267 research outputs found
General properties of response functions of nonequilibrium steady states
We derive general properties, which hold for both quantum and classical
systems, of response functions of nonequilibrium steady states. We clarify
differences from those of equilibrium states. In particular, sum rules and
asymptotic behaviors are derived, and their implications are discussed. Since
almost no assumptions are made, our results are applicable to diverse physical
systems. We also demonstrate our results by a molecular dynamics simulation of
a many-body interacting system.Comment: After publication of this paper, several typos were found, which have
been fixed in the erratum (J. Phys. Soc. Jpn., 80 (2011) 128001). All the
corrections have been made in this updated arXive version. 13 pages with 3
figure
Sum rule for response function in nonequilibrium Langevin systems
We derive general properties of the linear response functions of
nonequilibrium steady states in Langevin systems. These correspond to extension
of the results which were recently found in Hamiltonian systems [A. Shimizu and
T. Yuge, J. Phys. Soc. Jpn. {\bf 79}, 013002 (2010)]. We discuss one of the
properties, the sum rule for the response function, in particular detail. We
show that the sum rule for the response function of the velocity holds in the
underdamped case, whereas it is violated in the overdamped case. This implies
that the overdamped Langevin models should be used with great care. We also
investigate the relation of the sum rule to an equality on the energy
dissipation in nonequilibrium Langevin systems, which was derived by Harada and
Sasa.Comment: 8 page
Measurement of the Noise Spectrum Using a Multiple-Pulse Sequence
A method is proposed for obtaining the spectrum for noise that causes the
phase decoherence of a qubit directly from experimentally available data. The
method is based on a simple relationship between the spectrum and the coherence
time of the qubit in the presence of a pi-pulse sequence. The relationship is
found to hold for every system of a qubit interacting with the classical-noise,
bosonic, and spin baths.Comment: 8 pages (4 pages + 4 pages Supplemental material), 1 figur
Nonequilibrium Molecular Dynamics Simulation of Interacting Many Electrons Scattered by Lattice Vibrations
We propose a new model suitable for a nonequilibrium molecular dynamics (MD)
simulation of electrical conductors. The model consists of classical electrons
and atoms. The atoms compose a lattice vibration system. The electrons are
scattered by electron-electron and electron-atom interactions. Since the
scattering cross section is physically more important than the functional form
of a scattering potential, we propose to devise the electron-atom interaction
potential in such a way that its scattering cross section agrees with that of
quantum-mechanical one. To demonstrate advantages of the proposed model, we
perform a nonequilibrium MD simulation assuming a doped semiconductor at room
or higher temperature. In the linear response regime, we confirm Ohm's law, the
dispersion relations and the fluctuation-dissipation relation. Furthermore, we
obtain reasonable dependence of the electrical conductivity on temperature,
despite the fact that our model is a classical model.Comment: 21 pages, 11 figure
Indications of Universal Excess Fluctuations in Nonequilibrium Systems
The fluctuation in electric current in nonequilibrium steady states is
investigated by molecular dynamics simulation of macroscopically uniform
conductors. At low frequencies, appropriate decomposition of the spectral
intensity of current into thermal and excess fluctuations provides a simple
picture of excess fluctuations behaving as shot noise. This indicates that the
fluctuation-dissipation relation may be violated in a universal manner by the
appearance of shot noise for a wide range of systems with particle or momentum
transport.Comment: 4 pages, 4 figures; title changed, major revision; to appear in J.
Phys. Soc. Jp
Universal Properties of Nonlinear Response Functions of Nonequilibrium Steady States
We derive universal properties of nonlinear response functions of
nonequilibrium steady states. In particular, sum rules and asymptotic behaviors
are derived. Their consequences are illustrated for nonlinear optical materials
and nonlinear electrical conductors.Comment: 10 pages, 1 figure; added a few sentences and references to explain
detail
Long-Time Behavior of Velocity Autocorrelation Function for Interacting Particles in a Two-Dimensional Disordered System
The long-time behavior of the velocity autocorrelation function (VACF) is
investigated by the molecular dynamics simulation of a two-dimensional system
which has both a many-body interaction and a random potential. With
strengthening the random potential by increasing the density of impurities, a
crossover behavior of the VACF is observed from a positive tail, which is
proportional to t^{-1}, to a negative tail, proportional to -t^{-2}. The latter
tail exists even when the density of particles is the same order as the density
of impurities. The behavior of the VACF in a nonequilibrium steady state is
also studied. In the linear response regime the behavior is similar to that in
the equilibrium state, whereas it changes drastically in the nonlinear response
regime.Comment: 12 pages, 5 figure
Effects of Simulated Microgravity on Embryonic Stem Cells
There have been many studies on the biological effects of simulated microgravity (SMG) on differentiated cells or adult stem cells. However, there has been no systematic study on the effects of SMG on embryonic stem (ES) cells. In this study, we investigated various effects (including cell proliferation, cell cycle distribution, cell differentiation, cell adhesion, apoptosis, genomic integrity and DNA damage repair) of SMG on mouse embryonic stem (mES) cells. Mouse ES cells cultured under SMG condition had a significantly reduced total cell number compared with cells cultured under 1 g gravity (1G) condition. However, there was no significant difference in cell cycle distribution between SMG and 1G culture conditions, indicating that cell proliferation was not impaired significantly by SMG and was not a major factor contributing to the total cell number reduction. In contrast, a lower adhesion rate cultured under SMG condition contributed to the lower cell number in SMG. Our results also revealed that SMG alone could not induce DNA damage in mES cells while it could affect the repair of radiation-induced DNA lesions of mES cells. Taken together, mES cells were sensitive to SMG and the major alterations in cellular events were cell number expansion, adhesion rate decrease, increased apoptosis and delayed DNA repair progression, which are distinct from the responses of other types of cells to SMG
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