Temperature Dependence of Large Polaron Superconductivity.
Abstract
The theory based on covalent electron transfer (CET) between the d(sub x)2(sub-y)2 orbitals of mixed-valence Cu ions that was introduced previously to explain the variation of critical temperature Tc with chemical composition and the temperature dependence of high-Tc superconductor properties is modified to reflect lattice-vibration scattering of the large polarons that dominate the electrical conductivity of these compounds at low temperatures. One result of this refinement is a clearer picture of the dependence of electron hopping activation energy on crystal-field parameters. A generalization of the earlier exponential "two-fluid" model is introduced to describe more accurately the high rate of change in supercarrier density with temperatures in the regime where T approaches Tc. This new model can provide additional insights from the fitting of theory to data. At the lowest temperatures, supercarrier densities may be approximated by a linear function of temperature if Tc values are large. For lower-Tc superconductors the dependence is more typically exponential. With these improvements, precise fits to penetration depth versus temperature measurements for high-purity YBCO single crystals were obtained, as well as more accurate agreement with critical (transport) current density versus temperature data for thin films. Projections of critical current densities as a function of reduced temperature t = T/Tc are offered for the highest Tc materials predicted by this theory. Parameter values desired for optimum superconductor properties are reexamined. jg p.4
Document Details
- Document Type
- Technical Report
- Publication Date
- Jul 18, 1995
- Accession Number
- ADA297287
Entities
People
- Gerald F. Dionne
Organizations
- Massachusetts Institute of Technology