Superconducting State Of YBa2Cu3O7 Research Articles

Fluctuation conductivity in melt-textured YBaCuO samples under low magnetic fields

We have studied thermal fluctuations of the electrical conductivity in melt-textured YBa2Cu3O7 (YBCO) samples under low magnetic fields. Measurements were performed either for current applied parallel or perpendicular to the c-axis and fluctuation conductivity was studied in the proximity of the superconducting transition. Two melt processed samples were prepared with different concentrations of Y2Ba1Cu1O5 (Y211) phase. For the sample with lower concentration, 3D-Gaussian and genuine critical 3D-XY-E fluctuation regimes were identified in the conductivity parallel to the ab plane and a regime beyond 3D-XY was also observed. The 3D-XY-E scaling was also identified in the fluctuation conductivity along the c-axis. In the sample with higher concentration of the Y211 phase, disorder effects are relevant. The results indicate that the superconducting state in YBCO has a three-dimensional character, and suggest the presence of a sub-dominant order-parameter component that has an appreciable projection along the c-axis.

Comment on "Low-temperature charge ordering in the superconducting state of YBa2Cu3O(7-delta)"

Comment on "Low-temperature charge ordering in the superconducting state of YBa2Cu3O(7-delta)"

63,65Cu NQR Evidence for Low-Temperature Charge Ordering in the Superconducting State of YBa2Cu3O7−δ

Measurements of the nuclear quadrupole resonance (NQR) transverse relaxation rate and of the NQR spectral linewidths of 63,65Cu in the superconducting state of YBa2Cu3O7−δ reveal unusual features below T = 35 K. A narrow peak in the transverse relaxation rate and an increased quadrupolar line broadening with decreasing temperature are attributed to a charge density wave (CDW) ordered state below T = 35 K.

I. First Principles Cluster Calculations for the Electronic Structures of CuO7 Octahedron and CuO5 Pyramid

In this paper we first describe the multi-configuration self-consistent field method with configuration interaction (MCSCF-CI) which is a variational method most suitable for strongly-correlated cluster systems. Then the calculated results of CuO 6 cluster embedded in hole-doped La 2-x Sr x CuO 4 and of CuO 4 cluster in the electron-doped Nd 2-x Ce x CuO 4 are reviewed. Finally the calculated results on the electronic structure of CuO 5 pyramid embedded in the insulating YBa 2 Cu 3 O 6 and superconducting YBa2Cu3O7 are newly presented. In the latter case we will clarify an important role of the Cu-O chain in the electronic and superconducting state of YBa2Cu3O7 .

Effect of hydrostatic pressure on the resistance and critical temperature of YBa2Cu3O7−δ single crystals

The effect of hydrostatic pressure on the resistance and resistive transition to the superconducting state in YBa2Cu3O7−δ single crystals with different oxygen contents is studied experimentally. It is found that, following the application (or removal) of pressure, the resistance relaxes to an equilibrium value that depends on pressure, the relaxation time at room temperature being about two days. It is shown that the value of the superconducting transition temperature depends on the pressure, but the width and shape of the transition depend mainly on the degree of resistance relaxation to the equilibrium value. It is concluded that the decrease in resistance under hydrostatic pressure is associated with the ordering of labile oxygen in the Cu–O plane. The possible mechanisms of variation of the superconducting transition temperature are discussed.

Neutron scattering as a probe of unconventional superconductivity in YBa2Cu3O7 (invited) (abstract)

We have carried out a variety of neutron scattering experiments on the high temperature superconductor YBa2Cu3O7 which elucidate the unconventional microscopic properties of the cuprates and illustrate the power and versatility of neutron scattering as a probe of novel materials. Small angle magnetic neutron diffraction is thus far the only technique capable of revealing the structure of the vortex lattice in the cuprate superconductors in magnetic fields of several tesla. We have shown that in YBa2Cu3O7 the vortex lattice in this field range has an oblique structure when the field is applied along the c axis (perpendicular to the CuO2 layers). Recent theoretical work has attributed this unusual structure to the unconventional (d-wave) pairing state realized in the cuprates. We will also discuss the rich behavior of the vortex lattice when the field is tilted with respect to the c axis. We have used inelastic neutron scattering to elucidate the magnetic and lattice vibrational excitations in YBa2Cu3O7. Magnetic and phonon scattering were separated both by detailed calculations of phonon dynamical structure factors and by neutron polarization analysis. A novel resonant magnetic excitation at wave vector q=(π/a,π/a) and energy ℏω=40 meV appears in the superconducting state. The simplest model for this resonance relies on quasiparticle pair creation and implies a sign reversal of the energy gap function on the Fermi surface. Alternative models will also be discussed. We have also observed a strongly wave vector dependent renormalization of the energy and lifetime of a specific phonon in the superconducting state of YBa2Cu3O7. Since the phonon self-energy is sensitive to the anisotropies of the energy gap and Fermi surface via the electron–phonon interaction, our data contain important information about these parameters.

Investigations of the interlayer coupling in high-T c cuprates

Low-field magnetizationM(H) measurements can be used to probe the nature of the screening currents and the interlayer coupling in high-Tc cuprates. Here we compare theM(H) behaviour of single crystals of Bi2Sr2CaCu2O8 and fully oxygenated and oxygen reduced YBa2Cu3O7−∂. In YBa2Cu3O7, theM(H) behaviour is consistent with anisotropic 3D superconductivity whilst in Bi2Sr2CaCu2O8, the surface screening currents are strongly affected by the presence of vortices, implying that the CuO2 planes are coupled via a weak Josephson interaction. In oxygen-deficient YBa2Cu3O6.7 (Tc=63K), theM(H) behaviour at low temperatures is similar to that found for Bi2Sr2CaCu2O8, implying that the removal of oxygen from the chains has resulted in a dimensional crossover of the superconducting state in YBa2Cu3O7−∂. As the temperature approachesTc, the 3D behaviour is eventually restored as thec-axis coherence length ξc becomes comparable with the interlayer spacingd.

Line shapes of field-dependent nonresonant microwave and rf absorption in high-T c superconductors

The field-dependent nonresonant absorption at microwave (9.1 GHz) and radio (19.5 MHz) frequencies has been studied in the superconducting state of YBa2Cu3O7−x pellets and epitaxially grown c-axis oriented thin films using conventional continuous-wave electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectrometers. The line shapes of the signals from the thin-film samples are observed to depend sensitively on the operating frequency; at 19.5 MHz derivative looking signals saturating at low magnetic fields (∼10 Oe) are obtained while at 9.1 GHz, signals saturate only at fields of the order of a kOe. Two different possible origins of the losses, namely, diffraction reduction of junction currents in the weak links and the viscous motion of the fluxons are considered and compared. We find that the observed frequency dependence of the line shape can be explained only following the latter model.

Evidence for dx2-y2 pairing from nuclear-magnetic-resonance experiments in the superconducting state of YBa2Cu3O7

Evidence for dx2-y2 pairing from nuclear-magnetic-resonance experiments in the superconducting state of YBa2Cu3O7

The superconducting state of YBa2Cu3O7??

Many low temperature properties of high Tc superconductors deviate significantly from the detailed predictions of BCS theory. Here we discuss whether these effects could be caused by either: (a) an unconventional pairing state, or (b) local randomness in the gap function due to the intrinsic disorder. We review recent experiments pertinent to these questions: Josephson effects in (001) oriented planar junctions between YBa2Cu3O7-δ and classic superconductors and the temperature dependence of the a-b plane electro-magnetic penetration depth at low temperatures. We also calculate the density of states of s-wave superconductors with local quenched disorder in the gap function so as to determine whether s-wave pairing could be consistent with the low energy quasiparticle excitations seen in many experiments.

63Cu and17O NMR studies on the magnetic and superconducting properties of YBa2Cu3O7−y

Results of63Cu and17O NMR experiments in YBa2Cu3O7 and YBa2Cu3O6.63 are reviewed. The normal state data revealed two important aspects of the magnetic properties of these materials, namely, the temperature dependent antiferromagnetic Cu spin correlations and the spin gap behavior, the latter being observed in the reduced oxygen material. These features appear to be the general properties of many high-T c cuprates. Anomalous temperature dependence of the anisotropy of the Cu relaxation rate was found in the superconducting state of YBa2Cu3O7, which can be explained by a d-wave pairing model.

Intensity anomalies of Raman-active phonons in the superconducting state of YBa2Cu3O7− δ

We report anomalous intensity increases, by up to a factor of 2, of Raman-active phonons in YBa 2 Cu 3 O 7− δ below the superconducting transition temperature. This effect can be explained as a superconductivity-induced resonant Raman process stemming from the resonance of the phonons with the superconducting gap. In the normal state a continuous increase of the Raman efficiency was observed for most phonons upon cooling from 300 K to T c . All relative changes in intensity seem to be rather independent of the exciting laser energy used (1.92, 2.34, and 2.60 eV).

Ultrasonic attenuation in the high-Tcsuperconductor YBa2Cu3O7-δ

To explore the behaviour of the ultrasonic attenuation in the superconducting state of a high-Tc superconductor, measurements have been made of the attenuation of 5 MHz shear waves in YBa2Cu3O7- delta ceramic at temperatures below 100 K. The attenuation is found to increase linearly with temperature below 65 K in the superconducting state of YBa2Cu3O7- delta . Thermal hysteresis occurs at higher temperatures. Similar regimes of behaviour are found in sound velocity and AC susceptibility. Direct immersion of this ceramic superconductor in liquid coolants can result in spurious experimental artifacts in the acoustic response.