Effect of twin boundary density on the vortex melting transition in YBa2Cu3O7-δ single crystals

The authors present magnetotransport ρ(H,T) measurements in clean untwinned and twinned crystals of YBa2Cu3O7-δ in magnetic fields up to 8 Tesla for H parallel c. In untwinned crystals, they show strong support for a vortex solid to liquid first order melting transition at a temperature Tm, much below the mean field transition temperature Tc(H), characterized by an extremely sharp drop in resistivity ρ(Tm) and the appearance of hysteretic behavior at ρ(Hm). In densely twinned crystals, the sharp transition is replaced with a continuous transition at a higher temperature TBG > Tm. Furthermore, the cusp in TBG(θ) predicted by the Bose glass theory at fixed field is observed. In dilutely twinned crystals they demonstrate the competing effect of pinning and freezing at the phase transition. The vortex solid state at low fields is characterized by the appearance of a ''peak effect'' just below the melting temperature