49 research outputs found
Re-entrant resonant tunneling
We study the effect of electron-electron interactions on the
resonant-tunneling spectroscopy of the localized states in a barrier. Using a
simple model of three localized states, we show that, due to the Coulomb
interactions, a single state can give rise to two resonant peaks in the
conductance as a function of gate voltage, G(Vg). We also demonstrate that an
additional higher-order resonance with Vg-position in between these two peaks
becomes possibile when interactions are taken into account. The corresponding
resonant-tunneling process involves two-electron transitions. We have observed
both these effects in GaAs transistor microstructures by studying the time
evolution of three adjacent G(Vg) peaks caused by fluctuating occupation of an
isolated impurity (modulator). The heights of the two stronger peaks exibit
in-phase fluctuations. The phase of fluctuations of the smaller middle peak is
opposite. The two stronger peaks have their origin in the same localized state,
and the third one corresponds to a co-tunneling process.Comment: 9 pages, REVTeX, 4 figure
Enhanced fluctuations of the tunneling density of states near bottoms of Landau bands measured by a local spectrometer
We have found that the local density of states fluctuations (LDOSF) in a
disordered metal, detected using an impurity in the barrier as a spectrometer,
undergo enhanced (with respect to SdH and dHvA effects) oscillations in strong
magnetic fields, omega _c\tau > 1. We attribute this to the dominant role of
the states near bottoms of Landau bands which give the major contribution to
the LDOSF and are most strongly affected by disorder. We also demonstrate that
in intermediate fields the LDOSF increase with B in accordance with the results
obtained in the diffusion approximation.Comment: 4 pages, 4 figure
Effect of the Coulomb repulsion on the {\it ac} transport through a quantum dot
We calculate in a linear response the admittance of a quantum dot out of
equilibrium. The interaction between two electrons with opposite spins
simultaneously residing on the resonant level is modeled by an Anderson
Hamiltonian. The electron correlations lead to the appearence of a new feature
in the frequency dependence of the conductance. For certain parameter values
there are two crossover frequencies between a capacitive and an inductive
behavior of the imaginary part of the admittance. The experimental implications
of the obtained results are briefly discussed.Comment: 13 pages, REVTEX 3.0, 2 .ps figures from [email protected],
NUB-308
Correlation-function spectroscopy of inelastic lifetime in heavily doped GaAs heterostructures
Measurements of resonant tunneling through a localized impurity state are
used to probe fluctuations in the local density of states of heavily doped
GaAs. The measured differential conductance is analyzed in terms of correlation
functions with respect to voltage. A qualitative picture based on the scaling
theory of Thouless is developed to relate the observed fluctuations to the
statistics of single particle wavefunctions. In a quantitative theory
correlation functions are calculated. By comparing the experimental and
theoretical correlation functions the effective dimensionality of the emitter
is analyzed and the dependence of the inelastic lifetime on energy is
extracted.Comment: 41 pages, 14 figure
Acoustic Phonon-Assisted Resonant Tunneling via Single Impurities
We perform the investigations of the resonant tunneling via impurities
embedded in the AlAs barrier of a single GaAs/AlGaAs heterostructure. In the
characteristics measured at 30mK, the contribution of individual donors
is resolved and the fingerprints of phonon assistance in the tunneling process
are seen. The latter is confirmed by detailed analysis of the tunneling rates
and the modeling of the resonant tunneling contribution to the current.
Moreover, fluctuations of the local structure of the DOS (LDOS) and Fermi edge
singularities are observed.Comment: accepted in Phys. Rev.
An astrocyte-dependent mechanism for neuronal rhythmogenesis
Communication between neurons rests on their capacity to change their firing pattern to encode different messages. For several vital functions, such as respiration and mastication, neurons need to generate a rhythmic firing pattern. Here we show in the rat trigeminal sensori-motor circuit for mastication that this ability depends on regulation of the extracellular Ca2+ concentration ([Ca2+]e) by astrocytes. In this circuit, astrocytes respond to sensory stimuli that induce neuronal rhythmic activity, and their blockade with a Ca2+ chelator prevents neurons from generating a rhythmic bursting pattern. This ability is restored by adding S100b, an astrocytic Ca2+-binding protein, to the extracellular space, while application of an anti-S100b antibody prevents generation of rhythmic activity. These results indicate that astrocytes regulate a fundamental neuronal property: the capacity to change firing pattern. These findings may have broad implications for many other neural networks whose functions depend on the generation of rhythmic activity
Studies on the Nutrition of Macropodine Marsupials V.* Microbial Fermentation in the Forestomach of Thylogale thetis and Macropus eugenii
Rates of volatile fatty acid (VFA) and ammonia production at different sites along the forestomach of Thylogale thetis, the red-necked pademelon, and Macropus eugenii, the tammar wallaby, were measured both in vitro and in vivo. Estimates of the flow of microbial nitrogen from the stomach in vivo were also obtained. In both species VFA production was faster both in vitro (