A type-II superconductor is in the ''superclean'' regime when the spac
ing h omega(0) between energy levels in the vortex core greatly exceed
s the width of the levels h/tau. A key signature of this regime is a f
lux-flow Hall angle theta H that approaches +/-pi/2. We report the fir
st observation of this trend in a high-temperature superconductor. In
''60 K'' YBa(2)CU(3)0(6+y) crystals, \tan theta(H)\ is observed to inc
rease dramatically when the temperature decreases below similar to 40
K, attaining the magnitude 0.85 at 15 K. By extrapolation, we estimate
that omega(0) tau increases to 2-3 at 3 K. Below 40 K, the Hall condu
ctivity sigma(xy) is negative and decreases as 1/H at high fields.