J. P. Thiel

Microwave absorption of the high‐Tc superconductor YBa2Cu3O7

Using a resonant‐cavity perturbation technique, the microwave absorption at 16.8 GHz of the high‐Tc compound YBa2Cu3O7 is measured as a function of temperature (4–300 K) and magnetic field (0–7 T). At zero field the microwave loss shows a rapid decrease below 93 K, the onset of the superconducting transition. An important absorption, which is a decreasing function of temperature, is observed in the superconducting state.When a magnetic field is applied, the transition is broadened and shifted toward lower temperatures: an upper‐critical field Hc2(T=0)∼ 150 T is obtained via extrapolation of the data. In the superconducting state, the absorption is found to be highly dependent on the magnetic‐field intensity. For magnetic fields above 500 Oe, the absorption is a slowly increasing function and it is understood in terms of the temperature behavior of Hc2 ; for H<200 Oe, the absorption increases very rapidly with field and it is believed to be correlated to Hc1 and to the anisotropic character of the structure.

Microwave absorption of aligned crystalline grains of YBa2Cu3O7−x

We investigate here the anisotropic magnetic and electric properties of aligned crystalline grains of YBa2Cu3O7−x. The microwave absorption is indeed consistent with the expected temperature behaviors of the resistivity in the normal state along the c axis and in the a‐b plane. In the superconducting state, both the absorptions in pure rf electric and magnetic fields are high compared to the conventional superconductors; for the magnetic case activation energies are deduced and the dispersion is related to the diamagnetic susceptibility. These results are discussed qualitatively in relation to similar data obtained on single crystals and ceramic samples.

Spin-Lattice Relaxation in the Normal and Superconducting States of a High Tc Superconductor: La1.83Sr0.17CuO4

Polycrystalline samples of La1.83Sr0.17CuO4 were studied by pulsed NMR on the copper nucleus up to 70K. An extremely broad spectrum was observed. A Korringa constant of 14±± Ks and an energy gap of 2Δ/kBTc=1.3 were found. Homogeneous line narrowing was observed in the superconducting state. T2 data are presented for YBa2CuO7 as well.