Infrared Study of the Order Parameter in YBa2Cu3O7: Analogy to Short Range Ordering in Itinerant Ferromagnets
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 .
KeywordsDielectric Function Short Range Order Exchange Splitting Superfluid Density Superfluid Component
Unable to display preview. Download preview PDF.
- 3.H.-U. Habermeier and G. Mertens, Physica C 162–164: 601 (1989).Google Scholar
- 4.D. van der Marel, M. Bauer, E. H. Brandt, H.-U. Habermeier, D. Heitmann, W. König, and A. Wittlin, Phys. Rev. B, in print.Google Scholar
- D. van der Marel, H.-U. Habermeier, D. Heitmann, W. König, and A. Wittlin, Physica C, in print.Google Scholar
- 7.M. Tinkham, “Superconductivity”, McGraw-Hill, New York, 1965.Google Scholar
- 8.N.E. Bickers, D.J. Scalapino, R.T. Collins, and Z. Schlesinger, Phys. Rev. B 42: 67(1990).Google Scholar
- 10.R. T. Collins, Z. Schlesinger, F. Holtzberg, and C. Field, Phys. Rev. B, in print; Z. Schlesinger, these proceedings.Google Scholar
- 11.P.B. Littlewood and C. M. Varma, ”Phenomenology of the normal and superconducting states of a marginal Fermi liquid.”, presented at ”Theory of high Tcsyperconductivity”, Dubna, U.S.S.R., july 1990. These authors propose that in high T, materials the reflectivity feature corresponds to 4Δ, using clean limit/strong coupling arguments with an electronic excitation spectrum providing the ‘glue’ instead of phonons. However, the complete absence of any structure at 2Δ only results from treating the problem in the extreme limit, which is not the case according to our results.Google Scholar
- 12.An overview of the physics of long range order, both in superconductivity and in magnetism is given in ”Long range order in solids” by R. M. White and T. H. Geballe, Edited by H. Ehrenreich, F. Seitz and D. Turnbull, Supplement 15, Academic Press, New York (1979).Google Scholar
- Y. Bar-Yam, ibid. 2601 (1991).Google Scholar