10,140 research outputs found
Duality Between the Weak and Strong Interaction Limits for Randomly Interacting Fermions
We establish the existence of a duality transformation for generic models of
interacting fermions with two-body interactions. The eigenstates at weak and
strong interaction U possess similar statistical properties when expressed in
the U=0 and U=infinity eigenstates bases respectively. This implies the
existence of a duality point U_d where the eigenstates have the same spreading
in both bases. U_d is surrounded by an interval of finite width which is
characterized by a non Lorentzian spreading of the strength function in both
bases. Scaling arguments predict the survival of this intermediate regime as
the number of particles is increased.Comment: RevTex4, 4 pages, 4 figures. Accepted for publication at Phys. Rev.
Let
Loschmidt echoes in two-body random matrix ensembles
Fidelity decay is studied for quantum many-body systems with a dominant
independent particle Hamiltonian resulting e.g. from a mean field theory with a
weak two-body interaction. The diagonal terms of the interaction are included
in the unperturbed Hamiltonian, while the off-diagonal terms constitute the
perturbation that distorts the echo. We give the linear response solution for
this problem in a random matrix framework. While the ensemble average shows no
surprising behavior, we find that the typical ensemble member as represented by
the median displays a very slow fidelity decay known as ``freeze''. Numerical
calculations confirm this result and show, that the ground state even on
average displays the freeze. This may contribute to explanation of the
``unreasonable'' success of mean field theories.Comment: 9 pages, 5 figures (6 eps files), RevTex; v2: slight modifications
following referees' suggestion
1/f noise in the Two-Body Random Ensemble
We show that the spectral fluctuations of the Two-Body Random Ensemble (TBRE)
exhibit 1/f noise. This result supports a recent conjecture stating that
chaotic quantum systems are characterized by 1/f noise in their energy level
fluctuations. After suitable individual averaging, we also study the
distribution of the exponent \alpha in the 1/f^{\alpha} noise for the
individual members of the ensemble. Almost all the exponents lie inside a
narrow interval around \alpha=1 suggesting that also individual members exhibit
1/f noise, provided they are individually unfoldedComment: 4 pages, 3 figures, Accepted for publication in Phys. Rev.
Sexual coercion among Black and White teenagers: Sexual stereotypes and psycho-behavioral correlates
Cofactor regeneration by a soluble pyridine nucleotide transhydrogenase for biological production of hydromorphone
We have applied the soluble pyridine nucleotide transhydrogenase of Pseudomonas fluorescens to a cell-free system for the regeneration of the nicotinamide cofactors NAD and NADP in the biological production of the important semisynthetic opiate drug hydromorphone. The original recombinant whole-cell system suffered from cofactor depletion resulting from the action of an NADP(+)-dependent morphine dehydrogenase and an NADH-dependent morphinone reductase. By applying a soluble pyridine nucleotide transhydrogenase, which can transfer reducing equivalents between NAD and NADP, we demonstrate with a cell-free system that efficient cofactor cycling in the presence of catalytic amounts of cofactors occurs, resulting in high yields of hydromorphone. The ratio of morphine dehydrogenase, morphinone reductase, and soluble pyridine nucleotide transhydrogenase is critical for diminishing the production of the unwanted by-product dihydromorphine and for optimum hydromorphone yields. Application of the soluble pyridine nucleotide transhydrogenase to the whole-cell system resulted in an improved biocatalyst with an extended lifetime. These results demonstrate the usefulness of the soluble pyridine nucleotide transhydrogenase and its wider application as a tool in metabolic engineering and biocatalysis
Dynamics of Perfectly Wetting Drops under Gravity
We study the dynamics of small droplets of polydimethylsiloxane (PDMS)
silicone oil on a vertical, perfectly-wetting, silicon wafer. Interference
videomicroscopy allows us to capture the dynamics of these droplets. We use
droplets with a volumes typically ranging from 100 to 500 nanolitres
(viscosities from 10 to 1000 centistokes) to understand long time derivations
from classical solutions. Past researchers used one dimensional theory to
understand the typical scaling for the position of the tip of the
droplet in time . We observe this regime in experiment for intermediate
times and discover a two-dimensional, similarity solution of the shape of the
droplet. However, at long times our droplets start to move more slowly down the
plane than the scaling suggests and we observe deviations in droplet
shape from the similarity solution. We match experimental data with simulations
to show these deviations are consistent with retarded van der Waals forcing
which should become significant at the small heights observed
Statistical Theory of Parity Nonconservation in Compound Nuclei
We present the first application of statistical spectroscopy to study the
root-mean-square value of the parity nonconserving (PNC) interaction matrix
element M determined experimentally by scattering longitudinally polarized
neutrons from compound nuclei. Our effective PNC interaction consists of a
standard two-body meson-exchange piece and a doorway term to account for
spin-flip excitations. Strength functions are calculated using realistic
single-particle energies and a residual strong interaction adjusted to fit the
experimental density of states for the targets, ^{238} U for A\sim 230 and
^{104,105,106,108} Pd for A\sim 100. Using the standard Desplanques, Donoghue,
and Holstein estimates of the weak PNC meson-nucleon coupling constants, we
find that M is about a factor of 3 smaller than the experimental value for
^{238} U and about a factor of 1.7 smaller for Pd. The significance of this
result for refining the empirical determination of the weak coupling constants
is discussed.Comment: Latex file, no Fig
Intruder States and their Local Effect on Spectral Statistics
The effect on spectral statistics and on the revival probability of intruder
states in a random background is analysed numerically and with perturbative
methods. For random coupling the intruder does not affect the GOE spectral
statistics of the background significantly, while a constant coupling causes
very strong correlations at short range with a fourth power dependence of the
spectral two-point function at the origin.The revival probability is
significantly depressed for constant coupling as compared to random coupling.Comment: 18 pages, 10 Postscript figure
Half-lives and pre-supernova weak interaction rates for nuclei away from the stability line
A detailed model for the calculation of beta decay rates of the shell
nuclei for situations prevailing in pre-supernova and collapse phases of
evolution of the core of massive stars leading to supernova explosion has been
extended for electron-capture rates. It can also be used to determine the
half-lives of neutron-rich nuclei in the shell. The model uses an
averaged Gamow-Teller (GT) strength function. But it can also use the
experimental log ft values and GT strength function from reaction
studies wherever available. The calculated rate includes contributions from
each of the low-lying excited states of the mother including some specific
resonant states ("back resonance") having large GT matrix elements.Comment: 11 pages; Latex; no figs; version to appear in J. Phys.
Bicontinuous Soft Solids with a Gradient in Channel Size
We present examples of bicontinuous interfacially jammed emulsion gels
("bijels") with a designed gradient in the channel size along the sample. These
samples are created by quenching binary fluids which have a gradient in
particle concentration along the sample, since the channel size is determined
by the local particle concentration. A gradient in local particle concentration
is achieved using a two-stage loading process, with different particle volume
fractions in each stage. Confocal microscopy and image analysis were used to
quantitatively measure the channel size of the bijels. Bijels with a gradient
in channel size of up to 2.8%/mm have been created. Such tailored soft
materials could act as templates for energy materials optimised for both high
ionic transport rates (high power) and high interfacial area (high energy
density), potentially making them useful in novel energy applications.Comment: 9 pages, 5 figure
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