2,029 research outputs found
Local Lagrangian Formalism and Discretization of the Heisenberg Magnet Model
In this paper we develop the Lagrangian and multisymplectic structures of the
Heisenberg magnet (HM) model which are then used as the basis for geometric
discretizations of HM. Despite a topological obstruction to the existence of a
global Lagrangian density, a local variational formulation allows one to derive
local conservation laws using a version of N\"other's theorem from the formal
variational calculus of Gelfand-Dikii. Using the local Lagrangian form we
extend the method of Marsden, Patrick and Schkoller to derive local
multisymplectic discretizations directly from the variational principle. We
employ a version of the finite element method to discretize the space of
sections of the trivial magnetic spin bundle over an
appropriate space-time . Since sections do not form a vector space, the
usual FEM bases can be used only locally with coordinate transformations
intervening on element boundaries, and conservation properties are guaranteed
only within an element. We discuss possible ways of circumventing this problem,
including the use of a local version of the method of characteristics,
non-polynomial FEM bases and Lie-group discretization methods.Comment: 12 pages, accepted Math. and Comp. Simul., May 200
The Introduction of Yoga Recovery on Physiological and Psychological Stress and Performance in NCAA Athletes
NCAA student athletes face unique physiological and psychological stressors daily, which may contribute to overtraining, burnout, and other physical and mental health issues. However, NCAA institutions often leave recovery up to the individual athlete due to time restriction and Countable Athletic Related Activities (CARA) hour limitations on team mandated activities. Attention to methods to promote recovery from these training loads is increasing in NCAA institutions. A mind-body activity such as yoga is proposed to have physiological and psychological benefits for student athletes. The goal of this review is to identify which aspects of yoga promote the most effective recovery in measures such as: performance, physical biomarkers of stress, muscle damage, heart rate variability, sleep quality, mood state, anxiety, and depression. A recovery yoga protocol is presented, based on the current literature on the topic, to suggest a time- and cost-effective mind-body recovery modality for NCAA student athletes. The recommendation of this review concludes that, among other approaches, NCAA athletes are ideal candidates to undertake the practice of a yoga-based relaxation technique. As such, initial incorporation of yoga is recommended for 20 minutes twice weekly, with a third 60- minute practice each week for a more integrated mind-body yoga experience. From the literature, there is a scientific rationale to understand and anticipate physiological effects such as decreased muscle soreness, heart rate variability, and oxidative stress and psychological effects of improved mood, decreased anxiety, and depression in terms of yoga efficacy. Based on this rationale, introduction of yoga-based recovery is likely to improve some aspects of academic and athletic performance, as well as overall greater wellbeing upon completion of the recommended intervention. However, well-founded conclusions are tentative because explicit mechanistic research is sparse. Accordingly, outcomes based research is needed to confirm the extent to which introduction of these recovery-based approaches will benefit yoga novices
Non-Orthogonal Multiple Access for FSO Backhauling
We consider a free space optical (FSO) backhauling system which consists of
two base stations (BSs) and one central unit (CU). We propose to employ
non-orthogonal multiple access (NOMA) for FSO backhauling where both BSs
transmit at the same time and in the same frequency band to the same
photodetector at the CU. We develop a dynamic NOMA scheme which determines the
optimal decoding order as a function of the channel state information at the CU
and the quality of service requirements of the BSs, such that the outage
probabilities of both BSs are jointly minimized. Moreover, we analyze the
performance of the proposed NOMA scheme in terms of the outage probability over
Gamma-Gamma FSO turbulence channels. We further derive closed-form expressions
for the outage probability for the high signal-to-noise ratio regime. Our
simulation results confirm the analytical derivations and reveal that the
proposed dynamic NOMA scheme significantly outperforms orthogonal transmission
and existing NOMA schemes.Comment: This paper has been submitted to IEEE WCNC 201
Vibrational instability, two-level systems and Boson peak in glasses
We show that the same physical mechanism is fundamental for two seemingly
different phenomena such as the formation of two-level systems in glasses and
the Boson peak in the reduced density of low-frequency vibrational states
g(w)/w^2. This mechanism is the vibrational instability of weakly interacting
harmonic modes. Below some frequency w_c << w_0 (where w_0 is of the order of
Debye frequency) the instability, controlled by the anharmonicity, creates a
new stable universal spectrum of harmonic vibrations with a Boson peak feature
as well as double-well potentials with a wide distribution of barrier heights.
Both are determined by the strength of the interaction I ~ w_c between the
oscillators. Our theory predicts in a natural way a small value for the
important dimensionless parameter C ~ 10^{-4} for two-level systems in glasses.
We show that C ~ I^{-3} and decreases with increasing of the interaction
strength I. We show that the number of active two-level systems is very small,
less than one per ten million of oscillators, in a good agreement with
experiment. Within the unified approach developed in the present paper the
density of the tunneling states and the density of vibrational states at the
Boson peak frequency are interrelated.Comment: 28 pages, 3 figure
Statistical Modeling of FSO Fronthaul Channel for Drone-based Networks
We consider a drone-based communication network, where several drones hover
above an area and serve as mobile remote radio heads for a large number of
mobile users. We assume that the drones employ free space optical (FSO) links
for fronthauling of the users' data to a central unit. The main focus of this
paper is to quantify the geometric loss of the FSO channel arising from random
fluctuation of the position and orientation of the drones. In particular, we
derive upper and lower bounds, corresponding approximate expressions, and a
closed-form statistical model for the geometric loss. Simulation results
validate our derivations and quantify the FSO channel quality as a function of
the drone's instability, i.e., the variation of its position and orientation.Comment: This paper has been submitted to ICC 201
Field-induced structural aging in glasses at ultra low temperatures
In non-equilibrium experiments on the glasses Mylar and BK7, we measured the
excess dielectric response after the temporary application of a strong electric
bias field at mK--temperatures. A model recently developed describes the
observed long time decays qualitatively for Mylar [PRL 90, 105501, S. Ludwig,
P. Nalbach, D. Rosenberg, D. Osheroff], but fails for BK7. In contrast, our
results on both samples can be described by including an additional mechanism
to the mentioned model with temperature independent decay times of the excess
dielectric response. As the origin of this novel process beyond the "tunneling
model" we suggest bias field induced structural rearrangements of "tunneling
states" that decay by quantum mechanical tunneling.Comment: 4 pages, 4 figures, accepted at PRL, corrected typos in version
Pressure dependence of diffusion in simple glasses and supercooled liquids
Using molecular dynamics simulation, we have calculated the pressure
dependence of the diffusion constant in a binary Lennard-Jones Glass. We
observe four temperature regimes. The apparent activation volume drops from
high values in the hot liquid to a plateau value. Near the critical temperature
of the mode coupling theory it rises steeply, but in the glassy state we find
again small values, similar to the ones in the liquid. The peak of the
activation volume at the critical temperature is in agreement with the
prediction of mode coupling theory
Diffusion and jump-length distribution in liquid and amorphous CuZr
Using molecular dynamics simulation, we calculate the distribution of atomic
jum ps in CuZr in the liquid and glassy states. In both states
the distribution of jump lengths can be described by a temperature independent
exponential of the length and an effective activation energy plus a
contribution of elastic displacements at short distances. Upon cooling the
contribution of shorter jumps dominates. No indication of an enhanced
probability to jump over a nearest neighbor distance was found. We find a
smooth transition from flow in the liquid to jumps in the g lass. The
correlation factor of the diffusion constant decreases with decreasing
temperature, causing a drop of diffusion below the Arrhenius value, despite an
apparent Arrhenius law for the jump probability
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