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

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

2008-09-23

arxiv.org/abs/0809.4035v2

Phys. Rev. B 79, 024409 (2009)

Physics

Condensed Matter

Strongly Correlated Electrons

LaTeX 2e, 14 pages, 17 figures

Scientific paper

10.1103/PhysRevB.79.024409

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.

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