We calculate the thermodynamic Green's functions for a BCS superconductor with a wall of magnetic material cleaving it into two semi-infinite regions. This is a model of an interface between a superconductor and a perfectly insulating ferromagnet. We show that the scattering of superconducting quasiparticles from a spin-polarized wall leads to resonant pair breaking and the formation of subgap bands in the tunneling density of states. These bands bear a superficial resemblance to the bands observed in bulk superconductors containing dilute concentrations of magnetic impurities. In the absence of spin-orbit or spin-Qip scattering in the superconductor itself, the subgap bands accommodate only one spin of quasiparticle. The significance of this theory for superconductorferromagnetic insulator-superconductor tunneling is discussed.

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We present the first calculations of the electronic structure of the high-temperature superconductors in the disordered phase using the tight-binding coherent-potential approximation. These calculations are used to assess the influence of increasing oxygen vacancies when La is alloyed with M (~Sr or Ba) in the system \,2L2x M x Q\xO\y . Contrary to simple doping arguments, it is found that deoxygenation does not raise the Fermi level CEF). The increase in T c in the range 0.05 0.15 were ambiguous because of sample dependence, it has been established 1 that, although at equilibrium the system phase separates for x > 0.15, rapid cooling can produce single-phase material to much larger x. Samples become superconducting for x as small as 0.06, and T c peaks at x =0.15, then decreases to zero at x =0.3. The interpretation of this result is complicated by the discovery that, although the O vacancy concentration y is small (<0.03) for x < 0.15, for larger concentrations of M y increases, to near 0.10 for x =0.3 and to y =0.2 for x =0.6 [ Fig. (4) ofRef. 11. This complex, double disorder, behavior makes it unclear whether it is the increased alloying or the development of oxygen vacancies which kills T c . One interpretation 1 which is consistent with these data is that (l) alloying with the alkaline-earth ion lowers the Fermi level E? into the van Hove singularity, 2 " 4 increasing the density of states N(Ep) and enhancing T c , then (2) introduction of O vacancies overcompensates the Af-atom hole doping by electron doping, raising E? out of the van Hove singularity. We show below that this interpretation is inconsistent with calculations which account for the disorder.During the intense investigation of this system, several studies 2 " 4 have dealt with the band structure of the stoichiometric compound La2CuC>4 as well as the ordered compound 2 LaBaCuC>4. For the stoichiometric material the paramagnetic calculations do not correspond directly to the antiferromagnetic, and nonmetallic, 5 phase which is observed, but it has been assumed that a rigid-band extrapolation of the band structure into the metallic regime is realistic. This rigid-band picture predicts a peak in N(EF) precisely where T c peaks, as long a...

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