Granular superconductivity effects in polycrystalline samples of RuSr 2 (Gd 1.5 Ce 0.5)Cu 2 O 10−δ , as prepared (by a solid-state reaction method) and annealed (12 h at 845 • C) in pure oxygen at 30 atm, are presented. The resistive transition to the superconducting state of the as-prepared sample is found to be considerably affected by granularity. In particular, an evident kink in the temperature dependence of the resistance R(T) is seen at the temperature, T c0 ≈ 34 K, at which grains become superconducting. The resistive transition depends strongly on the applied current. The family of R(T) curves taken for different transport currents is branched with a branching point at T cJ ≈ 23.2 K. Below this temperature the intergrain Josephson coupling starts to develop. For low current, R decreases with decreasing temperature below T cJ as expected for the transition to the superconducting state, whereas R(T) curves for higher current form a minimum at T ≈ 17.3 K, showing a quasi-reentrant behaviour. The influence of the granular structure of the as-prepared sample shows itself also in the temperature behaviour of the magnetization, M(T), in low field. Application of low magnetic field (below 400 Oe) leads to a broadening of the resistive transitions below T cJ , similar to that caused by increasing the current. Both the current and magnetic field depress the Josephson coupling between the grains, producing a dramatically large effect on the resistive transition. The R(T) and M(T) dependences of the annealed sample show a fairly sharp superconducting transition far less affected by granularity. The results obtained imply that oxygen annealing improves the intergranular connection considerably, but it does not exert much influence on the intragrain superconductivity. No indication of intragrain granularity has

The analysis of quantum corrections to magnetoconductivity of thin Au films responsible for by the effect of weak electron localization has made it possible to determine the temperature dependences of electron phase relaxation time in the temperature range 0.5--50 K for different degrees of crystal lattice disorder. The disorder was enhanced by irradiating the films in vacuum with 3.5 keV Ar ions. The experimental data clearly demonstrate that the contribution of electron-electron interaction to electron phase relaxation increases with disorder and support the theoretical prediction that the frequency of electron-phonon scattering tends to diminish upon a decrease in electron mean free path. It is found that the spin-orbit scattering rate decreases with disorder. In our opinion, such unusual behavior can take place for thin films at decreasing the electron mean free path provided, that the surface electron scattering contributes significantly to the total spin-orbit scattering.Comment: 15 pages, REVTex, 6 Postscript figures, To be published in Phys. Rev.

The ultrasonic properties of La1−xCaxMnO3 (x ≈ 0.25) with the Curie temperature TC about 200 K are studied. Temperature dependences of longitudinal and transverse sound velocities were measured in zero magnetic field and for different constant magnetic fields as well. The ultrasonic study is supported by magnetic, resistive, magnetoresistive, structural and other measurements of the sample that facilitate interpretation of the results obtained. The magnetic field influence on sound properties found in this study presents some new features of the interplay between the elastic and magnetic properties of these compounds. It is shown that the paramagnetic-ferromagnetic transition in the sample studied is first order, but can become second order under the influence of applied magnetic field.

We compare the quasiparticle lifetime behavior in the unitary limit of two unconventional superconductors dressed by nonmagnetic impurity scattering to differentiate an anomalous functional behavior in its shape when the disorder concentration is changed in a triplet paired model with respect to the well behave singlet model. For singlet paired superconductors, the functional shape of the lifetime due to elastic scattering around the nodal regions in the reduced elastic scattering space, does not change with the disorder concentration, but for a triplet model with a tiny gap, an anomalous drop in shape is observed only when small values of disordering are added. We use a 2D tight-binding parametrization to study the reduced phase space of the first Brillouin zone, where the low-energy scattering is restricted to the nodal/quasinodal regions for two irreducible representations of the crystal lattice. A new class of Wigner probabilistic distribution functions is discussed for triplet superconductors.

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Resistive, magnetoresistive and magnetic properties of four types of pressed CrO2 powders synthesized from chromic anhydride by the hydrothermal method were investigated. The new synthesis method allowed controlling the thickness of dielectric shells. The powders consisted of rounded particles (≈120 nm in dia.) or acicular crystals (≈22.9 nm in dia. and 302 nm in length). The particles were covered with a surface dielectric shell of different thickness and type (e.g. β-CrOOH oxyhydroxide or chromium oxide Cr2O3). The influence of the properties and the thickness of intergranular dielectric layers as well as the shape of CrO2 particles on the tunneling resistance and magnetoresistance (MR) of the pressed powders was studied. It was found that at low temperatures all the investigated samples displayed a nonmetallic temperature dependence of resistance and a giant negative MR. The maximal values of MR at T ≈ 5 K were found to be approx. 37% in relatively low magnetic fields (0.5 T). The MR decreased rapidly with increasing temperature (down to approx. 1% in 1 T at T ≈ 200 K). At low temperatures the powders with acicular particles exhibited a new type of MR hysteresis and nonmonotonous dependence of MR with increasing magnetic field. A nonmonotonous temperature dependence Hp(T), where Hp is the field in which the resistance is maximal, mismatch between the values of Hp and coercive force Hc, and the anisotropy of MR as a function of mutual orientation of transport current and magnetic field were observed.

a b s t r a c tSubmicron powder of CrO 2 was prepared by hydrothermal synthesis method from chromium trioxide with use of special modifiers, which govern the growth of particles. Particles obtained were of rounded form with mean diameter about 120 nm. The powder (stabilized with thin surface layer of ␤-CrOOH) has been characterized by structural, X-ray and magnetic measurements. The powder under investigation (with Curie temperature about 385 K) was cold-pressed and its transport and magnetotransport properties have been measured in the temperature range 4-450 K in magnetic field up to 1.6 T. The samples studied is characterized by non-metallic temperature behavior of resistance and large negative magnetoresistance (MR) in low temperature range. At T = 5 K the MR magnitude has been −17% at H = 0.3 T and −20% at H = 1.4 T. Its magnitude decreased fast with increase in temperature reducing to 0.3% and less for T > 200 K. It is shown that this MR behavior is inherent for a system of magnetic grains with spindependent intergrain tunnelling. Some peculiarities of MR behavior in low-temperature range (below 40 K) can be associated with percolating character of tunnelling conductivity of this granular system under conditions of availability of only few conducting current paths through the sample.

The DC transport properties of and the radio-frequency (RF) wave absorption (at 2:525 MHz) in a sample of La 0:67 Sr 0:33 MnO 3 prepared by floating-zone method are measured. The Curie temperature, T c ; of the sample is about 374 K: Giant temperature and magnetic-field variations in RF absorption are found in the vicinity of T c : Relative change of the RF absorption in magnetic field (magnetoabsorption) is about 67% in field 2:1 kOe and about 55% in field 1 kOe: This giant magnetoabsorption effect can be used to develop RF devices controlled by temperature and low magnetic field. A weak temperature dependence of magnetoabsorption for the sample studied in the range from room temperature to about 350 K makes it especially attractive for practical use. The RF study supplemented with transport, magnetoresistive and magnetic measurements enables us to discuss the optimal properties of manganite samples for observation of giant magnetoabsorption in low field. r