[en] Nonmonotonic forms of resistive superconducting transition as a function of temperature and magnetic field have been observed in granular high-T_c ceramics of the electron-type family L_2-xMxCuO_4-y. The behavior is compared with that of low-temperature granular superconductors and is treated by the conventional terms of the energy gap and Josephson and quasiparticle tunnelings. The values of the intragranular upper critical field H_c2 perpendicular to the a-b plane of Sm-Ce-Cu-O are extracted. We find evidence for the appearance of superconductivity in individual grains at temperatures significantly higher than the resistance onset temperature

[en] We study the superconducting properties of the electron-doped compounds Ln_2-xM_xCuO_4-y (Ln = Pr, Nd, Sm, Eu; M = Ce, Th). Three characteristic temperatures of the superconducting transition are defined, corresponding to: the macroscopic resistive transition midpoint (T_r), the diamagnetic signal onset (T_d), and the intragrain transition temperature (T_g). T_d and T_g are found to vary weakly in the series of the studied compounds, in contrast with T_r, which is strongly reduced in Eu_1.85Ce_0.15CuO_4-y and Sm_1.85Th_0.15CuO_4-y samples

[en] Electron-beam deposition of an insulating granular aluminium or off-stoichiometric amorphous alumina layer on a SrTiO₃ surface is a simple way to get a metallic interface from insulating materials. No heating nor specific preparation of the SrTiO₃ surface is needed. In this paper, we investigate how the electrical properties of this interface can be tuned by the use of a back gate voltage (electrical field through the SrTiO₃ substrate). We demonstrate that the slow field-effect observed at room temperature can be used to tune in a controlled, reversible way the low temperature electrical properties of the interface. In particular, important parameters of a transistor such as the amplitude of the resistance response to gate voltage changes or the existence of an ‘on’ or an ‘off’ state at zero gate voltage and at low temperature can be adjusted in a single sample. This method should be applicable to any SrTiO₃-based interface in which oxygen vacancies are involved and might provide a powerful way to study the metal or superconductor insulator transition observed in such systems. (paper)

[en] It is shown that a metallic state can be induced on the surface of SrTiO₃ crystals by the electron beam evaporation of oxygen deficient alumina or insulating granular aluminium. No special preparation nor heating of the SrTiO₃ surface is needed. Final metallic or insulating states can be obtained depending on the oxygen pressure during the evaporation process. Photoconductivity and electrical field effect are also demonstrated. (paper)

[en] We propose an H-T phase diagram of granular superconductors based on the separate analyses of the intragranular critical fields and that of an assembly of Josephson junctions. The model is examined experimentally and its predictions are applied to several problems of high-temperature superconductivity like the H_c2(T) enhancement with a temperature reduction, the width of the R(T) transition under an applied magnetic field, and a double-step J_c(H) variation

[en] An unusual form of superconducting transition has been observed in some of the superconducting compounds L_2-xM_xCuO_4-y (L=Pr,Nd,Sm,Eu; M=Ce,Th) under low magnetic fields. When the temperature is reduced below the critical one, the sample's resistance first increases, then drops to a nonzero value, increases again, and finally decreases at low temperature, creating a characteristic double-peak transition. The behavior is explained in terms of intra- and intergranular superconducting transitions

[en] We have investigated a sintered Gd_1.85Th_0.15CuO₄ sample in a SQUID magnetometer under pressure up to 10 kbar. From high-field magnetic measurements, we have deduced a large decrease under pressure of the internal field associated to the weak ferromagnetism of the Cu sublattice. We have studied by magnetic susceptibility measurements in low field the pressure effect on Neel temperature, on the magnetic ordering temperature of the rare earth and on the intermediate spin reorientation temperature. Possible implications for electron-doped superconductivity of related compounds are discussed. (orig.)

[en] We focus on the slow relaxations observed in the conductance of disordered insulators at low temperature (especially granular aluminum films). They manifest themselves as a temporal logarithmic decrease of the conductance after a quench from high temperatures and the concomitant appearance of a field effect anomaly centered on the gate voltage maintained. We are first interested in ageing effects, i.e. the age dependence of the dynamical properties of the system. We stress that the formation of a second field effect anomaly at a different gate voltage is not a 'history free' logarithmic (lnt) process, but departs from lnt in a way which encodes the system's age. The apparent relaxation time distribution extracted from the observed relaxations is thus not 'constant' but evolves with time. We discuss what defines the age of the system and what external perturbation out of equilibrium does or does not rejuvenate it. We further discuss the problem of relaxation times and comment on the commonly used 'two dip' experimental protocol aimed at extracting 'characteristic times' for the glassy systems (granular aluminum, doped indium oxide...). We show that it is inoperable for systems like granular Al and probably highly doped InO_x where it provides a trivial value only determined by the experimental protocol. But in cases where different values are obtained like in lightly doped InO_x or some ultra thin metal films, potentially interesting information can be obtained, possibly about the 'short time' dynamics of the different systems. Present ideas about the effect of doping on the glassiness of disordered insulators may also have to be reconsidered.

[en] We report resistivity and AC susceptibility measurements under external magnetic field in NdCeCuO single crystals. From resistivity data we have estimated the upper critical fields, coherence lengths and anisotropy factor; AC susceptibility has allowed the extraction of the activation energy values, which turn out to be higher than the p-type HTSC ones. (orig.)

[en] The transport properties of the electron-doped cuprates M_2-xCe_xCuO₄ (M=Pr, Nd, and Sm) single crystals are investigated under hydrostatic pressure up to 2 GPa. The effects of pressure on both the c-axis (ρ_c) and the in-plane (ρ_ab) resistivity are measured from room temperature down to the superconducting state. It is found that at 300 K, lnρ_ab/dP∼d lnρ_c/dP∼-3% GPa^-1. The pressure does not change the temperature dependence of the normal-state resistivity, which at high temperature in both directions follows a ρ∼A(P)T² law, but the coefficient A(P) lowers as pressure is increased. Moreover, we have found that for any lanthanide host (Pr, Sm, and Nd), the onset of superconductivity does not change with pressure (P<2 GPa). These experimental results raise questions on the significance of the already proposed general behavior of lnT_c/dP on T_c and on the claimed ''antisymmetry'' of the pressure dependence of T_c in electron-doped and hole-doped high-temperature superconductors