D. Zech

Apex anharmonicity observed by Raman scattering in 18O substituted YBa2Cu3O6+x

Abstract IR excited (1.16 eV) Raman spectra of several YBa 2 Cu 3 O 6+ x metallic samples (0.48 x 16 O and 96% substituted 18 O have been performed. The dominant features of the spectra are two phonon bands around 340 and 490 cm -1 in 16 O samples. In comparing the energy shift of these two Raman bands upon isotopic substitution with the calculated ones, the “harmonic” approximation accounts well for the shift of the lower of the two, the O(2)/O(3) “dimpling” vibration, whereas the higher O(4) band behavior is consistent with the existence of a highly anharmonic potential along the c -axis, a property otherwise only found in ferroelectrics.

Is the splitting of the superconducting transition in YBa2Cu3O7−x (x<0.05) due to oxygen-diffusion limitations?

Abstract The oxygen self-diffusion coefficient in the strongly anisotropic process of the isotope exchange of chain sites in YBa 2 Cu 3 O 7− x (x

Doping dependence of the oxygen isotope effect in YBa2Cu3Ox

Measurements of the total oxygen isotope shift α0 and the condensate density of n of YBa2Cu3Ox (6.53 < x < 6.99) are presented. The Tc(n) dependence exhibits a two-plateau behavior which is associated with the ortho-I and ortho-II phase, respectively. Around optimum doping (x ≅6.9) and at the onset of the 60 K plateau where Tc only weakly depends on n the small α0 ≅ 0.025 is associated to changes in the phonon spectrum. A further decrease of Tc leads to a rapid increase of α0 ∝ ∂Tc/∂n suggesting that α0 is related to a change of the condensate density n upon isotope exchange.

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.

Muon-spin rotation (μSR) study of the temperature dependence of the London penetration depth in copper oxide superconductors

Abstract A μSR study of the temperature dependence of the London penetration depth λ in sintered samples of YBa 2 Cu 3 O z (with various oxygen contents x ), YBa 2 Cu 4 O 8 and Bi 2 Sr 2 CaCu 2 O 8 is presented. It is found that the temperature behavior of λ of all these cuprate superconductors is consistent with conventional s -wave pairing. However, there are significant differences concerning the exact temperature dependence of λ in these materials. In YBa 2 Cu 3 O z with high x , the behavior of λ( T ) is well described by the two-fluid model (strong coupling), whereas λ( T ) in YBa 2 Cu 3 O z with low x , YBa 2 Cu 4 O 8 and Bi 2 Sr 2 CaCu 2 O 8 is in better agreement with weak-coupling BCS theory. Possible reasons for the different temperature behavior of λ in these materials are discussed.

Indications for a Phase Separation in YBa2Cu3O7−x

Slope changes of the lattice constants and dc magnetization measurements as a function of the oxygen content of powder 123 samples, indicate the existence of three phases with phase boundaries at x ≅ 6.75 and x ≅ 6.91. In addition, a splitting of the derivative of the magnetization curve appears at x ≥ 6.95. These results are discussed in terms of a three-phase model. It is shown that oxygen inhomogeneities of the grains cannot be the reason for the double peaks in specific heat and magnetization.

Indications for a phase separation in YBa2Cu3O7-x (x<0.1)

Abstract A broadening of the magnetisation transition and lattice constant anomalies in the nearly oxygen stoichiometric YBa 2 Cu 3 O 7-x are observed. These anomalies together with others (resistivity, c p , muon depolarization rate) could be explained by the existence of two different phases in the overdoped range (x>6.90).

μSR investigation of the superconducting properties of YNi2B2C

Abstract Muon spin rotation (μSR) has been used to investigate the mixed state of the recently discovered superconductor YNi2B2C. The width of the μSR-lineshape at low temperature gives a measure of the superconducting penetration depth λ(T = 0), while the field dependence of the linewidth allows an estimate of the superconducting coherence length ξ(T = 0).

Site-selective oxygen isotope effect in highly doped YBa2Cu3Ox

Abstract New measurements of the oxygen ( 16 O vs. 18 O) isotope effect (OIS) in optimally doped YBa 2 Cu 3 O x ( x =6.95) are presented. A new two-step exchange process allowed the simultaneous preparation of fully isotope exchanged YBa 2 Cu 3 O x samples and site-selective substituted samples. this allows a direct comparison of the OIS of the different oxygen sites (site-selective OIS). In contract to a recent OIS study by Nickel et al. , our results demonstrate a conventional site-selective OIS. The observed site-selective OIS indicates that the CuO 2 plane sites play a significant role in the phonon-mediated part of the electron pairing.