50 research outputs found
Quantum-classical correspondence for local density of states and eigenfunctions of a chaotic periodic billiard
Classical-quantum correspondence for conservative chaotic Hamiltonians is
investigated in terms of the structure of the eigenfunctions and the local
density of states, using as a model a 2D rippled billiard in the regime of
global chaos. The influence of the observed localized and sparsed states in the
quantum-classical correspondence is discussed.Comment: 8 pages, 4 figure
Finite temperature effects in Coulomb blockade quantum dots and signatures of spectral scrambling
The conductance in Coulomb blockade quantum dots exhibits sharp peaks whose
spacings fluctuate with the number of electrons. We derive the
temperature-dependence of these fluctuations in the statistical regime and
compare with recent experimental results. The scrambling due to Coulomb
interactions of the single-particle spectrum with the addition of an electron
to the dot is shown to affect the temperature-dependence of the peak spacing
fluctuations. Spectral scrambling also leads to saturation in the temperature
dependence of the peak-to-peak correlator, in agreement with recent
experimental results. The signatures of scrambling are derived using discrete
Gaussian processes, which generalize the Gaussian ensembles of random matrices
to systems that depend on a discrete parameter -- in this case, the number of
electrons in the dot.Comment: 14 pages, 4 eps figures included, RevTe
Statistics of Coulomb Blockade Peak Spacings within the Hartree-Fock Approximation
We study the effect of electronic interactions on the addition spectra and on
the energy level distributions of two-dimensional quantum dots with weak
disorder using the self-consistent Hartree-Fock approximation for spinless
electrons. We show that the distribution of the conductance peak spacings is
Gaussian with large fluctuations that exceed, in agreement with experiments,
the mean level spacing of the non-interacting system. We analyze this
distribution on the basis of Koopmans' theorem. We show furthermore that the
occupied and unoccupied Hartree-Fock levels exhibit Wigner-Dyson statistics.Comment: 5 pages, 2 figures, submitted for publicatio
Quantum dots in magnetic fields: thermal response of broken symmetry phases
We investigate the thermal properties of circular semiconductor quantum dots
in high magnetic fields using finite temperature Hartree-Fock techniques. We
demonstrate that for a given magnetic field strength quantum dots undergo
various shape phase transitions as a function of temperature, and we outline
possible observable consequences.Comment: In Press, Phys. Rev. B (2001
Spin interactions and switching in vertically tunnel-coupled quantum dots
We determine the spin exchange coupling J between two electrons located in
two vertically tunnel-coupled quantum dots, and its variation when magnetic (B)
and electric (E) fields (both in-plane and perpendicular) are applied. We
predict a strong decrease of J as the in-plane B field is increased, mainly due
to orbital compression. Combined with the Zeeman splitting, this leads to a
singlet-triplet crossing, which can be observed as a pronounced jump in the
magnetization at in-plane fields of a few Tesla, and perpendicular fields of
the order of 10 Tesla for typical self-assembled dots. We use harmonic
potentials to model the confining of electrons, and calculate the exchange J
using the Heitler-London and Hund-Mulliken technique, including the long-range
Coulomb interaction. With our results we provide experimental criteria for the
distinction of singlet and triplet states and therefore for microscopic spin
measurements. In the case where dots of different sizes are coupled, we present
a simple method to switch on and off the spin coupling with exponential
sensitivity using an in-plane electric field. Switching the spin coupling is
essential for quantum computation using electronic spins as qubits.Comment: 13 pages, 9 figure
Voltage-tunable singlet-triplet transition in lateral quantum dots
Results of calculations and high source-drain transport measurements are
presented which demonstrate voltage-tunable entanglement of electron pairs in
lateral quantum dots. At a fixed magnetic field, the application of a
judiciously-chosen gate voltage alters the ground-state of an electron pair
from an entagled spin singlet to a spin triplet.Comment: 8.2 double-column pages, 10 eps figure
Non-linear response of a Kondo system: Perturbation approach to the time dependent Anderson impurity model
Nonlinear tunneling current through a quantum dot
(an Anderson impurity system) subject to both constant and alternating
electric fields is studied in the Kondo regime. A systematic diagram technique
is developed for perturbation study of the current in physical systems out of
equilibrium governed by time - dependent Hamiltonians of the Anderson and the
Kondo models. The ensuing calculations prove to be too complicated for the
Anderson model, and hence, a mapping on an effective Kondo problem is called
for. This is achieved by constructing a time - dependent version of the
Schrieffer - Wolff transformation. Perturbation expansion of the current is
then carried out up to third order in the Kondo coupling J yielding a set of
remarkably simple analytical expressions for the current. The zero - bias
anomaly of the direct current differential conductance is shown to be
suppressed by the alternating field while side peaks develop at finite source -
drain voltage. Both the direct component and the first harmonics of the time -
dependent response are equally enhanced due to the Kondo effect, while
amplitudes of higher harmonics are shown to be relatively small. A zero
alternating bias anomaly is found in the alternating current differential
conductance, that is, it peaks around zero alternating bias. This peak is
suppressed by the constant bias. No side peaks show up in the differential
alternating - conductance but their counterpart is found in the derivative of
the alternating current with respect to the direct bias. The results pertaining
to nonlinear response are shown to be valid also below the Kondo temperature.Comment: 55 latex pages 11 ps figure
Single-electron transport driven by surface acoustic waves: moving quantum dots versus short barriers
We have investigated the response of the acoustoelectric current driven by a
surface-acoustic wave through a quantum point contact in the closed-channel
regime. Under proper conditions, the current develops plateaus at integer
multiples of ef when the frequency f of the surface-acoustic wave or the gate
voltage Vg of the point contact is varied. A pronounced 1.1 MHz beat period of
the current indicates that the interference of the surface-acoustic wave with
reflected waves matters. This is supported by the results obtained after a
second independent beam of surface-acoustic wave was added, traveling in
opposite direction. We have found that two sub-intervals can be distinguished
within the 1.1 MHz modulation period, where two different sets of plateaus
dominate the acoustoelectric-current versus gate-voltage characteristics. In
some cases, both types of quantized steps appeared simultaneously, though at
different current values, as if they were superposed on each other. Their
presence could result from two independent quantization mechanisms for the
acoustoelectric current. We point out that short potential barriers determining
the properties of our nominally long constrictions could lead to an additional
quantization mechanism, independent from those described in the standard model
of 'moving quantum dots'.Comment: 25 pages, 12 figures, to be published in a special issue of J. Low
Temp. Phys. in honour of Prof. F. Pobel
Fano Resonances in Electronic Transport through a Single Electron Transistor
We have observed asymmetric Fano resonances in the conductance of a single
electron transistor resulting from interference between a resonant and a
nonresonant path through the system. The resonant component shows all the
features typical of quantum dots, but the origin of the non-resonant path is
unclear. A unique feature of this experimental system, compared to others that
show Fano line shapes, is that changing the voltages on various gates allows
one to alter the interference between the two paths.Comment: 8 pages, 6 figures. Submitted to PR