22,002 research outputs found
The equilibrium states of open quantum systems in the strong coupling regime
In this work we investigate the late-time stationary states of open quantum
systems coupled to a thermal reservoir in the strong coupling regime. In
general such systems do not necessarily relax to a Boltzmann distribution if
the coupling to the thermal reservoir is non-vanishing or equivalently if the
relaxation timescales are finite. Using a variety of non-equilibrium formalisms
valid for non-Markovian processes, we show that starting from a product state
of the closed system = system + environment, with the environment in its
thermal state, the open system which results from coarse graining the
environment will evolve towards an equilibrium state at late-times. This state
can be expressed as the reduced state of the closed system thermal state at the
temperature of the environment. For a linear (harmonic) system and environment,
which is exactly solvable, we are able to show in a rigorous way that all
multi-time correlations of the open system evolve towards those of the closed
system thermal state. Multi-time correlations are especially relevant in the
non-Markovian regime, since they cannot be generated by the dynamics of the
single-time correlations. For more general systems, which cannot be exactly
solved, we are able to provide a general proof that all single-time
correlations of the open system evolve to those of the closed system thermal
state, to first order in the relaxation rates. For the special case of a
zero-temperature reservoir, we are able to explicitly construct the reduced
closed system thermal state in terms of the environmental correlations.Comment: 20 pages, 2 figure
A computational study of stimulus driven epileptic seizure abatement
This is the final version of the article. Available from Public Library of Science via the DOI in this record.Active brain stimulation to abate epileptic seizures has shown mixed success. In spike-wave (SW) seizures, where the seizure and background state were proposed to coexist, single-pulse stimulations have been suggested to be able to terminate the seizure prematurely. However, several factors can impact success in such a bistable setting. The factors contributing to this have not been fully investigated on a theoretical and mechanistic basis. Our aim is to elucidate mechanisms that influence the success of single-pulse stimulation in noise-induced SW seizures. In this work, we study a neural population model of SW seizures that allows the reconstruction of the basin of attraction of the background activity as a four dimensional geometric object. For the deterministic (noise-free) case, we show how the success of response to stimuli depends on the amplitude and phase of the SW cycle, in addition to the direction of the stimulus in state space. In the case of spontaneous noise-induced seizures, the basin becomes probabilistic introducing some degree of uncertainty to the stimulation outcome while maintaining qualitative features of the noise-free case. Additionally, due to the different time scales involved in SW generation, there is substantial variation between SW cycles, implying that there may not be a fixed set of optimal stimulation parameters for SW seizures. In contrast, the model suggests an adaptive approach to find optimal stimulation parameters patient-specifically, based on real-time estimation of the position in state space. We discuss how the modelling work can be exploited to rationally design a successful stimulation protocol for the abatement of SW seizures using real-time SW detection.This work was supported by the EPSRC (EP/K026992/1), the BBSRC, the DTC for Systems Biology (University of Manchester), and the Nanyang Technological University Singapore. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
Magnetoresistance, Micromagnetism, and Domain Wall Scattering in Epitaxial hcp Co Films
Large negative magnetoresistance (MR) observed in transport measurements of
hcp Co films with stripe domains were recently reported and interpreted in
terms of a novel domain wall (DW) scattering mechanism. Here detailed MR
measurements, magnetic force microscopy, and micromagnetic calculations are
combined to elucidate the origin of MR in this material. The large negative
room temperature MR reported previously is shown to be due to ferromagnetic
resistivity anisotropy. Measurements of the resistivity for currents parallel
(CIW) and perpendicular to DWs (CPW) have been conducted as a function of
temperature. Low temperature results show that any intrinsic effect of DWs
scattering on MR of this material is very small compared to the anisotropic MR.Comment: 5 pages, 5 Figures, submitted to PR
Dynamics of D1-brane in I-brane Background
This paper is devoted to the study of the effective field theory description
of the probe D1-brane in the background of the system of two stacks of
fivebranes in type IIB theory that intersect on . We study the
properties of the Dirac-Born-Infeld action for D1-brane moving in this
background. We will argue that this action is invariant under an additional
symmetry in the near horizon limit and that this new symmetry is closely
related to the enhanced symmetry of the I-brane background considered recently
in [hep-th/0508025]. We also solve explicitly the equation of motion of
D1-brane in the near horizon limit.Comment: 21 pages, references adde
Time-dependent density-functional theory approach to nonlinear particle-solid interactions in comparison with scattering theory
An explicit expression for the quadratic density-response function of a
many-electron system is obtained in the framework of the time-dependent
density-functional theory, in terms of the linear and quadratic
density-response functions of noninteracting Kohn-Sham electrons and functional
derivatives of the time-dependent exchange-correlation potential. This is used
to evaluate the quadratic stopping power of a homogeneous electron gas for slow
ions, which is demonstrated to be equivalent to that obtained up to second
order in the ion charge in the framework of a fully nonlinear scattering
approach. Numerical calculations are reported, thereby exploring the range of
validity of quadratic-response theory.Comment: 14 pages, 3 figures. To appear in Journal of Physics: Condensed
Matte
Degenerate Dirac Neutrinos
A simple extension of the standard model is proposed in which all the three
generations of neutrinos are Dirac particles and are naturally light. We then
assume that the neutrino mass matrix is diagonal and degenerate, with a few eV
mass to solve the dark matter problem. The self energy radiative corrections,
however, remove this degeneracy and allow mixing of these neutrinos. The
electroweak radiative corrections then predict a lower bound on the mass difference which solves the solar neutrino problem through MSW
mechanism and also predict a lower bound on the mass
difference which is just enough to explain the atmospheric neutrino problem as
reported by super Kamiokande.Comment: 11 pages latex fil
Colour-electric spectral function at next-to-leading order
The spectral function related to the correlator of two colour-electric fields
along a Polyakov loop determines the momentum diffusion coefficient of a heavy
quark near rest with respect to a heat bath. We compute this spectral function
at next-to-leading order, O(alpha_s^2), in the weak-coupling expansion. The
high-frequency part of our result (omega >> T), which is shown to be
temperature-independent, is accurately determined thanks to asymptotic freedom;
the low-frequency part of our result (omega << T), in which Hard Thermal Loop
resummation is needed in order to cure infrared divergences, agrees with a
previously determined expression. Our result may help to calibrate the overall
normalization of a lattice-extracted spectral function in a perturbative
frequency domain T << omega << 1/a, paving the way for a non-perturbative
estimate of the momentum diffusion coefficient at omega -> 0. We also evaluate
the colour-electric Euclidean correlator, which could be directly compared with
lattice simulations. As an aside we determine the Euclidean correlator in the
lattice strong-coupling expansion, showing that through a limiting procedure it
can in principle be defined also in the confined phase of pure Yang-Mills
theory, even if a practical measurement could be very noisy there.Comment: 38 page
Generalized Massive Gravity and Galilean Conformal Algebra in two dimensions
Galilean conformal algebra (GCA) in two dimensions arises as contraction of
two copies of the centrally extended Virasoro algebra ( with ). The central charges of
GCA can be expressed in term of Virasoro central charges. For finite and
non-zero GCA central charges, the Virasoro central charges must behave as
asymmetric form . We propose that, the bulk
description for 2d GCA with asymmetric central charges is given by general
massive gravity (GMG) in three dimensions. It can be seen that, if the
gravitational Chern-Simons coupling behaves as of order
O() or (), the central charges
of GMG have the above dependence. So, in non-relativistic scaling
limit , we calculated GCA parameters and finite
entropy in term of gravity parameters mass and angular momentum of GMG.Comment: 9 page
Time-Dependent Partition-Free Approach in Resonant Tunneling Systems
An extended Keldysh formalism, well suited to properly take into account the
initial correlations, is used in order to deal with the time-dependent current
response of a resonant tunneling system. We use a \textit{partition-free}
approach by Cini in which the whole system is in equilibrium before an external
bias is switched on. No fictitious partitions are used. Besides the
steady-state responses one can also calculate physical dynamical responses. In
the noninteracting case we clarify under what circumstances a steady-state
current develops and compare our result with the one obtained in the
partitioned scheme. We prove a Theorem of asymptotic Equivalence between the
two schemes for arbitrary time-dependent disturbances. We also show that the
steady-state current is independent of the history of the external perturbation
(Memory Loss Theorem). In the so called wide-band limit an analytic result for
the time-dependent current is obtained. In the interacting case we propose an
exact non-equilibrium Green function approach based on Time Dependent Density
Functional Theory. The equations are no more difficult than an ordinary Mean
Field treatment. We show how the scattering-state scheme by Lang follows from
our formulation. An exact formula for the steady-state current of an arbitrary
interacting resonant tunneling system is obtained. As an example the
time-dependent current response is calculated in the Random Phase
Approximation.Comment: final version, 18 pages, 9 figure
Effect of a Domain Wall on the Conductance Quantization in a Ferromagnetic Nanowire
The effect of the domain wall (DW) on the conductance in a ballistic
ferromagnetic nanowire (FMNW) is revisited by exploiting a specific
perturbation theory which is effective for a thin DW; the thinness is often the
case in currently interested conductance measurements on FMNWs. Including the
Hund coupling between carrier spins and local spins in a DW, the conductance of
a FMNW in the presence of a very thin DW is calculated within the
Landauer-B\"{u}ttiker formalism. It is revealed that the conductance plateaus
are modified significantly, and the switching of the quantization unit from
to ``about '' is produced in a FMNW by the introduction of a
thin DW. This accounts well for recent observations in a FMNW.Comment: 5 pages, 2 figures, Corrected typos and added reference
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