444,628 research outputs found
Local order and magnetic field effects on the electronic properties of disordered binary alloys in the Quantum Site Percolation limit
Electronic properties of disordered binary alloys are studied via the
calculation of the average Density of States (DOS) in two and three dimensions.
We propose a new approximate scheme that allows for the inclusion of local
order effects in finite geometries and extrapolates the behavior of infinite
systems following `finite-size scaling' ideas. We particularly investigate the
limit of the Quantum Site Percolation regime described by a tight-binding
Hamiltonian. This limit was chosen to probe the role of short range order (SRO)
properties under extreme conditions. The method is numerically highly efficient
and asymptotically exact in important limits, predicting the correct DOS
structure as a function of the SRO parameters. Magnetic field effects can also
be included in our model to study the interplay of local order and the shifted
quantum interference driven by the field. The average DOS is highly sensitive
to changes in the SRO properties, and striking effects are observed when a
magnetic field is applied near the segregated regime. The new effects observed
are twofold: there is a reduction of the band width and the formation of a gap
in the middle of the band, both as a consequence of destructive interference of
electronic paths and the loss of coherence for particular values of the
magnetic field. The above phenomena are periodic in the magnetic flux. For
other limits that imply strong localization, the magnetic field produces minor
changes in the structure of the average DOS.Comment: 13 pages, 9 figures, 31 references, RevTex preprint, submitted to
Phys. Rev.
Molecular charge distribution of CO
The difference electron density of CO is studied by comparison of several calculations. It is shown that the Hartree-Fock-Slater and Hartree-Fock methods yield equally good charge-distributions and that the use of minimal basis sets should be avoided
Multiple resonance compensation for betatron coupling and its equivalence with matrix method
Analyses of betatron coupling can be broadly divided into two categories: the
matrix approach that decouples the single-turn matrix to reveal the normal
modes and the hamiltonian approach that evaluates the coupling in terms of the
action of resonances in perturbation theory. The latter is often regarded as
being less exact but good for physical insight. The common opinion is that the
correction of the two closest sum and difference resonances to the working
point is sufficient to reduce the off-axis terms in the 4X4 single-turn matrix,
but this is only partially true. The reason for this is explained, and a method
is developed that sums to infinity all coupling resonances and, in this way,
obtains results equivalent to the matrix approach. The two approaches is
discussed with reference to the dynamic aperture. Finally, the extension of the
summation method to resonances of all orders is outlined and the relative
importance of a single resonance compared to all resonances of a given order is
analytically described as a function of the working point.Comment: 22 pages, 10 figure
Pacman percolation: a model for enzyme gel degradation
We study a model for the gel degradation by an enzyme, where the gel is
schematized as a cubic lattice, and the enzyme as a random walker, that cuts
the bonds over which it passes. The model undergoes a (reverse) percolation
transition, which for low density of enzymes falls in a universality class
different from random percolation. In particular we have measured a gel
fraction critical exponent beta=1.0+-0.1, in excellent agreement with
experiments made on the real system.Comment: 4 pages, 7 eps figure
Why Tau First?
Electron neutrino has been the first neutral lepton to be foreseen and
discovered last century. The un-ordered muon and its neutrino arose later by
cosmic rays. The tau discover, the heaviest, the most unstable charged lepton,
was found surprisingly on 1975. Its neutrino was hardly revealed just on 2000.
So why High Energy Neutrino Astronomy should rise first via tau neutrino, the
last, the most rare one? The reasons are based on a chain of three favorable
coincidences found last decade: the neutrino masses and their flavor mixing,
the UHECR opacity on Cosmic Black Body (GZK cut off on BBR), the amplified tau
air-shower decaying in flight. Indeed guaranteed UHE GZK tau neutrinos, feed by
muon mixing, while skimming the Earth might lead to boosted UHE tau, mostly
horizontal ones. These UHE lepton decay in flight are spread, amplified, noise
free Air-Shower: a huge event for an unique particle. To be observed soon:
within Auger sky, in present decade. Its discover may sign of the first tau
appearance.Comment: 8 pages, 4 figure
Polarized semi-inclusive electroweak structure functions at next-to-leading-order
We present a next-to-leading order (NLO) computation of the full set of
polarized and unpolarized electroweak semi-inclusive DIS (SIDIS) structure
functions, whose knowledge is crucial for a precise extraction of polarized
parton distributions. We focus on the phenomenology of the polarized structure
functions for the kinematical conditions that could be reached in an
Electron-Ion-Collider.
We show that the NLO corrections are sizeable, particularly in the small-
range. We test the sensitivity of these structure functions on certain quark
distributions and compare it to the situation of inclusive DIS and
electromagnetic SIDIS.Comment: 17 pages, 5 figure
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