A high-resolution neutron powder diffraction study on high-Tc superconductor YBa2Cu3O7

A high-resolution neutron time-of-flight diffraction measurement of superconductive Y-Ba-Cu-O (T_co = 91 K) has been performed both at room temperature and at 77 K. Profile refinements of the measured data resulted in a unit cell of stoichiometry YBa₂Cu₃O₇. The positions of the oxygen deficiencies were also determined. Refinement of the anisotropic thermal parameters for the oxygen atoms seem to indicate some changes in the form of the thermal ellipsoids of these atoms when cooled below T.     

AC response of YBa2Cu3O7 thin film superconductor

Low-field ac measurements of magnetic susceptibility of YBa₂Cu₃O₇ high-temperature superconducting thin film were carried out over a wide range of temperatures and ac magnetic field amplitudes. A strong field dependence of the real χ′ and imaginary χ″ components was observed. The field dependence of the imaginary component is used to extract the temperature dependence of the critical current density in the sample using the critical state model. The exponent β of the power law relation J_c ～ (1 ? T/T_c)^β was determined from ac-susceptibility data and different values were found. Experimental results are compared with predictions of some existing theoretical models describing the ac response of superconducting thin film in perpendicular ac field.     

Photoemission studies of the high TC superconductor,YBa2Cu3O7

Considerable changes are observed in the spectra of the valence band and O(ls) regions in YBa₂Cu₃O₇, when the temperature is lowered from 300K to 80K. The results are interpreted in terms of a progressive formation of O^2- ₂-like species with decreasing temperature. This is accompanied by the transformation of Cu to Cu. It is suggested that the holes generated at the top of the valence band forming the O^2- ₂-like species may play a crucial role in the superconductivity of the oxide.     

Far-infrared reflectivity of sintered YBa2Cu3O7 in the normal and superconducting state

We report on a study of the far-infrared reflection for a sintered YBa₂Cu₃O₇ sample that contained a large portion of preferentially oriented crystallites with thea–b plane parallel to the surface and that showed extraordinary high far-infrared reflectivity. From experimental reflection data we determined, by Kramers-Kronig analysis, the dynamical conductivity and extracted the contributions due to free charge carriers and phonons, respectively. We find evidence for an anomalous behavior of the dynamical conductivity at temperatures aboveT _c ; the dynamical conductivity increases strongly for temperatures approachingT _c and is strongly frequency dependent. By use of the Mattis-Bardeen theory we obtain an estimate for the superconducting energy gap of 2/kT _c 4.6 (forTT _c ). We find that the lowest frequency infrared-active phonon mode is less damped in the superconducting state than in the normal state.     

Magnetic ordering below 10K in the high-Tc superconductor YBa2(Cu0.985Fe0.015)3O7−σ

The high-temperature superconductor with the nominal composition YBa₂Cu₃O_7?σ has been doped with iron 93% enriched in⁵⁷Fe. When 1.5 at % Fe substitutes for Cu, the transition temperature is reduced by about 30° K with full ac shielding (X′=?1/4π) achieved by ≈40 K. At room temperature the Mössbauer spectrum consists of three overlapping doublets. Below 10 K, hyperfine-field splitting is observed; relaxation still takes place at T=1.8 K.     

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.     

Thermal conductivity of YBa2Cu3O7 from 7 K to 260 K

Thermal conductivity of YBa₂Cu₃O₇ has been measured on samples of a few mm thickness. AboveT _c thermal conductivity is found to decrease with increase in temperature, pointing towards a contribution to thermal resistivity from three-phonon Umklapp processes. BelowT _c thermal conductivity increases rapidly before reaching a maximum at about 50 K and then falls towards zero at lower temperatures. The experimental set up is described and results discussed.     

Laser-induced surface reduction of the high-T c superconductor YBa2Cu3O7−x

The first investigations on the laser-induced reduction of YBa₂Cu₃O_7–x are presented. Here, the oxygen content of the material is diminished by local heating under cw Kr⁺ laser irradiation in H₂ atmosphere. The technique permits the superconducting properties of the material to be locally changed or destroyed. Laser-induced surface metallization may be useful for the electroding of such materials.     

NQR investigations of the high-Tc superconductor Ba2YCU3O7

NQR measurements of ^63,65Cu in both the normal and superconducting state are reported for the Cu1 and Cu2 sites in Ba₂YCu₃O₇. An anomalous behaviour in the vicinity and below T_c is observed: after a steep decrease at T_c the quadrupole frequency increases with decreasing temperature.