An investigation of the quantum J1−J2−J3J_1-J_2-J_3 model on the honeycomb lattice

We have investigated the quantum $J_1-J_2-J_3$ model on the honeycomb lattice with exact diagonalizations and linear spin-wave calculations for selected values of $J_{2}/J_{1}$, $J_{3}/J_{1}$ and antiferromagnetic ($J_{1}>0$) or ferromagnetic ($J_{1}<0$) nearest neighbor interactions. We found a variety of quantum effects: "order by disorder" selection of a N{\'e}el ordered ground-state, good candidates for non-classical ground-states with dimer long range order or spin-liquid like. The purely antiferromagnetic Heisenberg model is confirmed to be N{\'e}el ordered. Comparing these results with those observed on the square and triangular lattices, we enumerate some conjectures on the nature of the quantum phases in the isotropic models.Comment: 14 pages, 22 Postscript figures, uses svjour.cls and svepj.clo, submitted to European Physical Journal B: condensed matter physi

Field-induced gap in ordered Heisenberg antiferromagnets

Heisenberg antiferromagnets in a strong uniform magnetic field $H$ are expected to exhibit a gapless phase with a global O(2) symmetry. In many real magnets, a small energy gap is induced by additional interactions that can be viewed as a staggered transverse magnetic field $h = c H$, where $c$ is a small proportionality constant. We study the effects of such a perturbation, particularly for magnets with long-range order, by using several complimentary approaches: numerical diagonalizations of a model with long-range interactions, classical equations of motion, and scaling arguments. In an ordered state at zero temperature, the energy gap at first grows as $(cH)^{1/2}$ and then may dip to a smaller value, of order $(cH)^{2/3}$, at the quantum critical point separating the ``gapless'' phase from the gapped state with saturated magnetization. In one spatial dimension, the latter exponent changes to 4/5.Comment: 6 pages, 5 figure

Field theories of paramagnetic Mott insulators

This is a summary of a central argument in recent review articles by the author (cond-mat/0109419, cond-mat/0211005, and cond-mat/0211027). An effective field theory is derived for the low energy spin singlet excitations in a paramagnetic Mott insulator with collinear spin correlations.Comment: 12 pages, 4 figures, Proceedings of the International Conference on Theoretical Physics, Paris, UNESCO, July 200

Exact diagonalization Studies of Two-dimensional Frustrated Antiferromagnet Models

We describe the four kinds of behavior found in two-dimensional isotropic quantum antiferromagnets. Two of them display long range order at T=0: the N\'eel state and the Valence Bond Crystal. The last two are Spin-Liquids. Properties of these different states are shortly described and open questions are underlined.Comment: 7 pages; invited talk at "HFM 2000" (Waterloo, June 2000); submitted to Can. J. Phy

Integration of microsatellite (SSR) markers in Hevea spp. reference genetic map : a powerful tool for genetic analysis of qualitative and quantitative agronomic traits

In 2000, was published (Lespinasse et al. 2000) the 1st genetic map of rubber tree (Hevea spp.). This saturated map encompassed 717 loci, allowing an efficient coverage of the 18 chromosomes of the Hevea genome, and it can be considered as the reference map for the H. brasiliensis species. Nevertheless, the molecular markers used were manly RFLPs and AFLPs. RFLP markers proved to be very efficient in terms of genetic polymorphism and are sparsely distributed on rubber tree genome, but their applications to genotyping of large progenies are time and cost consuming. AFLPs were very useful PCR based markers, allowing rapid identification and mapping of hundreds of markers. But they are less informative than RFLPs as segregating AFLP bands are usually not locus specific and, in consequence, do not allow comparative alignment between independent maps. We thus developed Hevea specific microsatellite (alias SSRs for Simple Sequence Repeat) markers which potentially combine the advantages of RFLPs (polymorphism, abundance in plant genomes and locus specificity) and of AFLPs (high output genotyping thanks to PCR technique). Integration of SSRs in the Hevea reference map confirmed the usefulness of such molecular markers in terms of cost, rapidity, genome coverage and locus specificity. This last property means that selecting a set of SSR markers from the reference map allows obtaining a colinear map, whatever the Hevea clones used as progenitors of the mapping populations. Thus, the creation of a dense SSR map of rubber tree is very useful for genetic mapping and comparison of chromosome locations of Quantitative Traits Loci (QTLs) obtained from different mapping projects. (Résumé d'auteur

Ising phases of Heisenberg ladders in a magnetic field

We examine the influence of weak anisotropic interactions on the T=0 phase diagram of the frustrated two-leg Heisenberg ladder, a well-studied spin model exhibiting integer and fractional magnetization plateaux separated by gapless incommensurate states. We find that the Dzyaloshinskii--Moriya coupling may substantially modify the phase diagram so that the half-integer plateau and the surrounding gapless phases merge into a single Ising-ordered phase breaking the translational symmetry of the lattice. A different Ising order is found for a weakly frustrated ladder. Implications for experimental ladder and dimer systems are discussed.Comment: 5 pages, 5 figure

Field Induced Staggered Magnetization and Magnetic Ordering in Cu2(C5H12N2)2Cl4Cu_2 (C_5 H_{12} N_2)_2 Cl_4

We present a $^2$D NMR investigation of the gapped spin-1/2 compound $Cu_2 (C_5 H_{12} N_2)_2 Cl_4$. Our measurements reveal the presence of a magnetic field induced transverse staggered magnetization (TSM) which persists well below and above the field-induced 3D long-range magnetically ordered (FIMO) phase. The symmetry of this TSM is different from that of the TSM induced by the order parameter of the FIMO phase. Its origin, field dependence and symmetry can be explained by an intra-dimer Dzyaloshinskii-Moriya interaction, as shown by DMRG calculations on a spin-1/2 ladder. This leads us to predict that the transition into the FIMO phase is not in the BEC universality class.Comment: 4 page