Hall angle evidence for the superclean regime in 60 K YBa2Cu3O6+y

A type-II superconductor is in the "superclean" regime when the spacing ω0 between energy levels in the vortex core greatly exceeds the width of the levels τ. A key signature of this regime is a flux-flow Hall angle θH that approaches ±π2. We report the first observation of this trend in a high-temperature superconductor. In "60 K" YBa2Cu3O6+y crystals, |tanθH| is observed to increase dramatically when the temperature decreases below ∼40 K, attaining the magnitude 0.85 at 15 K. By extrapolation, we estimate that ω0τ increases to 2-3 at 3 K. Below 40 K, the Hall conductivity σxy is negative and decreases as 1H at high fields.