2,404 research outputs found
On Killing vectors in initial value problems for asymptotically flat space-times
The existence of symmetries in asymptotically flat space-times are studied
from the point of view of initial value problems. General necessary and
sufficient (implicit) conditions are given for the existence of Killing vector
fields in the asymptotic characteristic and in the hyperboloidal initial value
problem (both of them are formulated on the conformally compactified space-time
manifold)
Closing the gap between spatial and spin dynamics of electrons at the metal-to-insulator transition
We combine extensive precision measurements of the optically detected spin
dynamics and magneto-transport measurements in a contiguous set of n-doped bulk
GaAs structures in order to unambiguously unravel the intriguing but complex
contributions to the spin relaxation at the metal-to-insulator transition
(MIT). Just below the MIT, the interplay between hopping induced loss of spin
coherence and hyperfine interaction yields a maximum spin lifetime exceeding
800~ns. At slightly higher doping concentrations, however, the spin relaxation
deviates from the expected Dyakonov-Perel mechanism which is consistently
explained by a reduction of the effective motional narrowing with increasing
doping concentration. The reduction is attributed to the change of the dominant
momentum scattering mechanism in the metallic impurity band where scattering by
local conductivity domain boundaries due to the intrinsic random distribution
of donors becomes significant. Here, we fully identify and model all intricate
contributions of the relevant microscopic scattering mechanisms which allows
the complete quantitative modeling of the electron spin relaxation in the
entire regime from weakly interacting up to fully delocalized electrons
Electronic Theory for the Nonlinear Magneto-Optical Response of Transition-Metals at Surfaces and Interfaces: Dependence of the Kerr-Rotation on Polarization and on the Magnetic Easy Axis
We extend our previous study of the polarization dependence of the nonlinear
optical response to the case of magnetic surfaces and buried magnetic
interfaces. We calculate for the longitudinal and polar configuration the
nonlinear magneto-optical Kerr rotation angle. In particular, we show which
tensor elements of the susceptibilities are involved in the enhancement of the
Kerr rotation in nonlinear optics for different configurations and we
demonstrate by a detailed analysis how the direction of the magnetization and
thus the easy axis at surfaces and buried interfaces can be determined from the
polarization dependence of the nonlinear magneto-optical response, since the
nonlinear Kerr rotation is sensitive to the electromagnetic field components
instead of merely the intensities. We also prove from the microscopic treatment
of spin-orbit coupling that there is an intrinsic phase difference of
90 between tensor elements which are even or odd under magnetization
reversal in contrast to linear magneto-optics. Finally, we compare our results
with several experiments on Co/Cu films and on Co/Au and Fe/Cr multilayers. We
conclude that the nonlinear magneto-optical Kerr-effect determines uniquely the
magnetic structure and in particular the magnetic easy axis in films and at
multilayer interfaces.Comment: 23 pages Revtex, preprintstyle, 2 uuencoded figure
Ultrafast Spin Dynamics in Nickel
The spin dynamics in Ni is studied by an exact diagonalization method on the
ultrafast time scale. It is shown that the femtosecond relaxation of the
magneto-optical response results from exchange interaction and spin-orbit
coupling. Each of the two mechanisms affects the relaxation process
differently. We find that the intrinsic spin dynamics occurs during about 10 fs
while extrinsic effects such as laser-pulse duration and spectral width can
slow down the observed dynamics considerably. Thus, our theory indicates that
there is still room to accelerate the spin dynamics in experiments.Comment: 4 pages, Latex, 4 postscript figure
Electron spin relaxation in bulk GaAs for doping densities close to the metal-to-insulator transition
We have measured the electron spin relaxation rate and the integrated spin
noise power in n-doped GaAs for temperatures between 4 K and 80 K and for
doping concentrations ranging from 2.7 x 10^{-15} cm^{-3} to 8.8 x 10^{-16}
cm^{-3} using spin noise spectroscopy. The temperature dependent measurements
show a clear transition from localized to free electrons for the lower doped
samples and confirm mainly free electrons at all temperatures for the highest
doped sample. While the sample at the metal-insulator-transition shows the
longest spin relaxation time at low temperatures, a clear crossing of the spin
relaxation rates is observed at 70 K and the highest doped sample reveals the
longest spin relaxation time above 70 K.Comment: 6 pages, 4 figure
Theory for Spin-Polarized Oscillations in Nonlinear Magneto-Optics due to Quantum Well States
Using an electronic tight-binding theory we calculate the nonlinear
magneto-optical response from an x-Cu/1Fe/Cu(001) film as a function of
frequency and Cu overlayer thickness (x=3 ... 25). We find very strong
spin-polarized quantum well oscillations in the nonlinear magneto-optical Kerr
effect (NOLIMOKE). These are enhanced by the large density of Fe states
close to the Fermi level acting as intermediate states for frequency doubling.
In good agreement with experiment we find two oscillation periods of 6-7 and 11
monolayers the latter being more pronounced.Comment: 12 pages, Revtex, 3 postscript figure
Numerical evolution of axisymmetric, isolated systems in General Relativity
We describe in this article a new code for evolving axisymmetric isolated
systems in general relativity. Such systems are described by asymptotically
flat space-times which have the property that they admit a conformal extension.
We are working directly in the extended `conformal' manifold and solve
numerically Friedrich's conformal field equations, which state that Einstein's
equations hold in the physical space-time. Because of the compactness of the
conformal space-time the entire space-time can be calculated on a finite
numerical grid. We describe in detail the numerical scheme, especially the
treatment of the axisymmetry and the boundary.Comment: 10 pages, 8 figures, uses revtex4, replaced with revised versio
Measurement of heavy-hole spin dephasing in (InGa)As quantum dots
We measure the spin dephasing of holes localized in self-assembled (InGa)As
quantum dots by spin noise spectroscopy. The localized holes show a distinct
hyperfine interaction with the nuclear spin bath despite the p-type symmetry of
the valence band states. The experiments reveal a short spin relaxation time
{\tau}_{fast}^{hh} of 27 ns and a second, long spin relaxation time
{\tau}_{slow}^{hh} which exceeds the latter by more than one order of
magnitude. The two times are attributed to heavy hole spins aligned
perpendicular and parallel to the stochastic nuclear magnetic field. Intensity
dependent measurements and numerical simulations reveal that the long
relaxation time is still obscured by light absorption, despite low laser
intensity and large detuning. Off-resonant light absorption causes a
suppression of the spin noise signal due to the creation of a second hole
entailing a vanishing hole spin polarization.Comment: accepted to be published in AP
Nonlinear Magneto-Optical Response of - and -Wave Superconductors
The nonlinear magneto-optical response of - and -wave superconductors
is discussed. We carry out the symmetry analysis of the nonlinear
magneto-optical susceptibility in the superconducting state. Due to the surface
sensitivity of the nonlinear optical response for systems with bulk inversion
symmetry, we perform a group theoretical classification of the superconducting
order parameter close to a surface. For the first time, the mixing of singlet
and triplet pairing states induced by spin-orbit coupling is systematically
taken into account. We show that the interference of singlet and triplet
pairing states leads to an observable contribution of the nonlinear
magneto-optical Kerr effect. This effect is not only sensitive to the
anisotropy of the gap function but also to the symmetry itself. In view of the
current discussion of the order parameter symmetry of High-T
superconductors, results for a tetragonal system with bulk singlet pairing for
various pairing symmetries are discussed.Comment: 21 pages (REVTeX) with 8 figures (Postscript
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