NON-BCS BEHAVIOR OF THE SUPERCONDUCTING ORDER PARAMETER IN YBA2CU3O7 STUDIED WITH INFRARED-SPECTROSCOPY

We report on detailed measurements of the temperature dependence of the infrared reflectivity spectra of YBa2Cu3O7. It is shown that the prominent edge1 at 8k(B)T(c) shows practically no energy shift as a function of temperature, whereas the oscillator strength of this feature depends strongly on temperature with a marked critical behavior at the superconducting transition temperature. This behavior coincides with the temperature dependence of the superfluid fraction, which was determined from the spectra using two different methods. A precursor at the same energy position persists at temperatures above T(c). If the edge can be interpreted as the characteristic energy required to break up a pair, we arrive at the conclusion that the gap itself can not be considered as an order parameter in this case. The superfluid density on the other hand changes approximately as 1 - (T/T(c))4. We point out that a remarkable analogy exists to the situation in itinerant magnetism, where short range order persists above the Curie temperature resulting in an exchange splitting that does not vanish2 at T(c).