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Experimental Evidence of a Crossover in the Pinning of Vortices in Untwinned Single Crystals of YBa2Cu3O7−δ

Using a forced torsion pendulum we have investigated the frequency, amplitude and orientation dependence of the vortex response in untwinned single crystals of YBa2Cu3O7−δ. A new thermally activated dissipation peak which appears at high amplitude, accompanied by a crossover in the pinning, has been observed in addition to the one associated to the controversial vortex melting. The shape and mass anisotropy effects have been investigated.