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 and quantum effects in the organic compound (TMTSF)2PF6

The transverse magnetoresistance of (TMTSF)2PF6 has been investigated in magnetic fields up to 32 T, at low temperatures down to 4.2 K. Shubnikov-de-Haas oscillations are directly observed on the magnetoresistance in fields higher than 18 T, but there is no evidence of a threshold field since the disappearance of oscillations can be explained by the normal field-dependent damping. However these oscillations vanish very quickly between 5 and 4.2 K. This unusual behaviour could be related to a phase-transition. Finally an exploration of the fundamental field versus 0 is given (θ = angle between B and c*), showing the warping of the « Fermi tubes » along the c* axis.Nous avons étudié la magnétorésistance transverse du (TMTSF)2PF 6 pour des champs magnétiques allant jusqu'à 32 T, à basse température jusqu'à 4,2 K. Nous observons des oscillations de Shubnikov-de-Haas directement sur la magnétorésistance pour des champs supérieurs à 18 T, mais il n'est pas du tout certain qu'il existe un champ seuil car la disparition des oscillations peut être expliquée par l'amortissement normal fonction du champ magnétique. Par contre, celles-ci s'évanouissent très rapidement entre 5 et 4,2 K. Ce comportement inhabituel pourrait être dû à une transition de phase. Enfin nous donnons une exploration du champ fondamental en fonction de θ (0 = angle entre B et l'axe c*) qui montre le gondolement des « tubes de Fermi » le long de l'axe c*

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

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