High T c Oxide Superconductors
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ng states in the high Tc oxides is the underlying electronic structure—in particular, how it evolves as the insulating parent compounds are doped with électrons or holes to render them metallic and superconducting. The article by J.W. Allen and C.G. Olsen in this issue reviews direct and inverse photoemission spectroscopy studies of the electronic structure of high Tc oxide superconductors, with emphasis on distinguishing models for the normal state. Infrared absorption is an effective method for studying the superconducting energy gap as weU as the dynamics of highly correlated électron Systems in the normal state. Since the pénétration depth for electromagnetic radiation in the high Tc cuprates is about 1,500 Â, infrared measurements are essentially bulk probes in thèse materials. The article by Z. Schlesinger and R.T. Collins in this issue describes the current status of infrared studies of the superconducting energy gap and normal state dynamics of oxide superconductors. Electron runneling has proven to be a powerful technique for measuring the superconducting energy gap and obtaining information about the electronphonon interaction in conventional superconductors. The électron tunneling spectra of high Tc cuprates reveal a number of significant features, including a linear conductance in the normal
state that is apparently associated with électron corrélations, multiple structures which are presumed to be due to a gap (or gaps) in the electronic density of states, and features at high énergies that are reminiscent of the phonon structures in conventional superconductors. The article by J.M. Vallès Jr. and R.C. Dynes discusses électron tunneling measurements in the high Tc cuprates and bismuthates. The behavior of fluxoids in the vortex state of high Tc oxide superconductors, a problem of fundamental interest, also has important implications for the performance of superconducting materials in technological applications. A.P. Malozemoff considers thèse issues in connection with the rich and complex magnetic field-temperature phase diagram of the high Tc oxide superconductors. One of the most interesting of the high Tc oxides is Bai.xKxBi03, which exhibits superconductivity near 30 K for x = 0.4. This material is distinctively différent from the other high Tc oxides in the following respects: (1) it has a simple cubic perovskite crystal structure, (2) it does not contain copper, and (3) it does not exhibit antiferromagnetism in its insulating phase. The structural properties and various aspects of the superconducting state in the Ba^K^BiOa System are discussed in the article by D.G. Hinks. An important récent development on the materials front is the discovery of electron-doped superconductors of the type Ln2.xMJCCu04.y (Ln = Pr, Nd, Sm, Eu; M = Ce, Th; y = 0.02) for which Tc =S 25 K. Thèse materials are particularly interesting because they are the first examples of high Tc oxide superconductors in which the charge carriers involved in the superconductivity appear to be électrons rather than holes that réside in
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