62 research outputs found
Quantum Statistical Mechanics in Classical Phase Space. III. Mean Field Approximation Benchmarked for Interacting Lennard-Jones Particles
A Monte Carlo computer simulation algorithm in classical phase space is given
for the treatment of quantum systems. The non-commutativity of position and
momentum is accounted for by a mean field approach and instantaneous effective
harmonic oscillators. Wave function symmetrization is included at the dimer and
double dimer level. Quantitative tests are performed against benchmarks given
by Hernando and Van\'i\v{c}ek (2013) for spinless neon--parahydrogen, modeled
as interacting Lennard-Jones particles in a one dimensional harmonic trap. The
mean field approach is shown to be quantitatively accurate for high to moderate
temperatures , and moderate densities,
. Results for helium show that at the lowest temperature
studied, the average energy is about 4\% lower for bosons than for fermions. It
is argued that the mean field algorithm will perform better in three dimensions
than in one, and that it will scale sub-linearly with system size.Comment: 9 pages, 7 figures, 23 equations, 18 reference
Expansion for Quantum Statistical Mechanics Based on Wave Function Symmetrization
An expansion for quantum statistical mechanics is derived that gives
classical statistical mechanics as the leading term. Each quantum correction
comes from successively larger permutation loops, which arise from the
factorization of the symmetrization of the wave function with respect to
localized particle interchange. Explicit application of the theory yields the
full fugacity expansion for the quantum ideal gas, and the second fugacity
coefficient for interacting quantum particles, which agree with known results.
Compared to the Lee-Yang virial cluster expansion, the present expansion is
expected to be more rapidly converging and the individual terms appear to be
simpler to evaluate. The results obtained in this paper are intended for
practical computer simulation algorithms for terrestrial condensed matter
quantum systems.Comment: Quantum Physics. 40 pages. Version 2 clarifies Sec. IIB and App.
Quantum Monte Carlo in Classical Phase Space. Mean Field and Exact Results for a One Dimensional Harmonic Crystal
Monte Carlo simulations are performed in classical phase space for a
one-dimensional quantum harmonic crystal. Symmetrization effects for spinless
bosons and fermions are quantified. The algorithm is tested for a range of
parameters against exact results that use 20,000 energy levels. It is shown
that the singlet mean field approximation is very accurate at high
temperatures, and that the pair mean field approximation gives a systematic
improvement in the intermediate and low temperature regime. The latter is
derived from a cluster mean field approximation that accounts for the
non-commutativity of position and momentum, and that can be applied in three
dimensions.Comment: 10 pages, 5 figs, 4 sections. Total 1
Quantum Statistical Mechanics. I. Decoherence, Wave Function Collapse, and the von Neumann Density Matrix
The probability operator is derived from first principles for an equilibrium
quantum system. It is also shown that the superposition states collapse into a
mixture of states giving the conventional von Neumann trace form for the
quantum average. The mechanism for the collapse is found to be quite general:
it results from the conservation law for a conserved, exchangeable variable
(such as energy) and the entanglement of the total system wave function that
necessarily follows. The relevance of the present results to the einselection
mechanism for decoherence, to the quantum measurement problem, and to the
classical nature of the macroscopic world are discussed.Comment: 12 page
Quantum Statistical Mechanics. III. Equilibrium Probability
Given are a first principles derivation and formulation of the probabilistic
concepts that underly equilibrium quantum statistical mechanics. The transition
to non-equilibrium probability is traversed briefly.Comment: 22 page
Quantum Statistical Mechanics in Classical Phase Space. Expressions for the Multi-Particle Density, the Average Energy, and the Virial Pressure
Quantum statistical mechanics is formulated as an integral over classical
phase space. Some details of the commutation function for averages are
discussed, as is the factorization of the symmetrization function used for the
grand potential and for the multi-particle density. Three binary choices (eight
routes) for the average energy are shown to be mutually consistent. An
expression for the phase space function that gives the average virial pressure
is derived.Comment: 10 page
How to Measure Forces when the Atomic Force Microscope shows Non-Linear Compliance
A spreadsheet algorithm is given for the atomic force microscope that
accounts for non-linear behavior in the deflection of the cantilever and in the
photo-diode response. In addition, the data analysis algorithm takes into
account cantilever tilt, friction in contact, and base-line artifacts such as
drift, virtual deflection, and non-zero force. These are important for accurate
force measurement and also for calibration of the cantilever spring constant.
The zero of separation is determined automatically, avoiding human intervention
or bias. The method is illustrated by analyzing measured data for the
silica-silica drainage force and slip length.Comment: 20 pages, 11 figure
Quantum Statistical Mechanics in Classical Phase Space. V. Quantum Local, Average Global
One-particle energy eigenfunctions are used to obtain quantum averages in
many particle systems. These are based on the effective local field due to
fixed neighbors in classical phase space, while the averages account for the
non-commutativity of the position and momentum operators. Used in Monte Carlo
simulations for a one-dimensional Lennard-Jones fluid, the results prove more
reliable than a high temperature expansion and a harmonic local field approach,
and at intermediate temperatures agree with benchmark numerical results.
Results are presented for distinguishable particles, fermions, and bosons.Comment: 12 pages, 6 figures, 3 appendece
Design of Chemotaxis Devices Using Nano-Motors
Several designs for micro-devices for chemotaxis based on nano-motors are
proposed. The nano- or micro-motors are the conventional Janus rods or spheres
that are powered by the catalytic reaction of fuels such as hydrogen peroxide.
It is shown how these can be linked to make a device that can follow a
concentration gradient of the fuel. The feasibility of assembling the devices
using micromanipulation or metallic deposition is discussed. A possible design
principle is suggested for a device that follows the concentration gradient of
an analyte other than the fuel.Comment: 4 pages, 8 figure
More Reliable Measurements of the Slip Length with the Atomic Force Microscope
Further improvements are made to the non-linear data analysis algorithm for
the atomic force microscope [P. Attard, arXiv:1212.3019v2 (2012)]. The
algorithm is required when there is curvature in the compliance region due to
photo-diode non-linearity. Results are obtained for the hydrodynamic drainage
force, for three surfaces: hydrophilic silica (symmetric, Si-Si), hydrophobic
dichlorodimethylsilane (symmetric, DCDMS-DCDMS), and hydrophobic
octadecyltrichlorosilane (asymmetric, Si-OTS). The drainage force was measured
in the viscous liquid di-n-octylphthalate. The slip-lengths are found to be 3nm
for Si, 2nm for DCDMS, and 2nm for OTS, with an uncertainty on the order of a
nanometer. These slip lengths are a factor of 4--15 times smaller than those
obtained from previous analysis of the same raw data [L. Zhu et al., Langmuir,
27, 6712 (2011). Ibid, 28, 7768 (2012)].Comment: 18 pages, 11 figures (improved discussion of cantilever drag
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