  The infrared (900-1100 cm-1) Faraday rotation and circular dichroism are
measured in the normal state of underdoped High Tc superconductors and used to
study the magneto-transport. YBa2Cu3O6+x thin films are investigated in the
temperature range 10-300 K in magnetic fields up to 8 Tesla and as a function
of oxygen concentration. A dramatic increase of the Hall frequency is observed
for underdoped samples which is not consistent with the approach to a Mott
transition but is consistent with a partial gapping of the Fermi surface as
predicted in charge density wave models.
