126 research outputs found
Seismic and mineralogical structures of the lower mantle from probabilistic tomography
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95386/1/jgrb17106.pd
Visibility diagrams and experimental stripe structure in the quantum Hall effect
We analyze various properties of the visibility diagrams that can be used in
the context of modular symmetries and confront them to some recent experimental
developments in the Quantum Hall Effect. We show that a suitable physical
interpretation of the visibility diagrams which permits one to describe
successfully the observed architecture of the Quantum Hall states gives rise
naturally to a stripe structure reproducing some of the experimental features
that have been observed in the study of the quantum fluctuations of the Hall
conductance. Furthermore, we exhibit new properties of the visibility diagrams
stemming from the structure of subgroups of the full modular group.Comment: 8 pages in plain TeX, 7 figures in a single postscript fil
Charge Pumping in Carbon Nanotubes
We demonstrate charge pumping in semiconducting carbon nanotubes by a
traveling potential wave. From the observation of pumping in the nanotube
insulating state we deduce that transport occurs by packets of charge being
carried along by the wave. By tuning the potential of a side gate, transport of
either electron or hole packets can be realized. Prospects for the realization
of nanotube based single-electron pumps are discussed
Two-terminal conductance fluctuations in the integer quantum Hall regime
Motivated by recent experiments on the conductance fluctuations in mesoscopic
integr quantum Hall systems, we consider a model in which the Coulomb
interactions are incorporated into the picture of edge-state transport through
a single saddle-point. The occupancies of `classical' localised states in the
two-dimensional electron system change due to the interactions between
electrons when the gate voltage on top of the device is varied. The
electrostatic potential between the localised states and the saddle-point
causes fluctuations of the saddle-point potential and thus fluctuations of the
transmission probability of edge states. This simple model is studied
numerically and compared with the observation.Comment: 6 pages with 3 figures. To be published in Physical Review
Spin configurations of carbon nanotube in a nonuniform external potential
We study, theoretically, the ground state spin of a carbon nanotube in the
presence of an external potential. We find that when the external potential is
applied to a part of the nanotube, its variation changes the single electron
spectrum significantly. This, in combination with Coulomb repulsion and the
symmetry properties of a finite length armchair nanotube induces spin flips in
the ground state when the external potential is varied. We discuss the possible
application of our theory to recent measurements of Coulomb blocked peaks and
their dependence on a weak magnetic field in armchair carbon nanotubes.Comment: RevTeX, 5 pages + two figure
Magnetic-field-induced singularities in spin dependent tunneling through InAs quantum dots
Current steps attributed to resonant tunneling through individual InAs
quantum dots embedded in a GaAs-AlAs-GaAs tunneling device are investigated
experimentally in magnetic fields up to 28 T. The steps evolve into strongly
enhanced current peaks in high fields. This can be understood as a
field-induced Fermi-edge singularity due to the Coulomb interaction between the
tunneling electron on the quantum dot and the partly spin polarized Fermi sea
in the Landau quantized three-dimensional emitter.Comment: 5 pages, 4 figure
Solving rate equations for electron tunneling via discrete quantum states
We consider the form of the current-voltage curves generated when tunneling
spectroscopy is used to measure the energies of individual electronic energy
levels in nanometer-scale systems. We point out that the voltage positions of
the tunneling resonances can undergo temperature-dependent shifts, leading to
errors in spectroscopic measurements that are proportional to temperature. We
do this by solving the set of rate equations that can be used to describe
electron tunneling via discrete quantum states, for a number of cases important
for comparison to experiments, including (1) when just one spin-degenerate
level is accessible for transport, (2) when 2 spin-degenerate levels are
accessible, with no variation in electron-electron interactions between
eigenstates, and (3) when 2 spin-degenerate levels are accessible, but with
variations in electron-electron interactions. We also comment on the general
case with an arbitrary number of accessible levels. In each case we analyze the
voltage-positions, amplitudes, and widths of the current steps due to the
quantum states.Comment: REVTeX 4, 10 pages, 12 figures, submitted to Phys. Rev. B. Associated
programs available at http://www.ccmr.cornell.edu/~ralph
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
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