Abstract
We report on detailed measurements of the temperature dependence of the infrared reflectivity spectra of YBa 2Cu 3O 7. It is shown that the prominent edge 1 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 to 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 vanish 2 at T c.
Published Version
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