2,551 research outputs found
Report of the Terrestrial Bodies Science Working Group. Volume 4: The moon
A rationale for furture exploration of the moon is given. Topics discussed include the objectives of the lunar polar orbiter mission, the mission profile, and general characteristics of the spacraft to be used
Giant Electroresistance in Ferroelectric Tunnel Junctions
The interplay between the electron transport in metal/ferroelectric/metal
junctions with ultrathin ferroelectric barriers and the polarization state of a
barrier is investigated. Using a model which takes into account screening of
polarization charges in metallic electrodes and direct quantum tunneling across
a ferroelectric barrier we calculate the change in the tunneling conductance
associated with the polarization switching. We find the conductance change of a
few orders of magnitude for metallic electrodes with significantly different
screening lengths. This giant electroresistance effect is the consequence of a
different potential profile seen by transport electrons for the two opposite
polarization orientations.Comment: 4 page
Equidistribution of Heegner Points and Ternary Quadratic Forms
We prove new equidistribution results for Galois orbits of Heegner points
with respect to reduction maps at inert primes. The arguments are based on two
different techniques: primitive representations of integers by quadratic forms
and distribution relations for Heegner points. Our results generalize one of
the equidistribution theorems established by Cornut and Vatsal in the sense
that we allow both the fundamental discriminant and the conductor to grow.
Moreover, for fixed fundamental discriminant and variable conductor, we deduce
an effective surjectivity theorem for the reduction map from Heegner points to
supersingular points at a fixed inert prime. Our results are applicable to the
setting considered by Kolyvagin in the construction of the Heegner points Euler
system
A Matrix Approach to Numerical Solution of the DGLAP Evolution Equations
A matrix-based approach to numerical integration of the DGLAP evolution
equations is presented. The method arises naturally on discretisation of the
Bjorken x variable, a necessary procedure for numerical integration. Owing to
peculiar properties of the matrices involved, the resulting equations take on a
particularly simple form and may be solved in closed analytical form in the
variable t=ln(alpha_0/alpha). Such an approach affords parametrisation via data
x bins, rather than fixed functional forms. Thus, with the aid of the full
correlation matrix, appraisal of the behaviour in different x regions is
rendered more transparent and free of pollution from unphysical
cross-correlations inherent to functional parametrisations. Computationally,
the entire programme results in greater speed and stability; the matrix
representation developed is extremely compact. Moreover, since the parameter
dependence is linear, fitting is very stable and may be performed analytically
in a single pass over the data values.Comment: 13 pages, no figures, typeset with revtex4 and uses packages:
acromake, amssym
Systems, interactions and macrotheory
A significant proportion of early HCI research was guided by one very clear vision: that the existing theory base in psychology and cognitive science could be developed to yield engineering tools for use in the interdisciplinary context of HCI design. While interface technologies and heuristic methods for behavioral evaluation have rapidly advanced in both capability and breadth of application, progress toward deeper theory has been modest, and some now believe it to be unnecessary. A case is presented for developing new forms of theory, based around generic “systems of interactors.” An overlapping, layered structure of macro- and microtheories could then serve an explanatory role, and could also bind together contributions from the different disciplines. Novel routes to formalizing and applying such theories provide a host of interesting and tractable problems for future basic research in HCI
Purifying Hydrogen with Inorganic Silica Membranes at High Temperatures
Development of high quality membranes for industrial applications will lead to cost reductions over traditional separations processes. Silica membranes are a new technology for hydrogen separation that needs R&D specifically to apply them to industrial scales. Past work has shown a carbonised template silica membrane which offered hydrostability. This resulted in better stability under steam and high temperature conditions without compromising the permselectivity for small molecules. In this paper a hydrostable silica membrane was developed for hydrogen separation having a pore cut-off around 3 Angstron units. The carbon templates did not compromise the membrane's ability to permeate hydrogen selectively rather than other major gases in a synthesised coal gasifier mixture of CO, CO2 and N2. The selectivity of H2 to N2 was 26, whilst the hydrostable property of the carbonised template membrane was maintained. Computational fluid dynamics (CFD) can be used to develop membrane systems in tandem with these intrinsic improvements. CFD simulation studies were also conducted to gain better insight into the macroscopic flow parameters
Generalized Drude model: Unification of ballistic and diffusive electron transport
For electron transport in parallel-plane semiconducting structures, a model
is developed that unifies ballistic and diffusive transport and thus
generalizes the Drude model. The unified model is valid for arbitrary magnitude
of the mean free path and arbitrary shape of the conduction band edge profile.
Universal formulas are obtained for the current-voltage characteristic in the
nondegenerate case and for the zero-bias conductance in the degenerate case,
which describe in a transparent manner the interplay of ballistic and diffusive
transport. The semiclassical approach is adopted, but quantum corrections
allowing for tunneling are included. Examples are considered, in particular the
case of chains of grains in polycrystalline or microcrystalline semiconductors
with grain size comparable to, or smaller than, the mean free path. Substantial
deviations of the results of the unified model from those of the ballistic
thermionic-emission model and of the drift-diffusion model are found. The
formulation of the model is one-dimensional, but it is argued that its results
should not differ substantially from those of a fully three-dimensional
treatment.Comment: 14 pages, 5 figures, REVTEX file, to appear in J. Phys.: Condens.
Matte
Khinchin theorem for integral points on quadratic varieties
We prove an analogue the Khinchin theorem for the Diophantine approximation
by integer vectors lying on a quadratic variety. The proof is based on the
study of a dynamical system on a homogeneous space of the orthogonal group. We
show that in this system, generic trajectories visit a family of shrinking
subsets infinitely often.Comment: 19 page
Corrections to the universal behavior of the Coulomb-blockade peak splitting for quantum dots separated by a finite barrier
Building upon earlier work on the relation between the dimensionless interdot
channel conductance g and the fractional Coulomb-blockade peak splitting f for
two electrostatically equivalent dots, we calculate the leading correction that
results from an interdot tunneling barrier that is not a delta-function but,
rather, has a finite height V and a nonzero width xi and can be approximated as
parabolic near its peak. We develop a new treatment of the problem for g much
less than 1 that starts from the single-particle eigenstates for the full
coupled-dot system. The finiteness of the barrier leads to a small upward shift
of the f-versus-g curve at small values of g. The shift is a consequence of the
fact that the tunneling matrix elements vary exponentially with the energies of
the states connected. Therefore, when g is small, it can pay to tunnel to
intermediate states with single-particle energies above the barrier height V.
The correction to the zero-width behavior does not affect agreement with recent
experimental results but may be important in future experiments.Comment: Title changed from ``Non-universal...'' to ``Corrections to the
universal...'' No other changes. 10 pages, 1 RevTeX file with 2 postscript
figures included using eps
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