D. Wohlleben

Nature of the specific heat anomaly in the Bi and Tl high temperature superconductors

Abstract Specific heat, field cooling Meissner effect, isothermal magnetization, AC susceptibility and resistance were measured on sintered pellets of the Bi and Tl high temperature superconductors with nominal compositions BiPb 0.3 SrCaCu 2 O 5.8 and Tl 2 Ba 2 CaCu 2 O 8 , and for comparison on some pellets of the REBa 2 Cu 3 O 6.9 system. With decreasing field, the field cooling Meissner signals of the Bi and Tl systems approach the same magnitude and transition width (Δ T ≈ 2–3 K) as in high quality pellets of the REBa 2 Cu 3 O 6.9 . The field and temperature dependence of the Meissner signals is analysed in detail. At the transition temperatures of five superconducting phases of the Bi and Tl pellets, the specific heat shows no jumps nor maxima, within the accuracy of the measurement (Δ( C/T ) 2 )). There are however sharp breaks of the slope of C/T at the T c values, which are intrinsically as large as in the REBa 2 Cu 3 O 6.9 . The transitions in the Bi and Tl systems look more like third order transitions rather than like the second order transitions of the REBa 2 Cu 3 O 6.9 .

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.

Superconducting fluctuation in Bi2Sr2Ca2Cu3Ox

Measurements of the resistivity, of the DC Meissner effect, of the isothermal magnetisation, of the specific heat, of the thermal expansion and of the Hall effect are reported for a high quality polycrystalline sample of the 2223 phase of the BiSrCaCuO family of high temperature superconductors. They are discussed in terms of critical fluctuations within ± 10 K about Tc due to a very small coherence volume. Distinct features appear in the data at the Ginzburg-Landau transition temperature (TGL≈107 K) and at the Kosterlitz-Thouless like temperature (T∗KT≈106 K).

Fluctuation specific heat and thermal expansion of YBaCuO and DyBaCuO

Abstract We present high resolution specific heat measurements and thermal expansion data of YBa 2 Cu 3 O 7-δ polycrystalline samples in the temperature range 20–300 K. The specific heat anomaly at T c shows strong deviations from mean field behavior due to superconducting fluctuations. The fluctuation behaviour seems to be more complicated than simply Gaussian. The thermal expansion shows clear anomalies at the T c s of the samples. Additionally we give an estimate of the Gruneisen functions and discuss data of the low field DC Meissner effect in connection with the grain structure of the samples.

Low temperature phase transition and superconductivity in (La Nd)SrCuO

Abstract In ( La RE)SrCuO with RE = Nd, Eu, Gd, Dy, Tb we find besides the known tetragonal → orthorhombic transition a further low temperature structural phase transition to a tetragonal phase (probably P4 2 /ncm). A detailed phase diagram of La 1.85−x Nd x Sr 0.15 CuO 4 including structural and superconducting properties is presented. There is a competition between superconductivity and this low temperature phase transition: We observe no bulk superconductivity in the low temperature phase and no structural phase transition in the superconducting state.

ESR of single crystals of κ-(BEDT-TTF)2Cu[N(CN)2]X (X = Br and I)

Abstract Conduction electron spin resonance data are presented for single crystals of the two recently discovered organic superconductors κ-(BEDT-TTF)2Cu[N(CN)2]X, X = Br;I. The temperature and angular dependences of the parameters of the resonance line are analysed and arguments are put forward for the possible electronic structure and relaxation mechanisms.

Competition Between Structural and Superconducting Transition in (LaNd)-Sr-Cu-O

Experimental evidence is presented for a competition between the superconducting and a structural transition in La1.85−xNdxSr0.15CuO4 as a function of the Nd content. At high Nd concentration (x ≥ 0.18) a discontinuous, first-order structural transition is found, which destroys superconductivity and has a drastic influence on the normal-state transport properties. At lower Nd concentration (x < 0.18) a sharp superconducting transition occurs, which seems to suppress the structural phase transition.

EuPtP: a new mixed valent europium-system

The europium valency in hexagonal EuPtP, as examined byLIII-x-ray absorption and Moessbauer experiments, is a function of temperature and changes from 2.16 at 295 K to 2.40 at 4 K. In the region of the strongest temperature dependence of the valency, the compound undergoes two first order phase transitions atT1=235 K andT2=190 K, characterized by discontinuities in the lattice constants and in the magnetic susceptibility. In the europium Moessbauer spectra, several temperature dependent absorption lines were found, indicating a complex europium valency behaviour. Furthermore, we observe antiferromagnetic order at 8.6 K.

The equilibrium magnetization of granular high temperature superconductors in low fields: II. The model

In the spontaneous flux expulsion of granular high temperature superconductors, only 13 of the full Meissner effect is observed below Hc1, followed by full expulsion at H<Hm⪡Hc1. These deviations from the standard behavior of type-II superconductors are shown to be equilibrium effects caused by intrinsic and defect barriers with low conduction electron density. The influence of pinning is negligible. Hm is not an intrinsic property. Average sizes of the superconducting blocks between and of the Josephson loops around defect barriers can be extracted from the anomalous temperature and field dependence of the spontaneous flux expulsion.

Preparation and characterization of RE doped La2-xSrxCuO4

Abstract We report on the systhesis of (La,RE) 2- x Sr x CuO 4 for various RE=Pr, Nd, Sm, Eu, Gd, Dy, Ho and Er as a function of the RE and Sr concentrations. The sample characterization by means of powder X-ray diffraction and EDX is described. We present our results on the solubility limit of the various RE in the T structure of La 2- x Sr x CuO 4 and discuss the systematic influence of RE and Sr doping on the structural properties. We furthermore show that doping with RE does not influence the charge carrier concentration of the samples.