Thermal phase diagrams of columnar liquid crystals

In order to understand the possible sequence of transitions from the disordered columnar phase to the helical phase in hexa(hexylthio)triphenylene (HHTT), we study a three-dimensional planar model with octupolar interactions inscribed on a triangular lattice of columns. We obtain thermal phase diagrams using a mean-field approximation and Monte Carlo simulations. These two approaches give similar results, namely, in the quasi one-dimensional regime, as the temperature is lowered, the columns order with a linear polarization, whereas helical phases develop at lower temperatures. The helicity patterns of the helical phases are determined by the exact nature of the frustration in the system, itself related to the octupolar nature of the molecules.Comment: 12 pages, 9 figures, ReVTe

Rhodium Doped Manganites : Ferromagnetism and Metallicity

The possibility to induce ferromagnetism and insulator to metal transitions in small A site cation manganites Ln_{1-x}Ca_xMnO_3 by rhodium doping is shown for the first time. Colossal magnetoresistance (CMR) properties are evidenced for a large compositional range (0.35 \leq x < 0.60). The ability of rhodium to induce such properties is compared to the results obtained by chromium and ruthenium doping. Models are proposed to explain this behavior.Comment: 11 pages, 8 figure

Inferences on homogeneity of between-family components of variance and covariance among environments in balanced cross-classified designs

Estimation and testing of homogeneity of between-family components of variance and covariance among environments are investigated for balanced cross-classified designs. The variance-covariance structure of the residuals is assumed to be diagonal and heteroskedastic. The testing procedure for homogeneity of family components is based on the ratio of maximized log-restricted likelihoods for the reduced (hypothesis of homogeneity) and saturated models. An expectation-maximization (EM) algorithm is proposed for calculating restricted maximum likelihood (REML) estimates of the residual and between-family components of variance and covariance. The EM formulae to implement this are iterative and use the classical analysis of variance (ANOVA) statistics, ie the between- and within-family sums of squares and cross-products. They can be applied both to the saturated and reduced models and guarantee the solutions to be in the parameter space. Procedures presented in this paper are illustrated with the analysis of 5 vegetative and reproductive traits recorded in an experiment on 20 full-sib families of black medic (Medicago lupulina L) tested in 3 environments. Application to pure maximum likelihood procedures, extension to unbalanced designs and comparison with approaches relying on alternative models are also discussed.Cet article étudie les problèmes d’estimation et de test d’homogénéité des composantes familiales de variance et de covariance entre milieux dans des dispositifs factoriels équilibrés. La structure des variances et des covariances résiduelles est supposée diagonale et hétéroscédastique. La procédure de test d’homogénéité des composantes familiales repose sur le rapport des vraisemblances restreintes maximisées sous les modèles réduit (hypothèse d'homogénéité) et saturé. Un algorithme d’espérance-maximisation (EM) est proposé pour calculer les estimations du maximum de vraisemblance restreinte (REML) des composantes résiduelles et familiales de variance et de covariance. Les formules EM à appliquer sont itératives et utilisent les statistiques classiques de l’analyse de variance (ANOVA), c’est-à-dire les sommes de carrés et coproduits inter- et intrafamilles. Elles s’appliquent à la fois aux modèles réduit et saturé et garantissent l’appartenance des solutions à l’espace des paramètres. Les méthodes présentées dans cet article sont illustrées par l’analyse de 5 caractères végétatifs et reproductifs mesurés lors d’une expérience portant sur 20 familles de pleins frères testées dans 3 milieux chez la minette (Medicago lupulina L). L’application au maximum de vraisemblance stricto sensu, la généralisation à des dispositifs déséquilibrés ainsi que la comparaison à des approches reposant sur d’autres modèles sont également discutées

Exact Study of the 1D Boson Hubbard Model with a Superlattice Potential

We use Quantum Monte Carlo simulations and exact diagonalization to explore the phase diagram of the Bose-Hubbard model with an additional superlattice potential. We first analyze the properties of superfluid and insulating phases present in the hard-core limit where an exact analytic treatment is possible via the Jordan-Wigner transformation. The extension to finite on-site interaction is achieved by means of quantum Monte Carlo simulations. We determine insulator/superfluid phase diagrams as functions of the on-site repulsive interaction, superlattice potential strength, and filling, finding that insulators with fractional occupation numbers, which are present in the hard-core case, extend deep into the soft-core region. Furthermore, at integer fillings, we find that the competition between the on-site repulsion and the superlattice potential can produce a phase transition between a Mott insulator and a charge density wave insulator, with an intermediate superfluid phase. Our results are relevant to the behavior of ultracold atoms in optical superlattices which are beginning to be studied experimentally.Comment: 13 pages, 23 figure

Evaluation of far-field tsunami hazard in French Polynesia based on historical data and numerical simulations

International audienceThe first tsunami hazard map of French Polynesia is presented here on the basis of historical data, and numerical simulations. French Polynesia, because of its central position in the most tsunami prone ocean, the Pacific, is strongly exposed to far-field tsunamis. As no previous study on the area had been conducted, we compiled catalogues of all the historical observations (14 events), and tide gauges records (69 marigrams). The catalogues emphasise the higher hazard associated to the Marquesas archipelago, but also the deficiency of robust data in most other parts of French Polynesia. The recourse to numerical simulations allowed us to complement the existing records, and to test tsunami scenarii over different bathymetry and topography configurations, representative of the diversity of islands in French Polynesia. The tsunami hazard map assigns a high exposure level to the Marquesas and the island of Rurutu. Other islands of the Austral, and the Gambier archipelago have a elevated level of exposure, as well as three islands of Society: Tahiti, Moorea, and Huahine. All other islands of French Polynesia are considered as moderately exposed

Cathodoluminescence in a (S)TEM - Exploring Possibilities and Limits

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 - August 5, 201

Strongly correlated properties of the thermoelectric cobalt oxide Ca3Co4O9

We have performed both in-plane resistivity, Hall effect and specific heat measurements on the thermoelectric cobalt oxide Ca$_{3}$Co$_{4}$O$_{9}$. Four distinct transport regimes are found as a function of temperature, corresponding to a low temperature insulating one up to $T_{min}\approx$63 K, a strongly correlated Fermi liquid up to $T^*\approx$140 K, with $\rho=\rho_0+AT^2$ and $A\approx 3.63$ $10^{-2} \mu \Omega cm/K^{2}$, followed by an incoherent metal with $k_Fl\leq 1$ and a high temperature insulator above T$^{**}\approx$510 K . Specific heat Sommerfeld coefficient $\gamma = 93$ mJ/(mol.K$^{2}$) confirms a rather large value of the electronic effective mass and fulfils the Kadowaki-Woods ratio $A/\gamma^2 \approx 0.45$ 10$^{-5}$ $\mu \Omega cm.K^2/(mJ^2mol^{-2})$. Resistivity measurements under pressure reveal a decrease of the Fermi liquid transport coefficient A with an increase of $T^*$ as a function of pressure while the product $A(T^*)^2/a$ remains constant and of order $h/e^2$. Both thermodynamic and transport properties suggest a strong renormalization of the quasiparticles coherence scale of order $T^*$ that seems to govern also thermopower.Comment: 5 pages, 6 figures, accepted for publication in Physical Review

Field-Induced Magnetization Steps in Intermetallic Compounds and Manganese Oxides: The Martensitic Scenario

Field-induced magnetization jumps with similar characteristics are observed at low temperature for the intermetallic germanide Gd5Ge4and the mixed-valent manganite Pr0.6Ca0.4Mn0.96Ga0.04O3. We report that the field location -and even the existence- of these jumps depends critically on the magnetic field sweep rate used to record the data. It is proposed that, for both compounds, the martensitic character of their antiferromagnetic-to-ferromagnetic transitions is at the origin of the magnetization steps.Comment: 4 pages,4 figure