Vortex liquid-to-solid transition in underdoped YBa2Cu3O7-δ single crystals with B parallel ab

A nearly magnetic field independent vortex liquid-to-solid transition has been observed in both the ab-plane and in the c-axis resistivity of underdoped YBa2Cu3O7-δ single crystals for magnetic fields B parallel ab-plane. These observations suggest that intrinsic pinning caused by the superconducting layers do neither lead to a system of completely decoupled superconducting layers nor to a smectic vortex phase at high fields. A possible explanation may include a transition driven by changes in elastic properties (e.g., the shear modulus) of the vortex system