Z. Tarnawski

Low-temperature specific heat of REBa2Cu3O7 in magnetic fields up to 5 T (RE = Y, Pr, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu)

Abstract Specific heat measurements have been performed on a series of REBa 2 Cu 3 O 7 -compounds (RE = Y, Pr, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu) in magnetic fields up to 5 T in a temperature range between 1.4 and 40 K. The electronic and lattice contributions to the specific heat of GdBa 2 Cu 3 O 7 have been taken as a reference for the heavy rare-earth compounds in order to deduce the magnetic and crystal-field contributions to the specific heat. The low-temperature data in field of the Dy-, Er-, and Yb-compounds confirm a doublet ground state, that is splitted by the applied field. The crystal-field contributions to the specific heat in these compounds deviate from Schottky behaviour due to anisotropy effects. Values for gJ amount to 2.3, 3.2 and 1.6, respectively.

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.

Double anomaly in the specific heat of superconducting UPt3 and UPt3B0.11

Abstract The specific heat of several poly- and single-crystalline samples of UPt3 and UPt3B0.11 has been measured below 1 K, in order to study the substructure in the anomaly below the superconducting transition. A double anomaly has been observed for Czochralski-grown single-crystalline UPt3 as well as for poly-crystalline UPt3B0.11. The addition of boron increases the transition temperature with roughly 100 mK and does not suppress the second transition.

Unconventional angular dependence of the microscopic magnetic field distribution in single crystal Bi2Sr2CaCu2O8 from muon-spin rotation studies

Abstract The angular dependence of the microscopic magnetic field distribution in single-crystal Bi 2 Sr 2 CaCu 2 O 8 has been measured for a range of applied fields using the muon-spin rotation technique. The results show significant deviations from the predictions of the anisotropic 3D model, which may reflect the extremely anisotropic, quasi-2D nature of the flux structure. The temperature dependence of the field distribution at 400 mT has also been measured for a range of orientations, showing disagreement with the predictions of a simple two-fluid model.

Magnetic instabilities in high temperature superconductors

Abstract Magnetic instabilities in single-crystalline samples of high temperature superconductors have been studied by magnetocaloric and magnetization measurements under varying magnetic fields. Sharp repetitive jumps are observed and are interpreted as been caused by the avalanche-like flux flow and the transition from isothermal to adiabatic limit of the flux propagation. Periodicity, size and time duration of the instabilities are discussed.

Crystal-field interactions in ErBa2Cu3O7

The specific heat of single-crystalline superconducting SmBa2Cu3O7−x shows a broad Schottky-type anomaly at 1 K K due to the splitting of the Kramers-doublet ground-state. The results have been analyzed within the crystal-electricfield approach. A full set of CEF parameters is proposed which reproduces the level scheme obtained by inelasticneutron-scattering studies and provides a good agreement with specific-heat data, especially in applied magnetic field.

Specific heat of Nd1−xLuxMn2

Abstract Specific heat measurements have been performed on Nd1−xLuxMn2 compounds with x values of 0.0, 0.02, 0.05, 0.10 and 1.0 for temperatures between 1.5 and 300 K in magnetic fields up to 8 T. The electronic and lattice contributions to the specific heat of the isostructural paramagnetic compound LuMn2 have been taken as a reference for the Nd containing compounds in order to deduce the magnetic and crystal field contributions. By comparing the measured specific heat data with the calculated specific heat contribution from the Nd-ions, the Mn contribution is deduced. The variation of this Mn contribution with x is discussed.

PHYSICAL PROPERTIES OF SINGLE-PHASE HIGH-Tc SUPERCONDUCTOR YBa2Cu3O7

Transport and thermodynamical properties as well as structural investigations in the well-characterized 90 K-class high-Tc superconductor YBa2Cu3O7 are presented, especially in relation to the fundamental mechanism of the high-Tc superconductivity. High-precision specific heat measurements around Tc=91.8 K reveal that the value of ΔC/Tc is 48±2 mJ/mol K2. This value results in the fact that the value of ΔC/γTc is 1.33, which is close to 1.43, the weak-coupling BCS value, provided that the coefficient, γ, of the linear term in the specific heat is 36±2 mJ/mol K2.

Evidence for nonlinear flux diffusion from magnetization relaxation in Bi2Sr2CaCu2O8 single crystals

Abstract Using a sensitive Hall probe, the magnetization relaxation of Bi 2 Sr 2 CaCu 2 O 8 single crystals has been measured accurately. The sample was cooled down from T>T c in a large bias magnetic field and then relaxation has been registered after a small change of the magnetic field. The effects of the bias field value, ranging from almost zero up to H irrev (≈20 kOe), and of the amplitude of the field change were studied at T =20 K. We find that the results are well described by the model for the self-organized critical state: there are short- and long-time domains of the relaxation, both characterized by a power-law dependence on time. The crossover time depends on the bias field (i.e. on the critical current density) and on the amplitude of field change. The long-time relaxation is independent of the initial field change.

High-field magnetisation of Nd1−xLuxMn2 compounds

Abstract Nd 1− x Lu x Mn 2 compounds with x =0, 0.02, 0.03, 0.05, 0.10, 0.15, 0.20, 0.30 and 1.0 have been investigated in high-field magnetisation measurements up to 38 T at 4.2 K and in electrical resistivity, thermal expansion and specific heat experiments between 1.5 K and room temperature. The high-field magnetisation increases linearly with increasing field for LuMn 2 with a susceptibility of 11 × 10 -9 m 3 / mol , indicating a paramagnetic behaviour. No anomalies are observed in other experiments in the temperature range from 1.5 to 300 K, confirming this observation. For the compounds with x ⩽0.20, saturation is not reached in the highest field whereas around 10 T a transition in the magnetisation curve is observed. For the compound with x =0.30, the magnetisation shows a trend to saturation at higher field values. The value of the saturated moment in this case is close to that expected for Nd 3+ which suggests the absence of a Mn moment in this compound. This result is confirmed by other experimental data.