95 research outputs found
Correlation induced switching of local spatial charge distribution in two-level system
We present theoretical investigation of spatial charge distribution in the
two-level system with strong Coulomb correlations by means of Heisenberg
equations analysis for localized states total electron filling numbers taking
into account pair correlations of local electron density. It was found that
tunneling current through nanometer scale structure with strongly coupled
localized states causes Coulomb correlations induced spatial redistribution of
localized charges. Conditions for inverse occupation of two-level system in
particular range of applied bias caused by Coulomb correlations have been
revealed. We also discuss possibility of charge manipulation in the proposed
system.Comment: 6 pages, 4 figures Submitted to JETP Letter
Non-Universal Behavior of Finite Quantum Hall Systems as a Result of Weak Macroscopic Inhomogeneities
We show that, at low temperatures, macroscopic inhomogeneities of the
electron density in the interior of a finite sample cause a reduction in the
measured conductivity peak heights compared to the
universal values previously predicted for infinite homogeneous samples. This
effect is expected to occur for the conductivity peaks measured in standard
experimental geometries such as the Hall bar and the Corbino disc. At the
lowest temperatures, the decrease in is found to
saturate at values proportional to the difference between the adjacent plateaus
in , with a prefactor which depends on the particular realization
of disorder in the sample. We argue that this provides a possible explanation
of the ``non-universal scaling'' of observed in a
number of experiments. We also predict an enhancement of the ``non-local''
resistance due to the macroscopic inhomogeneities. We argue that, in the Hall
bar with a sharp edge, the enhanced ``non-local'' resistance and the size
corrections to the ``local'' resistance are directly related. Using
this relation, we suggest a method by which the finite-size corrections may be
eliminated from and in this case.Comment: REVTEX 3.0 file (38 pages) + 5 postscript figures in uuencoded
format. Revised version includes an additional figure showing unpublished
experimental dat
Phase diagram of aggregation of oppositely charged colloids in salty water
Aggregation of two oppositely charged colloids in salty water is studied. We
focus on the role of Coulomb interaction in strongly asymmetric systems in
which the charge and size of one colloid is much larger than the other one. In
the solution, each large colloid (macroion) attracts certain number of
oppositely charged small colloids (-ion) to form a complex. If the
concentration ratio of the two colloids is such that complexes are not strongly
charged, they condense in a macroscopic aggregate. As a result, the phase
diagram in a plane of concentrations of two colloids consists of an aggregation
domain sandwiched between two domains of stable solutions of complexes. The
aggregation domain has a central part of total aggregation and two wings
corresponding to partial aggregation. A quantitative theory of the phase
diagram in the presence of monovalent salt is developed. It is shown that as
the Debye-H\"{u}ckel screening radius decreases, the aggregation domain
grows, but the relative size of the partial aggregation domains becomes much
smaller. As an important application of the theory, we consider solutions of
long double-helix DNA with strongly charged positive spheres (artificial
chromatin). We also consider implications of our theory for in vitro
experiments with the natural chromatin. Finally, the effect of different shapes
of macroions on the phase diagram is discussed.Comment: 10 pages, 9 figures. The text is rewritten, but results are not
change
Relation between Barrier Conductance and Coulomb Blockade Peak Splitting for Tunnel-Coupled Quantum Dots
We study the relation between the barrier conductance and the Coulomb
blockade peak splitting for two electrostatically equivalent dots connected by
tunneling channels with bandwidths much larger than the dot charging energies.
We note that this problem is equivalent to a well-known single-dot problem and
present solutions for the relation between peak splitting and barrier
conductance in both the weak and strong coupling limits. Results are in good
qualitative agreement with the experimental findings of F. R. Waugh et al.Comment: 19 pages (REVTeX 3.0), 3 Postscript figure
Harmonic Solid Theory of Photoluminescence in the High Field Two-Dimensional Wigner Crystal
Motivated by recent experiments on radiative recombination of two-dimensional
electrons in acceptor doped GaAs-AlGaAs heterojunctions as well as the success
of a harmonic solid model in describing tunneling between two-dimensional
electron systems, we calculate within the harmonic approximation and the time
dependent perturbation theory the line shape of the photoluminescence spectrum
corresponding to the recombination of an electron with a hole bound to an
acceptor atom. The recombination process is modeled as a sudden perturbation of
the Hamiltonian for the in-plane degrees of freedom of the electron. We include
in the perturbation, in addition to changes in the equilibrium positions of
electrons, changes in the curvatures of the harmonically approximated
potential. The computed spectra have line shapes similar to that seen in a
recent experiment. The spectral width, however, is roughly a factor of 3
smaller than that seen in experiment if one assumes a perfect Wigner crystal
for the initial state state of the system, whereas a simple random disorder
model yields a width a factor of 3 too large. We speculate on the possible
mechanisms that may lead to better quantitative agreement with experiment.Comment: 22 pages, RevTex, 8 figures. Submitted to the Physical Review
Universality in an integer Quantum Hall transition
An integer Quantum Hall effect transition is studied in a modulation doped
p-SiGe sample. In contrast to most examples of such transitions the
longitudinal and Hall conductivities at the critical point are close to 0.5 and
1.5 (e^2/h), the theoretically expected values. This allows the extraction of a
scattering parameter, describing both conductivity components, which depends
exponentially on filling factor. The strong similarity of this functional form
to those observed for transitions into the Hall insulating state and for the
B=0 metal- insulator transition implies a universal quantum critical behaviour
for the transitions. The observation of this behaviour in the integer Quantum
Hall effect, for this particular sample, is attributed to the short-ranged
character of the potential associated with the dominant scatterers
Theory of Exciton Recombination from the Magnetically Induced Wigner Crystal
We study the theory of itinerant-hole photoluminescence of two-dimensional
electron systems in the regime of the magnetically induced Wigner crystal. We
show that the exciton recombination transition develops structure related to
the presence of the Wigner crystal. The form of this structure depends strongly
on the separation between the photo-excited hole and the plane of the
two-dimensional electron gas. When is small compared to the magnetic
length, additional peaks appear in the spectrum due to the recombination of
exciton states with wavevectors equal to the reciprocal lattice vectors of the
crystal. For larger than the magnetic length, the exciton becomes strongly
confined to an interstitial site of the lattice, and the structure in the
spectrum reflects the short-range correlations of the Wigner crystal. We derive
expressions for the energies and the radiative lifetimes of the states
contributing to photoluminescence, and discuss how the results of our analysis
compare with experimental observations.Comment: 10 pages, no figures, uses Revtex and multicol.st
Explanation for the Resistivity Law in Quantum Hall System
We consider a 2D electron system in a strong magnetic field, where the local
Hall resistivity is a function of position and
is small compared to . Particularly if the
correlations fall off slowly with distance, or if fluctuations exist on several
length scales, one finds that the macroscopic longitudinal resistivity
is only weakly dependent on and is approximately proportional to
the magnitude of fluctuations in . This may provide an explanation
of the empirical law where is
the Hall resistance, and is the magnetic field.Comment: 11 pages (REVTeX 3.0). Revised Version. Complete postscript file for
this paper is available on the World Wide Web at
http://cmtw.harvard.edu/~simon/ ; Preprint number HU-CMT-94S0
Transport Through Quantum Melts
We discuss superconductor to insulator and quantum Hall transitions which are
first order in the clean limit. Disorder creates a nearly percolating network
of the minority phase. Electrical transport is dominated by tunneling or
activation through the saddle point junctions, whose typical resistance is
calculated as a function of magnetic field. In the Boltzmann regime, this
approach yields resistivity laws which agree with recent experiments in both
classes of systems. We discuss the origin of dissipation at zero temperature.Comment: 4 pages, 1 figur
Thermoelectric Response of an Interacting Two-Dimensional Electron Gas in Quantizing Magnetic Field
We present a discussion of the linear thermoelectric response of an
interacting electron gas in a quantizing magnetic field. Boundary currents can
carry a significant fraction of the net current passing through the system. We
derive general expressions for the bulk and boundary components of the number
and energy currents. We show that the local current density may be described in
terms of ``transport'' and ``internal magnetization'' contributions. The latter
carry no net current and are not observable in standard transport experiments.
We show that although Onsager relations cannot be applied to the local current,
they are valid for the transport currents and hence for the currents observed
in standard transport experiments. We relate three of the four thermoelectric
response coefficients of a disorder-free interacting two-dimensional electron
gas to equilibrium thermodynamic quantities. In particular, we show that the
diffusion thermopower is proportional to the entropy per particle, and we
compare this result with recent experimental observations.Comment: 18 pages, 2 postscript figures included. Revtex with epsf.tex and
multicol.sty. In the revised version, the comparison with experimental
observations at is extended to include the possibility of
corrections due to weak impurity scattering. The conclusions that we reach
regarding the applicability of the composite fermion model at these filling
fractions are not affecte
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