Formation of two-dimensional weak localization in conducting Langmuir-Blodgett films

We report the magnetotransport properties up to 7 T in the organic highly conducting Langmuir-Blodgett(LB) films formed by a molecular association of the electroactive donor molecule bis(ethylendioxy)tetrathiafulvalene (BEDO-TTF) and stearic acid CH$_3$(CH$_2$)$_{16}$COOH. We show the logarithmic decrease of dc conductivity and the negative transverse magnetoresistance at low temperature. They are interpreted in the weak localization of two-dimensional (2D) electronic system based on the homogeneous conducting layer with the molecular size thickness of BEDO-TTF. The electronic length with phase memory is given at the mesoscopic scale, which provides for the first time evidence of the 2D coherent charge transport in the conducting LB films.Comment: 5 pages, 1 Table and 5 figure

High field magnetoresistance of silver containing rare-earth impurities

Nous présentons des mesures de magnétorésistance en champ pulsé (300 kG) sur des alliages Ag : Dy, Ag : Ho et Ag : Tm ainsi que l'analyse des contributions dues à la diffusion quadripolaire et à la diffusion d'échange.We present magnetoresistance measurements in pulsed fields (300 kG) on Ag : Dy, Ag : Ho and Ag : Tm alloys together with the analysis of the contributions from quadrupole and exchange scattering

Analytic expressions of the magnetoresistance due to localization and electron-electron interaction effects. - Application to the amorphous alloys La3Al and La3Ga

We start from the theoretical calculations of Alt'shuler et al. and Lee and Ramakrishnan for the contributions to magnetoresistance from weak localization and electron-electron interactions in a disordered system. We give analytic expressions which can be more easily handled by experimentalists. Our results on the amorphous alloys La3Al and La3Ga are presented and interpreted using these equations to get the best fit.Nous partons des calculs théoriques d'Alt'shuler et al. et de Lee et Ramakrishnan sur les contributions à la magnétorésistance de la localisation faible et des interactions électron-électron dans un système désordonné. Nous en donnons des formes analytiques d'utilisation facile pour les expérimentateurs. Nous présentons nos résultats expérimentaux sur les alliages amorphes La 3Al et La3Ga et nous les interprétons à partir des équations que nous avons obtenues

EXFOLIATION DU GRAPHITE PAR TRAITEMENT LASER D'UN COMPOSÉ INTERCALAIRE

L'exfoliation de graphite a été réalisée par chauffage d'un composé intercalaire (graphite naturel - acide nitrique) au moyen d'un laser continu à CO2 et d'un laser YAG à impulsions. Les surfaces résultantes sont homogènes et nettement plus étendues que celles obtenues avec le même composé intercalaire par les procédés habituels de chauffage. Les expériences font ressortir l'intérêt du laser dans la recherche des conditions optimales d'exfoliation du graphite et dans les études visant à une meilleure compréhension du phénomène lui-même ; elles montrent clairement l'importance du choc thermique.For the first time graphite has been exfoliated by laser heating of an intercalated compound (natural graphite-nitric acid). A continuous CO2 laser and a pulsed YAG laser were used. Homogeneous surfaces were prepared which are larger than those obtained by a conventional heating process with the intercalated compound considered. Laser heating appears quite suitable for a better understanding of the exfoliation mechanism itself and for the finding of the best exfoliation conditions. The importance of the thermal shock is clearly evidenced

HALL EFFECT IN SILVER - RARE EARTH AMORPHOUS ALLOYS

No abstract availabl

MAGNETORESISTIVITY OF AMORPHOUS ALLOYS CONTAINING RARE EARTHS

Nous analysons les résultats de mesures d'anisotropie de magnétorésistivité à basse température sous champ moyen (jusqu'à 45 kG) et sous champ pulsé (jusqu'à 300 kG) sur différentes séries d'alliages amorphes contenant des terres rares (TR) non S. Pour les alliages amorphes du type TR80Au20, la magnétorésistivité anisotrope (ƊρA) est négative pour toutes les TR lourdes. Pour les alliages du type TR67Co.33, ƊρA sous champ moyen présente le changement de signe entre le Ho et Er indiquant une contribution dominante des quadrupoles des TR. Par contre, à haut champ, ƊρA devient négative comme pour les alliages TR80-Au20. Pour les alliages dilués du type La80-xTRxAu20 (x valant quelques %), la contribution quadrupolaire à ƊρA est non équivoque. Le comportement de ƊρA en fonction du champ et de la température permet une identification du niveau fondamental pour Pr, Nd, Tb, Dy, Tm dans La80Au20 et une évaluation semi-quantitative de l'écart en énergie entre le fondamental et les premiers états excités. Les schémas obtenus indiquent une distribution étroite du paramètre d'asymétrie autour d'une valeur correspondant à une symétrie locale planaire.We report the results of low-temperature anisotropic magnetoresistivity (ƊρA) measurements in moderate steady field (up to 45 kG) and in high pulsed field (up to 300 kG) on various series of amorphous alloys containing non-S state Rare-Earths (RE). For RE80Au20 alloys, ƊρA is negative for all the heavy RE. For RE67Co33 alloys, ƊρA in moderate field exhibits between Ho and Er the change of sign expected for dominant contribution from RE quadrupole scattering. But, in high field, ƊρA becomes negative as for amorphous RE80Au20. For dilute La80-xRExAu20 alloys (x being a few %), the quadrupole contribution to ƊρA is unambiguous. From the field and temperature behavior of ƊρA, the nature of the ground state for Pr, Nd, Tb, Dy and Tm in amorphous La80Au20 is discussed together with an estimate of the energy gap between the ground state and the first excited states. The energy level schemes thus obtained indicate a narrow distribution of the asymmetry parameter around a mean value corresponding to a planar local symmetry around the RE ions

Upper critical field measurements in high-Tc superconducting oxides

We present upper critical field measurements on the superconducting oxides RE Ba2Cu3O7-δ (RE = Y, Gd) performed in a pulsed magnetic field up to 43 T. Values for Hc2 as high as 52 T and 77 T for Y and Gd respectively, are expected at 77 K. However, in order to observe no resistive behaviour up to 43 T the temperature must be decreased down to 50 K. In the case of oxygen deficient systems the magnetoresistance reveals two superconducting phases wich could be related to two different orders of oxygen vacancies

Negative spin-valve effect in Co65Fe35/Ag/(Co65Fe35)50Gd50 trilayers

International audienceA minimum of the magnetoresistance under low fields has been observed with a Co 65 Fe 35 /Ag/ Co 65 Fe 35 50 Gd 50 trilayer at 4.2 and 300 K. This negative spin-valve effect is explained by the scattering of the conduction electrons by the Co or Fe moments included in two different structures: a polycrystalline Co 65 Fe 35 and an amorphous one Co 65 Fe 35 50 Gd 50. The Gd moments allow one to control the magnetic configuration of the transition-metal moments of the layers but are inefficient for the magnetoresistance. This interpretation is confirmed by measurements using high pulsed fields

RESISTIVITY AND MAGNETORESISTANCE OF SILVER-RARE-EARTH AMORPHOUS ALLOYS

We have studied the resistivity and the magnetoresistance of Ag1-x Rx and Ag0.5 Lu0.5-x Rx amorphous alloys prepared by sputtering (R = rare-earth). Measurements have been performed in pulsed field up to 300 kG. We observe that magnetic ordering can yield positive or negative resistivity changes in these systems. We interpret the experimental results in terms of coherent exchange scattering and two-current conduction