The hopping conductivity of La2CuO4+δ single crystal with excess oxygen has been studied. It is found that the transition from variable-range hopping (with the temperature dependence of resistance R∝exp(T0/T)1/4) to simple activation conductivity (R∝exp(Δ/kT)) occurs at a temperature lower than 20 K. This transition is accompanied with significant changes in the behavior of magnetoresistance and current–voltage characteristics. It is shown that such a behavior of hopping conductivity below 20 K is due to the presence of superconducting inclusions in insulating sample due to phase separation of La2CuO4+δ. The observed peculiarities in the conducting properties are in good agreement with the known effects of competition between electron localization and superconductivity in homogeneous systems.

A bulk polycrystalline sample of YBa 2 Cu 3 O 7−δ (δ ≈ 0.1) has been irradiated by γ-rays with 60 Co source. Non-monotonic behavior of T c (defined as the temperature at which normal resistance is halved) with increasing irradiation dose Φ (up to about 220 MR) is observed: T c decreases at low doses (Φ ≤ 50 MR) from initial value (≈ 93 K) by about 2 K and then rises, forming minimum. At highest doses (Φ ≥ 120 MR) T c goes down again. The temperature width, δT c , of resistive transition increases rather sharp with dose below 75 MR and somewhat drops at higher dose. We believe that this effect is revealed for the first time at γ-irradiation of high-T c superconductor. The cross sections for the displacement of lattice atoms in YBCO by γ-rays due to the Compton process were calculated, and possible dpa values were estimated. The results obtained are discussed taking into account that sample studied is granular superconductor and, hence, the observed variations of superconducting properties should be connected primarily with the influence of γ-rays on intergrain Josephson coupling.

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.