552 research outputs found
Charge transport through weakly open one dimensional quantum wires
We consider resonant transmission through a finite-length quantum wire
connected to leads via finite transparency junctions. The coherent electron
transport is strongly modified by the Coulomb interaction. The low-temperature
current-voltage () curves show step-like dependence on the bias voltage
determined by the distance between the quantum levels inside the conductor, the
pattern being dependent on the ratio between the charging energy and level
spacing. If the system is tuned close to the resonance condition by the gate
voltage, the low-voltage curve is Ohmic. At large Coulomb energy and low
temperatures, the conductance is temperature-independent for any relationship
between temperature, level spacing, and coupling between the wire and the
leads
Quantum point contact due to Fermi-level pinning and doping profiles in semiconductor nanocolumns
We show that nanoscale doping profiles inside a nanocolumn in combination
with Fermi-level pinning at the surface give rise to the formation of a
saddle-point in the potential profile. Consequently, the lateral confinement
inside the channel varies along the transport direction, yielding an embedded
quantum point contact. An analytical estimation of the quantization energies
will be given
Negative 4-Probe Conductances of Mesoscopic Superconducting Wires
We analyze the longitudinal 4-probe conductance of mesoscopic normal and
superconducting wires and predict that in the superconducting case, large
negative values can arise for both the weakly disordered and localized regimes.
This contrasts sharply with the behaviour of the longitudinal 4-probe
conductance of normal wires, which in the localized limit is always
exponentially small and positive.Comment: Latex, 3 figures available on request to [email protected]
(Simon Robinson
Ballistic Composite Fermions in Semiconductor Nanostructures
We report the results of two fundamental transport measurements at a Landau
level filling factor of 1/2. The well known ballistic electron transport
phenomena of quenching of the Hall effect in a mesoscopic cross-junction and
negative magnetoresistance of a constriction are observed close to B~=~0 and
. The experimental results demonstrate semi-classical charge
transport by composite fermions, which consist of electrons bound to an even
number of flux quanta.Comment: 9 pages TeX 3.1415 C version 6.1, 3 PostScript figure
Evidence for a colour dependence in the size distribution of main belt asteroids
We present the results of a project to detect small (~1 km) main-belt
asteroids with the 3.6 meter Canada-France-Hawaii Telescope (CFHT). We observed
in 2 filters (MegaPrime g' and r') in order to compare the results in each
band. Owing to the observational cadence we did not observe the same asteroids
through each filter and thus do not have true colour information. However
strong differences in the size distributions as seen in the two filters point
to a colour-dependence at these sizes, perhaps to be expected in this regime
where asteroid cohesiveness begins to be dominated by physical strength and
composition rather than by gravity. The best fit slopes of the cumulative size
distributions (CSDs) in both filters tend towards lower values for smaller
asteroids, consistent with the results of previous studies. In addition to this
trend, the size distributions seen in the two filters are distinctly different,
with steeper slopes in r' than in g'. Breaking our sample up according to
semimajor axis, the difference between the filters in the inner belt is found
to be somewhat less pronounced than in the middle and outer belt, but the CSD
of those asteroids seen in the r' filter is consistently and significantly
steeper than in g' throughout. The CSD slopes also show variations with
semimajor axis within a given filter, particularly in r'. We conclude that the
size distribution of main belt asteroids is likely to be colour dependent at
kilometer sizes and that this dependence may vary across the belt.Comment: 28 pages, 5 figures, submitted to the Astronomical Journa
Quantum Point Contacts and Coherent Electron Focusing
I. Introduction
II. Electrons at the Fermi level
III. Conductance quantization of a quantum point contact
IV. Optical analogue of the conductance quantization
V. Classical electron focusing
VI. Electron focusing as a transmission problem
VII. Coherent electron focusing (Experiment, Skipping orbits and magnetic
edge states, Mode-interference and coherent electron focusing)
VIII. Other mode-interference phenomenaComment: #3 of a series of 4 legacy reviews on QPC'
A linear nonequilibrium thermodynamics approach to optimization of thermoelectric devices
Improvement of thermoelectric systems in terms of performance and range of
applications relies on progress in materials science and optimization of device
operation. In this chapter, we focuse on optimization by taking into account
the interaction of the system with its environment. For this purpose, we
consider the illustrative case of a thermoelectric generator coupled to two
temperature baths via heat exchangers characterized by a thermal resistance,
and we analyze its working conditions. Our main message is that both electrical
and thermal impedance matching conditions must be met for optimal device
performance. Our analysis is fundamentally based on linear nonequilibrium
thermodynamics using the force-flux formalism. An outlook on mesoscopic systems
is also given.Comment: Chapter 14 in "Thermoelectric Nanomaterials", Editors Kunihito
Koumoto and Takao Mori, Springer Series in Materials Science Volume 182
(2013
Crossover from mesoscopic to universal phase for electron transmission in quantum dots
Measuring phase in coherent electron systems (mesoscopic systems) provides
ample information not easily revealed by conductance measurements. Phase
measurements in relatively large quantum dots (QDs) recently demonstrated a
universal like phase evolution independent of dot size, shape, and occupancy.
Explicitly, in Coulomb blockaded QDs the transmission phase increased
monotonically by pi throughout each conductance peak, thereafter, in the
conductance valleys the phase returned sharply to its base value. Expected
mesoscopic features in the phase, related to spin degeneracy or to exchange
effects, were never observed. Presently, there is no satisfactory full
explanation for the observed phase universality. Unfortunately, the phase in a
few-electron QDs, where it can be better understood was never measured. Here we
report on such measurements on a small QD that occupy only 1-20 electrons. Such
dot was embedded in one arm of a two path electron interferometer, with an
electron counter near the dot. Unlike the repetitive behavior found in larger
dots we found now mesoscopic features for dot occupation of less than some 10
electrons. An unexpected feature in this regime is a clear observation of the
occupation of two different orbital states by the first two electrons -
contrary to the recent publications. As the occupation increased the phase
evolved and turned universal like for some 14 electrons and higher. The present
measurements allowed us to determine level occupancy and parity. More
importantly, they suggest that QDs go through a phase transition, from
mesoscopic to universal like behavior, as the occupancy increases. These
measurements help in singling out potential few theoretical models among the
many proposed.Comment: 12 pages, 6 figure
Sensitivity and back-action in charge qubit measurements by a strongly coupled single-electron transistor
We consider charge-qubit monitoring (continuous-in-time weak measurement) by
a single-electron transistor (SET) operating in the sequential-tunneling
regime. We show that commonly used master equations for this regime are not of
the Lindblad form that is necessary and sufficient for guaranteeing valid
physical states. In this paper we derive a Lindblad-form master equation and a
corresponding quantum trajectory model for continuous measurement of the charge
qubit by a SET. Our approach requires that the SET-qubit coupling be strong
compared to the SET tunnelling rates. We present an analysis of the quality of
the qubit measurement in this model (sensitivity versus back-action).
Typically, the strong coupling when the SET island is occupied causes
back-action on the qubit beyond the quantum back-action necessary for its
sensitivity, and hence the conditioned qubit state is mixed. However, in one
strongly coupled, asymmetric regime, the SET can approach the limit of an ideal
detector with an almost pure conditioned state. We also quantify the quality of
the SET using more traditional concepts such as the measurement time and
decoherence time, which we have generalized so as to treat the strongly
responding regime.Comment: About 11 pages, 6 figures. Changes in v2: we made general
improvements to the manuscript including, but not limited to(!), the removal
of one reference, and modification of the footnote
Non-equilibrium transport through a vertical quantum dot in the absence of spin-flip energy relaxation
We investigate non-equilibrium transport in the absence of spin-flip energy
relaxation in a few-electron quantum dot artificial atom. Novel non-equilibrium
tunneling processes involving high-spin states which cannot be excited from the
ground state because of spin-blockade, and other processes involving more than
two charge states are observed. These processes cannot be explained by orthodox
Coulomb blockade theory. The absence of effective spin relaxation induces
considerable fluctuation of the spin, charge, and total energy of the quantum
dot. Although these features are revealed clearly by pulse excitation
measurements, they are also observed in conventional dc current characteristics
of quantum dots.Comment: accepted for publication in Phys. Rev.Let
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