A. Kołodziejczyk

Properties, magnetic susceptibility, critical currents and irreversibility fields of (Tl0.5Pb0.5)Sr2 (Ca1−xGdx)Cu2Oz bulk superconductors

Bulk (Tl0.5Pb0.5)Sr2(Ca1−xGdx)Cu2Oz superconductors with x = 0.1,0.2 and 0.3 were prepared by a wet chemical gel technique followed by a special heat-treatment procedure. X-ray diffraction analysis revealed practically phase-pure samples with tetragonal structure (space group P/4mmm). The temperature dependence of the real and imaginary parts of the ac susceptibility and the dependence on the ac applied magnetic field were investigated to learn about the superconducting and magnetic properties of bulk (Tl0.5Pb0.5)Sr2(Ca1−xGdx)Cu2Oz superconductors. The critical current densities of the samples were derived from Beans model. The temperature dependence of the critical current was successfully fitted based on a power law from the thermally activated magnetic flux creep model. From magneto-resistance measurements the temperature dependences of the transport critical currents and the irreversibility fields were found. The dc magnetic field dependence of the width of superconducting transitions and the critical currents was also obtained. These properties of the superconductors were analyzed within available theoretical models.

Specific heat and NMR spin echo in the superconducting and magnetic region of the Y4Co3 compound

Abstract The specific heat and NMR of the magnetic superconductor Y4Co3 and other compounds close to this phase were neasured. We found that the electronic contribution to the specific heat is mainly caused by Co electrons but that the phonon contribution is mainly due to the Y lattice. The coexistence of magnetism and superconductivity was observed through changes of the Knight shift parameter of the NMR spectrum. The most striking result is that we found a negative Knight shift for some parts of the h-type Co atoms which might mean that superconductivity originates also from these atoms and not only from the Y band.

Electronic states of La1−xCaxMnO3 from photoelectron spectroscopy

Abstract The electron photoemission spectra from core-levels as well as from the valence bands of the manganese perovskites La 0.67 Ca 0.33 MnO 3 , La 0.63 Ca 0.37 Mn 0.92 Fe 0.08 O 3 and (La 0.57 Tb 0.10 )Ca 0.33 MnO 3 have been measured in insulating paramagnetic state at 300 K. The valence band spectra show a two-peak structure at about 3.0 and 5.7 eV below the upper edge of the valence band. Comparing the spectra with the high-resolution spectra measured at 110 eV synchrotron radiation the strong hybridisation of Mn 3d–O 2p states contributing to the valence band has been confirmed. The higher binding energy part of valence band spectrum of Tb-doped specimen shows a third feature whose physical origin is discussed. The values of the on-site Coulomb correlation energies in Mn 3d states U dd , in O 2p states U pp , the charge transfer energy Δ and the hybridisation energy between Mn 3d and O 2p states T were estimated from the Mn 2p spectra and Auger spectra analysed together with the relevant valence band spectra. Some results are compared with the 55 Mn nuclear magnetic resonance data.

Critical currents and magnetization of a (Tl0.5Pb0.5)(Sr0.85Ba0.15)2Ca2Cu3Oz film on silver substrate

A thin film of a (Tl0.5Pb0.5)(Sr0.85Ba0.15)2Ca2Cu3Oz superconductor was prepared on highly polished silver substrates. Its superconducting transition temperature (Tc,50%) is 114.7 ± 0.3 K and the critical current density is 11.4 × 103 A cm−2 at 77 K in the self-field. The transport critical currents densities were measured as a function of temperature for several applied magnetic fields. The magnetization versus the dc applied magnetic field and temperature were also measured and the critical current densities were calculated. The temperature dependence of the critical currents obeys the Ginzburg–Landau power-law. Reasons for this behaviour were discussed taking into account the pinning force and anisotropy of the critical currents associated with the c-axis grain orientation of the film. The magnetization as a function of the angle between the applied magnetic field and the a–b plane showed a periodic dependence due to changes of the demagnetizing field.

Intergrain magnetic properties and critical currents of T1-1223 bulk and tape superconductors

Abstract The magnetic field and the temperature dependencies of the dispersive and the absorption components of the low frequency ac magnetic susceptibility for Pb and Bi single doped bulk superconductors as well as for Pb and Bi co-doped Ag-sheathed superconducting T1-1223 tapes have been measured and analyzed. The studies were accompanied by optical and electron microscopic observations of the microstructure. The ac susceptibility measured at various ac magnetic field amplitudes showed both intergrain and intragrain features. The temperature dependencies of the intergrain critical current densities were extracted from the absorption susceptibility via a modified Bean model. They were at relevant temperatures one order of magnitude larger for the tape than for the bulk specimens.

Magnetic and transport properties of Fe-substituted manganites La0.67Ca0.33Mn1−xFexO3

Abstract Specific heat, magnetic susceptibility and magnetoresistivity measurements have been performed on the polycrystalline samples of Fe-substituted manganites. The transition temperatures derived from the specific heat anomalies for x=0,0.01,0.06 were in agreement with the position of the maximum slope of AC susceptibility. For compositions with x=0.10 and 0.15 no transition was observed in the specific heat. A sharp increase in resistivity below 135 and 125 K was observed in zero field for x=0.10, and 0.15, respectively.

Electronic States of UNi_2 from Photoemission Spectroscopy

We measured the angle-resolved ultraviolet and X-ray photoemission spectra of UNi2 single crystal. The valence band angle-resolved ultraviolet photoemission spectra were compared to the accessible band structure calculations. For UNi2 the lowest binding energy strong emission at about –0.1 eV comes from U 5f states and overlaps with the Fermi edge. A small feature at –0.6 eV was ascribed also to U 5f electrons with more localised character. The higher-energy two-peak structure at about –1.2 eV and –2.1 eV comes from Ni 3d states. X-ray photoemission spectroscopy shows the standard two-line spin–orbit splitting of the Ni 2p states and of the U 4f states. Some hybridisation of the Ni 3d and U 5f state was observed in the spectra. The pronounced satellites to the Ni 2p and 3p states, associated with the on-site Coulomb repulsion and other electron correlation energies, were observed.

Specific heat and electron photoemission of Pb- and Bi- doped Tl(Ba,Sr)CaCuO (1223) high-temperature superconductors

Abstract Specific heat of Tl 0.58 Pb 0.4 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O y , Tl 0.79 Bi 0.25 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O y and Tl 0.74 Bi 0.3 Sr 1.8 Ba 0.2 Ca 2 Cu 3 O y has been measured and the electronic specific heat coefficient γ and the specific heat jumps ΔC at T C have been calculated. The specific heat jumps are about three times smaller than for the YBa 2 Cu 3 O x samples. The core level and valence band spectra for Tl 0.58 Pb 0.4 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O y have been detected at room, at liquid nitrogen and at 125 K temperatures. The Fermi edge has not been observed though pronounced changes of the valence band shapes have been registered. This is most likely due to the larger anisotropy of electronic band structure of the thallium samples in comparison to the yttrium superconductors.