43,084 research outputs found
Interaction effects and charge quantization in single-particle quantum dot emitters
We discuss a theoretical model of an on-demand single-particle emitter that
employs a quantum dot, attached to an integer or fractional quantum Hall edge
state. Via an exact mapping of the model onto the spin-boson problem we show
that Coulomb interactions between the dot and the chiral quantum Hall edge
state, unavoidable in this setting, lead to a destruction of precise charge
quantization in the emitted wave-packet. Our findings cast doubts on the
viability of this set-up as a single-particle source of quantized charge
pulses. We further show how to use a spin-boson master equation approach to
explicitly calculate the current pulse shape in this set-up.Comment: 5+5 pages, 3 figures, fixed typos, update Supplement Material and
update figure
Partially resummed perturbation theory for multiple Andreev reflections in a short three-terminal Josephson junction
In a transparent three-terminal Josephson junction, modeling nonequilibrium
transport is numerically challenging, owing to the interplay between multiple
Andreev reflection (MAR) thresholds and multipair resonances in the pair
current. An approximate method, coined as "partially resummed perturbation
theory in the number of nonlocal Green's functions", is presented that can be
operational on a standard computer and demonstrates compatibility with results
existing in the literature. In a linear structure made of two neighboring
interfaces (with intermediate transparency) connected by a central
superconductor, tunneling through each of the interfaces separately is taken
into account to all orders. On the contrary, nonlocal processes connecting the
two interfaces are accounted for at the lowest relevant order. This yields
logarithmically divergent contributions at the gap edges, which are sufficient
as a semi-quantitative description. The method is able to describe the current
in the full two-dimensional voltage range, including commensurate as well as
incommensurate values. The results found for the multipair (for instance
quartet) current-phase characteristics as well as the MAR thresholds are
compatible with previous results. At intermediate transparency, the multipair
critical current is much larger than the background MAR current, which supports
an experimental observation of the quartet and multipair resonances. The paper
provides a proof of principle for addressing in the future the interplay
between quasiparticles and multipairs in four-terminal structures.Comment: 18 pages, 10 figures, improvements in the presentation, Eur. Phys. J.
B in pres
Metodologia Per la Caratterizzazione di amplificatori a basso rumore per UMTS
In questo lavoro si presenta una metodologia di
progettazione elettronica a livello di sistema,
affrontando il problema della caratterizzazione dello spazio di progetto dell' amplificatore a basso rumore costituente il primo stadio di un front end a conversione diretta per UMTS realizzato in tecnologia CMOS con lunghezza di canale .18u. La metodologia è sviluppata al fine di valutare in modo quantititativo le specifiche ottime di sistema per il front-end stesso e si basa sul concetto di Piattaforma Analogica, che prevede la costruzione di un modello di prestazioni per il blocco analogico basato su
campionamento statistico di indici di prestazioni del blocco stesso, misurati tramite simulazione di dimensionamenti dei componenti attivi e passivi soddisfacenti un set di equazioni specifico della topologia circuitale. Gli indici di prestazioni vengono successivamente ulizzati per parametrizzare modelli comportamentali utilizzati nelle fasi di ottimizzazione a livello di sistema. Modelli comportamentali atti a rappresentare i sistemi RF sono stati pertanto studiati per ottimizzare la scelta delle metriche di prestazioni. L'ottimizzazione dei set di
equazioni atti a selezionare le configurazione di
interesse per il campionamento ha al tempo stesso richiesto l'approfondimento dei modelli di dispositivi attivi validi in tutte le regioni di funzionamento, e lo studio dettagliato della progettazione degli amplificatori a basso rumore basati su degenerazione induttiva. Inoltre,
il problema della modellizzazione a livello di sistema degli effetti della comunicazione tra LNA e Mixer è stato affrontato proponendo e analizzando diverse soluzioni. Il lavoro ha permesso di condurre un'ottimizzazione del front-end UMTS, giungendo a specifiche ottime a livello di sistema per l'amplificatore stesso
Exponential and power-law renormalization in phonon-assisted tunneling
We investigate the spinless Anderson-Holstein model routinely employed to
describe the basic physics of phonon-assisted tunneling in molecular devices.
Our focus is on small to intermediate electron-phonon coupling; we complement a
recent strong coupling study [Phys.~Rev.~B {87}, 075319 (2013)]. The entire
crossover from the antiadiabatic regime to the adiabatic one is considered. Our
analysis using the essentially analytical functional renormalization group
approach backed-up by numerical renormalization group calculations goes beyond
lowest order perturbation theory in the electron-phonon coupling. In
particular, we provide an analytic expression for the effective tunneling
coupling at particle-hole symmetry valid for all ratios of the bare tunnel
coupling and the phonon frequency. It contains the exponential polaronic as
well as the power-law renormalization; the latter can be traced back to x-ray
edge-like physics. In the antiadiabatic and the adiabatic limit this expression
agrees with the known ones obtained by mapping to an effective interacting
resonant level model and lowest order perturbation theory, respectively. Away
from particle-hole symmetry, we discuss and compare results from several
approaches for the zero temperature electrical conductance of the model.Comment: 11 pages, 6 figures, Published versio
Non-Equilibrium Quantum Dissipation
Dissipative processes in non-equilibrium many-body systems are fundamentally
different than their equilibrium counterparts. Such processes are of great
importance for the understanding of relaxation in single molecule devices. As a
detailed case study, we investigate here a generic spin-fermion model, where a
two-level system couples to two metallic leads with different chemical
potentials. We present results for the spin relaxation rate in the nonadiabatic
limit for an arbitrary coupling to the leads, using both analytical and exact
numerical methods. The non-equilibrium dynamics is reflected by an exponential
relaxation at long times and via complex phase shifts, leading in some cases to
an "anti-orthogonality" effect. In the limit of strong system-lead coupling at
zero temperature we demonstrate the onset of a Marcus-like Gaussian decay with
{\it voltage difference} activation. This is analogous to the equilibrium
spin-boson model, where at strong coupling and high temperatures the spin
excitation rate manifests temperature activated Gaussian behavior. We find that
there is no simple linear relationship between the role of the temperature in
the bosonic system and a voltage drop in a non-equilibrium electronic case. The
two models also differ by the orthogonality-catastrophe factor existing in a
fermionic system, which modifies the resulting lineshapes. Implications for
current characteristics are discussed. We demonstrate the violation of
pair-wise Coulomb gas behavior for strong coupling to the leads. The results
presented in this paper form the basis of an exact, non-perturbative
description of steady-state quantum dissipative systems
Diagrammatic Monte Carlo simulation of non-equilibrium systems
We generalize the recently developed diagrammatic Monte Carlo techniques for
quantum impurity models from an imaginary time to a Keldysh formalism suitable
for real-time and nonequilibrium calculations. Both weak-coupling and
strong-coupling based methods are introduced, analysed and applied to the study
of transport and relaxation dynamics in interacting quantum dots
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
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