Magnetization plateaux and jumps in a class of frustrated ladders: A simple route to a complex behaviour

We study the occurrence of plateaux and jumps in the magnetization curves of a class of frustrated ladders for which the Hamiltonian can be written in terms of the total spin of a rung. We argue on the basis of exact diagonalization of finite clusters that the ground state energy as a function of magnetization can be obtained as the minimum - with Maxwell constructions if necessary - of the energies of a small set of spin chains with mixed spins. This allows us to predict with very elementary methods the existence of plateaux and jumps in the magnetization curves in a large parameter range, and to provide very accurate estimates of these magnetization curves from exact or DMRG results for the relevant spin chains.Comment: 14 pages REVTeX, 7 PostScript figures included using psfig.sty; this is the final version to appear in Eur. Phys. J B; some references added and a few other minor change

ENCORE: An Extended Contractor Renormalization algorithm

Contractor renormalization (CORE) is a real-space renormalization-group method to derive effective Hamiltionians for microscopic models. The original CORE method is based on a real-space decomposition of the lattice into small blocks and the effective degrees of freedom on the lattice are tensor products of those on the small blocks. We present an extension of the CORE method that overcomes this restriction. Our generalization allows the application of CORE to derive arbitrary effective models whose Hilbert space is not just a tensor product of local degrees of freedom. The method is especially well suited to search for microscopic models to emulate low-energy exotic models and can guide the design of quantum devices.Comment: 5 pages, 4 figure

Magnetocaloric effect in two-dimensional spin-1/2 antiferromagnets

The magnetocaloric effect is studied at the transition to saturation in the antiferromagnetic spin-1/2 Heisenberg model on the simplest two-dimensional lattices, namely the square and the triangular lattice. Numerical results are presented for the entropy which are consistent with identical universal properties. However, the absolute values of the entropy are bigger on the geometrically frustrated triangular lattice than on the non-frustrated square lattice, indicating that frustration improves the magnetocaloric properties.Comment: 2 pages, 2 figures included, to appear in Physica B (proceedings of SCES'05

Phase diagram of Bose-Fermi mixtures in one-dimensional optical lattices

The ground state phase diagram of the one-dimensional Bose-Fermi Hubbard model is studied in the canonical ensemble using a quantum Monte Carlo method. We focus on the case where both species have half filling in order to maximize the pairing correlations between the bosons and the fermions. In case of equal hopping we distinguish between phase separation, a Luttinger liquid phase and a phase characterized by strong singlet pairing between the species. True long-range density waves exist with unequal hopping amplitudes.Comment: 5 pages, 5 figures, replaced with published versio

Quench dynamics and non equilibrium phase diagram of the Bose-Hubbard model

We investigate the time evolution of correlations in the Bose-Hubbard model following a quench from the superfluid to the Mott insulating phase. For large values of the final interaction strength the system approaches a distinctly non-equilibrium steady state that bears strong memory of the initial conditions. In contrast, when the final interaction strength is comparable to the hopping, the correlations are rather well approximated by those at thermal equilibrium. The existence of two distinct non-equilibrium regimes is surprising given the non-integrability of the Bose-Hubbard model. We relate this phenomena to the role of quasi-particle interactions in the Mott insulating state