We take the Andreev reflection into account and apply a quantum-mechanical approach to studying spinpolarized transport in a ferromagnet/superconductor/ferromagnet double tunnel junction and its effect on superconductivity in the superconductor. It is found that in the presence of the Andreev reflection the tunneling magnetoresistance in the double tunnel junction depends strongly on barrier strength. In the antiferromagnetic alignment of the magnetizations, an increase in bias voltage will give rise to a first-order transition from the superconducting state to a normal one.

A macroscopic theory of giant magnetoresistance in granular magnetic materials is developed to improve on that of Rubinstein [Phys. Rev. B 50, 3830 (1994)]. By using a self-consistent method and introducing a useful parametrization, we show the magnetotransport in granular systems to be between those for currents in the plane of layers and currents perpendicular to the plane of the layers in multilayers. The theoretical result in the local limit is found to be in agreement with the observed singular dependence of the giant magnetoresistance on annealing temperature.

It is found that the sharp resistivity peak observed near the Curie temperature TC in the manganites R12xAxMnO3 is closely correlated to the residual resistivity r 0 of the sample, suggesting that nonmagnetic randomness plays an important role in determining their anomalous properties. Using the one-parameter scaling theory to study the electronic localization due to both the nonmagnetic randomness and the double exchange spin disorder, we show that the sharp resistivity peak is caused by the Anderson metal-insulator (M-I) transition and that r 0.r c (a critical value) is a prerequisite to the occurrence of the M-I transition. TC as a function of r 0 has also been calculated. These results are in good agreement with experimental measurements.

The conventional quasiclassical approach to magnetotransport in magnetic multilayers and magnetic granular solids is found to be suitable only for collinear magnetization configurations. A quantum treatment of electron spin is proposed to improve the quasiclassical theory to be applicable to arbitrary magnetization alignments, in which the electron distribution functions, as well as the electric fields and currents, need to be regarded as spinor matrices which are off-diagonal in spin space of conduction electrons. An extended Boltzmann equation has been used to derive the two-point spinor conductivity in magnetic inhomogeneous systems. The result obtained is found to be identical to that obtained from the real-space Kubo formula, indicating a close link between the reformed quasiclassical approach and the quantum theory. Angular dependence of the current-in-plane magnetoresistance in magnetic superlattices is discussed.

A localization model comprising spin disorder and nonmagnetic randomness is presented to account for novel magnetotransport properties in Mn oxides R12xAxMnO3. Localization length of electrons as a function of magnetization is determined by means of the transfer matrix method. Including the Coulomb interaction between electrons, the variable-range hopping resistivity is calculated as a function of temperature and magnetic field. The resulting sharp resistivity peak near the Curie temperature and, in particular, the magnitude of the colossal negative magnetoresistance are in good agreement with experimental measurements.

Recently it was proposed that a very small oxygen-isotope effect in the high-Tc Cu oxides can be understood in terms of a logarithmic singularity in the density of states, N (E), near the Fermi energy, along with a conventional BCS phonon-mediated pairing. In this paper we show that a relatively realistic N (E), derived from a tight-binding model on two-dimensional lattices, contains a logarithmic term plus a constant term. The calculated result indicates that the additional constant term in N (E) makes the minimum for the isotope-mass exponent a well exceed the corresponding experimental value (a～0-0.2). The interplanar hopping is shown to smear out the sharp peak of N (E) so that a shifts further towards the standard BCS value of 2/1. So it appears to us that an explanation of the isotope effect as a function of doping in terms of a Van Hove singularity is not tenable.

The method of the nonequilibrium statistical operator developed by Zubarev is employed to inves-tigate steady-state hot-electron transport in a strong electric field. The momentum and energy bal-ance equations which are nonlinear in the drift velocity Ve are derived to the lowest order in the electron-impurity and electron-phonon interactions. We show that these balance equations are ex-actly identical to those obtained by Lei and Ting. However, our equations cannot be reduced to those of Kalashnikov and Ferry, who previously applied a similar approach to study warm-electron transport. The origin of this difference will be demonstrated in detail. We show that our result yields better agreement with the experimental data in p-type Ge for hot heavy-hole transport.

To extend our previous Green's-function approach for steady-state hot-electron transport to the transient region, coupled differential and integral equations are constructed for the evolution of the drift velocity v (t) and the electron temperature Te(t). The effect of current fluctuations on these equations is included and discussed qualitatively. When the evolution equation for v(t) is linear-ized, a Langevin equation is obtained for the drift velocity. For electric fields with moderate strengths, we show that the calculated values for v(t) as a function of t with memory and without memory differ very little. Thus the memory effect is negligible and the original evolution equations reduce to a pair of differential and nonlinear equations for v(t) and Te(t). Numerical computation has been carried out for hole transport in p-type Ge. Scatterings due to both acoustic and nonpolar-optical phonons are considered. For a step field at t--0, v(t) shows the well-known ballis-tic and overshoot behavior. When a rectangular pulse field is applied, both overshoot and un-dershoot phenomena may appear in v (t).