N. Knauf

Preparation and characterisation of single-phase Bi-Pb-Sr-Ca-Cu-O high temperature superconductors

Abstract We identify the preparation procedures to obtain polycrystalline pellets of Bi-2212 and of Bi-2223 high-temperature superconductors, which slow only the superconducting properties and the X-ray diffraction pattern of one or the other phase (“single-phase” pellets). These procedures were found by systematic variation of the preparation, with substitution of PbO for Bi2O3 as well as with addition of PbO to the stoichiometric mixture of starting powders. Bi-2223 in single-phase form has a TZR of 107.5 ± 0.5 K, whereas single-phase pellets of “Bi-2212” can have sharp transitions with TZRs between 55–95 K, depending on the oxygen partial pressure of the atmosphere in the final annealing step.

Resistivity, Hall effect, Nernst effect and thermopower in the mixed state of Bi- and Tl-based high-Tc superconductors

Abstract We present measurements of the electric resistivity, the Hall effect, the Nernst effect and the thermopower in the mixed state of Bi 1.76 Pb 0.24 Sr 2 Ca 2 Cu 3 O δ and of Tl 2 Ba 2 Ca 2 Cu 3 O δ . We find a large broadening of the resistive transition independent of the current direction with respect to the magnetic field, thermally activated behavior of the resistivity ϱ at low temperatures and a strongly non-linear dependence of ϱ on the magnetic field. The Hall coefficient R H is strongly temperature-dependent and shows a pronounced sign change in small magnetic fields. We find a large Nernst effect, from which the entropy/heat transported by the vortices is estimated. The thermopower is larger by a factor of 1000 compared to what is expected from the conventional theory of flux motion. It is remarkably similar to the resistivity.

ESR study of the electronic properties of the new organic conductors κ-(BEDT-TTF)2Cu[N(CN)2]X, X = Br; I

Abstract The electron spin resonance data together with AC susceptibility and low field microwave absorption measurements are reported for the two organic conductors κ-(BEDT-TTF)2Cu[N(CN)2]X, with X = Br; I. The bromine containing salt is the recently discovered superconductor with Tonsetc = 11.6 K. In the compound with iodine only traces but not bulk superconductivity were found, with the onset at 9.3 K. Annealing resulted in an increase of the superconducting volume fraction but now with Tonsetc = 6.9 K. Possible interpretation of the data obtained and the discussion of the electronic structure of these compounds is given. The temperature dependences of the line width and the g-factor are discussed in connection with various mechanisms of interaction of conduction electrons with the lattice.

Inter- and intragranular effects in microwave absorption of (Bi, Pb)2Sr2Ca2Cu3Oy

Abstract Detailed low DC-field microwave absorption (LFMWA) and AC-susceptibility measurements were performed over a wide temperature range for high-quality bulk and fine powder samples of the (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O y compound. The features observed in fine powders are quite different from those observed in bulk samples. This allows one to make a separation of intra granular from inter granular effects. We find an intra granular critical current density of order 10 4 A/cm 2 , which is probably the intrinsic critical current density in the c -direction of the Bi-2223 compound due to the Josephson junctions formed by a repeat spacing of nearly insulating and highly conducting atomic planes along the c -direction of the crystal structure. The corresponding penetration depth λ c for fields parallel to the a , b -planes is about 35 μm. Furthermore, we can detect the presence of

The specific heat anomaly of Bi2Sr2CaCu2O8+δ at Tc in dependence on the oxygen content

Abstract Melt casting processed samples of Bi 2 Sr 2 CaCu 2 O 8+δ were prepared with oxygen contents ranging from δ = 0.126 to δ = 0.22 and characterized by electrical resistance and AC-susceptibility. The T c of this compound is shifted reversibly between 75 and 95 K by annealing in Ar and/or O 2 atmospheres for various times. The anomalies of the specific heat at T c were measured with a resolution of ΔC / C ≈ 5 × 10 −4 . They can be described by a superposition of a mean field step and a contribution due to strong, two-dimensional fluctuations. By a fit of a model with 2D gaussian fluctuations to the specific heat near T c we estimate Δ C and the in-plane coherence length ξ ∥ (0) of the superconductor.

Structure and superconductivity in La1.15-xPr0.85SrxCuO4

Abstract A phase diagram of Pr-doped La 2-x Sr x CuO 4 including structural and superconducting properties is presented. We find that Pr-doping shifts the phase boundary of the high temperature tetragonal to orthorhombic phase transition to higher Sr-concentrations without inducing the low temperature structural phase transition as other RE do. As superconductivity occurs in the same Sr-concentration range as in “pure” La 2-x Sr x CuO 4 (LSCO), the disappearance of superconductivity is not related to the high temperature structural transition, but is determined predominantly by the hole-concentration in the CuO-planes. The decrease of T c of ≈ 5–10 K in the Pr-doped samples with respect to the undoped ones can mainly be attributed to the increase of the octahedra tilting angle.

Paramagnetic Meissner effect in HTSC

For certain polycrystalline Bi-HTSC samples, a surprising feature was observed: instead of the usual diamagnetic response of the dc field-cooling (FC) signal, a paramagnetic moment develops below Tc for fields < 1 Oe. This effect correlates with an anomaly in the low-field microwave absorption. The data are consistent with spontaneous paramagnetic moments due to spontaneous orbital supercurrents. Such currents may originate in so called π-contacts in the weak link network of polycrystalline material.

Coupling constant versus density of states for La2 − xSrxCuO4 (EPR study)

Abstract By analyzing the EPR of Gd 3+ spin probes in La-Sr-Cu-O ceramics nonmonotonic dependence of the coupling constant g on the normal density of states at the Fermi level is discovered. Possible reasons for such behaviour are discussed.