The vortex state of a Bi2Sr2CaCu2O8+δ (Bi2212) single crystal was investigated by measuring the free torsional oscillation in magnetic field with special attention to the energy dissipation. The decay time and the oscillation period were measured at temperatures between 4.2 and 100 K under various fields up to 9 kOe. The temperature and field dependences of the energy dissipation due to flux depinning were obtained for the irreversible region. An abrupt increase of the energy dissipation was found around 1 kOe, suggesting the dimensional crossover from the 3D vortex solid to 2D pancakes with increasing field. A large damping anisotropy was also found and explained by a model of two-dimensional vortex pancakes.
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