Far-infrared magnetoconductivity of a high-{ital T}{sub {ital c}} superconducting film

Measurements of the far-infrared reflectivity {ital R} of a superconducting YBa{sub 2}Cu{sub 3}O{sub 7} thin film exhibit a strong dependence on magnetic field {ital B}. The ratios of {ital R}({ital B})/{ital R}(0) were used, in conjunction with a separate measurement of {ital R}(0), to obtain the absolute values of {ital R}({ital B}). A Kramers-Kronig analysis then gave the field-dependent conductivity which shows a broad resonance between 50 and 250 cm{sup {minus}1}. A theory involving vortex motion with pinning was able to fit the shape of the field-dependent conductivity resonance but not the magnitude.