763 research outputs found
Collision of one-dimensional fermion clusters
We study cluster-cluster collisions in one-dimensional Fermi systems with
particular emphasis on the non-trivial quantum effects of the collision
dynamics. We adopt the Fermi-Hubbard model and the time-dependent density
matrix renormalization group method to simulate collision dynamics between two
fermion clusters of different spin states with contact interaction. It is
elucidated that the quantum effects become extremely strong with the
interaction strength, leading to the transmittance much more enhanced than
expected from semiclassical approximation. We propose a concise model based on
one-to-one collisions, which unveils the origin of the quantum effects and also
explains the overall properties of the simulation results clearly. Our concise
model can quite widely describe the one-dimensional collision dynamics with
contact interaction. Some potential applications, such as repeated collisions,
are addressed.Comment: 5 pages, 5 figure
Unusual Low-Temperature Phase in VO Nanoparticles
We present a systematic investigation of the crystal and electronic structure
and the magnetic properties above and below the metal-insulator transition of
ball-milled VO nanoparticles and VO microparticles. For this research,
we performed a Rietveld analysis of synchrotron radiation x-ray diffraction
data, O x-ray absorption spectroscopy, V resonant inelastic x-ray
scattering, and magnetic susceptibility measurements. This study reveals an
unusual low-temperature phase that involves the formation of an elongated and
less-tilted V-V pair, a narrowed energy gap, and an induced paramagnetic
contribution from the nanoparticles. We show that the change in the crystal
structure is consistent with the change in the electronic states around the
Fermi level, which leads us to suggest that the Peierls mechanism contributes
to the energy splitting of the state. Furthermore, we find that the
high-temperature rutile structure of the nanoparticles is almost identical to
that of the microparticles.Comment: 7 pages, 8 figures, 2 table
The winemaker’s bug: From ancient wisdom to opening new vistas with frontier yeast science
The past three decades have seen a global wine glut. So far, well-intended but wasteful and expensive market-intervention has failed to drag the wine industry out of a chronic annual oversupply of roughly 15%. Can yeast research succeed where these approaches have failed by providing a means of improving wine quality, thereby making wine more appealing to consumers? To molecular biologists Saccharomyces cerevisiae is as intriguing as it is tractable. A simple unicellular eukaryote, it is an ideal model organism, enabling scientists to shed new light on some of the biggest scientific challenges such as the biology of cancer and aging. It is amenable to almost any modification that modern biology can throw at a cell, making it an ideal host for genetic manipulation, whether by the application of traditional or modern genetic techniques. To the winemaker, this yeast is integral to crafting wonderful, complex wines from simple, sugar-rich grape juice. Thus any improvements that we can make to wine, yeast fermentation performance or the sensory properties it imparts to wine will benefit winemakers and consumers. With this in mind, the application of frontier technologies, particularly the burgeoning fields of systems and synthetic biology, have much to offer in their pursuit of “novel” yeast strains to produce high quality wine. This paper discusses the nexus between yeast research and winemaking. It also addresses how winemakers and scientists face up to the challenges of consumer perceptions and opinions regarding the intervention of science and technology; the greater this intervention, the stronger the criticism that wine is no longer “natural.” How can wine researchers respond to the growing number of wine commentators and consumers who feel that scientific endeavors favor wine quantity over quality and “technical sophistication, fermentation reliability and product consistency” over “artisanal variation”? This paper seeks to present yeast research in a new light and a new context, and it raises important questions about the direction of yeast research, its contribution to science and the future of winemaking
Advances in delimiting the Hilbert-Schmidt separability probability of real two-qubit systems
We seek to derive the probability--expressed in terms of the Hilbert-Schmidt
(Euclidean or flat) metric--that a generic (nine-dimensional) real two-qubit
system is separable, by implementing the well-known Peres-Horodecki test on the
partial transposes (PT's) of the associated 4 x 4 density matrices). But the
full implementation of the test--requiring that the determinant of the PT be
nonnegative for separability to hold--appears to be, at least presently,
computationally intractable. So, we have previously implemented--using the
auxiliary concept of a diagonal-entry-parameterized separability function
(DESF)--the weaker implied test of nonnegativity of the six 2 x 2 principal
minors of the PT. This yielded an exact upper bound on the separability
probability of 1024/{135 pi^2} =0.76854$. Here, we piece together
(reflection-symmetric) results obtained by requiring that each of the four 3 x
3 principal minors of the PT, in turn, be nonnegative, giving an
improved/reduced upper bound of 22/35 = 0.628571. Then, we conclude that a
still further improved upper bound of 1129/2100 = 0.537619 can be found by
similarly piecing together the (reflection-symmetric) results of enforcing the
simultaneous nonnegativity of certain pairs of the four 3 x 3 principal minors.
In deriving our improved upper bounds, we rely repeatedly upon the use of
certain integrals over cubes that arise. Finally, we apply an independence
assumption to a pair of DESF's that comes close to reproducing our numerical
estimate of the true separability function.Comment: 16 pages, 9 figures, a few inadvertent misstatements made near the
end are correcte
Dielectric relaxations in poly(glycidyl phenyl ether): Effects of microstructure and cyclic topology
Cyclic and linear, isoregic and aregic, and isotactic and atactic poly(glycidyl phenyl ether) (PGPE) with molecular weights up to Mw = 5.5 kg/mol are synthesized by ring-opening polymerization of glycidyl phenyl ether. Initiation with tetrabutylammonium fluoride leads to isoregic linear polymers with ~95% regular linkages, and initiation with B(C6F5)3 and B(C6F5)3/water leads to aregic cyclic and linear polymers, respectively, with ~50% regular linkages as quantified by 13C NMR. Local, segmental, and chain dynamics in PGPE is investigated by broadband dielectric spectroscopy (10–2–106 Hz). The ß-relaxation for linear PGPE is separated into two contributions arising from the motions of side groups and end groups with activation energies of 35.4 and 23.8 kJ/mol, respectively. The ß-relaxation process for cyclic PGPE shows the same activation energy as that shown by the side-group contribution in linear PGPE, indicating that topology does not play a key role on the side-group local dynamics. Moreover, cyclic PGPE samples show higher calorimetric and dynamic glass transition temperatures as well as lower dynamic fragility compared to linear chains. Unexpectedly from topological considerations, cyclic PGPE shows low frequency dielectric contributions that can be attributed to short wavelength internal ring motions and that are detectable by dielectric relaxation due to the aregic nature of the rings.Peer ReviewedPostprint (author's final draft
An Improved Initialization Procedure for the Density-Matrix Renormalization Group
We propose an initialization procedure for the density-matrix renormalization
group (DMRG): {\it the recursive sweep method}. In a conventional DMRG
calculation, the infinite-algorithm, where two new sites are added to the
system at each step, has been used to reach the target system size. We then
need to obtain the ground state for a different system size for every site
addition, so 1) it is difficult to supply a good initial vector for the
numerical diagonalization for the ground state, and 2) when the system reduced
to a 1D system consists of an array of nonequivalent sites as in ladders or
Hubbard-Holstein model, special care has to be taken. Our procedure, which we
call the {\it recursive sweep method}, provides a solution to these problems
and in fact provides a faster algorithm for the Hubbard model as well as more
complicated ones such as the Hubbard-Holstein model.Comment: 4 pages, 4 figures, submitted to JPS
Relativistic instant-form approach to the structure of two-body composite systems
A new approach to the electroweak properties of two-particle composite
systems is developed. The approach is based on the use of the instant form of
relativistic Hamiltonian dynamics. The main novel feature of this approach is
the new method of construction of the matrix element of the electroweak current
operator. The electroweak current matrix element satisfies the relativistic
covariance conditions and in the case of the electromagnetic current also the
conservation law automatically. The properties of the system as well as the
approximations are formulated in terms of form factors. The approach makes it
possible to formulate relativistic impulse approximation in such a way that the
Lorentz-covariance of the current is ensured. In the electromagnetic case the
current conservation law is ensured, too. The results of the calculations are
unambiguous: they do not depend on the choice of the coordinate frame and on
the choice of "good" components of the current as it takes place in the
standard form of light--front dynamics. Our approach gives good results for the
pion electromagnetic form factor in the whole range of momentum transfers
available for experiments at present time, as well as for lepton decay constant
of pion.Comment: 26 pages, Revtex, 5 figure
Elastic Charge Form Factors of and K Mesons
The elastic charge form factors of the charged and mesons are
calculated in modified impulse approximation using instant form of relativistic
Hamiltonian dynamics. Our approach gives pion and kaon electromagnetic form
factors in the large range of momentum transfer. The results are in good
agreement with the available data. Relativistic effects are large at all values
of momentum transfers. The pion and kaon form factors at large depend
strongly on the choice of model. The experiments on pion form factor at large
momentum transfer planned at CEBAF will choose between such models. In the case
of kaon such a choosing may be performed only if supplemented by accurate
measurements of kaon MSR.Comment: 9 pages, LaTeX, 2 uuencoded PostScript figure
Hurst's Rescaled Range Statistical Analysis for Pseudorandom Number Generators used in Physical Simulations
The rescaled range statistical analysis (R/S) is proposed as a new method to
detect correlations in pseudorandom number generators used in Monte Carlo
simulations. In an extensive test it is demonstrated that the RS analysis
provides a very sensitive method to reveal hidden long run and short run
correlations. Several widely used and also some recently proposed pseudorandom
number generators are subjected to this test. In many generators correlations
are detected and quantified.Comment: 12 pages, 12 figures, 6 tables. Replaces previous version to correct
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