598 research outputs found
Trotter Derivation of Algorithms for Brownian and Dissipative Particle Dynamics
This paper focuses on the temporal discretization of the Langevin dynamics,
and on different resulting numerical integration schemes. Using a method based
on the exponentiation of time dependent operators, we carefully derive a
numerical scheme for the Langevin dynamics, that we found equivalent to the
proposal of Ermak, and not simply to the stochastic version of the
velocity-Verlet algorithm. However, we checked on numerical simulations that
both algorithms give similar results, and share the same ``weak order two''
accuracy. We then apply the same strategy to derive and test two numerical
schemes for the dissipative particle dynamics (DPD). The first one of them was
found to compare well, in terms of speed and accuracy, with the best currently
available algorithms.Comment: to be published in J.Chem.Phy
Injected Power Fluctuations in 1D Dissipative Systems
Using fermionic techniques, we compute exactly the large deviation function
(ldf) of the time-integrated injected power in several one-dimensional
dissipative systems of classical spins. The dynamics are T=0 Glauber dynamics
supplemented by an injection mechanism, which is taken as a Poissonian flipping
of one particular spin. We discuss the physical content of the results,
specifically the influence of the rate of the Poisson process on the properties
of the ldf.Comment: 18 pages, 8 figure
Entropic Elasticity of Phantom Percolation Networks
A new method is used to measure the stress and elastic constants of purely
entropic phantom networks, in which a fraction of neighbors are tethered by
inextensible bonds. We find that close to the percolation threshold the
shear modulus behaves as , where the exponent in two
dimensions, and in three dimensions, close to the corresponding
values of the conductivity exponent in random resistor networks. The components
of the stiffness tensor (elastic constants) of the spanning cluster follow a
power law , with an exponent and 2.6 in two and
three dimensions, respectively.Comment: submitted to the Europhys. Lett., 7 pages, 5 figure
Magnetic Fluctuations and Correlations in MnSi - Evidence for a Skyrmion Spin Liquid Phase
We present a comprehensive analysis of high resolution neutron scattering
data involving Neutron Spin Echo spectroscopy and Spherical Polarimetry which
confirm the first order nature of the helical transition and reveal the
existence of a new spin liquid skyrmion phase. Similar to the blue phases of
liquid crystals this phase appears in a very narrow temperature range between
the low temperature helical and the high temperature paramagnetic phases.Comment: 11 pages, 16 figure
The Effect of Thermal Fluctuations on Schulman Area Elasticity
We study the elastic properties of a two-dimensional fluctuating surface
whose area density is allowed to deviate from its optimal (Schulman) value. The
behavior of such a surface is determined by an interplay between the
area-dependent elastic energy, the curvature elasticity, and the entropy. We
identify three different elastic regimes depending on the ratio
between the projected (frame) and the saturated areas. We show that thermal
fluctuations modify the elastic energy of stretched surfaces (),
and dominate the elastic energy of compressed surfaces (). When
the elastic energy is not much affected by the fluctuations; the
frame area at which the surface tension vanishes becomes smaller than and
the area elasticity modulus increases.Comment: 12 pages, to appear in Euro. Phys. J.
VIS/NIR spectra and color parameters according to leaf age of some Eucalyptus species: influence on their classification and discrimination
Aim of study: The aim of this study was to verify the differences in VIS/NIR spectra and leaf color parameters of leaves of Eucalyptus badjensis, E. benthamii, E. dunnii, E. grandis, E. globulus and E. saligna, at four ages, and their influence on species discrimination. Area of study: São Mateus do Sul, Paraná, Brazil. Materials and methods: Seedlings of the six species, with four replicates for each, were acclimatized in the same environment, in Oc-tober 2015, in an entirely randomized design. Leaf samples were collected from plants that were 6, 8, 10 and 12 months old. Three leaves from each of four plants at each age were analyzed. Five parameters were recorded referring to the adaxial surface of each leaf, with a total of 15 records from repetitions and 60 per species at each age. The evaluation was performed in the spectral ranges from 360-740 nm (VIS) and 1000-2500 nm (NIR). Principal component analysis and linear discriminant analysis were performed. Main results: The influence of age differed within each species. In color data, the parameter with most variation among all samples was chromatic coordinate b*. In reflectance spectra (VIS), age of 12 months provided the best discrimination of species. Second derivative NIR spectra produced the best results of external prediction of Linear Discriminant Analysis models based on leaves of 12-month-old trees. Research highlights: Observation of color parameters and VIS/NIR spectroscopy have potential utility for discrimination of Eucalyptus species based on their green leaves
Quantum beats in the electric-field quenching of metastable hydrogen
The strong field-induced quantum beats observed in beam-foil studies of Ly- alpha radiation are obtained in a conventional metastable-hydrogen quenching experiment. The phase relation between the Stark shifted 2s 1/2- 2p 1/2 Lamb-shift oscillations and the much more rapid 2s 1-2p 3/2 fine-structure oscillations depends on the detailed way in which the quenching field is switched on. Apart from a phaseshift, the results agree with a non-perturbative theoretical calculation which assumes that the field is applied suddenly. Various frequency components of the time-dependent radiation intensity are identified with specific hyperfine transitions or groups of transitions. No adjustable parameters are used for the initial state amplitudes
A Chiral Paramagnetic Skyrmion-like Phase in MnSi
We present a comprehensive study of chiral fluctuations in the reference
helimagnet MnSi by polarized neutron scattering and Neutron Spin Echo
spectroscopy, which reveals the existence of a completely left-handed and
dynamically disordered phase. This phase may be identified as a spontaneous
skyrmion phase: it appears in a limited temperature range just above the
helical transition Tc and coexists with the helical phase at Tc.Comment: PRL accepte
Microscopic formulation of the Zimm-Bragg model for the helix-coil transition
A microscopic spin model is proposed for the phenomenological Zimm-Bragg
model for the helix-coil transition in biopolymers. This model is shown to
provide the same thermophysical properties of the original Zimm-Bragg model and
it allows a very convenient framework to compute statistical quantities.
Physical origins of this spin model are made transparent by an exact mapping
into a one-dimensional Ising model with an external field. However, the
dependence on temperature of the reduced external field turns out to differ
from the standard one-dimensional Ising model and hence it gives rise to
different thermophysical properties, despite the exact mapping connecting them.
We discuss how this point has been frequently overlooked in the recent
literature.Comment: 11 pages, 2 figure
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