Gap-modulation infrared spectroscopy of high transition temperature superconductors.

Conventional methods of determining the coupling factor alpha(2)(omega)F(omega) for the newly discovered high transition temperature (T(c)) cuprate superconductors by using tunneling and infrared measurements have thus far failed to show the cause of the very high T(c) of these compounds. This is due in part to difficulties in sample preparation for tunneling studies and to difficulties in obtaining good data at relatively high tunneling voltages. Also, in IR (infrared) measurements, small differences in absorptivity between the normal and superconducting state can be masked by changes in the phonon occupation at high and low temperatures. Here we propose a technique for determing the coupling constant, which should be less dependent on the surface quality of the sample than with tunneling and should allow measurements at higher energies with greater precision than do tunneling or simple IR observations. This should make possible a definitive determination of any possible exciton contribution to this coupling term, which would appear at energies well above the range where conventional IR or tunneling measurements are effective.