Hole concentrations, Hall number, and Tc relationships in substituted YBa2Cu3Oy.
One of the fundamental questions involving the high-temperature copper oxide containing superconductors is to understand the relationship between hole concentration and the superconducting transition temperature, T,. We help resolve this question by a study of Nd-, La-, Ga-, Ca-, and Zn-substituted YBa2Cu30~ with controlled oxygen stoichiometry. The total hole concentration is determined by a chemical method and trends in the mobile hole concentration by Hall measurements. By considering T, to be a function of the mobile sheet holes only, there is a maximum in T, (92 K) at a sheet-hole concentration of about 0.20-0.25 per Cu site. The importance of the hole concentration in Cu oxide containing superconductors is widely recognized and its relationship to the superconducting transition temperature (T, ) has been the object of numerous studies. ' '2 The hole concentration, which we call p, is an electron deficiency and can be determined chemically, as well as by Hall measurements. Although it is generally accepted that it is associated with the oxygen rather than the copper, i.e., 0, the chemistry of its determination is identical to that of Cu + or a hole in the copper-oxygen band. The Hall measurement is complimentary to the chemical measurement since it measures only the mobile holes. Previously, using both techniques we showed a rather linear relationship existed between T, and p in the La2 Sr„Cu04 system (for p &0.15) and discussed the possible T, vs p relationship in the 1:2:3-type materials, e.g. , YBa2Cu30». ' Our data suggested a possible relationship between the two systems if similar hole concentrations per sheet (CuOz), which we showed reached a maximum at about 0.15 in the one-sheet La2 „Sr,Cu04 system, could be produced in each of the two sheets of the 1:2:3 materials. Torrance et al. ' further studied the
Hole Density Dependence of the Critical Temperature and Coupling Constant in the Cuprate Superconductors
A bond valence sum (BVS) analysis was performed for the p-type cuprate superconductors. The superconducting critical temperature Tc versus in-plane Cu-O BVS correlation for copper is grouped into classes and subclasses. Only within a class or subclass for which the nonelectronic effect is constant does the variation of the in-plane Cu-O BVS reflect the corresponding change in the hole density nH of the CuO2 layers. This study strongly suggests that the Tc for every class or subclass of the superconductors is an inverted parabolic function of nH, and so is the coupling constant λ for Cooper pair formation.
Messung der Energieverlustspektren von Aluminium- und Silberfolien mit hoher Auflösung
The energy loss spectra of polycrystalline aluminium and silver foils have been measured with high resolution. For the plasmon energy of aluminium 14.97 eV is obtained. The half width of the plasmon peak is 0.60 eV corresponding to a relaxation time τ=1.1×10−15 sec in fair agreement with optical data. In silver films of certain thicknesses the surface plasmon peak and the plasmon peak can be separated in the energy loss spectrum. The values of the energy losses are 3.64 and 3.78 eV respectively.