218 research outputs found
Zero-bias anomalies of point contact resistance due to adiabatic electron renormalization of dynamical defects
We study effect of the adiabatic electron renormalization on the parameters
of the dynamical defects in the ballistic metallic point contact. The upper
energy states of the ``dressed'' defect are shown to give a smaller
contribution to a resistance of the contact than the lower energy ones. This
holds both for the "classical" renormalization related to defect coupling with
average local electron density and for the "mesoscopic" renormalization caused
by the mesoscopic fluctuations of electronic density the dynamical defects are
coupled with. In the case of mesoscopic renormalization one may treat the
dynamical defect as coupled with Friedel oscillations originated by the other
defects, both static and mobile. Such coupling lifts the energy degeneracy of
the states of the dynamical defects giving different mesoscopic contribution to
resistance, and provides a new model for the fluctuator as for the object
originated by the electronic mesoscopic disorder rather than by the structural
one. The correlation between the defect energy and the defect contribution to
the resistance leads to zero-temperature and zero-bias anomalies of the point
contact resistance.
A comparison of these anomalies with those predicted by the Two Channel Kondo
Model (TCKM) is made. It is shown, that although the proposed model is based on
a completely different from TCKM physical background, it leads to a zero-bias
anomalies of the point contact resistance, which are qualitatively similar to
TCKM predictions.Comment: 6 pages, to be published in Phys. Rev.
Study of Genotype X Environment Interaction in Alfalfa Forage Yield
The response of alfalfa (Medicago sativa L.) forage yield to eight Alberta test sites was studied for the 1990 and 1991 production years. Cluster analysis was used to group locations and cultivars. Analyses of variances indicated genotype x environment (location) interaction for the first cut yield, the total yield and the difference between first and second cut yields. The Brooks, Bow Island (irrigation), Bow Island (dryland) and Provost locations always clustered together indicating that three of these four test sites may be eliminated without sacrificing reliability
Magnetoresistance of p-GaAs/AlGaAs structures in the vicinity of metal-insulator transition: Effect of superconducting leads
Experimental and theoretical studies on transport in semiconductor samples
with superconducting electrodes are reported. We focus on the samples close to
metal-insulator transition. In metallic samples, a peak of negative
magnetoresistance at fields lower than critical magnetic field of the leads was
observed. This peak is attributed to restoration of a single-particle tunneling
emerging with suppression of superconductivity. The experimental results allow
us to estimate tunneling transparency of the boundary between superconductor
and metal. In contrast, for the insulating samples no such a peak was observed.
We explain this behavior as related to properties of transport through the
contact between superconductor and hopping conductor. This effect can be used
to discriminate between weak localization and strong localization regimes.Comment: 10 pages, 3 fi
Point contact spectroscopy of hopping transport: effects of a magnetic field
The conductance of a point contact between two hopping insulators is expected
to be dominated by the individual localized states in its vicinity. Here we
study the additional effects due to an external magnetic field. Combined with
the measured conductance, the measured magnetoresistance provides detailed
information on these states (e.g. their localization length, the energy
difference and the hopping distance between them). We also calculate the
statistics of this magnetoresistance, which can be collected by changing the
gate voltage in a single device. Since the conductance is dominated by the
quantum interference of particular mesoscopic structures near the point
contact, it is predicted to exhibit Aharonov-Bohm oscillations, which yield
information on the geometry of these structures. These oscillations also depend
on local spin accumulation and correlations, which can be modified by the
external field. Finally, we also estimate the mesoscopic Hall voltage due to
these structures.Comment: 7 pages, 5 figur
Magnetoresistance in semiconductor structures with hopping conductivity: effects of random potential and generalization for the case of acceptor states
We reconsider the theory of magnetoresistance in hopping semiconductors.
First, we have shown that the random potential of the background impurities
affects significantly preexponential factor of the tunneling amplitude which
becomes to be a short-range one in contrast to the long-range one for purely
Coulomb hopping centers. This factor to some extent suppresses the negative
interference magnetoresistance and can lead to its decrease with temperature
decrease which is in agreement with earlier experimental observations. We have
also extended the theoretical models of positive spin magnetoresistance, in
particular, related to a presence of doubly occupied states (corresponding to
the upper Hubbard band) to the case of acceptor states in 2D structures. We
have shown that this mechanism can dominate over classical wave-shrinkage
magnetoresistance at low temperatures. Our results are in semi-quantitative
agreement with experimental data.Comment: 19 pages, 3 figure
Nonlinear absorption of surface acoustic waves by composite fermions
Absorption of surface acoustic waves by a two-dimensional electron gas in a
perpendicular magnetic field is considered. The structure of such system at the
filling factor close to 1/2 can be understood as a gas of {\em composite
fermions}. It is shown that the absorption at can be strongly
nonlinear, while small deviation form 1/2 will restore the linear absorption.
Study of nonlinear absorption allows one to determine the force acting upon the
composite fermions from the acoustic wave at turning points of their
trajectories.Comment: 7 pages, 1 figure, submitted to Europhysics letter
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