FAR-INFRARED REFLECTANCE OF YBI2CU3O7-DELTA AT HIGH MAGNETIC-FIELDS

We report on low temperature far infrared reflection measurements on t hin film YBa2Cu3O7 at magnetic fields up to 15.5 Tesla. Our measuremen ts show for the superconductive state a clear edge in the reflectance near 400 cm(-1), which develops into a distinct kink if the temperatur e is lowered to 1.2 K. We observed practically no influence of the mag netic field on the infrared reflectance of YBa2Cu3O7. In particular we find no field induced shift of the 400 cm(-1) edge, as would be expec ted for the effect of pairbreaking or Zeeman splitting on a single gap structure. The absence of distinct field-dependent features indicates a very anisotropic gap in the ab plane.