1,224 research outputs found
Recommended from our members
Tailor-made composite functions as tools in model choice: the case of sigmoidal vs bi-linear growth profiles
BACKGROUND: Roots are the classical model system to study the organization and dynamics of organ growth zones. Profiles of the velocity of root elements relative to the apex have generally been considered to be sigmoidal. However, recent high-resolution measurements have yielded bi-linear profiles, suggesting that sigmoidal profiles may be artifacts caused by insufficient spatio-temporal resolution. The decision whether an empirical velocity profile follows a sigmoidal or bi-linear distribution has consequences for the interpretation of the underlying biological processes. However, distinguishing between sigmoidal and bi-linear curves is notoriously problematic. A mathematical function that can describe both types of curve equally well would allow them to be distinguished by automated curve-fitting. RESULTS: On the basis of the mathematical requirements defined, we created a composite function and tested it by fitting it to sigmoidal and bi-linear models with different noise levels (Monte-Carlo datasets) and to three experimental datasets from roots of Gypsophila elegans, Aurinia saxatilis, and Arabidopsis thaliana. Fits of the function proved robust with respect to noise and yielded statistically sound results if care was taken to identify reasonable initial coefficient values to start the automated fitting procedure. Descriptions of experimental datasets were significantly better than those provided by the Richards function, the most flexible of the classical growth equations, even in cases in which the data followed a smooth sigmoidal distribution. CONCLUSION: Fits of the composite function introduced here provide an independent criterion for distinguishing sigmoidal and bi-linear growth profiles, but without forcing a dichotomous decision, as intermediate solutions are possible. Our function thus facilitates an unbiased, multiple-working hypothesis approach. While our discussion focusses on kinematic growth analysis, this and similar tailor-made functions will be useful tools wherever models of steadily or abruptly changing dependencies between empirical parameters are to be compared
Properties of 1D two-barrier quantum pump with harmonically oscillating barriers
We study a one-dimensional quantum pump composed of two oscillating
delta-functional barriers. The linear and non-linear regimes are considered.
The harmonic signal applied to any or both barriers causes the stationary
current. The direction and value of the current depend on the frequency,
distance between barriers, value of stationary and oscillating parts of barrier
potential and the phase shift between alternating voltages.Comment: 7 pages, 8 figure
Conductivity of 2D many-component electron gas, partially-quantized by magnetic field
The 2D semimetal consisting of heavy holes and light electrons is studied.
The consideration is based on assumption that electrons are quantized by
magnetic field while holes remain classical. We assume also that the
interaction between components is weak and the conversion between components is
absent. The kinetic equation for holes colliding with quantized electrons is
utilized. It has been stated that the inter-component friction and
corresponding correction to the dissipative conductivity {\it do
not vanish at zero temperature} due to degeneracy of the Landau levels. This
correction arises when the Fermi level crosses the Landau level.
The limits of kinetic equation applicability were found. We also study the
situation of kinetic memory when particles repeatedly return to the points of
their meeting.Comment: 13 pages, 1 figur
On the effect of far impurities on the density of states of two-dimensional electron gas in a strong magnetic field
The effect of impurities situated at different distances from a
two-dimensional electron gas on the density of states in a strong magnetic
field is analyzed. Based on the exact result of Brezin, Gross, and Itzykson, we
calculate the density of states in the whole energy range, assuming the Poisson
distribution of impurities in the bulk. It is shown that in the case of small
impurity concentration the density of states is qualitatively different from
the model case when all impurities are located in the plane of the
two-dimensional electron gas.Comment: 6 pages, 1 figure, submitted to JETP Letter
Two regimes for effects of surface disorder on the zero-bias conductance peak of tunnel junctions involving d-wave superconductors
Impurity-induced quasiparticle bound states on a pair-breaking surface of a
d-wave superconductor are theoretically described, taking into account
hybridization of impurity- and surface-induced Andreev states. Further a theory
for effects of surface disorder (of thin impurity surface layer) on the
low-bias conductance of tunnel junctions is developed. We find a threshold
for surface impurity concentration , which separates the two regimes
for surface impurity effects on the zero-bias conductance peak (ZBCP). Below
the threshold, surface impurities do not broaden the ZBCP, but effectively
reduce its weight and generate impurity bands. For low impurity bands can
be, in principle, resolved experimentally, being centered at energies of bound
states induced by an isolated impurity on the surface. For larger
impurity bands are distorted, move to lower energies and, beginning with the
threshold concentration , become centered at zero energy. With
increasing above the threshold, the ZBCP is quickly destroyed in the case
of strong scatterers, while it is gradually suppressed and broaden in the
presence of weak impurity potentials. More realistic cases, taking into account
additional broadening, not related to the surface disorder, are also
considered.Comment: 9 pages, 7 figure
Photocurrent in nanostructures with asymmetric antidots
The steady current induced by electromagnetic field in a 2D system with
asymmetric scatterers is studied. The scatterers are assumed to be oriented
cuts with one diffusive and another specular sides. Besides, the existence of
isotropic impurity scatterers is assumed. This simple model simulates the
lattice of half-disk which have been studied numerically recently. The model
allows the exact solution in the framework of the kinetic equation. The static
current response in the second order of electric field is obtained. The
photogalvanic tensor contains both responses to linear and circular
polarization of electromagnetic field. The model possesses non-analyticity with
regards to the rate of impurity scattering.Comment: 9 pages, 6 figure
Density of States and Conductivity of Granular Metal or Array of Quantum Dots
The conductivity of a granular metal or an array of quantum dots usually has
the temperature dependence associated with variable range hopping within the
soft Coulomb gap of density of states. This is difficult to explain because
neutral dots have a hard charging gap at the Fermi level. We show that
uncontrolled or intentional doping of the insulator around dots by donors leads
to random charging of dots and finite bare density of states at the Fermi
level. Then Coulomb interactions between electrons of distant dots results in
the a soft Coulomb gap. We show that in a sparse array of dots the bare density
of states oscillates as a function of concentration of donors and causes
periodic changes in the temperature dependence of conductivity. In a dense
array of dots the bare density of states is totally smeared if there are
several donors per dot in the insulator.Comment: 13 pages, 15 figures. Some misprints are fixed. Some figures are
dropped. Some small changes are given to improve the organizatio
Giant Magnetoresistance Oscillations Induced by Microwave Radiation and a Zero-Resistance State in a 2D Electron System with a Moderate Mobility
The effect of a microwave field in the frequency range from 54 to 140
on the magnetotransport in a GaAs quantum well with AlAs/GaAs
superlattice barriers and with an electron mobility no higher than
is investigated. In the given two-dimensional system under
the effect of microwave radiation, giant resistance oscillations are observed
with their positions in magnetic field being determined by the ratio of the
radiation frequency to the cyclotron frequency. Earlier, such oscillations had
only been observed in GaAs/AlGaAs heterostructures with much higher mobilities.
When the samples under study are irradiated with a 140- microwave
field, the resistance corresponding to the main oscillation minimum, which
occurs near the cyclotron resonance, appears to be close to zero. The results
of the study suggest that a mobility value lower than
does not prevent the formation of zero-resistance states in magnetic field in a
two-dimensional system under the effect of microwave radiation.Comment: 4 pages, 2 figur
Transport in two dimensional periodic magnetic fields
Ballistic transport properties in a two dimensional electron gas are studied
numerically, where magnetic fields are perpendicular to the plane of two
dimensional electron systemsand periodically modulated both in and
directions. We show that there are three types of trajectories of classical
electron motions in this system; chaotic, pinned and runaway trajectories. It
is found that the runaway trajectories can explain the peaks of
magnetoresistance as a function of external magnetic fields, which is believed
to be related to the commensurability effect between the classical cyclotron
diameter and the period of magnetic modulation. The similarity with and
difference from the results in the antidot lattice are discussed.Comment: 4 pages, 7 figures, to appear in J. Phys. Soc. Jpn., vol. 67 (1998)
Novembe
Short--range impurity in the vicinity of a saddle point and the levitation of the 2D delocalized states in a magnetic field
The effect of a short--range impurity on the transmission through a
saddle--point potential for an electron, moving in a strong magnetic field, is
studied. It is demonstrated that for a random position of an impurity and
random sign of its potential the impurity--induced mixing of the Landau levels
diminishes {\em on average} the transmission coefficient. This results in an
upward shift (levitation) of the energy position of the delocalized state in a
smooth potential. The magnitude of the shift is estimated. It increases with
decreasing magnetic field as .Comment: LaTeX, 20 page
- …