394 research outputs found
Effective mass in quasi two-dimensional systems
The effective mass of the quasiparticle excitations in quasi two-dimensional
systems is calculated analytically. It is shown that the effective mass
increases sharply when the density approaches the critical one of
metal-insulator transition. This suggests a Mott type of transition rather than
an Anderson like transition.Comment: 3 pages 3 figure
Conductivity in quasi two-dimensional systems
The conductivity in quasi two-dimensional systems is calculated using the
quantum kinetic equation. Linearizing the Lenard-Balescu collision integral
with the extension to include external field dependences allows one to
calculate the conductivity with diagrams beyond the GW approximation including
maximally crossed lines. Consequently the weak localization correction as an
interference effect appears here from the field dependence of the collision
integral (the latter dependence sometimes called intra-collisional field
effect). It is shown that this weak localization correction has the same origin
as the Debye-Onsager relaxation effect in plasma physics. The approximation is
applied to a system of quasi two-dimensional electrons in hetero-junctions
which interact with charged and neutral impurities and the low temperature
correction to the conductivity is calculated analytically. It turns out that
the dynamical screening due to charged impurities leads to a linear temperature
dependence, while the scattering from neutral impurities leads to the usual
Fermi-liquid behavior. By considering an appropriate mass action law to
determine the ratio of charged to neutral impurities we can describe the
experimental metal-insulator transition at low temperatures as a Mott-Hubbard
transition.Comment: 7 pages 7 pages appendix 11 figure
In-medium two-nucleon properties in high electric fields
The quantum mechanical two - particle problem is considered in hot dense
nuclear matter under the influence of a strong electric field such as the field
of the residual nucleus in heavy - ion reactions. A generalized
Galitskii-Bethe-Salpeter equation is derived and solved which includes
retardation and field effects. Compared with the in-medium properties in the
zero-field case, bound states are turned into resonances and the scattering
phase shifts are modified. Four effects are observed due to the applied field:
(i) A suppression of the Pauli-blocking below nuclear matter densities, (ii)
the onset of pairing occurs already at higher temperatures due to the field,
(iii) a field dependent finite lifetime of deuterons and (iv) the imaginary
part of the quasiparticle self-energy changes its sign for special values of
density and temperatures indicating a phase instability. The latter effect may
influence the fragmentation processes. The lifetime of deuterons in a strong
Coulomb field is given explicitly.Comment: ps file + 7 figures (eps
Decay estimates for variable coefficient wave equations in exterior domains
In this article we consider variable coefficient, time dependent wave
equations in exterior domains. We prove localized energy estimates if the
domain is star-shaped and global in time Strichartz estimates if the domain is
strictly convex.Comment: 15 pages. In the new version, some typos are fixed and a minor
correction was made to the proof of Lemma 1
Stability and Instability of Extreme Reissner-Nordstr\"om Black Hole Spacetimes for Linear Scalar Perturbations I
We study the problem of stability and instability of extreme
Reissner-Nordstrom spacetimes for linear scalar perturbations. Specifically, we
consider solutions to the linear wave equation on a suitable globally
hyperbolic subset of such a spacetime, arising from regular initial data
prescribed on a Cauchy hypersurface crossing the future event horizon. We
obtain boundedness, decay and non-decay results. Our estimates hold up to and
including the horizon. The fundamental new aspect of this problem is the
degeneracy of the redshift on the event horizon. Several new analytical
features of degenerate horizons are also presented.Comment: 37 pages, 11 figures; published version of results contained in the
first part of arXiv:1006.0283, various new results adde
Time management and nectar flow: flower handling and suction feeding in long-proboscid flies (Nemestrinidae: Prosoeca)
A well-developed suction pump in the head represents an important adaptation for nectar-feeding insects, such as Hymenoptera, Lepidoptera and Diptera. This pumping organ creates a pressure gradient along the proboscis, which is responsible for nectar uptake. The extremely elongated proboscis of the genus Prosoeca (Nemestrinidae) evolved as an adaptation to feeding from long, tubular flowers. According to the functional constraint hypothesis, nectar uptake through a disproportionately elongated, straw-like proboscis increases flower handling time and consequently lowers the energy intake rate. Due to the conspicuous length variation of the proboscis of Prosoeca, individuals with longer proboscides are hypothesised to have longer handling times. To test this hypothesis, we used field video analyses of flower-visiting behaviour, detailed examinations of the suction pump morphology and correlations of proboscis length with body length and suction pump dimensions. Using a biomechanical framework described for nectar-feeding Lepidoptera in relation to proboscis length and suction pump musculature, we describe and contrast the system in long-proboscid flies. Flies with longer proboscides spent significantly more time drinking from flowers. In addition, proboscis length and body length showed a positive allometric relationship. Furthermore, adaptations of the suction pump included an allometric relationship between proboscis length and suction pump muscle volume and a combination of two pumping organs. Overall, the study gives detailed insight into the adaptations required for long-proboscid nectar feeding, and comparisons with other nectar-sucking insects allow further considerations of the evolution of the suction pump in insects with sucking mouthparts. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00114-013-1114-6) contains supplementary material, which is available to authorized users
Quasiparticle transport equation with collision delay. II. Microscopic Theory
For a system of non-interacting electrons scattered by neutral impurities, we
derive a modified Boltzmann equation that includes quasiparticle and virial
corrections. We start from quasiclassical transport equation for
non-equilibrium Green's functions and apply limit of small scattering rates.
Resulting transport equation for quasiparticles has gradient corrections to
scattering integrals. These gradient corrections are rearranged into a form
characteristic for virial corrections
Exactly solvable model of three interacting particles in an external magnetic field
The quantum mechanical problem of three identical particles, moving in a
plane and interacting pairwise via a spring potential, is solved exactly in the
presence of a magnetic field. Calculations of the pair--correlation function,
mean distance and the cluster area show a quantization of these parameters.
Especially the pair-correlation function exhibits a certain number of maxima
given by a quantum number. We obtain Jastrow pre-factors which lead to an
exchange correlation hole of liquid type, even in the presence of the
attractive interaction between the identical electrons.Comment: 8 pages 3 figure
Off shell behaviour of the in medium nucleon-nucleon cross section
The properties of nucleon-nucleon scattering inside dense nuclear matter are
investigated. We use the relativistic Brueckner-Hartree-Fock model to determine
on-shell and half off-shell in-medium transition amplitudes and cross sections.
At finite densities the on-shell cross sections are generally suppressed. This
reduction is, however, less pronounced than found in previous works. In the
case that the outgoing momenta are allowed to be off energy shell the
amplitudes show a strong variation with momentum. This description allows to
determine in-medium cross sections beyond the quasi-particle approximation
accounting thereby for the finite width which nucleons acquire in the dense
nuclear medium. For reasonable choices of the in-medium nuclear spectral width,
i.e. MeV, the resulting total cross sections are, however,
reduced by not more than about 25% compared to the on-shell values. Off-shell
effect are generally more pronounced at large nuclear matter densities.Comment: 31 pages Revtex, 12 figures, typos corrected, to appear in Phys. Rev.
Bernoulli potential in type-I and weak type-II supercoductors: II. Surface dipole
The Budd-Vannimenus theorem is modified to apply to superconductors in the
Meissner state. The obtained identity links the surface value of the
electrostatic potential to the density of free energy at the surface which
allows one to evaluate the electrostatic potential observed via the capacitive
pickup without the explicit solution of the charge profile.Comment: 7 pages, 1 figur
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