F-19 NMR AS A PROBE OF FLUX-LINE MOTION IN FLUORINATED YBA2CU4O8

The F-19 NMR linewidth and nuclear spin-lattice relaxation rate T-1(-1 ) has been measured in the temperature interval 4.2-300 K and at diffe rent external magnetic fields in an oriented powder of the superconduc tor YBa2Cu4O8:F (T-c = 81 K). It is observed that below T-c both the F -19 linewidth and T-1(-1) are affected by the thermal fluctuations of the flux lines demonstrating the possibility of using F-19 nuclei to p robe the flux-line dynamics. An anomalous field dependence of both the linewidth and of T-1(-1) is also reported. (C) 1997 Elsevier Science B.V.