From quantum criticality to enhanced thermopower in strongly correlated layered cobalt oxide

We report on susceptibility measurements in the strongly correlated layered cobalt oxide [BiBa0.66K0.36O2]CoO2, which demonstrate the existence of a magnetic quantum critical point (QCP) governing the electronic properties. The investigated low frequency susceptibility displays a scaling behavior with both the temperature T and the magnetic field B ranging from the high-T non-Fermi liquid down to the low-T Fermi liquid. Whereas the inferred scaling form can be discussed within the standard framework of the quantum critical phenomena, the determined critical exponents suggest an unconventional magnetic QCP of a potentially generic type. Accordingly, these quantum critical fluctuations account for the anomalous logarithmic temperature dependence of the thermopower. This result allows us to conjecture that quantum criticality can be an efficient source of enhanced thermopower

Large increase of the Curie temperature by orbital ordering control

Using first principle calculations we showed that the Curie temperature of manganites thin films can be increased by far more than an order of magnitude by applying appropriate strains. Our main breakthrough is that the control of the orbital ordering responsible for the spectacular $T_C$ increase cannot be imposed by the substrate only. Indeed, the strains, first applied by the substrate, need to be maintained over the growth direction by the alternation of the manganite layers with another appropriate material. Following these theoretical findings, we synthesized such super-lattices and verified our theoretical predictions

Magnetoresistance scaling in the layered cobaltate Ca3Co4O9

We investigate the low temperature magnetic field dependences of both the resistivity and the magnetization in the misfit cobaltate Ca3Co4O9 from 60 K down to 2 K. The measured negative magnetoresistance reveals a scaling behavior with the magnetization which demonstrates a spin dependent diffusion mechanism. This scaling is also found to be consistent with a shadowed metalliclike conduction over the whole temperature range. By explaining the observed transport crossover, this result shed a new light on the nature of the elementary excitations relevant to the transport

Hallucination politique des écrans et création artistique contemporaine

L'hallucination politique des écrans, thème de la 3e rencontre interdisciplinaire internationale du groupe TELAA (Ecrans électroniques, littérature & arts audiovisuels), à l’Université nationale de Brasília, est une réalité qui s'impose aux citoyens comme aux artistes. L'utilisateur des écrans a accès aujourd'hui à une richesse de contenus qu'il peut croiser, devenant ainsi à son tour le monteur des images et des textes qu'il reçoit et qui sont déjà, eux-mêmes, des constructions de la réalité effectuées par d'autres. Issu de la conférence inaugurale de ces rencontres, cet article veut ouvrir des pistes de réflexions sur les enjeux esthétiques et politiques de la création contemporaine dans ce contexte. Comment l'artiste peut-il questionner le monde sachant que ses propres œuvres subiront un traitement identique ? Quelles résonnances les œuvres d'art peuvent-elles avoir dans un monde de l'image en perpetuelle construction qui évacue un présent nécessaire à la réflexion émancipatrice ? Quels rapports directs l'artiste peut-il nouer avec ses publics sans altérer son processus créatif par des dispositifs lui assurant la maîtrise du sens de ses œuvres ?L'hallucination politique des écrans est une réalité qui s'impose aux citoyens comme aux artistes et aux écrivains. L'utilisateur des écrans a accès aujourd'hui à une richesse de contenus qu'il peut croiser, devenant ainsi à son tour le monteur des images et des textes qu'il reçoit et qui sont déjà, eux-mêmes, des constructions de la réalité effectuées par d'autres. Cet article ouvre des pistes de réflexions sur les enjeux esthétiques et politiques de la création contemporaine dans le contexte des travaux du groupe TELAA (Écrans électroniques, littérature & arts audiovisuels) à l’Université nationale de Brasília. Comment l'artiste peut-il questionner le monde sachant que ses propres oeuvres subiront un traitement identique? Quelles résonances les oeuvres d'art peuvent-elles avoir dans un monde de l'image en perpétuelle construction qui évacue un présent nécessaire à la réflexion émancipatrice? Quels rapports directs l'artiste peut-il nouer avec ses publics sans altérer son processus créatif par des dispositifs lui assurant la maîtrise du sens de ses oeuvres

X-ray Dichroism and the Pseudogap Phase of Cuprates

A recent polarized x-ray absorption experiment on the high temperature cuprate superconductor Bi2Sr2CaCu2O8 indicates the presence of broken parity symmetry below the temperature, T*, where a pseudogap appears in photoemission. We critically analyze the x-ray data, and conclude that a parity-breaking signal of the kind suggested is unlikely based on the crystal structures reported in the literature. Possible other origins of the observed dichroism signal are discussed. We propose x-ray scattering experiments that can be done in order to determine whether such alternative interpretations are valid or not.Comment: final version to be published in Phys Rev B: some calculational details added, clarification of XNLD contamination and biaxiality, more discussion on possible space groups and previous optics result

Alucinação política das telas e criação artística contemporânea

A alucinação política das telas é uma realidade que se impõe aos cidadãos, artistas e escritores. O usuário de telas hoje tem acesso a uma riqueza de conteúdos que ele pode atravessar, tornando-se o editor das imagens e textos que recebe e que já são, eles próprios, construções da realidade feita por outros. Este artigo abre caminhos para a reflexão sobre as questões estéticas e políticas da criação contemporânea no contexto do trabalho do grupo TELAA (Telas Eletrônicas, Literatura e Artes Audiovisuais) da Universidade de Brasília. Como o artista pode questionar o mundo sabendo que suas próprias obras serão submetidas ao mesmo tratamento? Que ressonâncias as obras de arte podem ter em um mundo de imagens em constante construção que evacua um presente necessário para a reflexão emancipatória? Que relações diretas o artista pode estabelecer com seu público sem alterar seu processo criativo por meio do controle sobre o significado de suas obras

New electronic orderings observed in cobaltates under the influence of misfit periodicities

We study with ARPES the electronic structure of CoO2 slabs, stacked with rock-salt (RS) layers exhibiting a different (misfit) periodicity. Fermi Surfaces (FS) in phases with different doping and/or periodicities reveal the influence of the RS potential on the electronic structure. We show that these RS potentials are well ordered, even in incommensurate phases, where STM images reveal broad stripes with width as large as 80\AA. The anomalous evolution of the FS area at low dopings is consistent with the localization of a fraction of the electrons. We propose that this is a new form of electronic ordering, induced by the potential of the stacked layers (RS or Na in NaxCoO2) when the FS becomes smaller than the Brillouin Zone of the stacked structure

Influence of pulsed laser deposition growth conditions on the thermoelectric properties of Ca3Co4O9 thin films

Thin films of the misfit cobaltite Ca3Co4O9 were grown on (0001)-oriented (c-cut) sapphire substrates, using the pulsed-laser deposition techniques. The dependence of the thermoelectric/transport properties on the film growth conditions was investigatedComment: To be published in J. Appl. Phy

Experimental study of the incoherent spectral weight in the photoemission spectra of the misfit cobaltate [Bi2Ba2O4][CoO2]2

Previous ARPES experiments in NaxCoO2 reported both a strongly renormalized bandwidth near the Fermi level and moderately renormalized Fermi velocities, leaving it unclear whether the correlations are weak or strong and how they could be quantified. We explain why this situation occurs and solve the problem by extracting clearly the coherent and incoherent parts of the band crossing the Fermi level. We show that one can use their relative weight to estimate self-consistently the quasiparticle weight Z, which turns out to be very small Z=0.15 +/- 0.05. We suggest this method could be a reliable way to study the evolution of correlations in cobaltates and for comparison with other strongly correlated systems