T. Skośkiewicz

Low temperature minimum of the electrical resistance in NiCu and NiCuH alloys

Abstract The electrical resistance of some NiCu alloys samples has been investigated continuously in the temperature range 4.2–80°K. In alloys containing more than 60% at. of copper a minima appears at low temperatures. The alloy with 50% at. copper exhibits a resistance minimum after suitable hydrogen alloying. The decay of ferromagnetism in Ni and NiCu alloys has been found as an essential condition for appearance of the above anomalies. For the Ni 0.33 Cu 0.67 alloy the magnetic contribution to the electrical resistance has been proved to fulfil Kondos interpretation.

Magnetic properties of the high Tc superconductor YBa2Cu3O7−δ

Abstract Magnetic measurements have been performed on the YBa 2 Cu 6 O 7−δ superconducting system in the temperature range 4.2–320 K. A hystereris loop indicating a coexistence of the superconducting and magnetic ordering below H c1 at 4.2 K has been recorded.

Colossal magnetoresistance and superconductivity in REAMnO/YBCO heterostructures

Abstract We report on the fabrication of RE1−xSrxMnO3/YBa2Cu3O7 ( RE = Nd , La ) multilayers grown by high pressure DC sputtering on LaAlO3 substrates. We show negative magnetoresistance in the multilayer structures above the superconducting transition temperature Tc. Magnetic moment measurements suggest a superconducting coupling through the ferromagnetic spacer as well as a ferromagnetic coupling through the superconducting spacer in this heterostructures.

Millikelvin studies of mixed-valence HgSe : Fe

Resistivity and magnetic susceptibility measurements of HgSe:Fe have been performed down to 30 mK and in the magnetic field range 50 mOe-3 kOe. Previously observed high conductivities of HgSe:Fe in the mixed valence region of Fe donors have been found to remain unchanged down to the lowest measured temperatures, indicating a minor influence of the resonant scattering and Kondo effect. This points to a weak k-d hybridization and substantiates classification of HgSe:Fe as an inhomogeneous mixed-valence system in which intersite Coulomb interactions impose spatial correlations in positions of charged centers. A resistive and inductive indication of superconductivity has been detected in some samples. The phenomenon is assigned to the presence of Hg-related superconducting inclusions and the associated proximity effect. This interpretation explains the field and temperature dependence of the resistivity and magnetic susceptibility, as well as being consistent with a sensitivity of the effect on deviations from stoichiometry and on annealing in Se vapor.

Flux pinning by anisotropic arrays of antidots in superconducting thin films

Anisotropic arrays of antidots have been prepared on Y-Ba-Cu-O and Nb thin films by electron-beam lithography. Fast-Fourier-Transform analysis of the magnetization curves for Y-Ba-Cu-O thin film reveals their periodical character which corresponds to the matching effect of the vortex lattice. For niobium thin film, matching peaks can be observed on the decreasing branches of the magnetization curves. Magneto-optical investigations at lower temperatures show unidirectional flux entry and exit along the shorter antidot lattice parameter. In higher magnetic fields, an anisotropic critical state is observed. Our observations strongly suggest formation of anisotropic vortex lattices in the patterned thin films.

Ferromagnetism and superconductivity in La0.7Sr0.3MnO3/YB2Cu3O7 superlattices

Abstract We report on the fabrication of La 0.7 Sr 0.3 MnO 3 / YBa 2 Cu 3 O 7 , (LSMO/YBCO) multilayers grown by high pressure dc sputtering on LaAlO 3 substrates. X-ray diffraction analysis of the single and bilayer films confirms c-axis epitaxial growth; superlattice structure shows second order satellite peaks. Magnetic moment measurements show both superconducting and ferromagnetic order below the superconducting transitions T c 0 . Superconducting transitions for a series of multilayers with fixed YBCO thickness d YBCO = 5.8 nm and increasing thickness of LSMO layers d LSMO from 1.2 to 3.6 nm show monotonic decrease of T c 0 .

PRESSURE EFFECTS ON NEW HIGH- T c SUPERCONDUCTORS

Abstract The pressure dependence of the superconducting transition temperature T c for La 1.8 Sr 0.2 CuO 4 , YBa 2 Cu 3 O 7-δ and EuBa 2 Cu 3 O 7−δ systems was measured under hydrostatic pressure up to 10 kbar. It is found that T c linearly increases with pressure for the La and Eu based systems. A nonlinear decrease of T c has been observed for the Y system.

Angular Dependence of Flux Pinning in High-T c Superconductor Thin Films and Superconductor-Ferromagnetic Heterostructures

We report on the results of measurements of the angular dependence of magnetization of high-T c superconducting YBa2Cu3O7−δ, and La1.85Sr0.15CuO4 thin films and superconducting/ferromagnetic YBa2Cu3O7−δ/Nd0.67Sr0.33MnO3 superlattices. We compare our present results to our results, previously obtained for YBa2Cu3O7−δ and Bi2Sr2CaCu2O8 single crystals. We discuss the influence of the presence of Nd0.67Sr0.33MnO3 sublayers in YBa2Cu3O7−δ/Nd0.67Sr0.33MnO3 superlattices on the critical temperature, flux pinning, and the critical field for the first vortex penetration.

Fabrication and magnetoconductance studies on submicron wires and films of MBE grown CdTe:In

Abstract Epitaxial CdTe:In films of the thickness down to 0.1 μm were grown and doped by MBE. The carrier concentration was controlled in the range from 8 × 1014 to 2 × 1018 cm−3 by varying the In flux. Free standing, conducting wires with the linewidth down to 0.3 μm were fabricated by means of electron-beam lithography followed by wet etching, and visualized by means of scanning-electron and atomic-force microscopies. Magnetoresistance measurements of the films and wires were carried out at temperatures down to 60 mK. A comparison of the data with theoretical predictions for the magnetoconductance in the weakly localized regime demonstrated the presence of temperature-induced dimensional crossovers in the studied structures. The electron phase-coherence length was determined for 1, 2, and 3 dimensional systems and showed to be limited by electron-electron interactions.

Magnetic properties of oxygen-deficient GdBa2Cu3Oy single crystals

Abstract Magnetisation measurements have been performed on a series of GdBa2Cu3Oy crystals with varying oxygen content y = 6.2 − 6.4. The measurements have been done at a temperature range T = 5 − 60 K for two magnetic field orientations: H ⊥ ab and H ⊥ ab. We have observed two contributions to the measured dc magnetisation: a paramagnetic contribution related to the Gd ions and a diamagnetic contribution related to superconductivity with Tc = 26 − 40 K. The diamagnetic contribution is strongly anisotropic. we discuss a possibility of existence of the 2D-type superconductivity in the studied samples. Another possible explanation of our results is an inhomogeneous oxygen distribution in the studied crystals.