Nearly field-independent in-plane vortex solid-to-liquid transition in the c-axis resistivity of oxygen deficient single crystals of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$

The c-axis resistivity has been measured in oxygen deficient single crystals of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (anisotropy $\gamma \approx 24–29)$ with the magnetic field B accurately aligned parallel to the ab plane. An almost field-independent vortex solid-to-liquid transition is observed at high fields in contrast to the usual field-dependent one, which is recovered for a small misalignment between B and ab. This observation challenges previous interpretations of the same effect in the ab resistivity as coming from a smectic vortex state or a decoupling of the superconducting planes.