288 research outputs found
On Ultrafast Spin Dynamics: Spin Dependent Fast Response of Hot Electrons, of Band--Structure
Different energy shifts for majority and minority electrons occur. Thus, for
example in case of (laser) excited ferromagnetic metals majority and minority
electrons may respond differently in time during closing the exchange
splitting. Spin flip transitions of the hot electrons due to electron
interactions cause quasi hybridization of the spin split states. This is also
the case in itinerant ferromagnetic metals due to hopping between sites having
magnetic moments pointing in direction of the magnetization (+) and opposite
direction (-) and with energy levels and
. For energetic reasons the molecular field acts
asymmetrically on the spins of the electrons and on spin flip transitions and
thus causes different lifetimes of minority and majority electrons and spin
dependent electron energy shifts. Quite general minority hot electrons in spin
split states may respond faster than majority electrons at non--equilibrium.
The molecular field acting on the spins delays spin flip transitions and thus a response of the hot majority electrons and
their energy levels. The closing of the exchange splitting in the electron
spectrum of ferromagnetic transition and rare--earth metals, ferromagnetic
semiconductors, spin split quantum well states in thin ferromagnetic films,
etc. will reflect this. The time and spin dependent energy shifts of electrons
at non--equilibrium may cause interesting behavior, in particular of magnetic
tunnel junctions, spin currents etc.. In ferromagnets the moment reversal
lifetime of (local) magnetic moments parallel to the global magnetization is
larger than of moments pointing in opposite direction.Comment: 25pages,8 Fig
Theory for the Nonlinear Optical Response of Transition-Metals: Polarization Dependence as a Fingerprint of the Electronic Structure at Surfaces and Interfaces
We show that the nonlinear optical response reflects sensitively the
electronic structure of transition-metal surfaces and interfaces. and
electrons may contribute rather differently to the second harmonic generation
(SHG) signal. This results from the different sensitivity of and
electrons to surface and symmetry changes. Consequently, SHG for noble metals
shows a by far larger dependence on the polarization of the incoming light than
for transition metals like Fe, Co, Ni, in particular at lower frequencies. The
theoretical results are compared with recent measurements. We conclude that the
SHG yield is in addition to the nonlinear magneto-optical Kerr-effect a
sensitive fingerprint of the electronic structure at surfaces and interfaces.Comment: 20 pages+ 4 postscript-formatted figures,Revtex, submitted to Phys.
Rev. B 08/02/199
Nonlinear magneto-optical Kerr spectra of thin ferromagnetic iron films calculated with ab initio theory
Using a spin-polarized full-potential linear muffin-tin orbital method we
present calculations of the nonlinear magneto-optical Kerr effect for thin bcc
Fe films within a slab geometry. Film layer dependent contributions to the Kerr
spectrum are determined. Thus, we calculate the magneto-optical Kerr spectra
for the linear and nonlinear case. Our results show clearly that the Kerr
spectra of thin films are characteristicly different from those at surfaces of
bulk materials. In the case of linear Kerr spectra of Au/Fe(bcc)/Au(001) films
our theoretical results are in good agreement with observed frequency- and
thickness-dependent spectra.Comment: 13 pages, latex (revtex), 8 uuencoded postscript figure
Analysis of the elementary excitations in high-T_c cuprates: explanation of the new energy scale observed by ARPES
Using the Hubbard Hamiltonian we analyze the energy- and momentum-dependence
of the elementary excitations in high-T superconductors resulting from the
coupling to spin fluctuations. As a result of the energy dependence of the
self-energy , characteristic features occur in the
spectral density explaining the 'kink' in recent ARPES experiments. We present
results for the spectral density resulting from the
crossover from Im to Im , for the feedback of superconductivity on the
excitations, and for the superconducting order parameter . These results relate also to inelastic neutron scattering and
tunneling experiments and shed important light on the essential ingredients a
theory of the elementary excitations in the cuprates must contain.Comment: 5 pages, 4 figure, misprints correcte
Theory for Nonlinear Mie-Scattering from Spherical Metal Clusters
Using classical electrodynamics we determine the angular dependence of the
light intensities radiated in second and third harmonic generation by spherical
metal clusters. Forward and backward scattering is analyzed in detail. Also
resonance effects in the integrated intensities are studied. Our work treats
the case of intermediate cluster sizes. Thus it completes the scattering theory
fo spherical clusters between Rayleigh-type analysis for small spheres and
geometrical optics for spheres much larger than the wavelength for nonlinear
optics. Since the particle size sensitivity of Mie-scattering is increased by
nonlinearity, the results can be used to extract sizes of small particles from
nonlinear optics. (submitted to Phys. Rev. B)Comment: 34 pages with 13 figures (postscript code avalaible upon request),
RevTe
Theoretical analysis of Cooper-pair phase fluctuations in underdoped cuprates: a spin-fluctuation exchange study
We study Cooper-pair phase fluctuations in cuprate superconductors for a spin
fluctuation pairing interaction. Using an electronic theory we calculate in
particular for the underdoped cuprate superconductors the superfluid density
, the superconducting transition temperature below
which phase coherent Cooper-pairs occur, and where the phase
incoherent Cooper-pairs disappear. Also we present results for the penetration
depth and for the weak pseudogap temperature at which
a gap structure occurs in the spectral density. A Mei{\ss}ner effect is
obtained only for . We find that phase fluctuations become
increasingly important in the underdoped regime and lead to a reduction of
in good agreement with the experimental situation.Comment: 4 pages, 5 figures. One reference adde
Theory for the nonlinear optical response of a nonspherical metal cluster
Using classical electrodynamics we determine the higher harmonic radiation by
a nonspherical metal cluster in form of a matrix formalism. Extending the
theory for the source of the higher harmonic radiation for spherical metal
clusters as introduced by \"Ostling et al. [Z. Phys. D {\bf 28}, 169 (1993)] we
calculate the sources for nonspherical particles. Employing these sources we
develop the nonlinear Mie theory and determine the radiated higher harmonic
fields generated by the cluster. Our theory is valid for arbitrary shape and
arbitrary complex refractive index for cluster sizes much smaller and
comparable to the wavelength of the incident light.Comment: 6 pages, Revte
Theory for the Dependence of Optical Second Harmonic Generation Intensity on Non-equilibrium Electron Temperatures at Metal Surfaces
We present a theory for the electron-temperature dependence of
optical second harmonic generation (SHG). Such an analysis is required to study
the dynamics of metallic systems with many hot electrons not at equilibrium
with the lattice. Using a tight-binding theory for the nonlinear susceptibility
\cwtel and the Fresnel coefficients we present results for the SHG intensity
\iwtel for a Cu monolayer. In the case of linear optical response we find that
the intensity will decrease monotonously for increasing . In agreement
with experiment we find a frequency range where \iwtel may be enhanced or
reduced depending on electron temperature. Note, \cwtel rather than the Fresnel
coefficients determines essentially the temperature dependence. Our theory
yields also that SHG probes effects due to hot electrons more sensitively than
linear optics. We also discuss the -dependence of SHG for Au and Ag.Comment: 17 pages, Revtex, 10 postscript figures, tar'ed together, compressed
with 'gzip', encoded with 'uuendcode
Dynamical Mean Field Theory for Perovskites
Using the Hubbard Hamiltonian for transition metal-3d and oxygen-2p states
with perovskite geometry, we present a dynamical mean field theory which
becomes exactin the limit of large coordination numbers or equivalently large
spatial dimensions . The theory is based on a new description of these
systems for large using a selective treatment of different hopping
processes which can not be generated by a unique scaling of the hopping
element. The model is solved using a perturbational approach and an extended
non-crossing approximation. We discuss the breakdown of the perturbation theory
near half filling, the origin of the various 3d and 2p bands, the doping
dependence of its spectral weight, and the evolution of quasi particles at the
Fermi-level upon doping, leading to interesting insight into the dynamical
character of the charge carriers near the metal insulator instability of
transition metal oxide systems, three dimensional perovskites and other
strongly correlated transition metal oxides.Comment: 9 pages, Revtex, 8 postscript figure
Theory for underdoped high-T_c superconductors: effects of phase fluctuations
In underdoped cuprates, T_c is thought to be determined by Cooper pair phase
fluctuations because of the small superfluid density n_s. Experimentally, T_c
is found to scale with n_s. The fluctuation-exchange approximation (FLEX) in
its standard form fails to predict this behavior of T_c since it does not
include phase fluctuations. We therefore extend the FLEX to include them
selfconsistently. We present results for T_c[n_s,x], where x is the doping.Comment: (5 pages, 3 figures included, minor changes, to appear in JLTP
proceedings MOS99
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