Hall Anomaly and Vortex-Lattice Melting in Superconducting Single Crystal ${{\mathrm{YBa}}_2{\mathrm{Cu}}_{3}O}_{7-\mathit{\delta }}$

Subnanovolt resolution longitudinal and Hall voltages are measured in an ultrapure ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ single crystal. The Hall anomaly and the first-order vortex-lattice melting transition are observed simultaneously. Changes in the dynamic behavior of the vortex solid and liquid are correlated with features of the Hall conductivity ${\sigma }_{\mathrm{xy}}$. With the magnetic field oriented at an angle from the twin boundaries, the Hall conductivity sharply decreases toward large negative values at the vortex-lattice melting transition.