23,971 research outputs found
Bursts and Shocks in a Continuum Shell Model
We study a "burst" event, i. e. the evolution of an initial condition having
support only in a finite interval of k-space, in the continuum shell model due
to Parisi. We show that the continuum equation without forcing or dissipation
can be explicitly written in characteristic form and that the right and left
moving parts can be solved exactly. When this is supplemented by the
appropriate shock condition it is possible to find the asymptotic form of the
burst.Comment: 15 pages, 2 eps figures included, Latex 2e. Contribution to the
proceedings of the conference: Disorder and Chaos, in honour of Giovanni
Paladin, September 22-24, 1997, in Rom
Application of satellite data for snow mapping in Norway
The author has identified the following significant results. A close quantitative relationship was found between snow covered areas and subsequent runoff for different parts of the country despite climate differences. Digital LANDSAT data can be used for areas down to approximately 10 sq km to 20 sq km for accurate measurement of snow cover extent. On large watersheds (more than 500 sq km), digital NOAA/TIROS imagery can be used for snow mapping if the area/runoff relationship is determined by using observations from previous years
Phase fluctuations in atomic Bose gases
We improve on the Popov theory for partially Bose-Einstein condensed atomic
gases by treating the phase fluctuations exactly. As a result, the theory
becomes valid in arbitrary dimensions and is able to describe the
low-temperature crossover between three, two and one-dimensional Bose gases,
which is currently being explored experimentally. We consider both homogeneous
and trapped Bose gases.Comment: 4 pages. Title changed Major changes involve extension of theory to
include trapped Bose gases. Deletion of reference to and comparison with
hydrogen experiment. Due to these changes, second author added. Modified
manuscript accepted for PR
Spin susceptibility of underdoped cuprates: the case of Ortho-II YBa_2Cu_3O_{6.5}
Recent inelastic neutron scattering measurements found that the spin
susceptibility of detwinned and highly ordered ortho-II YBa_2Cu_3O_{6.5}
exhibits, in both the normal and superconducting states, one-dimensional
incommensurate modulations at low energies which were interpreted as a
signature of dynamic stripes. We propose an alternative model based on
quasiparticle transitions between the arcs of a truncated Fermi surface. Such
transitions are resonantly enhanced by scattering to the triplet spin
resonance. We show that the anisotropy in the experimental spin response is
consistent with this model if the gap at the saddle points is anisotropic.Comment: 5 fives, 3 postscript figure
Collective Modes of Massive Dirac Fermions in Armchair Graphene Nanoribbons
We report the plasmon dispersion characteristics of intrinsic and extrinsic
armchair graphene nanoribbons of atomic width N = 5 using a p_z-orbital tight
binding model with third-nearest-neighbor (3nn) coupling. The coupling
parameters are obtained by fitting the 3nn dispersions to that of an extended
Huckel theory. The resultant massive Dirac Fermion system has a band gap E_g
\approx 64 meV. The extrinsic plasmon dispersion relation is found to approach
a common dispersion curve as the chemical potential increases, whereas
the intrinsic plasmon dispersion relation is found to have both energy and
momentum thresholds. We also report an analytical model for the extrinsic
plasmon group velocity in the q \rightarrow 0 limit
Large-scale electronic structure theory for simulating nanostructure process
Fundamental theories and practical methods for large-scale electronic
structure calculations are given, in which the computational cost is
proportional to the system size. Accuracy controlling methods for microscopic
freedoms are focused on two practical solver methods, Krylov-subspace method
and generalized-Wannier-state method. A general theory called the
'multi-solver' scheme is also formulated, as a hybrid between different solver
methods. Practical examples are carried out in several insulating and metallic
systems with 10^3-10^5 atoms. All the theories provide general guiding
principles of constructing an optimal calculation for simulating nanostructure
processes, since a nanostructured system consists of several competitive
regions, such as bulk and surface regions, and the simulation is designed to
reproduce the competition with an optimal computational cost.Comment: 19 pages, 6 figures. To appear in J. Phys. Cond. Matt. A preprint PDF
file in better graphics is available at
http://fujimac.t.u-tokyo.ac.jp/lses/index_e.htm
Experimental investigation of the Landau-Pomeranchuk-Migdal effect in low-Z targets
In the CERN NA63 collaboration we have addressed the question of the
potential inadequacy of the commonly used Migdal formulation of the
Landau-Pomeranchuk-Migdal (LPM) effect by measuring the photon emission by 20
and 178 GeV electrons in the range 100 MeV - 4 GeV, in targets of
LowDensityPolyEthylene (LDPE), C, Al, Ti, Fe, Cu, Mo and, as a reference
target, Ta. For each target and energy, a comparison between simulated values
based on the LPM suppression of incoherent bremsstrahlung is shown, taking
multi-photon effects into account. For these targets and energies, we find that
Migdal's theoretical formulation is adequate to a precision of better than
about 5%, irrespective of the target substance.Comment: 8 pages, 13 figure
Maps of complex motion selectivity in the superior temporal cortex of the alert macaque monkey: a double-label 2-deoxyglucose study
The superior temporal sulcus (STS) of the macaque monkey contains multiple visual areas. Many neurons within these regions respond selectively to motion direction and to more complex motion patterns, such as expansion, contraction and rotation. Single-unit recording and optical recording studies in MT/MST suggest that cells with similar tuning properties are clustered into columns extending through multiple cortical layers. In this study, we used a double-label 2-deoxyglucose technique in awake, behaving macaque monkeys to clarify this functional organization. This technique allowed us to label, in a single animal, two populations of neurons responding to two different visual stimuli. In one monkey we compared expansion with contraction; in a second monkey we compared expansion with clockwise rotation. Within the STS we found a patchy arrangement of cortical columns with alternating stimulus selectivity: columns of neurons preferring expansion versus contraction were more widely separated than those selective for expansion versus rotation. This mosaic of interdigitating columns on the floor and posterior bank of the STS included area MT and some neighboring regions of cortex, perhaps including area MST
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