1,610 research outputs found
Experimental evidence of delocalized states in random dimer superlattices
We study the electronic properties of GaAs-AlGaAs superlattices with
intentional correlated disorder by means of photoluminescence and vertical dc
resistance. The results are compared to those obtained in ordered and
uncorrelated disordered superlattices. We report the first experimental
evidence that spatial correlations inhibit localization of states in disordered
low-dimensional systems, as our previous theoretical calculations suggested, in
contrast to the earlier belief that all eigenstates are localized.Comment: 4 pages, 5 figures. Physical Review Letters (in press
Quark spin coupling in baryons - revisited
A direct connection can be made between mixing angles in negative parity
baryons and the spin coupling of constituent quarks. The mixing angles do not
depend on spectral data. These angles are recalculated for gluon exchange and
pion exchange between quarks. For pion exchange the results of Glozman and
Riska are corrected. The experimental data on mixing are very similar to those
derived from gluon exchange but substantially different from the values
obtained for pion exchange.Comment: 10 pages, RevTex; a sign error is corrected, spin-orbit results are
include
Electronic structure and vertical transport in random dimer GaAs-Al_xGa_(1-x)As superlattices
We report a systematic study of several GaAs-AlxGa1-xAs semiconductor superlattices grown by molecular-beam epitaxy specifically designed to explore the existence of extended states in random dimer superlattices. We have confirmed our previous results [V. Bellani et al., Phys. Rev. Lett. 82, 2159 (1999)] with much additional evidence that allows us to lay claim to a clear-cut experimental verification of the presence of extended states in random dimer superlattices due to the short-range correlations (dimers) that inhibit the localization effects of the disorder
S_3 and the L=1 Baryons in the Quark Model and the Chiral Quark Model
The S_3 symmetry corresponding to permuting the positions of the quarks
within a baryon allows us to study the 70-plet of L=1 baryons without an
explicit choice for the spatial part of the quark wave functions: given a set
of operators with definite transformation properties under the spin-flavor
group SU(3) x SU(2) and under this S_3, the masses of the baryons can be
expressed in terms of a small number of unknown parameters which are fit to the
observed L=1 baryon mass spectrum. This approach is applied to study both the
quark model and chiral constituent quark model. The latter theory leads to a
set of mass perturbations which more satisfactorily fits the observed L=1
baryon mass spectrum (though we can say nothing, within our approach, about the
physical reasonableness of the parameters in the fit). Predictions for the
mixing angles and the unobserved baryon masses are given for both models as
well as a discussion of specific baryons.Comment: 24 pages, requires picte
Quantum transport and momentum conserving dephasing
We study numerically the influence of momentum-conserving dephasing on the
transport in a disordered chain of scatterers. Loss of phase memory is caused
by coupling the transport channels to dephasing reservoirs. In contrast to
previously used models, the dephasing reservoirs are linked to the transport
channels between the scatterers, and momentum conserving dephasing can be
investigated. Our setup provides a model for nanosystems exhibiting conductance
quantization at higher temperatures in spite of the presence of phononic
interaction. We are able to confirm numerically some theoretical predictions.Comment: 7 pages, 4 figure
One-Loop Factorization of the Nucleon g_2-Structure Function in the Non-Singlet Case
We consider the one-loop factorization of the simplest twist-three process:
inclusive deep-inelastic scattering of longitudinally-polarized leptons on a
transversely-polarized nucleon target. By studying the Compton amplitudes for
certain quark and gluon states at one loop, we find the coefficient functions
for the non-singlet twist-three distributions in the factorization formula of
g_2(x_B,Q^2). The result marks the first step towards a next-to-leading order
(NLO) formalism for this transverse-spin-dependent structure function of the
nucleon.Comment: 14 pages, revtex4, four figures included, minor change
Cellular automaton supercolliders
Gliders in one-dimensional cellular automata are compact groups of
non-quiescent and non-ether patterns (ether represents a periodic background)
translating along automaton lattice. They are cellular-automaton analogous of
localizations or quasi-local collective excitations travelling in a spatially
extended non-linear medium. They can be considered as binary strings or symbols
travelling along a one-dimensional ring, interacting with each other and
changing their states, or symbolic values, as a result of interactions. We
analyse what types of interaction occur between gliders travelling on a
cellular automaton `cyclotron' and build a catalog of the most common
reactions. We demonstrate that collisions between gliders emulate the basic
types of interaction that occur between localizations in non-linear media:
fusion, elastic collision, and soliton-like collision. Computational outcomes
of a swarm of gliders circling on a one-dimensional torus are analysed via
implementation of cyclic tag systems
Theranostics in Boron neutron capture therapy
Boron neutron capture therapy (BNCT) has the potential to specifically destroy tumor cells without damaging the tissues infiltrated by the tumor. BNCT is a binary treatment method based on the combination of two agents that have no effect when applied individually:10B and thermal neutrons. Exclusively, the combination of both produces an effect, whose extent depends on the amount of10B in the tumor but also on the organs at risk. It is not yet possible to determine the10B concentration in a specific tissue using non-invasive methods. At present, it is only possible to measure the10B concentration in blood and to estimate the boron concentration in tissues based on the assumption that there is a fixed uptake of10B from the blood into tissues. On this imprecise assumption, BNCT can hardly be developed further. A therapeutic approach, combining the boron carrier for therapeutic purposes with an imaging tool, might allow us to determine the10B concentration in a specific tissue using a non-invasive method. This review provides an overview of the current clinical protocols and preclinical experiments and results on how innovative drug development for boron delivery systems can also incorporate concurrent imaging. The last section focuses on the importance of proteomics for further optimization of BNCT, a highly precise and personalized therapeutic approach
Delays Associated with Elementary Processes in Nuclear Reaction Simulations
Scatterings, particularly those involving resonances, and other elementary
processes do not happen instantaneously. In the context of semiclassical
nuclear reaction simulations, we consider delays associated with an interaction
for incident quantum wave-packets. As a consequence, we express delays
associated with elementary processes in terms of elements of the scattering
matrix and phase shifts for elastic scattering. We show that, to within the
second order in density, the simulation must account for delays in scattering
consistently with the mean field in order to properly model thermodynamic
properties such as pressure and free-energy density. The delays associated with
nucleon-nucleon and pion-nucleon scattering in free space are analysed with
their nontrivial energy dependence. Finally, an example of s-channel scattering
of massless partons is studied, and scattering schemes in nuclear reaction
simulations are investigated in the context of scattering delays.Comment: 45 pages, 5 uuencoded Postscript figure
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