950 research outputs found
A search for soft X-ray emission associated with prominent high-velocity-cloud complexes
We correlate the ROSAT 1/4 keV all-sky survey with the Leiden/Dwingeloo HI
survey, looking for soft X-ray signatures of prominent high-velocity-cloud
(HVC) complexes. We study the transfer of 1/4 keV photons through the
interstellar medium in order to distinguish variations in the soft X-ray
background (SXRB) intensity caused by photoelectric absorption effects from
those due to excess X-ray emission. The X-ray data are modelled as a
combination of emission from the Local Hot Bubble (LHB) and emission from a
distant plasma in the galactic halo and extragalactic sources. The X-ray
radiation intensity of the galactic halo and extragalactic X-ray background is
modulated by the photoelectric absorption of the intervening galactic
interstellar matter. We show that large- and small-scale intensity variations
of the 1/4 keV SXRB are caused by photoelectric absorption which is
predominantly traced by the total N(HI) distribution. The extensive coverage of
the two surveys supports evidence for a hot, X-ray emitting corona. We show
that this leads to a good representation of the SXRB observations. For four
large areas on the sky, we search for regions where the modelled and observed
X-ray emission differ. We find that there is excess X-ray emission towards
regions near HVC complexes C, D, and GCN. We suggest that the excess X-ray
emission is positionally correlated with the high-velocity clouds. Some lines
of sight towards HVCs also pass through significant amounts of
intermediate-velocity gas, so we cannot constrain the possible role played by
IVC gas in these directions of HVC and IVC overlap, in determining the X-ray
excesses.Comment: 16 pages, 8 figures, accepted for publication in Astronomy &
Astrophysics main journa
Current-induced non-adiabatic spin torques and domain wall motion with spin relaxation in a ferromagnetic metallic wire
Within the s-d model description, we derive the current-driven spin torque in
a ferromagnet, taking explicitly into account a spin-relaxing Caldeira-Leggett
bath coupling to the s-electrons. We derive Bloch-Redfield equations of motion
for the s-electron spin dynamics, and formulate a systematic gradient expansion
to obtain non-adiabatic (higher-order) corrections to the well-known adiabatic
(first-order) spin torque. We provide simple analytical expressions for the
second-order spin torque. The theory is applied to current-driven domain wall
motion. Second-order contributions imply a deformation of a transverse
tail-to-tail domain wall. The wall center still moves with a constant velocity
that now depends on the spin-polarized current in a non-trivial manner.Comment: Phys. Rev. B, in press, replaced with published versio
Applying voltage sources to a Luttinger liquid with arbitrary transmission
The Landauer approach to transport in mesoscopic conductors has been
generalized to allow for strong electronic correlations in a single-channel
quantum wire. We describe in detail how to account for external voltage sources
in adiabatic contact with a quantum wire containing a backscatterer of
arbitrary strength. Assuming that the quantum wire is in the Luttinger liquid
state, voltage sources lead to radiative boundary conditions applied to the
displacement field employed in the bosonization scheme. We present the exact
solution of the transport problem for arbitrary backscattering strength at the
special Coulomb interaction parameter g=1/2.Comment: 9 pages REVTeX, incl 2 fig
Electron injection in a nanotube with leads: finite frequency noise-correlations and anomalous charges
The non-equilibrium transport properties of a carbon nanotube which is
connected to Fermi liquid leads, where electrons are injected in the bulk, are
computed. A previous work which considered an infinite nanotube showed that the
zero frequency noise correlations, measured at opposite ends of the nanotube,
could be used to extract the anomalous charges of the chiral excitations which
propagate in the nanotube. Here, the presence of the leads have the effect that
such-noise cross-correlations vanish at zero frequency. Nevertheless,
information concerning the anomalous charges can be recovered when considering
the spectral density of noise correlations at finite frequencies, which is
computed perturbatively in the tunneling amplitude. The spectrum of the noise
cross-correlations is shown to depend crucially on the ratio of the time of
flight of quasiparticles traveling in the nanotube to the ``voltage'' time
which defines the width of the quasiparticle wave-packets injected when an
electron tunnels. Potential applications toward the measurement of such
anomalous charges in non-chiral Luttinger liquids (nanotubes or semiconductor
quantum wires) are discussed.Comment: 11 pages, 5 figure
External voltage sources and Tunneling in quantum wires
We (re) consider in this paper the problem of tunneling through an impurity
in a quantum wire with arbitrary Luttinger interaction parameter. By combining
the integrable approach developed in the case of Quantum Hall edge states with
the introduction of radiative boundary conditions to describe the adiabatic
coupling to reservoirs, we are able to obtain the exact equilibrium and non
equilibrium current. One of the most striking features observed is the
appearance of negative differential conductances out of equilibrium in the
strongly interacting regime g <=.2. In spite of the various charging effects, a
remarkable form of duality is still observed.
New results on the computation of transport properties in integrable impurity
problems are gathered in appendices. In particular, we prove that the TBA
results satisfy a remarkable relation, originally derived using the Keldysh
formalism, between the order T^2 correction to the current out of equilibrium
and the second derivative of this current at T=0 with respect to the voltage.Comment: 16 pages, 7 figure
Dynamics of Dissipative Quantum Systems--from Path Integrals to Master Equations
The path integral approach offers not only an exact expression for the non-
equilibrium dynamics of dissipative quantum systems, but is also a convenient
starting point for perturbative treatments. An alternative way to explore the
influence of friction in the quantum realm is based on master equations which
require, however, in one or the other aspect approximations. Here it is
discussed under which conditions and limitations Markovian master equations can
be derived from exact path integrals thus providing a firm basis for their
applicability.Comment: 10 pages, 1 figur
Asymptotic tunneling conductance in Luttinger liquids
Conductance through weak constrictions in Luttinger liquids is shown to
vanish with frequency as , where
is a dimensionless parameter characterizing the Luttinger liquid phase, and
and are nonuniversal constants. The first term arises from the ^^
Coulomb blockade' effect and dominates for , whereas the second
results from eliminating high-energy modes and dominates for .Comment: Latex file + one appended postcript figur
Quantum Monte Carlo simulation for the conductance of one-dimensional quantum spin systems
Recently, the stochastic series expansion (SSE) has been proposed as a
powerful MC-method, which allows simulations at low for quantum-spin
systems. We show that the SSE allows to compute the magnetic conductance for
various one-dimensional spin systems without further approximations. We
consider various modifications of the anisotropic Heisenberg chain. We recover
the Kane-Fisher scaling for one impurity in a Luttinger-liquid and study the
influence of non-interacting leads for the conductance of an interacting
system.Comment: 8 pages, 9 figure
Strong coupling theory for driven tunneling and vibrational relaxation
We investigate on a unified basis tunneling and vibrational relaxation in
driven dissipative multistable systems described by their N lowest lying
unperturbed levels. By use of the discrete variable representation we derive a
set of coupled non-Markovian master equations. We present analytical treatments
that describe the dynamics in the regime of strong system-bath coupling. Our
findings are corroborated by ``ab-initio'' real-time path integral
calculations.Comment: 4 LaTeX pages including 3 figure
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