We study the ground-state phase diagram of the quantum J 1 − J 2 model on the honeycomb lattice by means of an entangled-plaquette variational ansatz. Values of energy and relevant order parameters are computed in the range 0 J 2 /J 1 1. The system displays classical order for J 2 /J 1 0.2 (Néel) and for J 2 /J 1 0.4 (collinear). In the intermediate region, the ground state is disordered. Our results show that the reduction of the half-filled Hubbard model to the model studied here does not yield accurate predictions.

We present results of a theoretical study of para-hydrogen and ortho-deuterium clusters at low temperature (0.5 K < T < 3.5 K), based on Path Integral Monte Carlo simulations. Clusters of size up to N=21 para-hydrogen molecules are nearly entirely superfluid at T < 1 K. For 21 < N < 30, the superfluid response displays strong variations with N, reflecting structural changes that occur on adding or removing even a single molecule. Some clusters in this size range display quantum melting, going from solid- to liquid-like as T tends to 0. Melting is caused by quantum exchanges of molecules. The largest para-hydrogen cluster for which a significant superfluid response is observed comprises 27 molecules. Evidence of a finite superfluid response is presented for ortho-deuterium clusters of size up to 14 molecules. Magic numbers are observed, at which both types of clusters feature pronounced stability.Comment: 11 pages, 16 figure

Abstract. We propose a new ansatz for the ground-state wave function of quantum many-body systems on a lattice. The key idea is to cover the lattice with plaquettes and obtain a state whose configurational weights can be optimized by means of a variational Monte Carlo algorithm. Such a scheme applies to any dimension, without any 'sign' instability. We show results for various twodimensional spin models (including frustrated ones). A detailed comparison with available exact results, as well as with variational methods based on different ansatze, is offered. In particular, our numerical estimates are in quite good agreement with exact ones for unfrustrated systems, and compare favorably to other methods for frustrated ones.

Structural and superfluid properties of p-H2 clusters of size up to N=40 molecules, are studied at low temperature (0.5 K

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