Electronic Correlations in CoO2, the Parent Compound of Triangular Cobaltates

A 59Co NMR study of CoO2, the x=0 end member of AxCoO2 (A = Na, Li...) cobaltates, reveals a metallic ground state, though with clear signs of strong electron correlations: low-energy spin fluctuations develop at wave vectors q different from 0 and a crossover to a Fermi-liquid regime occurs below a characteristic temperature T*~7 K. Despite some uncertainty over the exact cobalt oxidation state n this material, the results show that electronic correlations are revealed as x is reduced below 0.3. The data are consistent with NaxCoO2 being close to the Mott transition in the x -> 0 limit.Comment: 4 pages, submitte

Similar glassy features in the NMR response of pure and disordered La1.88Sr0.12CuO4

High Tc superconductivity in La2-xSrxCuO4 coexists with (striped and glassy) magnetic order. Here, we report NMR measurements of the 139La spin-lattice relaxation, which displays a stretched-exponential time dependence, in both pure and disordered x=0.12 single crystals. An analysis in terms of a distribution of relaxation rates T1^-1 indicates that i) the spin-freezing temperature is spatially inhomogeneous with an onset at Tg(onset)=20 K for the pristine samples, and ii) the width of the T1^-1 distribution in the vicinity of Tg(onset) is insensitive to an ~1% level of atomic disorder in CuO2 planes. This suggests that the stretched-exponential 139La relaxation, considered as a manifestation of the systems glassiness, may not arise from quenched disorder.Comment: 7 pages, to be published in Phys. Rev.

Nonmagnetic insulator state in Na1CoO2 and phase separation of Na vacancies

Crystallographic, magnetic, and NMR properties of a NaxCoO2 single crystal with x≃1 are presented. We identify the stoichiometric Na1CoO2 phase, which is shown to be a nonmagnetic insulator, as expected for homogeneous planes of Co3+ ions with S=0. In addition, we present evidence that, because of slight average Na deficiency, chemical and electronic phase separation leads to a segregation of Na vacancies into the well-defined, magnetic, Na0.8CoO2 phase. The importance of phase separation is discussed in the context of magnetic order for x≃0.8 and the occurrence of a metal-insulator transition for x→1

Large enhancement of the thermopower in Nax_xCoO2_2 at high Na doping

Research on the oxide perovskites has uncovered electronic properties that are strikingly enhanced compared with those in conventional metals. Examples are the high critical temperatures of the cuprate superconductors and the colossal magnetoresistance in the manganites. The conducting layered cobaltate $\rm Na_xCoO_2$ displays several interesting electronic phases as $x$ is varied including water-induced superconductivity and an insulating state that is destroyed by field. Initial measurements showed that, in the as-grown composition, $\rm Na_xCoO_2$ displays moderately large thermopower $S$ and conductivity $\sigma$. However, the prospects for thermoelectric cooling applications faded when the figure of merit $Z$ was found to be small at this composition (0.6$<x<$0.7). Here we report that, in the poorly-explored high-doping region $x>$0.75, $S$ undergoes an even steeper enhancement. At the critical doping $x_p\sim$ 0.85, $Z$ (at 80 K) reaches values $\sim$40 times larger than in the as-grown crystals. We discuss prospects for low-temperature thermoelectric applications.Comment: 6 pages, 7 figure

Un nouveau système d'exploitation hybride pour les réseaux de capteurs sans fil

The Operating Systems (OS) dedicated to Wireless Sensor Network (WSN) are becoming an important research topic in recent years. This kind of OS has two natures: the resource-aware design and the wireless routing capability. This paper presents a Novel Hybrid Operating System (NHOS) dedicated to WSN. It consists of two parts: a hybrid real-time micro-kernel: LIMOS (LIghtweight Multi-Threading Operating System); a dedicated hybrid wireless protocol stack: CIVIC (Cooperative Inter-Vehicle Intelligent Communication). LIMOS combines the advantages of TinyOS and SDREAM. It has tiny resource consumption and supports the distributed real-time multi-thread applications. CIVIC is a context-aware routing protocol which is implemented to support the infrastructure and ad-hoc network, the hybrid MANET protocol and the multi-support wireless access mediums. This paper demonstrates the key features of NHOS

Investigation of Mg2(Si,Sn) thin films for integrated thermoelectric devices

International audienceThe ongoing miniaturization of thermoelectric (TE) modules requires scaling down to thin films of TE materials with high efficiency. Moreover, thin film-based integrated devices contain interfaces (between TE materials and dielectric or conductive components) that must show chemical and mechanical stability in the whole range of the operating temperature. In search for such materials, thin films of Mg2(Si,Sn) solid solutions have been deposited by microwave plasma-assisted co-sputtering method with a fine control over their composition. Three types of substrates were chosen (SiO2/Si, glass and Ni substrates) to examine their potential use as insulators and electrodes in a miniaturized thermoelectric module based on Mg2(Si,Sn). The electrical conductivity and thermo-mechanical properties, as well as the thermal stability, of the thin films have been investigated in the intermediate range of temperature (300–700 K). It is shown that the deposition process, as well as the substrates on which the films are grown, determine the subsequent adherence of the films. Also, the metastability of the Mg2Si0.4Sn0.6 solid solution for small variations in composition (possibly bordering the edge of the miscibility gap in the phase diagram) has been observed, which can lead to a separation into 2 phases during the first annealing treatment at intermediate temperatures

LIMOS : un micro noyau temps réel léger pour les objets sans fil

TinyOS is a natural event-driven single-task system. It fits for the single-task specific-application but not for the real-time multi-task application; while, SDREAM is a typical distributed real-time micro-kernel. It supports realtime multi-task applications but normally consumes more resources. This paper presents a novel hybrid real-time micro-kernel: LIMOS (LIghtweight Multi-threading Operating System), which combines the features of TinyOS and SDREAM. LIMOS adopts the event & thread two-level architecture and correspondingly has a two-level scheduling policy: event-driven' (high level scheduling for events) and priority-based preemptive' (low level scheduling for threads). LIMOS adopts the LINDA concept: tuple space and IN & OUT primitive-pair for system synchronization &communication. At present, LIMOS has been applied into a wireless application with the integration of multi-wireless techniques (RFID, ZigBee, and WiFi)

LiveFile : un système compact pour le stockage de données

Wireless sensor performance is highly correlated with available resources use. Resource awareness is more and more a key concept in WSN (Wireless Sensor Network). The increase of available on chip memory reveals the necessity to manage it efficiently. This article presents LiveFile (LIMOS Versatile Embedded File System) an interrogative micro-file system integrated to LIMOS (LIghtweight Multi-threading Operating System). LiveFile merges a micro-file system and an interrogative data system. It applies essentially on Flash memory