Lifshitz transition in Kondo alloys

We study the low energy states of Kondo alloys as function of the magnetic impurity concentration per site, x, and the conduction electron average site occupation, nc. Using two complementary approaches, the mean-field coherent potential approximation and the strong coupling limit, we identify and characterize two different Fermi liquid regimes. We propose that both regimes are separated by a Lifshitz transition at x = nc. Indeed, we predict a discontinuity of the number of quasiparticles which are enclosed in the Fermi surface. This feature could provide a scenario for the non-Fermi liquid properties that were recently observed in Kondo alloy systems around x = nc.Comment: 5 pages, 2 figure

Ising Like Order by Disorder In The Pyrochlore Antiferromagnet with Dzyaloshinskii-Moriya Interactions

It is shown that the mechanism of order out of disorder is at work in the antisymmetric pyrochlore antiferromagnet. Quantum as well as thermal fluctuations break the continuous degeneracy of the classical ground state manifold and reduce its symmetry to $\mathbb{Z}_3 \times \mathbb{Z}_2$. The role of anisotropic symmetric exchange is also investigated and we conclude that this discrete like ordering is robust with respect to these second order like interactions. The antisymmetric pyrochlore antiferromagnet is therefore expected to order at low temperatures, whatever the symmetry type of its interactions, in both the classical and semi classical limits.Comment: 6 pages. 9 figure

s- and d-wave superconductivity in a two-band model

Superconductivity in strongly correlated systems is a remarkable phenomenon that attracts a huge interest. The study of this problem is relevant for materials as the high Tc oxides, pnictides and heavy fermions. In this work we study a realistic model that includes the relevant physics of superconductivity in the presence of strong Coulomb correlations. We consider a two-band model, since most of these correlated systems have electrons from at least two different atomic orbitals coexisting at their Fermi surface. The Coulomb repulsion is taken into account through a local repulsive interaction. Pairing is considered among quasi- particles in neighbouring sites and we allow for different symmetries of the order parameter. In order to deal with the strong local correlations, we use the well known slave boson approach that has proved very successful for this problem. Here we are interested in obtaining the zero temperature properties of the model, specifically its phase diagram and the existence and nature of superconducting quantum critical points. We show that these can arise by increasing the mixing between the two bands. Since this can be controlled by external pressure or doping, our results have a direct relation with experiments. We show that the superconductor-to-normal transition can be either to a metal, a correlated metal or to an insulator. Also we compare the relative stability of s and d-wave paired states for different regions of parameter space and investigate the BCS- BEC crossover in the two-band lattice model as function of the strength of the pairing interaction.Comment: 21 pages, 14 figure

Mortalité adulte et longévité exceptionnelle au Québec ancien

Ce mémoire présente, dans une première partie, une étude détaillée de la mortalité adulte des Canadiens-français nés entre 1620 et 1749. Des résultats inédits sont présentés sous forme de tables de mortalité abrégées avec entrées échelonnées. La comparaison de celles-ci permet de décrire les différences de comportement observées entre les hommes et les femmes, les populations de régions urbaines et rurales ainsi que trois groupes de générations s’étant succédé au cours de la période étudiée. Dans une deuxième partie, une étude de cas visant à confirmer l’influence du caractère familial sur la longévité est présentée. Une famille se distinguant par les durées de vie exceptionnellement longues de plusieurs de ses membres est comparée à une famille fictive de référence, représentative de la population moyenne de l’époque. Les résultats opposant ces deux familles consistent en des âges moyens au décès et des proportions de survivants à différents âges. Cette deuxième étude s’inscrit dans le prolongement des recherches effectuées avec les ascendances de Jeanne Calment et Marie-Louise Meilleur. Les informations tirées des registres paroissiaux du Québec ancien et consolidées informatiquement par le Programme de recherche en démographie historique (PRDH) dans une base de données appelée Registre de la population du Québec ancien (RPQA) constituent la source exploitée pour la réalisation des deux parties du mémoire.This thesis presents, in the first part, a detailed study of adult mortality of French-Canadians born between 1620 and 1749. New time published results are presented in the form of abridged life tables with staggered entries. The life tables are compared to ascertain behavioural differences between men and women, between residents of urban and rural regions and across three generations during the period studied. In the second part, the thesis presents a case study conducted to test the influence of inheritance on human longevity. A family distinguished by the exceptional longevity of several of its members is compared to a reference fictional family representative of the average population of the time. Evidence provided on these two families consists of mean ages at death and proportions of survivors at different ages. This case study builds on research conducted with the ancestry of Jeanne Calment and Marie Louise Meilleur. Information drawn from the parish registers of old Quebec computerized by the Programme de recherche en démographie historique (PRDH) in a database called Registre de la population du Québec ancien (RPQA) is the primary source used for the life tables and case study

Metamagnetic transition in the two ff orbitals Kondo lattice model

In this work, we study the effects of a transverse magnetic field in a Kondo lattice model with two $f$ orbitals interacting with the conduction electrons. The $f$ electrons that are present on the same site interact through Hund's coupling, while on neighboring sites they interact through intersite exchange. We consider here that part of $f$ electrons are localized (orbital 1) while another part (orbital 2) are delocalized, as it is frequent in uranium systems. Then, only electrons in the localized orbital 1 interact through exchange interaction with the neighboring ones, while electrons in orbital 2 are coupled with conduction electrons through a Kondo interaction. We obtain a solution where ferromagnetism and Kondo effect coexist for small values of an applied transverse magnetic field for $T\rightarrow0$. Increasing the transverse field, two situations can be obtained when Kondo coupling vanishes: first, a metamagnetic transition occurs just before or at the same time of the fully polarized state, and second, a metamagnetic transition occurs when the spins are already pointing out along the magnetic field.Comment: 15 pages, 7 figure

Effect of anisotropy in the S=1S=1 underscreened Kondo lattice

We study the effect of crystal field anisotropy in the underscreened $S=1$ Kondo lattice model. Starting from the two orbital Anderson lattice model and including a local anisotropy term, we show, through Schrieffer-Wolff transformation, that local anisotropy is equivalent to an anisotropic Kondo interaction ($J_{\parallel} \neq{J_{\perp}}$). The competition and coexistence between ferromagnetism and Kondo effect in this effective model is studied within a generalized mean-field approximation. Several regimes are obtained, depending on the parameters, exhibiting or not coexistence of magnetic order and Kondo effect. Particularly, we show that a re-entrant Kondo phase at low temperature can be obtained. We are also able to describe phases where the Kondo temperature is smaller than the Curie temperature ($T_K<T_C$). We propose that some aspects of uranium and neptunium compounds that present coexistence of Kondo effect and ferromagnetism, can be understood within this model.Comment: 7 pages, 3 figure

Interplay between spin-glass clusters and geometrical frustration

International audienceThe presence of spin-glass (SG) order in highly geometrically frustrated systems is analyzed in a cluster SG model. The model considers infinite-range disordered interactions among cluster magnetic moments and the J1−J2 model couplings between Ising spins of the same cluster. This model can introduce two sources of frustration: one coming from the disordered interactions and another coming from the J1−J2 intracluster interactions (intrinsic frustration). The framework of one-step replica symmetry breaking is adopted to obtain a one-cluster problem that is exactly solved. As a main result we create phase diagrams of the temperature T versus intensity of the disorder J, where the paramagnetic-SG phase transition occurs at Tf when T decreases for high-J values. For low-J values, the SG order is absent for antiferromagnetic clusters without intrinsic frustration. However, the SG order can be observed within the intracluster intrinsic frustration regime even for lower intensity of disorder. In particular, the results indicate that the presence of small clusters in geometrically frustrated antiferromagnetic systems can help stabilize the SG order within a weak disorder

Ordering in the pyrochlore antiferromagnet due to Dzyaloshinsky-Moriya interactions

The Heisenberg nearest neighbour antiferromagnet on the pyrochlore (3D) lattice is highly frustrated and does not order at low temperature where spin-spin correlations remain short ranged. Dzyaloshinsky-Moriya interactions (DMI) may be present in pyrochlore compounds as is shown, and the consequences of such interactions on the magnetic properties are investigated through mean field approximation and monte carlo simulations. It is found that DMI (if present) tremendously change the low temperature behaviour of the system. At a temperature of the order of the DMI a phase transition to a long range ordered state takes place. The ordered magnetic structures are explicited for the different possible DMI which are introduced on the basis of symmetry arguments. The relevance of such a scenario for pyrochlore compounds in which an ordered magnetic structure is observed experimentally is dicussed