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Interference effects on the transport through two localized tunnel junctions on the surface of a well-grounded sample reveal intrinsic spatial correlations characteristic of the uncoupled sample. Differential conductances of the twojunction probe are related to the spatial correlations of both normal and superconducting samples. For a superconducting sample the gap anisotropy strongly affects the results. This may serve as a sensitive probe of the order parameter in high-temperature superconductors.

The electronic structure near a single classical magnetic impurity in a superconductor is determined using a fully self-consistent Koster-Slater algorithm.Localized excited states are found within the energy gap which are half electron and half hole. Within a jellium model we find the new result that the spatial structure of the positive-frequency (electron-like) spectral weight (or local density of states), can differ strongly from that of the negative frequency (hole-like) spectral weight. The effect of the impurity on the continuum states above the energy gap is calculated with good spectral resolution for the first time. This is also the first three-dimensional self-consistent calculation for a strong magnetic impurity potential.

We examine the effect of an impurity on the nearby tunneling conductance in an anisotropically gapped superconductor. The variation of the conductance has pronounced spatial dependence which depends strongly on the Fermi surface location of gap extrema. In particular, different gap symmetries produce profoundly different spatial features in the conductance. These effects may be detectable with a scanning-tunneling-microscope study of the surface of a high-temperature superconductor.PACS numbers: 74.50.+r, 74.80.-g Any well-developed theory of high-temperature superconductivity must predict the symmetry of the energy gap. Some proposals, including the strongly coupled phonon-mediated pairing which causes superconductivity in ordinary metals [1], yield isotropic or nearly isotropic gaps. The dominant characteristic of the pairing interaction in this model is retardation. In the low-temperature superconductors this is reflected in the frequency dependence of the superconducting gap and clearly shows that phonons mediate the pairing.For a nearly half-filled Hubbard model, approximations involving the exchange of para-antiferromagnetic spin fluctuations give rise to an effective electron-electron interaction which has a characteristic momentum dependence, peaking near the antiferromagnetic wave vector [2,3]. A similar momentum dependence has been found in Monte Carlo simulations [4]. Such an interaction favors d x 2_ y 2 pairing and gives rise to a momentum-dependent gap which has four nodes.Gap-measurement techniques that were introduced and developed for use on electron-phonon superconductors measure accurately the frequency structure of the gap. Among these are the voltage dependence of the tunneling I{V) characteristic into the homogeneous superconductor [5] and the frequency dependence of electromagnetic absorption [6]. If, however, the gap has strong momentum dependence these measurements depend on some momentum-averaged value of the gap. Most of these probes [7,8] now indicate pronounced gap anisotropy or gapless superconductivity in the hightemperature superconductors.In the cuprate-oxide superconductors it would be useful to have experimental information on the momentum dependence of the gap. Certain techniques exist which are designed to detect gap nodes on the Fermi surface. These include measurements of the low-temperature dependence of thermodynamic and transport properties, such as the specific heat [9] and the magnetic penetration depth [10]. Here power-law dependencies on temperature typically imply nodes, in contrast to the exponential behavior associated with a fully gapped Fermi surface. Measurements of these quantities proved extremely illuminating in studies of heavy-fermion materials [11]. Recent results [10] on YBa2Cu307 indicate a low-temperature penetration depth proportional to T, suggesting the existence of nodes.At this time, however, only a few experiments exist that directly measure the gap as a function of momentum on the Fermi surface. Angle-resolved photoemission (ARPES) on Bi2Sr 2 CaC...

The behavior of spin diffusion in doped semiconductors is shown to be qualitatively different than in undoped (intrinsic) ones. Whereas a spin packet in an intrinsic semiconductor must be a multiple-band disturbance, involving inhomogeneous distributions of both electrons and holes, in a doped semiconductor a single-band disturbance is possible. For n-doped nonmagnetic semiconductors the enhancement of diffusion due to a degenerate electron sea in the conduction band is much larger for these single-band spin packets than for charge packets, and can exceed an order of magnitude at low temperatures even for equilibrium dopings as small as 10 16 cm −3 . In n-doped ferromagnetic and semimagnetic semiconductors the motion of spin packets polarized antiparallel to the equilibrium carrier spin polarization is predicted to be an order of magnitude faster than for parallel polarized spin packets. These results are reversed for p-doped semiconductors.