1,938 research outputs found
Frequency dependent heat capacity within a kinetic model of glassy dynamics
There has been renewed interest in the frequency dependent specific heat of
supercooled liquids in recent years with computer simulation studies exploring
the whole frequency range of relaxation. The simulation studies can thus
supplement the existing experimental results to provide an insight into the
energy landscape dynamics. We here investigate a kinetic model of cooperative
dynamics within the landscape paradigm for the dynamic heat capacity behavior.
In this picture, the beta-process is modeled as a thermally activated event in
a two-level system and the alpha-process is described as a beta-relaxation
mediated cooperative transition in a double well. The model provides a
description of the activated hopping in the energy landscape in close relation
with the cooperative nature of the hopping event. For suitable choice of
parameters, the model predicts a frequency dependent heat capacity that
reflects the two-step relaxation behavior. Although experimentally obtained
specific heat spectra of supercooled liquids till date could not capture the
two-step relaxation behavior, this has been observed in a computer simulation
study by Scheidler et. al. [Phys. Rev. B 63, 104204 (2001)]. The temperature
dependence of the position of the low-frequency peak, due to the
alpha-relaxation, shows a non-Arrhenius behavior as observed experimentally by
Birge and Nagel [Phys. Rev. Lett. 54, 2674 (1985)]. The shape of the alpha-peak
is, however, found to be temperature independent, in agreement with the
simulation result. The high-frequency peak appears with considerably larger
amplitude than the alpha-peak. We attempt a plausible reason for this
observation that is in contrast with the general feature revealed by the
dielectric spectroscopy.Comment: 10 pages, 10 figure
Proteinopathy, oxidative stress and mitochondrial dysfunction: cross talk in alzheimer’s disease and parkinson’s disease
Alzheimer's disease and Parkinson's disease are two common neurodegenerative diseases of the elderly people that have devastating effects in terms of morbidity and mortality. The predominant form of the disease in either case is sporadic with uncertain etiology. The clinical features of Parkinson's disease are primarily motor deficits, while the patients of Alzheimer's disease present with dementia and cognitive impairment. Though neuronal death is a common element in both the disorders, the postmortem histopathology of the brain is very characteristic in each case and different from each other. In terms of molecular pathogenesis, however, both the diseases have a significant commonality, and proteinopathy (abnormal accumulation of misfolded proteins), mitochondrial dysfunction and oxidative stress are the cardinal features in either case. These three damage mechanisms work in concert, reinforcing each other to drive the pathology in the aging brain for both the diseases; very interestingly, the nature of interactions among these three damage mechanisms is very similar in both the diseases, and this review attempts to highlight these aspects. In the case of Alzheimer's disease, the peptide amyloid beta (A beta) is responsible for the proteinopathy, while alpha-synuclein plays a similar role in Parkinson's disease. The expression levels of these two proteins and their aggregation processes are modulated by reactive oxygen radicals and transition metal ions in a similar manner. In turn, these proteins - as oligomers or in aggregated forms - cause mitochondrial impairment by apparently following similar mechanisms. Understanding the common nature of these interactions may, therefore, help us to identify putative neuroprotective strategies that would be beneficial in both the clinical conditions
Class Transitions and Two Component Accretion Flow in GRS 1915+105
The light curve of the galactic micro-quasar GRS 1915+105 changes in at least
thirteen different ways which are called classes. We present examples of the
transitions from one class to another as observed by the IXAE instrument aboard
the Indian Satellite IRS-P3. We find that the transitions are associated with
changes in photon counts over a time-scale of only a few hours and they take
place through unknown classes. Assuming that the transitions are caused by
variation of the accretion rates, this implies that a significant fraction of
the matter must be nearly freely falling in order to have such dramatic changes
in such a short time.Comment: 9 pages, 6 figures, Astronomy and Astrophys. (in press
Phases, many-body entropy measures and coherence of interacting bosons in optical lattices
Already a few bosons with contact interparticle interactions in small optical
lattices feature a variety of quantum phases: superfluid, Mott-insulator and
fermionized Tonks gases can be probed in such systems. To detect these phases
-- pivotal for both experiment and theory -- as well as their many-body
properties we analyze several distinct measures for the one-body and many-body
Shannon information entropies. We exemplify the connection of these entropies
with spatial correlations in the many-body state by contrasting them to the
Glauber normalized correlation functions. To obtain the ground-state for
lattices with commensurate filling (i.e. an integer number of particles per
site) for the full range of repulsive interparticle interactions we utilize the
multiconfigurational time-dependent Hartree method for bosons (MCTDHB) in order
to solve the many-boson Schr\"odinger equation. We demonstrate that all
emergent phases -- the superfluid, the Mott insulator, and the fermionized gas
can be characterized equivalently by our many-body entropy measures and by
Glauber's normalized correlation functions. In contrast to our many-body
entropy measures, single-particle entropy cannot capture these transitions.Comment: 11 pages, 7 figures, software available at http://ultracold.or
Probing geomagnetic storm-driven magnetosphere-ionosphere dynamics in D-region via propagation characteristics of very low frequency radio signals
The amplitude and phase of VLF/LF radio signals are sensitive to changes in electrical conductivity of the lower ionosphere which imprints its signature on the Earth-ionosphere waveguide. This characteristics makes it useful in studying sudden ionospheric disturbances, especially those related to prompt X-ray flux output from solar flares and gamma ray bursts (GRBs). However, strong geomagnetic disturbance and storm conditions are known to produce large and global ionospheric disturbances, which can significantly affect VLF radio propagation in the D region of the ionosphere. In this paper, using the data of three propagation paths at mid-latitudes (40° – 54°), we analyze the trend of aspects of VLF diurnal signal under varying solar and geomagnetic space environmental conditions in order to identify possible geomagnetic footprints on the D region characteristics. We found that the trend of variations generally reflect the prevailing space weather conditions in various time scales. In particular, the ‘dipping’ of mid-day signal amplitude (MDP) of VLF always occurs after geomagnetic perturbed or storm conditions in the time scale of 1–2 days. The mean signal before sunrise (MBSR) and mean signal after sunset (MASS) also exhibit storm-induced dipping, but they appear to be influenced by event's exact occurrence time and highly variable conditions of dusk-to-dawn ionosphere. We observed fewer cases of the signals rise (e.g., MDP, MBSR or MASS) following a significant geomagnetic event, though this effect may be related to storms associated phenomena or effects arising from sources other than solar origin. The magnitude of induced dipping (or rise) significantly depends on the intensity and duration of event(s), as well as the propagation path of the signal. The post-storm day signal (following a main event, with lesser or significantly reduced geomagnetic activity), exhibited a tendency of recovery to pre-storm day level. In the present analysis, We do not see a well defined trend of the variations of the post-storm sunrise terminator (SRT) and sunset terminator (SST). The SRT and SST signals show more post-storm dipping in GQD-A118 propagation path but generally an increase along DHO-A118 propagation path. Thus the result could be propagation path dependent and detailed modeling is required to understand these phenomena
Viscosity in spherically symmetric accretion
The influence of viscosity on the flow behaviour in spherically symmetric
accretion, has been studied here. The governing equation chosen has been the
Navier-Stokes equation. It has been found that at least for the transonic
solution, viscosity acts as a mechanism that detracts from the effectiveness of
gravity. This has been conjectured to set up a limiting scale of length for
gravity to bring about accretion, and the physical interpretation of such a
length-scale has been compared with the conventional understanding of the
so-called "accretion radius" for spherically symmetric accretion. For a
perturbative presence of viscosity, it has also been pointed out that the
critical points for inflows and outflows are not identical, which is a
consequence of the fact that under the Navier-Stokes prescription, there is a
breakdown of the invariance of the stationary inflow and outflow solutions --
an invariance that holds good under inviscid conditions. For inflows, the
critical point gets shifted deeper within the gravitational potential well.
Finally, a linear stability analysis of the stationary inflow solutions, under
the influence of a perturbation that is in the nature of a standing wave, has
indicated that the presence of viscosity induces greater stability in the
system, than has been seen for the case of inviscid spherically symmetric
inflows.Comment: 7 pages. Minor changes made in the version published in MNRA
Temperature-controlled interlayer exchange coupling in strong/weak ferromagnetic multilayers: a thermo-magnetic Curie-switch
We investigate a novel type of interlayer exchange coupling based on driving
a strong/weak/strong ferromagnetic tri-layer through the Curie point of the
weakly ferromagnetic spacer, with the exchange coupling between the strongly
ferromagnetic outer layers that can be switched, on and off, or varied
continuously in magnitude by controlling the temperature of the material. We
use Ni-Cu alloy of varied composition as the spacer material and model the
effects of proximity-induced magnetism and the interlayer exchange coupling
through the spacer from first principles, taking into account not only thermal
spin-disorder but also the dependence of the atomic moment of Ni on the
nearest-neighbor concentration of the non-magnetic Cu. We propose and
demonstrate a gradient-composition spacer, with a lower Ni-concentration at the
interfaces, for greatly improved effective-exchange uniformity and
significantly improved thermo-magnetic switching in the structure. The reported
magnetic multilayer materials can form the base for a variety of novel magnetic
devices, such as sensors, oscillators, and memory elements based on
thermo-magnetic Curie-switching in the device.Comment: 15 pages, 5 figure
Defect generation in a spin-1/2 transverse XY chain under repeated quenching of the transverse field
We study the quenching dynamics of a one-dimensional spin-1/2 model in a
transverse field when the transverse field is quenched repeatedly
between and . A single passage from to or the other way around is referred to as a half-period of
quenching. For an even number of half-periods, the transverse field is brought
back to the initial value of ; in the case of an odd number of
half-periods, the dynamics is stopped at . The density of
defects produced due to the non-adiabatic transitions is calculated by mapping
the many-particle system to an equivalent Landau-Zener problem and is generally
found to vary as for large ; however, the magnitude is
found to depend on the number of half-periods of quenching. For two successive
half-periods, the defect density is found to decrease in comparison to a single
half-period, suggesting the existence of a corrective mechanism in the reverse
path. A similar behavior of the density of defects and the local entropy is
observed for repeated quenching. The defect density decays as
for large for any number of half-periods, and shows a increase in kink
density for small for an even number; the entropy shows qualitatively
the same behavior for any number of half-periods. The probability of
non-adiabatic transitions and the local entropy saturate to 1/2 and ,
respectively, for a large number of repeated quenching.Comment: 5 pages, 3 figure
Adiabatic multicritical quantum quenches: Continuously varying exponents depending on the direction of quenching
We study adiabatic quantum quenches across a quantum multicritical point
(MCP) using a quenching scheme that enables the system to hit the MCP along
different paths. We show that the power-law scaling of the defect density with
the rate of driving depends non-trivially on the path, i.e., the exponent
varies continuously with the parameter that defines the path, up to a
critical value ; on the other hand for , the scaling exponent saturates to a constant value. We show that
dynamically generated and {\it path()-dependent} effective critical
exponents associated with the quasicritical points lying close to the MCP (on
the ferromagnetic side), where the energy-gap is minimum, lead to this
continuously varying exponent. The scaling relations are established using the
integrable transverse XY spin chain and generalized to a MCP associated with a
-dimensional quantum many-body systems (not reducible to two-level systems)
using adiabatic perturbation theory. We also calculate the effective {\it
path-dependent} dimensional shift (or the shift in center of the
impulse region) that appears in the scaling relation for special paths lying
entirely in the paramagnetic phase. Numerically obtained results are in good
agreement with analytical predictions.Comment: 5 pages, 4 figure
Accretion of Chaplygin gas upon black holes: Formation of faster outflowing winds
We study the accretion of modified Chaplygin gas upon different types of
black hole. Modified Chaplygin gas is one of the best candidates for a combined
model of dark matter and dark energy. In addition, from a field theoretical
point of view the modified Chaplygin gas model is equivalent to that of a
scalar field having a self-interacting potential. We formulate the equations
related to both spherical accretion and disc accretion, and respective winds.
The corresponding numerical solutions of the flow, particularly of velocity,
are presented and are analyzed. We show that the accretion-wind system of
modified Chaplygin gas dramatically alters the wind solutions, producing faster
winds, upon changes in physical parameters, while accretion solutions
qualitatively remain unaffected. This implies that modified Chaplygin gas is
more prone to produce outflow which is the natural consequence of the dark
energy into the system.Comment: 21 pages including 7 figures; published in Classical and Quantum
Gravit
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