Flux Line Dynamics in YBa_2Cu_4O_8 From ^{89}Y NMR

Authors: F. TABAK, M. CORTI, A. RIGAMONTI

Abstract: ^{89}Y NMR linewidth \Delta \nu and spin lattice relaxation rate T_1^{-1} measurements have been performed in high T_c superconductor YBa_2Cu_4O_8 (T_c=81K) for both H_0 \Vert c and H_0 \perp c at H_0= 9.4 T and H_0= 5.9 T. T_1^{-1} shows an anomalous enhancement with a maximum above the irreversibility line T_{\rm irr}. The maximum shifts to higher temperature for lower magnetic field similar to the behaviour of T_{\rm irr}(H), indicating that the thermal motion of flux lines is the cause of the anomalous enhancement together with the extra narrowing of ^{89}Y NMR line for T\to T_c.

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