462 research outputs found

    Spin-flip effects on the current-in-plane magnetotransport in magnetic multilayers with arbitrary magnetization alignments

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    An extended Boltzmann equation approach, with nondiagonal components of the electron distribution, function taken into account, is proposed to study spin-flip effects on the magnetoresistance (MR) in magnetic inhomogeneous systems with arbitrary magnetization alignments. The presence of spin-flip scattering is found to reduce the MR and to decrease deviation of the MR from linear dependence on sin 2(θ/2) where θ is the angle between the magnetizations of successive magnetic films.published_or_final_versio

    Mixed (s+id)-wave order parameters in the Van Hove scenario

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    In the Van Hove scenario including orthorhombic distortion effect, we develop a pair of coupled gap equations for the mixed (s+id)-wave order parameter. It is found that a mixed s+id symmetry state is realized in a certain range of relative strength of the s and d interactions, and there are two second-order transitions between the mixed and the pure symmetry states. Particular attention is paid to the temperature dependence of two components in the mixed order parameter as well as their evolution from a pure s to a pure d symmetry state.published_or_final_versio

    Transport theory in metallic films: Crossover from the classical to the quantum regime

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    Using the quantum-statistical approach, we have developed a unified transport theory of metallic films. A general description for the conductivity in metallic films has been rigorously formulated in the presence of both impurity scattering and surface roughness. An explicit connection between the quasiclassical and present quantum approaches is also presented. We show that the quasiclassical theory by Fuchs [Proc. Cambridge Philos. Soc. 34, 100 (1938)] and Sondheimer [Adv. PHys. 1, 1 (1952)] can be reformed to be applicable to ultrathin metallic films by introducing a treatment of the surface via angle-dependent specularity parameters and including the quantum size effect. Moreover, to the lowest-order approximation in the theory, the previous quantum-approach results and discussions have naturally been recovered. © 1995 The American Physical Society.published_or_final_versio

    Quasiclassical approach to magnetotransport in magnetic inhomogeneous systems

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    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.published_or_final_versio

    Vortex dynamics in twinned superconductors

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    We numerically solve the overdamped equation of vortex motion in a twin-boundary (TB) superconductor, in which the applied Lorentz force F L, the pinning forces due to TB's and point defects, and the intervortex interacting force are taken into account. Our simulations show that TB's act as easy flow channels for the vortex motion parallel to the TB's and obstructive barriers for that normal to the TB's. Due to the barrier effect, the transverse velocity of vortices increases with F L, but if F L is strong enough, the vortices can cross through the TB's so that the transverse velocity vs F L curve exhibits peak behavior.published_or_final_versio

    Phase diagram of an extended Kondo lattice model for manganites: The Schwinger-boson mean-field approach

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    We investigate the phase diagram of an extended Kondo lattice model for doped manganese oxides in the presence of strong but finite Hund's coupling and on-site Coulomb interaction. By means of the Schwinger-boson mean-field approach, it is found that, besides magnetic ordering, there will be nonuniform charge distributions, such as charge ordering and phase separation, if the interaction between electrons prevails over the hybridization. Which of the charge ordering and phase separation appears is determined by a competition between effective repulsive and attractive interactions due to virtual processes of electron hopping. Calculated results show that strong electron correlations caused by the on-site Coulomb interaction as well as the finite Hund's coupling play an important role in the magnetic ordering and charge distribution at low temperatures. ©2000 The American Physical Society.published_or_final_versio

    Spin current carried by magnons

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    A spin current is usually carried by electrons and generated due to the imbalance of up-spin and down-spin. Here we investigate another type of spin current, which is carried by magnons. Using nonequilibrium Green's-function technique, we have derived a Landauer-Büttiker-type formula for spin current transport. The spin current satisfies conservation condition and can be expressed in terms of the magnon Green's functions of the mesoscopic ferromagnetic isolating system. As an application of this theory, we study the magnon transport properties of a two-level magnon quantum dot in the presence of the magnon-magnon scattering. By solving the self-consistent equations, we obtain the nonlinear spin current as a function of the magnetochemical potential. The spin current generated by using a parametric quantum pumping mechanism is also discussed.published_or_final_versio

    Thermoelectric power of hot carriers in the nonequilibrium-statistical- operator approach

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    The thermoelectric power of charge carriers heated under a strong applied electric field in semiconductors is obtained by use of the nonequilibrium- statistical-operator (NSO) method. The balance equations are derived in terms of the NSO density matrix and the force-force correlation functions which can easily be calculated for a system with electron-impurity and electron-phonon interactions. A numerical study has been performed for hole-doped Ge. It is shown that the hot-electron thermoelectric power is sensitively affected by the applied electric field and that its sign is reversed at higher electric fields. © 1995 The American Physical Society.published_or_final_versio

    Semiclassical transport theory of inhomogeneous systems

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    Based on the probability-conserved Boltzmann equation, we develop a formal and general transport theory for the conductivity in inhomogeneous systems. In particular, we show that the local current density inside the sample can be expressed as a boundary value integral, so that the local electric field need not be calculated explicitly. The theory is first applied to multilayer systems and shown to recover the previous theory. More importantly, by including spin-dependent interface scattering and bulk scattering, we employ our theory successfully to account for the giant magnetoresistance in magnetic granular systems.published_or_final_versio

    Conductance oscillation of a mesoscopic normal metal spanning unconventional and conventional superconductors

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    We present a theory for the conductance of a mesoscopic normal metal spanning two superconductors, in which an analytical expression of the conductance is formulated. It is found that the conductance oscillates with the phase difference of two superconductors periodically. When one of the superconductors has a d-wave symmetry, the 2π-period component of the conductance oscillation decays with the misorientation angle α and vanishes at α = π/4 in contradiction to the s-wave case, from which a method is proposed to identify unambiguously the pairing symmetry of the high-Tc superconductors.published_or_final_versio
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