965 research outputs found
FIBONACCI SUPERLATTICES OF NARROW-GAP III-V SEMICONDUCTORS
We report theoretical electronic structure of Fibonacci superlattices of
narrow-gap III-V semiconductors. Electron dynamics is accurately described
within the envelope-function approximation in a two-band model.
Quasiperiodicity is introduced by considering two different III-V semiconductor
layers and arranging them according to the Fibonacci series along the growth
direction. The resulting energy spectrum is then found by solving exactly the
corresponding effective-mass (Dirac-like) wave equation using tranfer-matrix
techniques. We find that a self-similar electronic spectrum can be seen in the
band structure. Electronic transport properties of samples are also studied and
related to the degree of spatial localization of electronic envelope-functions
via Landauer resistance and Lyapunov coefficient. As a working example, we
consider type II InAs/GaSb superlattices and discuss in detail our results in
this system.Comment: REVTeX 3.0, 16 pages, 8 figures available upon request. To appear in
Semiconductor Science and Technolog
Nonlinear instability in flagellar dynamics: a notel modulation mechanism in sperm migration
Throughout biology, cells and organisms use flagella and cilia to propel fluid and achieve motility. The beating of these organelles, and the corresponding ability to sense, respond to and modulate this beat is central to many processes in health and disease. While the mechanics of flagellum–fluid interaction has been the subject of extensive mathematical studies, these models have been restricted to being geometrically linear or weakly nonlinear, despite the high curvatures observed physiologically. We study the effect of geometrical nonlinearity, focusing on the spermatozoon flagellum. For a wide range of physiologically relevant parameters, the nonlinear model predicts that flagellar compression by the internal forces initiates an effective buckling behaviour, leading to a symmetry-breaking bifurcation that causes profound and complicated changes in the waveform and swimming trajectory, as well as the breakdown of the linear theory. The emergent waveform also induces curved swimming in an otherwise symmetric system, with the swimming trajectory being sensitive to head shape—no signalling or asymmetric forces are required. We conclude that nonlinear models are essential in understanding the flagellar waveform in migratory human sperm; these models will also be invaluable in understanding motile flagella and cilia in other systems
Dynamical approach to chains of scatterers
Linear chains of quantum scatterers are studied in the process of
lengthening, which is treated and analysed as a discrete dynamical system
defined over the manifold of scattering matrices. Elementary properties of such
dynamics relate the transport through the chain to the spectral properties of
individual scatterers. For a single-scattering channel case some new light is
shed on known transport properties of disordered and noisy chains, whereas
translationally invariant case can be studied analytically in terms of a simple
deterministic dynamical map. The many-channel case was studied numerically by
examining the statistical properties of scatterers that correspond to a certain
type of transport of the chain i.e. ballistic or (partially) localised.Comment: 16 pages, 7 figure
One-dimensional models of disordered quantum wires: general formalism
In this work we describe, compile and generalize a set of tools that can be
used to analyse the electronic properties (distribution of states, nature of
states, ...) of one-dimensional disordered compositions of potentials. In
particular, we derive an ensemble of universal functional equations which
characterize the thermodynamic limit of all one-dimensional models and which
only depend formally on the distributions that define the disorder. The
equations are useful to obtain relevant quantities of the system such as
density of states or localization length in the thermodynamic limit
THE IMPACT OF TEAM EMPOWERMENT ON VIRTUAL TEAM PERFORMANCE: THE MODERATING ROLE OF FACE-TO-FACE INTERACTION.
We investigated the relationship between team empowerment and virtual team performance and the moderating role of the extent of face-to-face interaction using 35 sales and service virtual teams in a high-technology organization. Team empowerment was positively related to two independent assessments of virtual team performance--process improvement and customer satisfaction. Further, the number of face-to-face meetings moderated the relationship between team empowerment and process improvement: team empowerment was a stronger predictor for teams that met face-to-face less, rather than more, frequently
Improved Experimental Limits on the Production of Magnetic Monopoles
We present new limits on low mass accelerator-produced point-like Dirac
magnetic monopoles trapped and bound in matter surrounding the D\O collision
region of the Tevatron at Fermilab (experiment E-882). In the context of a
Drell-Yan mechanism, we obtain cross section limits for the production of
monopoles with magnetic charge values of 1, 2, 3, and 6 times the minimum Dirac
charge of the order of picobarns, some hundred times smaller than found in
similar previous Fermilab searches. Mass limits inferred from these cross
section limits are presented.Comment: 5 pages, 4 eps figures, REVTe
Persistent Currents in Multichannel Interacting Systems
Persistent currents of disordered multichannel mesoscopic rings of spinless
interacting fermions threaded by a magnetic flux are calculated using exact
diagonalizations and self-consistent Hartree-Fock methods. The validity of the
Hartree-Fock approximation is controled by a direct comparison with the exact
results on small clusters. For sufficiently large disorder
(diffusive regime), the effect of repulsive interactions on the current
distribution is to slightly decrease its width (mean square current) but to
{\it increase} its mean value (mean current). This effect is stronger in the
case of a long range repulsion. Our results suggest that the coupling between
the chains is essential to understand the large currents observed
experimentally.Comment: Revised version, uuencoded compressed file including fig
Statistics of the One-Electron Current in a One-Dimensional Mesoscopic Ring at Arbitrary Magnetic Fields
The set of moments and the distribution function of the one-electron current
in a one-dimensional disordered ring with arbitrary magnetic flux are
calculated.Comment: 10 pages; Plain TeX; IFUM 448/FT; to appear in J. Stat. Phy
Multivisceral intestinal transplantation: Surgical pathology
We report the diagnostic surgical pathology of two children who underwent multivisceral abdominal transplantation and survived for 1 month and 6 months. There is little relevant literature, and diagnostic criteria for the various clinical possibilities are not established; this is made more complicated by the simultaneous occurrence of more than one process. We based our interpretations on conventional histology, augmented with immunohistology, including HLA staining that distinguished graft from host cells in situ. In some instances functional analysis of T cells propagated from the same biopsies was available and was used to corroborate morphological interpretations. A wide spectrum of changes was encountered. Graft-versus-host disease, a prime concern before surgery, was not seen. Rejection was severe in 1 patient, not present in the other, and both had evidence of lymphoproliferative disease, which was related to Epstein-Barr virus. Bacterial translocation through the gut wall was also a feature in both children. This paper documents and illustrates the various diagnostic possibilities.. © 1989 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted
Suppression of Persistent Currents in 1-D Disordered Rings by Coulomb Interaction
Effects of Coulomb interaction on persistent currents in disordered
one-dimensional rings are numerically investigated. First of all effectiveness
of the Hartree-Fock approximation is established on small systems. Then the
calculations are done for systems with 40 electrons in 100 sites. It is found
that the amplitude of the average persistent current in the diffusive regime is
suppressed as the strength of the Coulomb interaction increases. The
suppression of the current is stronger in larger rings than in smaller ones.
The enhancement of the current by the electron-electron interaction was not
observed in the diffusive regime.Comment: 9 pages (RevTeX), 4 figures available upon request
([email protected]), KCMG-preprint-HK
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