Abstract:

We report on the second peak line H-sp and the possible phases of vortex matter near T-c for a detwinned underdoped YBa2Cu3O7-delta single crystal. Our local ac and dc permeability measurements revealed that the second peak line H-sp does not terminate either at the critical temperature or on the irreversibility line H-irr. Pronounced hysteretic behavior is observed in the regime below the second peak line H-sp. Below (above) a characteristic field the end point of the second peak differs from (coincides with) the irreversibility point, H(end point)not equalH(irr) (H(end point)equivalent toH(irr)). A possible reentrance of a dynamically ordered solid phase in the high-temperature regime near the irreversibility line is discussed.