of the system. We then compare various coverings of the square lattice with plaquettes, dimers and other degrees of freedom, and show that only the {\it symmetric plaquette} covering, which reproduces the original Bravais lattice, leads to the known phase diagram. The intermediate quantum paramagnetic phase is shown to be a (singlet) {\it plaquette crystal}, connected with the neighboring N\'eel phase by a continuous phase transition. We also introduce fluctuations around the hierarchical mean-field solutions, and demonstrate that in the paramagnetic phase the ground and first excited states are separated by a finite gap, which closes in the N\'eel and columnar phases. Our results suggest that the quantum phase transition between N\'eel and paramagnetic phases can be properly described within the Ginzburg-Landau-Wilson paradigm.