645 research outputs found
On the derivation of Fourier's law in stochastic energy exchange systems
We present a detailed derivation of Fourier's law in a class of stochastic
energy exchange systems that naturally characterize two-dimensional mechanical
systems of locally confined particles in interaction. The stochastic systems
consist of an array of energy variables which can be partially exchanged among
nearest neighbours at variable rates. We provide two independent derivations of
the thermal conductivity and prove this quantity is identical to the frequency
of energy exchanges. The first derivation relies on the diffusion of the
Helfand moment, which is determined solely by static averages. The second
approach relies on a gradient expansion of the probability measure around a
non-equilibrium stationary state. The linear part of the heat current is
determined by local thermal equilibrium distributions which solve a
Boltzmann-like equation. A numerical scheme is presented with computations of
the conductivity along our two methods. The results are in excellent agreement
with our theory.Comment: 19 pages, 5 figures, to appear in Journal of Statistical Mechanics
(JSTAT
On the Notion of Pressure in a Canonical Ensemble
Recently a controversy has arisen between J. De Boer and H. S. Green concerning the notion of pressure in a canonical ensemble. According to Green, only classically is the pressure as derived from the partition function equal to that obtained from the virial theorem, while at low temperatures, at which quantum effects become important, there will be considerable deviations between the two. De Boer attempts to prove that the two pressures are actually identical. We have come to the same conclusion, and shall show this in several ways; first, by considering a simple example (Section 2), and then in general using the energy representation (Section 3). We believe that the discrepancy between the two pressures which Green has found is in fact due to improper handling of the effect of the wall of the vessel in which the particles are contained. Finally, we are of the opinion that Green's criticism of De Boer's calculation is not justified, and in the last section arguments are given to show that the traces of all commutators of interest in quantum statistical mechanics are zero.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69356/2/JCPSA6-18-8-1066-1.pd
On the Theory of the Virial Development of the Equation of State of Monoatomic Gases
The problem of the condensation of a gas is intimately related to the asymptotic behavior of the virial coefficients, Bm, as m→∞. The problem of the evaluation of the virial coefficients may be divided into two distinctly different ones. The first of these, which is purely combinatorial in nature and is independent of the intermolecular force law, is that of determining the number of a certain type of connected graphs of l points and k lines which are called ``stars.'' This problem is solved by means of generating functions, with the result that the total number of such stars is asymptotically equal to(12l(l−1)k),for almost all k. Arguments are also presented which indicate that the total number of topologically different stars is1l!(12l(l−1)k).With these results the combinatorial problem is essentially solved.The second problem is that of evaluating certain integrals of functions which depend on the intermolecular potential. This problem is not so near to a solution. For a purely repulsive force, asymptotic expressions are obtained for k=l, and k=l+1. The partial contributions to the virial coefficient in these two cases are:(−1)l⋅53(52π)12(2b)l−1(l−1)l5∕2,and(−1)l2⋅5324π3(2b)l−1,respectively. Results for some simple one‐dimensional rigid lines are also given.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69794/2/JCPSA6-21-11-2056-1.pd
The kinetic method in statistical mechanics
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32617/1/0000760.pd
Successive approximation methods in classical statistical mechanics
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32400/1/0000475.pd
Perturbation theory for a stochastic process with Ornstein-Uhlenbeck noise
The Ornstein-Uhlenbeck process may be used to generate a noise signal with a
finite correlation time. If a one-dimensional stochastic process is driven by
such a noise source, it may be analysed by solving a Fokker-Planck equation in
two dimensions. In the case of motion in the vicinity of an attractive fixed
point, it is shown how the solution of this equation can be developed as a
power series. The coefficients are determined exactly by using algebraic
properties of a system of annihilation and creation operators.Comment: 7 pages, 0 figure
Stationary and Transient Work-Fluctuation Theorems for a Dragged Brownian Particle
Recently Wang et al. carried out a laboratory experiment, where a Brownian
particle was dragged through a fluid by a harmonic force with constant velocity
of its center. This experiment confirmed a theoretically predicted work related
integrated (I) Transient Fluctuation Theorem (ITFT), which gives an expression
for the ratio for the probability to find positive or negative values for the
fluctuations of the total work done on the system in a given time in a
transient state. The corresponding integrated stationary state fluctuation
theorem (ISSFT) was not observed. Using an overdamped Langevin equation and an
arbitrary motion for the center of the harmonic force, all quantities of
interest for these theorems and the corresponding non-integrated ones (TFT and
SSFT, resp.) are theoretically explicitly obtained in this paper. While the
(I)TFT is satisfied for all times, the (I)SSFT only holds asymptotically in
time. Suggestions for further experiments with arbitrary velocity of the
harmonic force and in which also the ISSFT could be observed, are given. In
addition, a non-trivial long-time relation between the ITFT and the ISSFT was
discovered, which could be observed experimentally, especially in the case of a
resonant circular motion of the center of the harmonic force.Comment: 20 pages, 3 figure
Fluid/solid transition in a hard-core system
We prove that a system of particles in the plane, interacting only with a
certain hard-core constraint, undergoes a fluid/solid phase transition
Quantum many-body theory of qubit decoherence in a finite-size spin bath
Decoherence of a center spin or qubit in a spin bath is essentially
determined by the many-body bath evolution. We develop a cluster-correlation
expansion (CCE) theory for the spin bath dynamics relevant to the qubit
decoherence problem. A cluster correlation term is recursively defined as the
evolution of a group of bath spins divided by the cluster correlations of all
the subgroups. The so-defined correlation accounts for the authentic collective
excitations within a given group. The bath propagator is the product of all
possible cluster correlation terms. For a finite-time evolution as in the qubit
decoherence problem, a convergent result can be obtained by truncating the
expansion up to a certain cluster size. The two-spin cluster truncation of the
CCE corresponds to the pair-correlation approximation [PRB 74,195301(2006)]. In
terms of the standard linked cluster expansion, a cluster correlation term is
the infinite summation of all the connected diagrams with all and only the
spins in the group flip-flopped, and thus the expansion is exact whenever
converges. When the individual contribution of each higher-order correlation
term to the decoherence is small, as the usual case for relatively large baths
where the decoherence could complete well within the bath spin flip-flop time,
the CCE coincides with the cluster expansion [PRB 74,035322(2006)]. For small
baths, however, the qubit decoherence may not complete within the bath spin
flip-flop timescale and thus individual higher-order cluster correlations could
grow significant. In such cases, only the CCE converges to the exact coherent
dynamics of multi-spin clusters. We check the accuracy of the CCE in an exactly
solvable spin-chain model.Comment: 13 pages, 13 figure
Adventures of the Coupled Yang-Mills Oscillators: I. Semiclassical Expansion
We study the quantum mechanical motion in the potentials with
, which arise in the spatially homogeneous limit of the Yang-Mills (YM)
equations. These systems show strong stochasticity in the classical limit
() and exhibit a quantum mechanical confinement feature. We
calculate the partition function going beyond the Thomas-Fermi (TF)
approximation by means of the semiclassical expansion using the Wigner-Kirkwood
(WK) method. We derive a novel compact form of the differential equation for
the WK function. After separating the motion in the channels of the
equipotential surface from the motion in the central region, we show that the
leading higher-order corrections to the TF term vanish up to eighth order in
, if we treat the quantum motion in the hyperbolic channels correctly by
adiabatic separation of the degrees of freedom. Finally, we obtain an
asymptotic expansion of the partition function in terms of the parameter
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