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
Magnetic penetration depth and critical current in YBaCuO thin films
The ab-plane magnetic penetration depth lambda in c-axis-oriented YBaCuO thin films was estimated from inductance measurements. The temperature dependence of lambda was shown to be consistent with the Bardeen-Cooper-Schrieffer (BCS) theory under the local limit condition. It was also shown that the experimentally obtained relationship among lambda (T=0), the resistivity p, and the critical temperature T/sub c/ was consistent with the theoretical one obtained from the local limit BCS theory. From that relationship, the value of lambda (T=0) could be deduced from p and T/sub c/. Moreover, the critical current was studied on the basis of the bias current dependence of lambda .<<ETX>>
Formation and Strength of Magnesia Whiskers
The formation of single-crystal magnesia whiskers during the compressive deformation of single crystals at room temperatures is described. These whiskers can have strengths of the order of millions of pounds per square inch. They are formed during fracture as a result of the discontinuous propagation of cracks which are mechanically relieving internal stresses resulting from plastic flow