[en] Experimentally were investigated volt-ampere characteristics (VAC) of contacts at a microcrack in whiskers Bi-Sr-Ca-Cu-O (2/2/1/2 phase) with the critical temperature 60 K ≤ T_c ≤ 81 K. In contact VAC's was found a well reproduced gap structure which permitted to determine the gap parameter Δ. It has been established that in the investigated range of T_c the 2Δ(0)/kT_c ratio for whiskers Bi-Sr-Ca-Cu-O (2/2/1/2 phase) is 7.1±0.2. 23 refs., 3 figs., 1 tab

No abstract available

No abstract available

No abstract available

[en] In Bi derived HT_c-cuprates the question of how many excitations occur near the Fermi surface is not yet completely answered. There are hints that more than the two peaks derived from bilayer-splitting are hidden in the well-known peak-dip-hump structure. In our group it was previously argued that an additional polarisation dependent double-peak structure arises in the one- and two-layer Bi-cuprate. In Bi(Pb)-2201 this can be traced unequivocally since there are no superstructure or bilayer effects possibly concealing this excitation. In Bi(Pb)-2212 the intensity ratio of the peak-dip-hump structure is energy dependent which could be a tool to uncover split excitations. In this contribution the photon energy dependence of the excitations near the Fermi energy of Bi(Pb)-2212 and Bi(Pb)-2201 is studied. (orig.)

No abstract available

[en] In order to study the influence of fluorine on the superconducting properties of YBa₂Cu₃O_7-δ in more detail samples of YBa₂Cu₃F_xO_y with x = 0, 0.25, 0.50, 0.75, 1.0, 1.25, 1.5, 1.75, 1.8, 1.9, 2.0, 2.1, 2.2, and 2.5 have been prepared. The Ohmic resistance (R) and the diamagnetic susceptibility (χ) as functions of temperature and the onset temperature (T₀), zero-resistance temperature (T_c), R_120K, and χ_77K as functions of the chemical composition are plotted and discussed. Results show that small amounts of fluorine (x ≤ 1) have only a weak influence on the superconductivity of YBa₂Cu₃F_xO_y. For x = 2.5 the samples become semiconducting. The compounds with 1.25 ≤ x ≤ 2.5 show new properties. Considering a percolation model of superconducting and non-superconducting crystallites it becomes clear that an increasing part of BaF₂ is followed by decreasing χ and decreasing T_c

[en] With scanning tunneling microscopy (STM) we have performed a detailed and systematic structural analysis of optimally lanthanum doped single-layered bismuth cuprates, Bi_2-yPb_ySr_2-xLa_xCuO_6+δ (x=0.4), as a function of the Pb content. As expected, the periodicity of the well-known (5 x 1) superstructure varies with increasing the amount of Pb. For about y=0.4 the superstructure is almost suppressed but, unexpectedly, new modulations occur in the electron density which might influence the electronic properties of these cuprates. In addition, this could affect the charge transfer between the carrier reservoir (BiO-SrO) and the CuO₂ plane as suggested for Pb-Bi2212 by Shi et al. (orig.)

[en] Due to doping with lead it is well known that the about (1 x 5) superstructure of Bi cuprate superconductors will be suppressed. Nevertheless, a Fermi surface map of Bi_2-yPb_ySr_2-xLa_xCuO_6+δ with x=0.4 and y=0.4 determined by angular resolved photoemission (ARPES) revealed additional Fermi surface (FS) features. Here a La content of x=0.4 means optimum hole doping for a maximum value of Tc and for a Pb amount of y=0.4 one commonly expect complete suppression of the superstructure. Low energy electron diffraction of these samples showed no sign of a superstructure. Scanning tunnelling microscopy, on the other hand, revealed directly two modulations of the electron density of much weaker amplitude, one long-range modulation of about (1 x 32) periodicity and a second of about (12 x 12). By taking into account the wave vectors and intensities of these two modulations the corresponding Fermi surface has been simulated, which agrees strikingly good with the experimental one. The occurrence of modulations in these high-Tc superconductors will be further discussed. (orig.)

No abstract available