5,823 research outputs found
Double Exchange model for nanoscopic clusters
We solve the double exchange model on nanoscopic clusters exactly, and
specifically consider a six-site benzene-like nanocluster. This simple model is
an ideal testbed for studying magnetism in nanoclusters and for validating
approximations such as the dynamical mean field theory (DMFT). Non-local
correlations arise between neighboring localized spins due to the Hund's rule
coupling, favoring a short-range magnetic order of ferro- or antiferromagnetic
type. For a geometry with more neighboring sites or a sufficiently strong
hybridization between leads and the nanocluster, these non-local correlations
are less relevant, and DMFT can be applied reliably.Comment: 9 pages, 9 figures, 1 tabl
Quantum interference from remotely trapped ions
We observe quantum interference of photons emitted by two continuously
laser-excited single ions, independently trapped in distinct vacuum vessels.
High contrast two-photon interference is observed in two experiments with
different ion species, calcium and barium. Our experimental findings are
quantitatively reproduced by Bloch equation calculations. In particular, we
show that the coherence of the individual resonance fluorescence light field is
determined from the observed interference
New PbSnTe heterojunction laser diode structures with improved performance
Several recent advances in the state-of-the-art of lead tin telluride double heterojunction laser diodes are summarized. Continuous Wave operation to 120 K and pulsed operation to 166 K with single, lowest order transverse mode emission to in excess of four times threshold at 80 K were achieved in buried stripe lasers fabricated by liquid phase epitaxy in the lattice-matched system, lead-tin telluride-lead telluride selenide. At the same time, liquid phase epitaxy was used to produce PbSnTe distributed feedback lasers with much broader continuous single mode tuning ranges than are available from Fabry-Perot lasers. The physics and philosophy behind these advances is as important as the structures and performance of the specific devices embodying the advances, particularly since structures are continually being evolved and the performance continues to be improved
Coherent transport through graphene nanoribbons in the presence of edge disorder
We simulate electron transport through graphene nanoribbons of experimentally
realizable size (length L up to 2 micrometer, width W approximately 40 nm) in
the presence of scattering at rough edges. Our numerical approach is based on a
modular recursive Green's function technique that features sub-linear scaling
with L of the computational effort. We identify the influence of the broken A-B
sublattice (or chiral) symmetry and of K-K' scattering by Fourier spectroscopy
of individual scattering states. For long ribbons we find Anderson-localized
scattering states with a well-defined exponential decay over 10 orders of
magnitude in amplitude.Comment: 8 pages, 6 Figure
Magnetic excitations in multiferroic LuMnO3 studied by inelastic neutron scattering
We present data on the magnetic and magneto-elastic coupling in the hexagonal
multiferroic manganite LuMnO3 from inelastic neutron scattering, magnetization
and thermal expansion measurements. We measured the magnon dispersion along the
main symmetry directions and used this data to determine the principal exchange
parameters from a spin-wave model. An analysis of the magnetic anisotropy in
terms of the crystal field acting on the Mn is presented. We compare the
results for LuMnO3 with data on other hexagonal RMnO3 compounds.Comment: 7 pages, 8 figures, typo correcte
Ab initio lattice dynamics simulations and inelastic neutron scattering spectra for studying phonons in BaFe2As2: Effect of structural phase transition, structural relaxation and magnetic ordering
We have performed extensive ab initio calculations to investigate phonon
dynamics and their possible role in superconductivity in BaFe2As2 and related
systems. The calculations are compared to inelastic neutron scattering data
that offer improved resolution over published data [Mittal et al., PRB 78
104514 (2008)], in particular at low frequencies. Effects of structural phase
transition and full/partial structural relaxation, with and without magnetic
ordering, on the calculated vibrational density of states are reported. Phonons
are best reproduced using either the relaxed magnetic structures or the
experimental cell. Several phonon branches are affected by the subtle
structural changes associated with the transition from the tetragonal to the
orthorhombic phase. Effects of phonon induced distortions on the electronic and
spin structure have been investigated. It is found that for some vibrational
modes, there is a significant change of the electronic distribution and spin
populations around the Fermi level. A peak at 20 meV in the experimental data
falls into the pseudo-gap region of the calculation. This was also the case
reported in our recent work combined with an empirical parametric calculation
[Mittal et al., PRB 78 104514 (2008)]. The combined evidence for the coupling
of electronic and spin degrees of freedom with phonons is relevant to the
current interest in superconductivity in BaFe2As2 and related systems
Pump-induced Exceptional Points in Lasers
We demonstrate that the above-threshold behavior of a laser can be strongly
affected by exceptional points which are induced by pumping the laser
nonuniformly. At these singularities, the eigenstates of the non-Hermitian
operator which describes the lasing modes coalesce. In their vicinity, the
laser may turn off even when the overall pump power deposited in the system is
increased. Such signatures of a pump- induced exceptional point can be
experimentally probed with coupled ridge or microdisk lasers.Comment: 4.5 pages, 4 figures, final version including additional FDTD dat
Conductance of Open Quantum Billiards and Classical Trajectories
We analyse the transport phenomena of 2D quantum billiards with convex
boundary of different shape. The quantum mechanical analysis is performed by
means of the poles of the S-matrix while the classical analysis is based on the
motion of a free particle inside the cavity along trajectories with a different
number of bounces at the boundary. The value of the conductance depends on the
manner the leads are attached to the cavity. The Fourier transform of the
transmission amplitudes is compared with the length of the classical paths.
There is good agreement between classical and quantum mechanical results when
the conductance is achieved mainly by special short-lived states such as
whispering gallery modes (WGM) and bouncing ball modes (BBM). In these cases,
also the localization of the wave functions agrees with the picture of the
classical paths. The S-matrix is calculated classically and compared with the
transmission coefficients of the quantum mechanical calculations for five modes
in each lead. The number of modes coupled to the special states is effectively
reduced.Comment: 19 pages, 6 figures (jpg), 2 table
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