Phase analysis of quantum oscillations in graphite

The quantum de Haas van Alphen (dHvA) and Shubnikov de Haas (SdH) oscillations measured in graphite were decomposed by pass-band filtering onto contributions from three different groups of carriers. We develop the two-dimensional phase analysis method which allows to identify these carriers as (i) minority holes having two-dimensional (2D) parabolic massive spectrum, (ii) majority electrons, also massive but with intermediate 2D-3D spectrum, and (iii) majority holes with 2D Dirac-like spectrum which seems to be responsible for the unusual strongly-correlated electronic phenomena in graphite.Comment: latest version as was published in PR

Comment on "Consistent Interpretation of the Low-Temperature Magnetotransport in Graphite Using the Slonczewski-Weiss-McClure 3D Band-Structure Calculations" (arXiv:0902.1925)

In 2004 we have shown that substantial part of conductivity in graphite is provided by holes with massless linear spectrum - Dirac Fermions that coexist with massive normal carriers - electrons. In a recent Letter [Phys. Rev. Lett. 102, 166403 (2009), arXiv:0902.1925] Schneider et al. revised our conclusion pointed that both types of carriers are massive. Since both groups use the same method of phase determination of Shubnikov de Haas oscillation we comment here that the controversy originates from the improper treatment of experimental results in Schneider2009 et al

Multidomain switching in the ferroelectric nanodots

Controlling the polarization switching in the ferroelectric nanocrystals, nanowires and nanodots has an inherent specificity related to the emergence of depolarization field that is associated with the spontaneous polarization. This field splits the finite-size ferroelectric sample into polarization domains. Here, based on 3D numerical simulations, we study the formation of 180$^{\circ }$ polarization domains in a nanoplatelet, made of uniaxial ferroelectric material, and show that in addition to the polarized monodomain state, the multidomain structures, notably of stripe and cylindrical shapes, can arise and compete during the switching process. The multibit switching protocol between these configurations may be realized by temperature and field variations

Théorie des domaines et des textures non uniformes dans les ferroélectriques

La phrase favorite de Landau, Personne ne peut annuler la loi de Coulomb , est souvent négligée dans la compréhension des ferroélectriques car paradoxalement, ils devraient être déstabilisés par le champ électrostatique dépolarisant induit par la discontinuité de la polarisation en surface. Comme l'ont proposé Landau (1935) et Kittel (1946), cette énigme peut être résolue dans les ferromagnetiques par la division de l'echantillon en domaines d'aimantation orientés différemment. On pensait traditionnellement que la formation des domaines n'était pas pertinente dans les ferroélectriques où le champ dépolarisant peut être écranté par les charges libres semiconductrices. C est seulement dans les années 2000 qu il a été expérimentalement confirmé que les textures de domaines apparaissaient dans les échantillons <500nm. Le but de la Thèse est de proposer une méthode d'analyse pour modéliser la texture de polarisation à l'échelle nanoscopique. En nous basant sur l'approche de Kittel à basse température et sur le formalisme de Ginzburg-Landau valable près de la température de transition, nous étudions les paramètres de la texture en domaines des films ferroélectriques uniaxiaux et leur dépendance en température, champ appliqué et épaisseur. Nous étudions ensuite la distribution de la polarisation dans les ferroélectriques multicomposantes cubiques où le degré de liberté est de type quasi-Goldstone. Nous montrons que la condition d énergie de dépolarisation minimale conduit à la formation de vortex topologique ou de skyrmion chiral. Leur formation au cours du processus de renversement peut résoudre le paradoxe de Landauer concernant le faible champ coercitif des ferroélectriques.The Landau's sentence "Nobody can cancel the Coulomb's law" is often overlooked in understanding of ferroelectric materials that, paradoxically, should be destabilized by the backward depolarizing electrostatic field produced by the charge of the polarization surface breakdown. As was proposed by Landau (1935) and Kittel (1946) this puzzle can be resolved for ferromagnetics with similar magnetostatic problem by sample segregation onto differently oriented magnetization domains. It was thought however that domain formation mechanism is irrelevant for ferroelectrics where the depolarization field can be screened by free semiconducting charges. Only in 2000's it was recognized and experimentally confirmed that the regular domain patterns do appear in nanoscopic samples <500nm. The objective of the Thesis is to propose the analytical method for modeling of non-uniform polarization texture in nano-scale samples caused by depolarization field. Basing on the generalized Kittel low-temperature approach and on the Ginzburg-Landau formalism valid close to transition temperature we study the principal parameters of the domain structure in uniaxial ferroelectric films and their dependance on temperature, applied field and film thickness. We investigate next the distribution of polarization in cubic multicomponent ferroelectrics having the quasi-Goldstone (rotational) polarization degree of freedom. We show that the requirement of minimization of the depolarization energy leads to formation of unconventional topological vortices or chiral skyrmions. Their formation during the switching process can resolve the long-standing Landauer paradox of small coercive field in ferroelectrics.AMIENS-BU Sciences (800212103) / SudocSudocFranceF

[Invited] On chiral skyrmions in ferroelectrics

International audienc

Formation and evolution of exotic polarization textures in nanoferroelectrics

International audienc