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.

By means of local Hall probe ac and dc permeability measurements, we investigate the phase diagram of vortex matter for the HgBa2CuO4+delta superconductor in the regime near critical temperature. The second peak line H-sp, in contrast to what is usually assumed, does not terminate at the critical temperature. Our local ac permeability measurements reveal a pronounced hysteretic behavior and thermomagnetic history effects near the onset of the second peak, giving evidence of a phase transition of vortex matter from an ordered qausilattice state to a disordered glass.