4,124 research outputs found
Electron correlations in MnGaAs as seen by resonant electron spectroscopy and dynamical mean field theory
After two decades from the discovery of ferromagnetism in Mn-doped GaAs, its
origin is still debated, and many doubts are related to the electronic
structure. Here we report an experimental and theoretical study of the valence
electron spectrum of Mn-doped GaAs. The experimental data are obtained through
the differences between off- and on-resonance photo-emission data. The
theoretical spectrum is calculated by means of a combination of
density-functional theory in the local density approximation and dynamical
mean-field theory (LDA+DMFT), using exact diagonalisation as impurity solver.
Theory is found to accurately reproduce measured data, and illustrates the
importance of correlation effects. Our results demonstrate that the Mn states
extend over a broad range of energy, including the top of the valence band, and
that no impurity band splits off from the valence band edge, while the induced
holes seem located primarily around the Mn impurity.Comment: 5 pages, 4 figure
Polarized Neutron Laue Diffraction on a Crystal Containing Dynamically Polarized Proton Spins
We report on a polarized-neutron Laue diffraction experiment on a single
crystal of neodynium doped lanthanum magnesium nitrate hydrate containing
polarized proton spins. By using dynamic nuclear polarization to polarize the
proton spins, we demonstrate that the intensities of the Bragg peaks can be
enhanced or diminished significantly, whilst the incoherent background, due to
proton spin disorder, is reduced. It follows that the method offers unique
possibilities to tune continuously the contrast of the Bragg reflections and
thereby represents a new tool for increasing substantially the signal-to-noise
ratio in neutron diffraction patterns of hydrogenous matter.Comment: 5 pages, 3 figure
ELVIS - ELectromagnetic Vector Information Sensor
The ELVIS instrument was recently proposed by the authors for the Indian
Chandrayaan-1 mission to the Moon and is presently under consideration by the
Indian Space Research Organisation (ISRO). The scientific objective of ELVIS is
to explore the electromagnetic environment of the moon. ELVIS samples the full
three-dimensional (3D) electric field vector, E(x,t), up to 18 MHz, with
selective Nyqvist frequency bandwidths down to 5 kHz, and one component of the
magnetic field vector, B(x,t), from a few Hz up to 100 kHz.As a transient
detector, ELVIS is capable of detecting pulses with a minimum pulse width of 5
ns. The instrument comprises three orthogonal electric dipole antennas, one
magnetic search coil antenna and a four-channel digital sampling system,
utilising flexible digital down conversion and filtering together with
state-of-the-art onboard digital signal processing.Comment: 8 pages, 3 figures. Submitted to the DGLR Int. Symposium "To Moon and
Beyond", Bremen, Germany, 2005. Companion paper to arXiv:astro-ph/050921
Can we always get the entanglement entropy from the Kadanoff-Baym equations? The case of the T-matrix approximation
We study the time-dependent transmission of entanglement entropy through an
out-of-equilibrium model interacting device in a quantum transport set-up. The
dynamics is performed via the Kadanoff-Baym equations within many-body
perturbation theory. The double occupancy , needed to determine the entanglement entropy, is obtained from
the equations of motion of the single-particle Green's function. A remarkable
result of our calculations is that can become negative, thus not permitting to evaluate the
entanglement entropy. This is a shortcoming of approximate, and yet conserving,
many-body self-energies. Among the tested perturbation schemes, the -matrix
approximation stands out for two reasons: it compares well to exact results in
the low density regime and it always provides a non-negative . For the second part of this statement, we
give an analytical proof. Finally, the transmission of entanglement across the
device is diminished by interactions but can be amplified by a current flowing
through the system.Comment: 6 pages, 6 figure
Evidence of nonadiabatic exciton-phonon interaction probed by second-order LO-phonon replicas of single quantum dots
A Mach-Zehnder interferometer based on orbital angular momentum for improved vortex coronagraph efficiency
The Annular Groove Phase Mask (AGPM) is a vectorial vortex phase mask. It acts as a half-wave plate with a radial fast axis orientation operating in the mid infrared domain. When placed at the focus of a telescope element provides a continuous helical phase ramp for an on axis sources, which creates the orbital angular momentum. Thanks to that phase, the intensity of the central source is canceled by a down-stream pupil stop, while the off axis sources are not affected. However due to experimental conditions the nulling is hardly perfect. To improve the null, a Mach-Zehnder interferometer containing Dove prisms differently oriented can be proposed to sort out light based on its orbital angular momentum (OAM). Thanks to the differential rotation of the beam, a π phase shift is achieved for the on axis light affected by a non zero OAM. Therefore the contrast between the star and its faint companion is enhanced. Nevertheless, due the Dove prisms birefringence, the performance of the interferometer is relatively poor. To solve this problem, we propose to add a birefringent wave-plate in each arm to compensate this birefringence. In this paper, we will develop the mathematical model of the wave front using the Jones formalism. The performance of the interferometer is at first computed for the simple version without the birefringent plate. Then the effect of the birefringent plate is be mathematically described and the performance is re-computed
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