Magnetic-field effects on submillimeter absorptivity in epitaxial thin films of YBa2Cu3O7.

Abstract

We have studied the dependence of the directly measured absorptivities in epitaxial films of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ on magnetic field. These high-quality films are characterized by their low microwave surface resistance measured at 4 K near 10 GHz. The field-dependent absorptivity measurements were made at 2 K in fields up to 3 T and for frequencies between 40 and 200 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. We observe a region of enhanced absorption in the presence of a magnetic field near 60 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (8 meV) for all films. The shape and location of this structure varies significantly from sample to sample. We have used the model proposed recently by Hsu and also the model of Karrai et al. to fit our data. These two models suggest different interpretations of the enhanced absorption feature. The fits are unable to constrain the more physically meaningful Hsu model, indicating large systematic differences between the data and the Hsu model.