16 research outputs found
Nonequilibrium Kondo Effect in a Quantum Dot Coupled to Ferromagnetic Leads
We study the Kondo effect in the electron transport through a quantum dot
coupled to ferromagnetic leads, using a real-time diagrammatic technique which
provides a systematic description of the nonequilibrium dynamics of a system
with strong local electron correlations. We evaluate the theory in an extension
of the `resonant tunneling approximation', introduced earlier, by introducing
the self-energy of the off-diagonal component of the reduced propagator in spin
space. In this way we develop a charge and spin conserving approximation that
accounts not only for Kondo correlations but also for the spin splitting and
spin accumulation out of equilibrium. We show that the Kondo resonances, split
by the applied bias voltage, may be spin polarized. A left-right asymmetry in
the coupling strength and/or spin polarization of the electrodes significantly
affects both the spin accumulation and the weight of the split Kondo resonances
out of equilibrium. The effects are observable in the nonlinear differential
conductance. We also discuss the influence of decoherence on the Kondo
resonance in the frame of the real-time formulation.Comment: 13 pages, 13 figure
Theory of Orbital Ordering, Fluctuation and Resonant X-ray Scattering in Manganites
A theory of resonant x-ray scattering in perovskite manganites is developed
by applying the group theory to the correlation functions of the pseudospin
operators for the orbital degree of freedom. It is shown that static and
dynamical informations of the orbital state are directly obtained from the
elastic, diffuse and inelastic scatterings due to the tensor character of the
scattering factor. We propose that the interaction and its anisotropy between
orbitals are directly identified by the intensity contour of the diffuse
scattering in the momentum space.Comment: 4 pages, 1 figur
Nonlinear Optical Response in two-dimensional Mott Insulators
We study the third-order nonlinear optical susceptibility and
photoexcited states of two-dimensional (2D) Mott insulators by using an
effective model in the strong-coupling limit of a half-filled Hubbard model. In
the numerically exact diagonalization calculations on finite-size clusters, we
find that the coupling of charge and spin degrees of freedom plays a crucial
role in the distribution of the dipole-allowed states with odd parity and the
dipole-forbidden states with even parity in the photoexcited states. This is in
contrast with the photoexcited states in one dimension, where the charge and
spin degrees of freedom are decoupled. In the third-harmonic generation (THG)
spectrum, main contribution is found to come from the process of three-photon
resonance associated with the odd-parity states. As a result, the two-photon
resonance process is less pronounced in the THG spectrum. The calculated THG
spectrum is compared with recent experimental data. We also find that
with cross-polarized configuration of pump and probe photons shows
spectral distributions similar to with co-polarized configuration,
although the weight is small. These findings will help the analyses of the
experimental data of in the 2D Mott insulators.Comment: 9 pages,5 figures,RevTeX
Linear-response theory of the longitudinal spin Seebeck effect
We theoretically investigate the longitudinal spin Seebeck effect, in which
the spin current is injected from a ferromagnet into an attached nonmagnetic
metal in a direction parallel to the temperature gradient. Using the fact that
the phonon heat current flows intensely into the attached nonmagnetic metal in
this particular configuration, we show that the sign of the spin injection
signal in the longitudinal spin Seebeck effect can be opposite to that in the
conventional transverse spin Seebeck effect when the electron-phonon
interaction in the nonmagnetic metal is sufficiently large. Our linear-response
approach can explain the sign reversal of the spin injection signal recently
observed in the longitudinal spin Seebeck effect.Comment: Proc. of ICM 2012 (Accepted for publication in J. Korean Phys. Soc.),
typos correcte
``Flux'' state in double exchange model
We study the ground state properties of the double-exchange systems. The
phase factor of the hopping matrix elements arises from spin texture
in two or more dimensions. A novel ``flux'' state is stabilized against the
canted antiferromagnetic and spiral spin states. In a certain range of hole
doping, the phase separation occurs between the ``flux'' state and
antiferromagnetic states. Constructing a trial state which provides the
rigorous upper bound on the ground state, we show that the metallic canted
antiferromagnetic state is not stable in the double exchange model.Comment: REVTEX, 8 pages and 4 PS figure
Polarization Dependence of Anomalous X-ray Scattering in Orbital Ordered Manganites
In order to determine types of the orbital ordering in manganites, we study
theoretically the polarization dependence of the anomalous X-ray scattering
which is caused by the anisotropy of the scattering factor. The general
formulae of the scattering intensity in the experimental optical system is
derived and the atomic scattering factor is calculated in the microscopic
electronic model. By using the results, the X-ray scattering intensity in
several types of the orbital ordering is numerically calculated as a function
of azimuthal and analyzer angles.Comment: 9 pages, 7 figure
Andreev Reflection in Ferromagnet/Superconductor/Ferromagnet Double Junction Systems
We present a theory of Andreev reflection in a
ferromagnet/superconductor/ferromagnet double junction system. The spin
polarized quasiparticles penetrate to the superconductor in the range of
penetration depth from the interface by the Andreev reflection. When the
thickness of the superconductor is comparable to or smaller than the
penetration depth, the spin polarized quasiparticles pass through the
superconductor and therefore the electric current depends on the relative
orientation of magnetizations of the ferromagnets. The dependences of the
magnetoresistance on the thickness of the superconductor, temperature, the
exchange field of the ferromagnets and the height of the interfacial barriers
are analyzed. Our theory explains recent experimental results well.Comment: 8 pages, 9 figures, submitted to Phys. Rev.
Extremely long quasiparticle spin lifetimes in superconducting aluminium using MgO tunnel spin injectors
There has been an intense search in recent years for long-lived
spin-polarized carriers for spintronic and quantum-computing devices. Here we
report that spin polarized quasi-particles in superconducting aluminum layers
have surprisingly long spin-lifetimes, nearly a million times longer than in
their normal state. The lifetime is determined from the suppression of the
aluminum's superconductivity resulting from the accumulation of spin polarized
carriers in the aluminum layer using tunnel spin injectors. A Hanle effect,
observed in the presence of small in-plane orthogonal fields, is shown to be
quantitatively consistent with the presence of long-lived spin polarized
quasi-particles. Our experiments show that the superconducting state can be
significantly modified by small electric currents, much smaller than the
critical current, which is potentially useful for devices involving
superconducting qubits
Angle-resolved photoemission in high Tc cuprates from theoretical viewpoints
The angle-resolved photoemission (ARPES) technique has been developed rapidly
over the last decay, accompanied by the improvement of energy and momentum
resolutions. This technique has been established as the most powerful tool to
investigate the high Tc cuprate superconductors. We review recent ARPES data on
the cuprates from a theoretical point of view, with emphasis on the systematic
evolution of the spectral weight near the momentum (pi,0) from insulator to
overdoped systems. The effects of charge stripes on the ARPES spectra are also
reviewed. Some recent experimental and theoretical efforts to understand the
superconducting state and the pseudogap phenomenon are discussed.Comment: Review, 25 pages, with 22 GIF figures. To appear in Supercond. Sci.
Technol. Vol. 13 April 2000. A version including PS figures can be found at
http://www.maekawa-lab.imr.tohoku.ac.jp/TOHYAMA/tohyama.ps.g
Resonant X-ray Scattering in Manganites - Study of Orbital Degree of Freedom -
Orbital degree of freedom of electrons and its interplay with spin, charge
and lattice degrees of freedom are one of the central issues in colossal
magnetoresistive manganites. The orbital degree of freedom has until recently
remained hidden, since it does not couple directly to most of experimental
probes. Development of synchrotron light sources has changed the situation; by
the resonant x-ray scattering (RXS) technique the orbital ordering has
successfully been observed . In this article, we review progress in the recent
studies of RXS in manganites. We start with a detailed review of the RXS
experiments applied to the orbital ordered manganites and other correlated
electron systems. We derive the scattering cross section of RXS where the
tensor character of the atomic scattering factor (ASF) with respect to the
x-ray polarization is stressed. Microscopic mechanisms of the anisotropic
tensor character of ASF is introduced and numerical results of ASF and the
scattering intensity are presented. The azimuthal angle scan is a unique
experimental method to identify RXS from the orbital degree of freedom. A
theory of the azimuthal angle and polarization dependence of the RXS intensity
is presented. The theoretical results show good agreement with the experiments
in manganites. Apart from the microscopic description of ASF, a theoretical
framework of RXS to relate directly to the 3d orbital is presented. The
scattering cross section is represented by the correlation function of the
pseudo-spin operator for the orbital degree of freedom. A theory is extended to
the resonant inelastic x-ray scattering and methods to observe excitations of
the orbital degree of freedom are proposed.Comment: 47 pages, 24 figures, submitted to Rep. Prog. Phy