No abstract

The superfluid response of single-crystal YBa 2 Cu 3 O 72x and YBa 2 Cu 4 O 8 shows a strong anisotropy in the a-b plane as revealed by far infrared spectroscopy. The value of the London penetration depth in both systems is considerably smaller in the direction along the Cu-O chains, suggesting that a substantial portion of the superconducting condensate is on the chains.

The authors have used ' Y NMR to measure the internal magnetic field of a sample of YBa:Cu30& z (T, =90 K) in the superconducting state to correct for the effects of the Meissner shielding currents. They used this information to separate the magnetic-shift tensor into orbital (chemical-shift) and spin (Knight-shift) contributions. They find that the temperature dependence of the chain Cu Knight shift fits the classic Yosida function of weak-coupling, orbital-s-state, spinsinglet BCS theory. For the planes, the Knight shift also requires a spin singlet, but with a strongcoupling Yosida function. The best fit is for an orbital s state, but an orbital d state is also possible.They find a zero-temperature gap b(0) of 1.76k& T, for the chains. For the planes, 6(0) can range from 1.9k8T, to 3.lk~T, .

We have measured the resistivity tensor in one nearly twin-free crystal and one twin-free crystal of YBa2Cu307 z. The first crystal was grown with a large twin-free region. We removed the twins in the second crystal by applying a uniaxial stress. Using a modified Montgomery technique, we measured the electrical resistivities in all three crystal directions and found them to be linear in temperature above the superconducting transition temperature T, . Our results for p, and pb at room temperature are among the lowest that have been reported in the literature, indicating that our samples are of high quality. The ratio p, /pb is independent of temperature between 150 and 275 K, and its value, 2.2+0.2, indicates that the Cu-0 chains contribute 60% of the current when the electric field is parallel to them, with the rest of the current being contributed by the Cu-0 planes. This result, at zero frequency, is in good agreement with the recent conductivity at infrared frequencies obtained by Schlesinger et al.