2,589 research outputs found
Properties of Turbulence in the Very Local Interstellar Clouds
We have investigated the degree to which turbulence in the Very Local
Interstellar Clouds resembles the highly-studied turbulence in the solar corona
and the solar wind. The turbulence diagnostics for the Local Clouds are the
absorption line widths measured along 32 lines of sight to nearby stars,
yielding measurements for 53 absorption components (Redfield and Linsky 2004).
We have tested whether the Local Cloud turbulence has the following properties
of turbulence in the solar corona or the solar wind: (a) velocity fluctuations
mainly perpendicular to the average magnetic field, (b) a temperature
anisotropy in the sense that the perpendicular temperature is larger than the
parallel temperature (or at least enhanced relative to expectation), and (c) an
ion temperature which is dependent on the ion Larmor radius, in the sense that
more massive ions have higher temperatures. Our analysis of the data does not
show compelling evidence for any of these properties in Local Cloud turbulence,
indicating possible differences with heliospheric plasmas. In the case of
anisotropy of velocity fluctuations, although the expected observational
signature is not seen, we cannot exclude the possibility of relatively high
degrees of anisotropy (anisotropy parameter ), if
some other process in the the Local Clouds is causing variations in the
turbulent line width from one line of sight to another. We briefly consider
possible reasons for differences between coronal and solar wind turbulence and
that in the Local Clouds.Comment: Submitted to the Astrophysical Journa
Ion-Neutral Collisions in the Interstellar Medium: Wave Damping and Elimination of Collisionless Processes
Most phases of the interstellar medium contain neutral atoms in addition to
ions and electrons. This introduces differences in plasma physics processes in
those media relative to the solar corona and the solar wind at a heliocentric
distance of 1 astronomical unit. In this paper, we consider two well-diagnosed,
partially-ionized interstellar plasmas. The first is the Diffuse Ionized Gas
(DIG) which is probably the extensive phase in terms of volume. The second is
the gas that makes up the Local Clouds of the Very Local Interstellar Medium
(VLISM). Ion-neutral interactions seem to be important in both media. In the
DIG, ion-neutral collisions are relatively rare, but sufficiently frequent to
damp magnetohydrodynamic (MHD) waves (as well as propagating MHD eddies) within
less than a parsec of the site of generation. This result raises interesting
questions about the sources of turbulence in the DIG. In the case of the VLISM,
the ion-neutral collision frequency is higher than that in the DIG, because the
hydrogen is partially neutral rather than fully ionized. We present results
showing that prominent features of coronal and solar wind turbulence seem to be
absent in VLISM turbulence. For example, ion temperature does not depend on ion
mass. This difference may be attributable to ion-neutral collisions, which
distribute power from more effectively heated massive ions such as iron to
other ion species and neutral atoms.Comment: Submitted to American Institute of Physics Conference Proceedings for
conference "Partially Ionized Plasmas Throughout the Cosmos", Dastgeer
Shaikh, edito
Time-convolutionless master equation for quantum dots: Perturbative expansion to arbitrary order
The master equation describing the non-equilibrium dynamics of a quantum dot
coupled to metallic leads is considered. Employing a superoperator approach, we
derive an exact time-convolutionless master equation for the probabilities of
dot states, i.e., a time-convolutionless Pauli master equation. The generator
of this master equation is derived order by order in the hybridization between
dot and leads. Although the generator turns out to be closely related to the
T-matrix expressions for the transition rates, which are plagued by
divergences, in the time-convolutionless generator all divergences cancel order
by order. The time-convolutionless and T-matrix master equations are contrasted
to the Nakajima-Zwanzig version. The absence of divergences in the
Nakajima-Zwanzig master equation due to the nonexistence of secular reducible
contributions becomes rather transparent in our approach, which explicitly
projects out these contributions. We also show that the time-convolutionless
generator contains the generator of the Nakajima-Zwanzig master equation in the
Markov approximation plus corrections, which we make explicit. Furthermore, it
is shown that the stationary solutions of the time-convolutionless and the
Nakajima-Zwanzig master equations are identical. However, this identity neither
extends to perturbative expansions truncated at finite order nor to dynamical
solutions. We discuss the conditions under which the Nakajima-Zwanzig-Markov
master equation nevertheless yields good results.Comment: 13 pages + appendice
Observational Tests of the Properties of Turbulence in the Very Local Interstellar Medium
The Very Local Interstellar Medium (VLISM) contains clouds which consist of
partially-ionized plasma. These clouds can be effectively diagnosed via high
resolution optical and ultraviolet spectroscopy of the absorption lines they
form in the spectra of nearby stars. Among the information provided by these
spectroscopic measurements are the root-mean-square velocity fluctuation due to
turbulence in these clouds and the ion temperature, which may be partially
determined by dissipation of turbulence. We consider whether this turbulence
resembles the extensively studied and well-diagnosed turbulence in the solar
wind and solar corona. Published observations are used to determine if the
velocity fluctuations are primarily transverse to a large-scale magnetic field,
whether the temperature perpendicular to the large scale field is larger than
that parallel to the field, and whether ions with larger Larmor radii have
higher temperatures than smaller gyroradius ions. Although a thorough
investigation of the data is underway, a preliminary examination of the
published data shows neither evidence for anisotropy of the velocity
fluctuations or temperature, nor Larmor radius-dependent heating. These results
indicate differences between solar wind and Local Cloud turbulence.Comment: Paper submitted to Nonlinear Processes in Geophysic
Excitation energy transfer: Study with non-Markovian dynamics
In this paper, we investigate the non-Markovian dynamics of a model to mimic
the excitation energy transfer (EET) between chromophores in photosynthesis
systems. The numerical path integral method is used. This method includes the
non-Markovian effects of the environmental affects and it does not need the
perturbation approximation in solving the dynamics of systems of interest. It
implies that the coherence helps the EET between chromophores through lasting
the transfer time rather than enhances the transfer rate of the EET. In
particular, the non-Markovian environment greatly increase the efficiency of
the EET in the photosynthesis systems.Comment: 5 pages, 5 figure
Multiple-time correlation functions for non-Markovian interaction: Beyond the Quantum Regression Theorem
Multiple time correlation functions are found in the dynamical description of
different phenomena. They encode and describe the fluctuations of the dynamical
variables of a system. In this paper we formulate a theory of non-Markovian
multiple-time correlation functions (MTCF) for a wide class of systems. We
derive the dynamical equation of the {\it reduced propagator}, an object that
evolve state vectors of the system conditioned to the dynamics of its
environment, which is not necessarily at the vacuum state at the initial time.
Such reduced propagator is the essential piece to obtain multiple-time
correlation functions. An average over the different environmental histories of
the reduced propagator permits us to obtain the evolution equations of the
multiple-time correlation functions. We also study the evolution of MTCF within
the weak coupling limit and it is shown that the multiple-time correlation
function of some observables satisfy the Quantum Regression Theorem (QRT),
whereas other correlations do not. We set the conditions under which the
correlations satisfy the QRT. We illustrate the theory in two different cases;
first, solving an exact model for which the MTCF are explicitly given, and
second, presenting the results of a numerical integration for a system coupled
with a dissipative environment through a non-diagonal interaction.Comment: Submitted (04 Jul 04
Decoherence in a superconducting flux qubit with a pi-junction
We consider the use of a pi-junction for flux qubits to realize degenerate
quantum levels without external magnetic field. On the basis of the
Caldeira-Leggett model, we derive an effective spin-Boson model, and study
decoherece of this type of qubits. We estimate the dephasing time by using
parameters from recent experiments of SIFS junctions, and show that high
critical current and large subgap resistance are required for the pi-junction
to realize a long coherent time.Comment: 5 pages, 2 figure
Dynamics of quantum dissipation systems interacting with bosonic canonical bath: Hierarchical equations of motion approach
A nonperturbative theory is developed, aiming at an exact and efficient
evaluation of a general quantum system interacting with arbitrary bath
environment at any temperature and in the presence of arbitrary time-dependent
external fields. An exact hierarchical equations of motion formalism is
constructed on the basis of calculus-on-path-integral algorithm, via the
auxiliary influence generating functionals related to the interaction bath
correlation functions in a parametrization expansion form. The corresponding
continued-fraction Green's functions formalism for quantum dissipation is also
presented. Proposed further is the principle of residue correction, not just
for truncating the infinite hierarchy, but also for incorporating the small
residue dissipation that may arise from the practical difference between the
true and the parametrized bath correlation functions. The final
residue-corrected hierarchical equations of motion can therefore be used
practically for the evaluation of arbitrary dissipative quantum systems.Comment: 12 pages, submitted to PR
Circuit theory for decoherence in superconducting charge qubits
Based on a network graph analysis of the underlying circuit, a quantum theory
of arbitrary superconducting charge qubits is derived. Describing the
dissipative elements of the circuit with a Caldeira-Leggett model, we calculate
the decoherence and leakage rates of a charge qubit. The analysis includes
decoherence due to a dissipative circuit element such as a voltage source or
the quasiparticle resistances of the Josephson junctions in the circuit. The
theory presented here is dual to the quantum circuit theory for superconducting
flux qubits. In contrast to spin-boson models, the full Hilbert space structure
of the qubit and its coupling to the dissipative environment is taken into
account. Moreover, both self and mutual inductances of the circuit are fully
included.Comment: 8 pages, 3 figures; v2: published version; typo in Eq.(30) corrected,
minor changes, reference adde
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