415 research outputs found
Sources of negative tunneling magneto-resistance in multilevel quantum dots with ferromagnetic contacts
We analyze distinct sources of spin-dependent energy level shifts and their
impact on the tunneling magnetoresistance (TMR) of interacting quantum dots
coupled to collinearly polarized ferromagnetic leads. Level shifts due to
virtual charge fluctuations can be quantitatively evaluated within a
diagrammatic representation of our transport theory. The theory is valid for
multilevel quantum dot systems and we exemplarily apply it to carbon nanotube
quantum dots, where we show that the presence of many levels can qualitatively
influence the TMR effect.Comment: 4 pages, 2 figures, supplemental materia
Slave-boson Keldysh field theory for the Kondo effect in quantum dots
We present a {\it nonequilibrium nonperturbative} field theory for the Kondo
effect in strongly interacting quantum dots at finite temperatures. Unifying
the slave-boson representation with the Keldysh field integral an effective
Keldysh action is derived and explored in the vicinity of the zero
slave-bosonic field configuration. The theory properly reflects the essential
features of the Kondo physics and at the same time significantly simplifies a
field-theoretic treatment of the phenomenon, avoiding complicated saddle point
analysis or 1/N expansions, used so far. Importantly, our theory admits a {\it
closed analytical} solution which explains the mechanism of the Kondo effect in
terms of an interplay between the real and imaginary parts of the slave-bosonic
self-energy. It thus provides a convenient nonperturbative building block,
playing the role of a "free propagator", for more advanced theories. We finally
demonstrate that already this simplest possible field theory is able to
correctly reproduce experimental data on the Kondo peak observed in the
differential conductance, correctly predicts the Kondo temperature and, within
its applicability range, has the same universal temperature dependence of the
conductance as the one obtained in numerical renormalization group
calculations.Comment: published versio
Phase diffusion as a model for coherent suppression of tunneling in the presence of noise
We study the stabilization of coherent suppression of tunneling in a driven
double-well system subject to random periodic function ``kicks''. We
model dissipation due to this stochastic process as a phase diffusion process
for an effective two-level system and derive a corresponding set of Bloch
equations with phase damping terms that agree with the periodically kicked
system at discrete times. We demonstrate that the ability of noise to localize
the system on either side of the double-well potenital arises from overdamping
of the phase of oscillation and not from any cooperative effect between the
noise and the driving field. The model is investigated with a square wave
drive, which has qualitatively similar features to the widely studied
cosinusoidal drive, but has the additional advantage of allowing one to derive
exact analytic expressions.Comment: 17 pages, 4 figures, submitted to Phys. Rev.
Spectrum and Franck-Condon factors of interacting suspended single-wall carbon nanotubes
A low energy theory of suspended carbon nanotube quantum dots in weak
tunnelling coupling with metallic leads is presented. The focus is put on the
dependence of the spectrum and the Franck-Condon factors on the geometry of the
junction including several vibronic modes. The relative size and the relative
position of the dot and its associated vibrons strongly influence the
electromechanical properties of the system. A detailed analysis of the complete
parameters space reveals different regimes: in the short vibron regime the
tunnelling of an electron into the nanotube generates a plasmon-vibron
excitation while in the long vibron regime polaron excitations dominate the
scenario. The small, position dependent Franck-Condon couplings of the small
vibron regime convert into uniform, large couplings in the long vibron regime.
Selection rules for the excitations of the different plasmon-vibron modes via
electronic tunnelling events are also derived.Comment: 23 pages, 8 figures, new version according to the published on
Decoherence and Relaxation of a Quantum Bit in the Presence of Rabi Oscillations
Dissipative dynamics of a quantum bit driven by a strong resonant field and
interacting with a heat bath is investigated. We derive generalized Bloch
equations and find modifications of the qubit's damping rates caused by Rabi
oscillations. Nonequilibrium decoherence of a phase qubit inductively coupled
to a LC-circuit is considered as an illustration of the general results. It is
argued that recent experimental results give a clear evidence of effective
suppression of decoherence in a strongly driven flux qubit.Comment: 14 pages; misprints correcte
Transport properties of double-walled carbon nanotube quantum dots
The transport properties of quantum dot (QD) systems based on double-walled
carbon nanotube (DWCNT) are investigated. The interplay between microscopic
structure and strong Coulomb interaction is treated within a bosonization
framework. The linear and nonlinear G-V-V_g characteristics of the QD system is
calculated by starting from the Liouville equation for the reduced density
matrix. Depending on the intershell couplings, an 8-electron periodicity of the
Coulomb blockade peak spacing in the case of commensurate DWCNT QDs and a
4-electron periodicity in the incommensurate case are predicted. The
contribution of excited states of DWCNTs to the nonlinear transport is
investigated as well.Comment: 18 pages, 7 figure
Correlated sequential tunneling through a double barrier for interacting one-dimensional electrons
The problem of resonant tunneling through a quantum dot weakly coupled to
spinless Tomonaga-Luttinger liquids has been studied. We compute the linear
conductance due to sequential tunneling processes upon employing a master
equation approach. Besides the previously used lowest-order golden rule rates
describing uncorrelated sequential tunneling (UST) processes, we systematically
include higher-order correlated sequential tunneling (CST) diagrams within the
standard Weisskopf-Wigner approximation. We provide estimates for the parameter
regions where CST effects can be important. Focusing mainly on the temperature
dependence of the peak conductance, we discuss the relation of these findings
to previous theoretical and experimental results.Comment: replaced with the published versio
Spin channel Keldysh field theory for weakly interacting quantum dots
We develop a low-energy nonequilibrium field theory for weakly interacting
quantum dots. The theory is based on the Keldysh field integral in the spin
channel of the quantum dot described by the single impurity Anderson
Hamiltonian. The effective Keldysh action is a functional of the
Hubbard-Stratonovich magnetization field decoupling the quantum dot spin
channel. We expand this action up to the second order with respect to the
magnetization field, which allows to describe nonequilibrium interacting
quantum dots at low temperatures and weak electron-electron interactions,up to
the contacts-dot coupling energy. Besides its simplicity, an additional
advantage of the theory is that it correctly describes the unitary limit giving
the correct result for the conductance maximum. Thus our theory establishes an
alternative simple method relevant for investigation of weakly interacting
nonequilibrium nanodevices.Comment: published versio
Iterative algorithm versus analytic solutions of the parametrically driven dissipative quantum harmonic oscillator
We consider the Brownian motion of a quantum mechanical particle in a
one-dimensional parabolic potential with periodically modulated curvature under
the influence of a thermal heat bath. Analytic expressions for the
time-dependent position and momentum variances are compared with results of an
iterative algorithm, the so-called quasiadiabatic propagator path integral
algorithm (QUAPI). We obtain good agreement over an extended range of
parameters for this spatially continuous quantum system. These findings
indicate the reliability of the algorithm also in cases for which analytic
results may not be available a priori.Comment: 15 pages including 11 figures, one reference added, minor typos
correcte
Landscape, Environmental Sustainability, and Climate Instability—The EDUSCAPE Project: University Research for Innovation in School Education
This article presents the main contents, methods, and results of the European project EDUSCAPE (Erasmus+) developed by a team of international researchers from four countries, just over a year and a half after its launch with a focus on SAAD/UNICAM contributions. Into the scientific-disciplinary frame of environmental education and climate adaptation, EDUSCAPE aims to integrate the polysemic, transversal, and multidisciplinary concept of landscape and its decline, as a promoter of new forms of knowledge in response to emerging dynamics, within the educational offer of school programs (6-15 years). This paper presents the general structure of the project, the methodology experimented (PBL educational approach), and the qualitative and quantitative intermediate results obtained so far (literature review, curriculum analysis, and needs analysis). To integrate landscape into school curricula and renew them, EDUSCAPE is preparing Didactic Units (DUs) as the final result of the project to provide theoretical foundations and practical solutions supporting teaching which will be tested in the schools of the partnership network. In general, this paper explores the possibility to disseminate the pedagogical and social role of the landscape through a new way of teaching based on a deeper exploration of the theme that can stimulate critical thinking in current and future generations regarding the global/local challenges of the 21st century
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