6,876 research outputs found
Improved chlorate candle provides concentrated oxygen source
Improved chlorate candle is used as a solid, portable source of oxygen in emergency situations. It contains sodium chlorate, iron, barium peroxide, and glass mixed in powdered form. The oxygen evolves from the decomposition of the sodium chlorate when an ignition pellet is electrically initiated
Competition between Kondo screening and quantum Hall edge reconstruction
We report on a Kondo correlated quantum dot connected to two-dimensional
leads where we demonstrate the renormalization of the g-factor in the pure
Zeeman case i.e, for magnetic fields parallel to the plane of the quantum dot.
For the same system we study the influence of orbital effects by investigating
the quantum Hall regime i.e. a perpendicular magnetic field is applied. In this
case an unusual behaviour of the suppression of the Kondo effect and of the
split zero-bias anomaly is observed. The splitting decreases with magnetic
field and shows discontinuous changes which are attributed to the intricate
interplay between Kondo screening and the quantum Hall edge structure
originating from electrostatic screening. This edge structure made up of
compressible and incompressible stripes strongly affects the Kondo temperature
of the quantum dot and thereby influences the renormalized g-factor
Optical properties of current carrying molecular wires
We consider several fundamental optical phenomena involving single molecules
in biased metal-molecule-metal junctions. The molecule is represented by its
highest occupied and lowest unoccupied molecular orbitals, and the analysis
involves the simultaneous consideration of three coupled fluxes: the electronic
current through the molecule, energy flow between the molecule and
electron-hole excitations in the leads and the incident and/or emitted photon
flux. Using a unified theoretical approach based on the non-equilibrium Green
function method we derive expressions for the absorption lineshape (not an
observable but a ueful reference for considering yields of other optical
processes) and for the current induced molecular emission in such junctions. We
also consider conditions under which resonance radiation can induce electronic
current in an unbiased junction. We find that current driven molecular emission
and resonant light induced electronic currents in single molecule junctions can
be of observable magnitude under appropriate realizable conditions. In
particular, light induced current should be observed in junctions involving
molecular bridges that are characterized by strong charge transfer optical
transitions. For observing current induced molecular emission we find that in
addition to the familiar need to control the damping of molecular excitations
into the metal substrate the phenomenon is also sensitive to the way in which
the potential bias si distributed on the junction.Comment: 56 pages, 8 figures; submitted to JC
Kinetics of electron-positron pair plasmas using an adaptive Monte Carlo method
A new algorithm for implementing the adaptive Monte Carlo method is given. It
is used to solve the relativistic Boltzmann equations that describe the time
evolution of a nonequilibrium electron-positron pair plasma containing
high-energy photons and pairs. The collision kernels for the photons as well as
pairs are constructed for Compton scattering, pair annihilation and creation,
bremsstrahlung, and Bhabha & Moller scattering. For a homogeneous and isotropic
plasma, analytical equilibrium solutions are obtained in terms of the initial
conditions. For two non-equilibrium models, the time evolution of the photon
and pair spectra is determined using the new method. The asymptotic numerical
solutions are found to be in a good agreement with the analytical equilibrium
states. Astrophysical applications of this scheme are discussed.Comment: 43 pages, 7 postscript figures, to appear in the Astrophysical
Journa
Quantum transport in chains with noisy off-diagonal couplings
We present a model for conductivity and energy diffusion in a linear chain
described by a quadratic Hamiltonian with Gaussian noise. We show that when the
correlation matrix is diagonal, the noise-averaged Liouville-von Neumann
equation governing the time-evolution of the system reduces to the Lindblad
equation with Hermitian Lindblad operators. We show that the noise-averaged
density matrix for the system expectation values of the energy density and the
number density satisfy discrete versions of the heat and diffusion equations.
Transport coefficients are given in terms of model Hamiltonian parameters. We
discuss conditions on the Hamiltonian under which the noise-averaged
expectation value of the total energy remains constant. For chains placed
between two heat reservoirs, the gradient of the energy density along the chain
is linear.Comment: 6 pages, to appear in J. Chem. Phy
Ultrafast Magnetization Dynamics in Diluted Magnetic Semiconductors
We present a dynamical model that successfully explains the observed time
evolution of the magnetization in diluted magnetic semiconductor quantum wells
after weak laser excitation. Based on the pseudo-fermion formalism and a second
order many-particle expansion of the exact p-d exchange interaction, our
approach goes beyond the usual mean-field approximation. It includes both the
sub-picosecond demagnetization dynamics and the slower relaxation processes
which restore the initial ferromagnetic order in a nanosecond time scale. In
agreement with experimental results, our numerical simulations show that,
depending on the value of the initial lattice temperature, a subsequent
enhancement of the total magnetization may be observed within a time scale of
few hundreds of picoseconds.Comment: Submitted to PR
Signatures of spin in the n=1/3 Fractional Quantum Hall Effect
The activation gap Delta of the fractional quantum Hall state at constant
filling n =1/3 is measured in wide range of perpendicular magnetic field B.
Despite the full spin polarization of the incompressible ground state, we
observe a sharp crossover between a low-field linear dependence of Delta on B
associated to spin texture excitations and a Coulomb-like behavior at large B.
From the global gap-reduction we get information about the mobility edges in
the fractional quantum Hall regime.Comment: 4 pages, 3 figure
Theoretical study of electronic transport through a small quantum dot with a magnetic impurity
We model a small quantum dot with a magnetic impurity by the Anderson
Hamiltonian with a supplementary exchange interaction term. The transport
calculations are performed by means of the Green functions within the equation
of motion scheme, in which two decoupling procedures are proposed, for high and
low temperatures, respectively. The paper focuses on the charge fluctuations
for such a system, aspect not addressed before, as well as on the Kondo
resonance. We show a specific role of the excited state, which can be observed
in transport and in spin-spin correlations. Our studies show on a new many-body
feature of the phase shift of transmitted electrons, which is manifested in a
specific dip. In the Kondo regime, our calculations complement existing
theoretical results. The system shows three Kondo peaks in the density of
states: one at the Fermi energy and two side peaks, at a distance corresponding
to the singlet-triplet level spacing. The existence of the central peak is
conditioned by a degenerate state (the triplet) below the Fermi energy.Comment: 12 pages, 4 figure
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