222 research outputs found
Simulation of a non-invasive charge detector for quantum cellular automata
Information in a Quantum Cellular Automata architecture is encoded in the
polarizazion state of a cell, i.e., in the occupation numbers of the quantum
dots of which the cell is made up. Non-invasive charge detectors of single
electrons in a quantum dot are therefore needed, and recent experiments have
shown that a quantum constriction electrostatically coupled to the quantum dot
may be a viable solution. We have performed a numerical simulation of a system
made of a quantum dot and a nearby quantum point contact defined, by means of
depleting metal gates, in a two-dimensional electron gas at a GaAs/AlGaAs
heterointerface. We have computed the occupancy of each dot and the resistance
of the quantum wire as a function of the voltage applied to the plunger gate,
and have derived design criteria for achieving optimal sensitivity.Comment: 8 pages, RevTeX, epsf, 5 figure
Operation of Quantum Cellular Automaton cells with more than two electrons
We present evidence that operation of QCA (Quantum Cellular Automaton) cells
with four dots is possible with an occupancy of 4N+2 electrons per cell (N
being an integer). We show that interaction between cells can be described in
terms of a revised formula for cell polarization, which is based only on the
difference between diagonal occupancies. We validate our conjectures with full
quantum simulations of QCA cells for a number of electrons varying from 2 to 6,
using the Configuration-Interaction method.Comment: 4 pages, 4 figures included, submitted to AP
Shot noise in resonant tunneling structures
We propose a quantum mechanical approach to noise in resonant tunneling
structures, that can be applied in the whole range of transport regimes, from
completely coherent to completely incoherent. In both limiting cases, well
known results which have appeared in the literature are recovered. Shot noise
reduction due to both Pauli exclusion and Coulomb repulsion, and their combined
effect, are studied as a function of the rate of incoherent processes in the
well (which are taken into account by means of a phenomenological relaxation
time), and of temperature. Our approach allows the study of noise in a variety
of operating conditions (i.e., equilibrium, sub-peak voltages, second resonance
voltages), and as a function of temperature, explaining experimental results
and predicting interesting new results.Comment: RevTeX file, 26 pages, 3 Postscript figures, uses epsf.sty. submitted
to Phys. Rev.
Symmetry causes a huge conductance peak in double quantum dots
We predict a huge interference effect contributing to the conductance through
large ultra-clean quantum dots of chaotic shape. When a double-dot structure is
made such that the dots are the mirror-image of each other, constructive
interference can make a tunnel barrier located on the symmetry axis effectively
transparent. We show (via theoretical analysis and numerical simulation) that
this effect can be orders of magnitude larger than the well-known universal
conductance fluctuations and weak-localization (both less than a conductance
quantum). A small magnetic field destroys the effect, massively reducing the
double-dot conductance; thus a magnetic field detector is obtained, with a
similar sensitivity to a SQUID, but requiring no superconductors.Comment: 5pages 3 figures and an appendix ONLY in arXiv versio
Geometry-dependent conductance and noise behavior of a graphene ribbon with a series of randomly spaced potential barriers
We perform an envelope-function based numerical analysis of the effect of a sequence of randomly spaced potential barriers on the conductance and shot noise of an armchair graphene ribbon. The behavior is dominated by Klein tunneling and by resonant tunneling and strongly depends on the geometrical details of the device. Klein tunneling effectively filters the modes that can propagate through the device. For a large number of cascaded barriers, this gives rise to different transport regimes for metallic and semiconducting ribbons, with diverging shot noise behaviors. Resonant tunneling is instead energy selective and has quite a different effect depending on whether the barriers are identical or not. We also explore the effect of tilting the barriers with respect to the ribbon edges, observing a transition toward a diffusive transport regime and a one-third shot noise suppression. We investigate this effect, and we find that it takes place also in more traditional semiconducting materials. The results of our analysis could be instrumental for the fabrication of mode-filtering and energy-filtering graphene-based nanodevices. Moreover, our study highlights the importance of the measurement of shot noise as a probe for the nature of the transport regime
Thermal behavior of Quantum Cellular Automaton wires
We investigate the effect of a finite temperature on the behavior of logic
circuits based on the principle of Quantum Cellular Automata (QCA) and of
ground state computation. In particular, we focus on the error probability for
a wire of QCA cells that propagates a logic state. A numerical model and an
analytical, more approximate, model are presented for the evaluation of the
partition function of such a system and, consequently, of the desired
probabilities. We compare the results of the two models, assessing the limits
of validity of the analytical approach, and provide estimates for the maximum
operating temperature.Comment: 15 pages, 7 figures, uses revte
Gauge invariant grid discretization of Schr\"odinger equation
Using the Wilson formulation of lattice gauge theories, a gauge invariant
grid discretization of a one-particle Hamiltonian in the presence of an
external electromagnetic field is proposed. This Hamiltonian is compared both
with that obtained by a straightforward discretization of the continuous
Hamiltonian by means of balanced difference methods, and with a tight-binding
Hamiltonian. The proposed Hamiltonian and the balanced difference one are used
to compute the energy spectrum of a charged particle in a two-dimensional
parabolic potential in the presence of a perpendicular, constant magnetic
field. With this example we point out how a "naive" discretization gives rise
to an explicit breaking of the gauge invariance and to large errors in the
computed eigenvalues and corresponding probability densities; in particular,
the error on the eigenfunctions may lead to very poor estimates of the mean
values of some relevant physical quantities on the corresponding states. On the
contrary, the proposed discretized Hamiltonian allows a reliable computation of
both the energy spectrum and the probability densities.Comment: 7 pages, 4 figures, discussion about tight-binding Hamiltonians adde
Enhanced shot noise in resonant tunneling: theory and experiment
We show that shot noise in a resonant tunneling diode biased in the negative
differential resistance regions of the I-V characteristic is enhanced with
respect to ``full'' shot noise. We provide experimental results showing a Fano
factor up to 6.6, and show that it is a dramatic effect caused by
electron-electron interaction through Coulomb force, enhanced by the particular
shape of the density of states in the well. We also present numerical results
from the proposed theory, which are in agreement with the experiment,
demonstrating that the model accounts for the relevant physics involved in the
phenomenon.Comment: 4 pages, 4 figure
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