268 research outputs found
Ultra-high-frequency piecewise-linear chaos using delayed feedback loops
We report on an ultra-high-frequency (> 1 GHz), piecewise-linear chaotic
system designed from low-cost, commercially available electronic components.
The system is composed of two electronic time-delayed feedback loops: A primary
analog loop with a variable gain that produces multi-mode oscillations centered
around 2 GHz and a secondary loop that switches the variable gain between two
different values by means of a digital-like signal. We demonstrate
experimentally and numerically that such an approach allows for the
simultaneous generation of analog and digital chaos, where the digital chaos
can be used to partition the system's attractor, forming the foundation for a
symbolic dynamics with potential applications in noise-resilient communications
and radar
Competing mechanisms for singlet-triplet transition in artificial molecules
We study the magnetic field induced singlet/triplet transition for two
electrons in vertically coupled quantum dots by exact diagonalization of the
Coulomb interaction. We identify the different mechanisms occurring in the
transition, involving either in-plane correlations or localization in opposite
dots, depending on the field direction. Therefore, both spin and orbital
degrees of freedom can be manipulated by field strength and direction. The
phase diagram of realistic devices is determined.Comment: To appear in Phys. Rev. B - Rapid Comm. - 5 pages, 3 figure
Full configuration interaction approach to the few-electron problem in artificial atoms
We present a new high-performance configuration interaction code optimally
designed for the calculation of the lowest energy eigenstates of a few
electrons in semiconductor quantum dots (also called artificial atoms) in the
strong interaction regime. The implementation relies on a single-particle
representation, but it is independent of the choice of the single-particle
basis and, therefore, of the details of the device and configuration of
external fields. Assuming no truncation of the Fock space of Slater
determinants generated from the chosen single-particle basis, the code may
tackle regimes where Coulomb interaction very effectively mixes many
determinants. Typical strongly correlated systems lead to very large
diagonalization problems; in our implementation, the secular equation is
reduced to its minimal rank by exploiting the symmetry of the effective-mass
interacting Hamiltonian, including square total spin. The resulting Hamiltonian
is diagonalized via parallel implementation of the Lanczos algorithm. The code
gives access to both wave functions and energies of first excited states.
Excellent code scalability in a parallel environment is demonstrated; accuracy
is tested for the case of up to eight electrons confined in a two-dimensional
harmonic trap as the density is progressively diluted and correlation becomes
dominant. Comparison with previous Quantum Monte Carlo simulations in the
Wigner regime demonstrates power and flexibility of the method.Comment: RevTeX 4.0, 18 pages, 6 tables, 9 postscript b/w figures. Final
version with new material. Section 6 on the excitation spectrum has been
added. Some material has been moved to two appendices, which appear in the
EPAPS web depository in the published versio
A monolayer transition-metal dichalcogenide as a topological excitonic insulator
Monolayer transition-metal dichalcogenides in the T\u2032 phase could enable the realization of the quantum spin Hall effect1 at room temperature, because they exhibit a prominent spin\u2013orbit gap between inverted bands in the bulk2,3. Here we show that the binding energy of electron\u2013hole pairs excited through this gap is larger than the gap itself in the paradigmatic case of monolayer T\u2032 MoS2, which we investigate from first principles using many-body perturbation theory4. This paradoxical result hints at the instability of the T\u2032 phase in the presence of spontaneous generation of excitons, and we predict that it will give rise to a reconstructed \u2018excitonic insulator\u2019 ground state5\u20137. Importantly, we show that in this monolayer system, topological and excitonic order cooperatively enhance the bulk gap by breaking the crystal inversion symmetry, in contrast to the case of bilayers8\u201316 where the frustration between the two orders is relieved by breaking time reversal symmetry13,15,16. The excitonic topological insulator is distinct from the bare topological phase because it lifts the band spin degeneracy, which results in circular dichroism. A moderate biaxial strain applied to the system leads to two additional excitonic phases, different in their topological character but both ferroelectric17,18 as an effect of electron\u2013electron interaction
Observation and Spectroscopy of a Two-Electron Wigner Molecule in an Ultra-Clean Carbon Nanotube
Coulomb interactions can have a decisive effect on the ground state of
electronic systems. The simplest system in which interactions can play an
interesting role is that of two electrons on a string. In the presence of
strong interactions the two electrons are predicted to form a Wigner molecule,
separating to the ends of the string due to their mutual repulsion. This
spatial structure is believed to be clearly imprinted on the energy spectrum,
yet to date a direct measurement of such a spectrum in a controllable
one-dimensional setting is still missing. Here we use an ultra-clean suspended
carbon nanotube to realize this system in a tunable potential. Using tunneling
spectroscopy we measure the excitation spectra of two interacting carriers,
electrons or holes, and identify seven low-energy states characterized by their
spin and isospin quantum numbers. These states fall into two multiplets
according to their exchange symmetries. The formation of a strongly-interacting
Wigner molecule is evident from the small energy splitting measured between the
two multiplets, that is quenched by an order of magnitude compared to the
non-interacting value. Our ability to tune the two-electron state in space and
to study it for both electrons and holes provides an unambiguous demonstration
of the fundamental Wigner molecule state.Comment: SP and FK contributed equally to this wor
Molecular phases in coupled quantum dots
We present excitation energy spectra of few-electron vertically coupled
quantum dots for strong and intermediate inter-dot coupling. By applying a
magnetic field, we induce ground state transitions and identify the
corresponding quantum numbers by comparison with few-body calculations. In
addition to atomic-like states, we find novel "molecular-like" phases. The
isospin index characterizes the nature of the bond of the artificial molecule
and this we control. Like spin in a single quantum dot, transitions in isospin
leading to full polarization are observed with increasing magnetic field.Comment: PDF file only, 28 pages, 3 tables, 4 color figures, 2 appendices. To
appear in Physical Review B, Scheduled 15 Feb 2004, Vol. 69, Issue
Effect of electron-electron interaction on the phonon-mediated spin relaxation in quantum dots
We estimate the spin relaxation rate due to spin-orbit coupling and acoustic
phonon scattering in weakly-confined quantum dots with up to five interacting
electrons. The Full Configuration Interaction approach is used to account for
the inter-electron repulsion, and Rashba and Dresselhaus spin-orbit couplings
are exactly diagonalized. We show that electron-electron interaction strongly
affects spin-orbit admixture in the sample. Consequently, relaxation rates
strongly depend on the number of carriers confined in the dot. We identify the
mechanisms which may lead to improved spin stability in few electron (>2)
quantum dots as compared to the usual one and two electron devices. Finally, we
discuss recent experiments on triplet-singlet transitions in GaAs dots subject
to external magnetic fields. Our simulations are in good agreement with the
experimental findings, and support the interpretation of the observed spin
relaxation as being due to spin-orbit coupling assisted by acoustic phonon
emission.Comment: 12 pages, 10 figures. Revised version. Changes in section V
(simulation of PRL 98, 126601 experiment
Probing the spin states of three interacting electrons in quantum dots
We observe a low-lying sharp spin mode of three interacting electrons in an
array of nanofabricated AlGaAs/GaAs quantum dots by means of resonant inelastic
light scattering. The finding is enabled by a suppression of the inhomogeneous
contribution to the excitation spectra obtained by reducing the number of
optically-probed quantum dots. Supported by configuration-interaction
calculations we argue that the observed spin mode offers a direct probe of
Stoner ferromagnetism in the simplest case of three interacting spin one-half
fermions
Transient scaling and resurgence of chimera states in networks of Boolean phase oscillators
We study networks of non-locally coupled electronic oscillators that can be
described approximately by a Kuramoto-like model. The experimental networks
show long complex transients from random initial conditions on the route to
network synchronization. The transients display complex behaviors, including
resurgence of chimera states, which are network dynamics where order and
disorder coexists. The spatial domain of the chimera state moves around the
network and alternates with desynchronized dynamics. The fast timescale of our
oscillators (on the order of ) allows us to study the scaling
of the transient time of large networks of more than a hundred nodes, which has
not yet been confirmed previously in an experiment and could potentially be
important in many natural networks. We find that the average transient time
increases exponentially with the network size and can be modeled as a Poisson
process in experiment and simulation. This exponential scaling is a result of a
synchronization rate that follows a power law of the phase-space volume.Comment: http://journals.aps.org/pre/abstract/10.1103/PhysRevE.90.03090
Spin picture of the one-dimensional Hubbard model: Two-fluid structure and phase dynamics
We propose a scheme for investigating the quantum dynamics of interacting
electron models by means of time-dependent variational principle and spin
coherent states of space lattice operators. We apply such a scheme to the
one-dimensional hubbard model, and solve the resulting equations in different
regimes. In particular, we find that at low densities the dynamics is mapped
into two coupled nonlinear Schroedinger equations, whereas near half-filling
the model is described by two coupled Josephson junction arrays. Focusing then
to the case in which only the phases of the spin variables are dynamically
active, we examine a number of different solutions corresponding to the
excitations of few macroscopic modes. Based on fixed point equation of the
simpler among them, we show that the standard one-band ground state phase space
is found.Comment: 10 pages, 1 figure, to appear on Phys. Rev.
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