2,017 research outputs found
Path Integral Monte Carlo Simulations for Fermion Systems: Pairing in the Electron-Hole Plasma
We review the path integral method wherein quantum systems are mapped with
Feynman's path integrals onto a classical system of "ring-polymers" and then
simulated with the Monte Carlo technique. Bose or Fermi statistics correspond
to possible "cross-linking" of polymers. As proposed by Feynman, superfluidity
and Bose condensation result from macroscopic exchange of bosons. To map
fermions onto a positive probability distribution, one must restrict the paths
to lie in regions where the fermion density matrix is positive. We discuss a
recent application to the two-component electron-hole plasma. At low
temperature excitons and bi-excitons form. We have used nodal surfaces
incorporating paired fermions and see evidence of a Bose condensation in the
energy, specific heat and superfluid density. In the restricted path integral
picture, pairing appears as intertwined electron-hole paths. Bose condensation
occurs when these intertwined paths wind around the periodic boundaries.Comment: 14 pages, 7 figures Prepared for the 1999 International Conference on
Strongly Coupled Coulomb Systems, Saint-Malo, Franc
COW-CALF PRODUCERS' PERCEIVED PROFIT MAXIMIZATION OBJECTIVE: A LOGIT ANALYSIS
A logistic regression (logit) model was developed to examine how socioeconomic characteristics of cow-calf producers influenced their perceptions of themselves as profit maximizers. Amount of pasture acreage, percent of income earned from the cow-calf operation, and desire to increase net worth and efficiently use labor significantly increased the producer's probability of claiming to be in the business primarily to maximize profits. Some sociological reasons for owning cattle significantly reduced the probability of the producer claiming to be a profit maximizer while others significantly increased the probability.Livestock Production/Industries,
Can Individual Investors Beat the Market?
We document strong persistence in the performance of trades of individual investors. Investors classified in the top 10 percent place other trades that on average earn excess returns of 15 basis points per day. A rolling-forward strategy of going long firms purchased by previously successful investors and shorting firms purchased by previously unsuccessful investors results in excess returns of 5 basis points per day. These returns are not confined to small stocks nor to stocks in which the investors are likely to have inside information. Our results suggest that skillful individual investors exploit market inefficiencies to earn abnormal profits, above and beyond any profits available from well-known strategies based upon size, value, or momentum.Individual Investors, Market Efficiency, Performance Persistence
Biexciton recombination rates in self-assembled quantum dots
The radiative recombination rates of interacting electron-hole pairs in a
quantum dot are strongly affected by quantum correlations among electrons and
holes in the dot. Recent measurements of the biexciton recombination rate in
single self-assembled quantum dots have found values spanning from two times
the single exciton recombination rate to values well below the exciton decay
rate. In this paper, a Feynman path-integral formulation is developed to
calculate recombination rates including thermal and many-body effects. Using
real-space Monte Carlo integration, the path-integral expressions for realistic
three-dimensional models of InGaAs/GaAs, CdSe/ZnSe, and InP/InGaP dots are
evaluated, including anisotropic effective masses. Depending on size, radiative
rates of typical dots lie in the regime between strong and intermediate
confinement. The results compare favorably to recent experiments and
calculations on related dot systems. Configuration interaction calculations
using uncorrelated basis sets are found to be severely limited in calculating
decay rates.Comment: 11 pages, 4 figure
Farm Growth, Consolidation, and Diversification: Washington Dairy Industry
Farm Management, Q12,
Autocorrelations in the totally asymmetric simple exclusion process and Nagel-Schreckenberg model
We study via Monte Carlo simulation the dynamics of the Nagel-Schreckenberg
model on a finite system of length L with open boundary conditions and parallel
updates. We find numerically that in both the high and low density regimes the
autocorrelation function of the system density behaves like 1-|t|/tau with a
finite support [-tau,tau]. This is in contrast to the usual exponential decay
typical of equilibrium systems. Furthermore, our results suggest that in fact
tau=L/c, and in the special case of maximum velocity 1 (corresponding to the
totally asymmetric simple exclusion process) we can identify the exact
dependence of c on the input, output and hopping rates. We also emphasize that
the parameter tau corresponds to the integrated autocorrelation time, which
plays a fundamental role in quantifying the statistical errors in Monte Carlo
simulations of these models.Comment: 7 pages, 6 figure
Optical spectroscopy of single quantum dots at tunable positive, neutral and negative charge states
We report on the observation of photoluminescence from positive, neutral and
negative charge states of single semiconductor quantum dots. For this purpose
we designed a structure enabling optical injection of a controlled unequal
number of negative electrons and positive holes into an isolated InGaAs quantum
dot embedded in a GaAs matrix. Thereby, we optically produced the charge states
-3, -2, -1, 0, +1 and +2. The injected carriers form confined collective
'artificial atoms and molecules' states in the quantum dot. We resolve
spectrally and temporally the photoluminescence from an optically excited
quantum dot and use it to identify collective states, which contain charge of
one type, coupled to few charges of the other type. These states can be viewed
as the artificial analog of charged atoms such as H, H, H,
and charged molecules such as H and H. Unlike higher
dimensionality systems, where negative or positive charging always results in
reduction of the emission energy due to electron-hole pair recombination, in
our dots, negative charging reduces the emission energy, relative to the
charge-neutral case, while positive charging increases it. Pseudopotential
model calculations reveal that the enhanced spatial localization of the
hole-wavefunction, relative to that of the electron in these dots, is the
reason for this effect.Comment: 5 figure
Path integral study of the role of correlation in exchange coupling of spins in double quantum dots and optical lattices
We explore exchange coupling of a pair of spins in a double dot and in an
optical lattice. Our algorithm uses the frequency of exchanges in a bosonic
path integral, evaluated with Monte Carlo. This algorithm is simple enough to
be a "black box" calculator, yet gives insights into the role of correlation
through two-particle probability densities, visualization of instantons, and
pair correlation functions. We map the problem to Hubbard model and see that
exchange and correlation renormalize the effective parameters, dramatically
lowering U at larger separations.Comment: 5 pages, 6 figure
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The matter between them
A collection of short stories about the Gardner family, spanning several generations, but focusing on the children of Len and Laura Gardner, Brooke, the second oldest, in particular. Set primarily in Utah, the collection tells the story of family members' struggles with one another, with their religious faith, and with the demands of the world beyond their community
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