Temperature dependence of the magnetic penetration depth in YBa2Cu3O6.95

We report precision transverse fieldSR measurements of the internal field distribution in the vortex state of crystalline YBa₂Cu₃O_6.95. A novel low background apparatus was used to study a mosaic sample of three high quality single crystals (T_c=93.3K). The observed frequency spectra in magnetic fields of 5kG and 15 kG applied along the c-axis have the characteristic features expected for a regular vortex lattice with some additional broadening. From a preliminary analysis we find that [(0)/(T)]² has a linear temperature coefficient forT<30 K. Such a term is inconsistent with simple s-wave pairing in the superconducting state. These results support recent microwave measurements of(T) on similar crystals in zero applied field but differ significantly from previousSR reports on sintered powders and crystals with lower T_c.This research was supported by the Natural Sciences and Engineering Research Council of Canada.     

Dispersion of magnetic excitations in optimally doped superconducting YBa2Cu3O6.95

Detailed neutron scattering measurements of YBa2Cu3O6.95 found that the resonance peak and incommensurate magnetic scattering induced by superconductivity represent the same physical phenomenon: two dispersive branches that converge near 41 meV and the in-plane wave vector q(AF)=(pi/a,pi/a) to form the resonance peak. One branch has a circular symmetry around q(AF) and quadratic downward dispersion from approximately 41 meV to the spin gap of 33+/-1 meV. The other, of lower intensity, disperses from approximately 41 meV to at least 55 meV. Our results exclude a quartet of vertical incommensurate rods in q-omega space expected from spin waves produced by dynamical charge stripes as an origin of the observed incommensurate scattering in optimally doped YBCO.     

The superconducting gap and an unidentified 110–190 K phase of YBa2Cu3O7 observed by far-infrared ellipsometry

We have measured the complex dielectric function of pure and Fe-doped YBa₂Cu₃O₇ ceramics, between 10 and 150 cm^–1 and 20 and 300 K, with a far-infrared, rotating-analyzer laser ellipsometer. The superconducting phase transition and the gap energy are clearly observable. The undoped ceramic exhibits remarkable dielectric hysteresis between 110 K and 190 K. This points to a still unidentified phase which we observe to be sensitive to a weak magnetic field.     

Cu nuclear spin-spin relaxation in YBa2Cu3O6.98 and YBa2Cu4O8

We have measured the temperature (T) dependence of the transverse relaxation rate (T _G ^–1 ) of the Cu(1) nuclear spin in YBa₂Cu₃O_6.98 (T _c=92 K) and YBa₂Cu₄O₈ (T _c=82 K). From the scaling ratio ofT _G ^–2 (Cu1) toT _G ^–2 (Cu2), we have estimated the strength of a covalent bonding between the CuO₂ plane and the CuO chain to be B0.38×A _zz. The experimentalT _G ^–1 (Cu1) in YBa₂Cu₄O₈ was of the same order of magnitude as the estimated one fromT _G ^–1 (Cu2). These results appear to indicate that the electrons in the CuO₂ plane fairly spread out of the plane in both compounds.     

Ac-Josephson effect in YBa2Cu3O7/YBa2Cu3O7 point contact junctions

The ac-Josephson effect could be demonstrated up to dc voltages of 20 mV on the I–V characteristic of adjustable YBa₂Cu₃O₇/YBa₂Cu₃O₇ point contacts at 4.2 K. A detailed analysis of the rf power dependence of the size of microwave-induced constant voltage steps is given for microwave frequencies of 70 and 90 GHz. The results prove that electron pairing is responsible for the superconductivity in YBa₂Cu₃O₇.     

The collective nature of a flux creep in high-T c superconductor YBa2Cu3O x as observed in microwave absorption experiments

Studies of the flux lattice dynamics via microwave absorption measurements with the help of an ESR spectrometer are presented. The experiments give evidence that for the temperatures below 35 K the absorbed power depends on the input microwave power in a non-linear way. For the case of absorption due to the motion of vortexes via thermally activated flux creep this corresponds to the dependence of the activation barrier on the current density. The measurements lead to the conclusion, that the activation barrier should grow with decreasing current. Fitting of the expression for the dissipated power obtained in terms of the collective creep theory to the obtained experimental data enable us to determine the character of pinning and creep at small amplitudes of the external exciting current. The obtained numerical values give evidence that the flux lattice is frozen in a vortex-glass state and in the limit of zero current the resistance at finite temperatures vanishes.     

Determining the Wiedemann-Franz ratio from the thermal hall conductivity: application to Cu and YBa2Cu3O6.95

The Wiedemann-Franz (WF) ratio compares the thermal and electrical conductivities in a metal. We describe a new way to determine its value, based on the thermal Hall conductivity. The technique is applied to copper and to untwinned YBaCuO. In the latter, we uncover a T-linear dependence and suppression of the Hall-channel WF ratio. We discuss the implications of this suppression. The general suppression of the WF ratio in systems with predominant electron-electron scattering is discussed.     

Theoretical study ofμ + site in YBa2Cu3O x site in YBa2Cu3O x

The muon is a useful probe of magnetic fields in superconductors, but knowing the field seen by the muon is often of limited value until we know where the muon is in the crystal lattice. Here we employ two independent theoretical methods to search for candidate muon sites:the potential energy field method, which seeks the minimum of the electrostatic potential of theμ ⁺, and themagnetic dipolar field method, which compares the calculated magnetic field (due to host electronic or nuclear dipolar fields) with the observed local fields at the muon. Work supported by Canadian NRC and NSERC.