Possible reordering of vortex matter near the end point of the second peak line in theYBa2Cu3O7−δcompound

Abstract

We report on the second peak line ${H}_{\mathrm{sp}}$ and the possible phases of vortex matter near ${T}_{c}$ for a detwinned underdoped ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystal. Our local ac and dc permeability measurements revealed that the second peak line ${H}_{\mathrm{sp}}$ does not terminate either at the critical temperature or on the irreversibility line ${H}_{\mathrm{irr}}.$ Pronounced hysteretic behavior is observed in the regime below the second peak line ${H}_{\mathrm{sp}}.$ Below (above) a characteristic field the end point of the second peak differs from (coincides with) the irreversibility point, ${H}_{\mathrm{end}\mathrm{}\mathrm{point}}\ensuremath{\ne}{H}_{\mathrm{irr}}$ ${(H}_{\mathrm{end}\mathrm{}\mathrm{point}}\ensuremath{\equiv}{H}_{\mathrm{irr}}).$ A possible reentrance of a dynamically ordered solid phase in the high-temperature regime near the irreversibility line is discussed.