Cu NQR-Echo in Superconducting YBa2Cu3O7-x in Weak External Magnetic Field
The behaviour of the 63Cu NQR-echo amplitude in weak external magnetic field Bo ≲ 2 mT is studied for powdered samples of the high-TC superconductor (HTSC) YBa2Cu3O7-x. We worked out the calculations of the NQR-ecno x amplitudes for the case of a polycrystalline sample and an arbitrary resonance offset Above TC the NQR signals of the Cu(1) sites (CuO-chains) can be explained according to this theory confirming the value ≀=1 of the asymmetry parameter of the electrical field gradient (EFG). At the Cu(2) sites (CuO2 planes, ≀ = 0), however, the external BO seems to create an inhomogeneous local magnetic field, the reason of which is not yet clear. In the superconducting phase the amplitudes of the NQR echo of both Cu(1) and Cu(2) sites with increasing external magnetic field decrease nearly without oscillations, which can be explained by local magnetic field inhomogeneities caused by induced supercurrents.