Accession Number : ADA297287

Title :   Temperature Dependence of Large Polaron Superconductivity.

Descriptive Note : Technical rept.,

Corporate Author : MASSACHUSETTS INST OF TECH LEXINGTON LINCOLN LAB

Personal Author(s) : Dionne, Gerald F.

PDF Url : ADA297287

Report Date : 18 JUL 1995

Pagination or Media Count : 27

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

Descriptors :   *TEMPERATURE, *CRYSTAL LATTICES, *HIGH TEMPERATURE SUPERCONDUCTORS, THERMAL PROPERTIES, IONS, MEASUREMENT, SCATTERING, LOW TEMPERATURE, VIBRATION, OPTIMIZATION, HIGH RATE, MODELS, COVALENT BONDS, DEFORMATION, THIN FILMS, CURRENT DENSITY, ATOMIC ORBITALS, ELECTRICAL CONDUCTIVITY, CRITICAL TEMPERATURE, REDUCTION, CHEMICAL COMPOSITION, PENETRATION, OXIDES, COPPER, LINEARITY, VALENCE, ELECTRON TRANSFER, BARIUM, FUNCTIONS(MATHEMATICS), CHEMICAL COMPOUNDS, YTTRIUM, ACTIVATION ENERGY.

Subject Categories : Inorganic Chemistry
      Physical Chemistry
      Crystallography
      Electricity and Magnetism
      Atomic and Molecular Physics and Spectroscopy

Distribution Statement : APPROVED FOR PUBLIC RELEASE