110 research outputs found
Controlled dephasing of a quantum dot in the Kondo regime
Kondo correlation in a spin polarized quantum dot (QD) results from the
dynamical formation of a spin singlet between the dot's net spin and a Kondo
cloud of electrons in the leads, leading to enhanced coherent transport through
the QD. We demonstrate here significant dephasing of such transport by coupling
the QD and its leads to potential fluctuations in a near by 'potential
detector'. The qualitative dephasing is similar to that of a QD in the Coulomb
Blockade regime in spite of the fact that the mechanism of transport is quite
different. A much stronger than expected suppression of coherent transport is
measured, suggesting that dephasing is induced mostly in the 'Kondo cloud' of
electrons within the leads and not in the QD.Comment: to be published in PR
Fano Effect in a Few-Electron Quantum Dot
We have studied the Fano effect in a few-electron quantum dot side-coupled to
a quantum wire. The conductance of the wire, which shows an ordinal
staircase-like quantization without the dot, is modified through the
interference (the Fano effect) and the charging effects. These effects are
utilized to verify the exhaustion of electrons in the dot. The "addition energy
spectrum" of the dot shows a shell structure, indicating that the electron
confinement potential is fairly circular. A rapid sign inversion of the Fano
parameter on the first conductance plateau with the change of the wire gate
voltage has been observed, and explained by introducing a finite width of
dot-wire coupling.Comment: 11 pages, 7 figure
Dephasing and Measurement Efficiency via a Quantum Dot Detector
We study charge detection and controlled dephasing of a mesoscopic system via
a quantum dot detector (QDD), where the mesoscopic system and the QDD are
capacitively coupled. The QDD is considered to have coherent resonant
tunnelling via a single level. It is found that the dephasing rate is
proportional to the square of the conductance of the QDD for the Breit-Wigner
model, showing that the dephasing is completely different from the shot noise
of the detector. The measurement rate, on the other hand, shows a dip near the
resonance. Our findings are peculiar especially for a symmetric detector in the
following aspect: The dephasing rate is maximum at resonance of the QDD where
the detector conductance is insensitive to the charge state of the mesoscopic
system. As a result, the efficiency of the detector shows a dip and vanishes at
resonance, in contrast to the single-channel symmetric non-resonant detector
that has always a maximum efficiency. We find that this difference originates
from a very general property of the scattering matrix: The abrupt phase change
exists in the scattering amplitudes in the presence of the symmetry, which is
insensitive to the detector current but {\em stores} the information of the
quantum state of the mesoscopic system.Comment: 7 pages, 3 figure
The current polarization rectification of the integer quantized Hall effect
We report on our theoretical investigation considering the widths of
quantized Hall plateaus (QHPs) depending on the density asymmetry induced by
the large current within the out-of-linear response regime. We solve the
Schrodinger equation within the Hartree type mean field approximation using
Thomas Fermi Poisson nonlinear screening theory. We observe that the two
dimensional electron system splits into compressible and incompressible regions
for certain magnetic field intervals, where the Hall resistance is quantized
and the longitudinal resistance vanishes, if an external current is imposed. We
found that the strong current imposed, induces an asymmetry on the IS width
depending linearly on the current intensity.Comment: 5 pages, 2 figur
Controlled Dephasing of Electrons by Non-Gaussian Shot Noise
In a 'controlled dephasing' experiment [1-3], an interferometer loses its
coherence due to entanglement with a controlled quantum system ('which path'
detector). In experiments that were conducted thus far in mesoscopic systems
only partial dephasing was achieved. This was due to weak interactions between
many detector electrons and the interfering electron, resulting in a Gaussian
phase randomizing process [4-10]. Here, we report the opposite extreme: a
complete destruction of the interference via strong phase randomization only by
a few electrons in the detector. The realization was based on interfering edge
channels (in the integer quantum Hall effect regime, filling factor 2) in a
Mach-Zehnder electronic interferometer, with an inner edge channel serving as a
detector. Unexpectedly, the visibility quenched in a periodic lobe-type form as
the detector current increased; namely, it periodically decreased as the
detector current, and thus the detector's efficiency, increased. Moreover, the
visibility had a V-shape dependence on the partitioning of the detector
current, and not the expected dependence on the second moment of the shot
noise, T(1-T), with T the partitioning. We ascribe these unexpected features to
the strong detector-interferometer coupling, allowing only 1-3 electrons in the
detector to fully dephase the interfering electron. Consequently, in this work
we explored the non-Gaussian nature of noise [11], namely, the direct effect of
the shot noise full counting statistics [12-15].Comment: 14 pages, 4 figure
Crossover from mesoscopic to universal phase for electron transmission in quantum dots
Measuring phase in coherent electron systems (mesoscopic systems) provides
ample information not easily revealed by conductance measurements. Phase
measurements in relatively large quantum dots (QDs) recently demonstrated a
universal like phase evolution independent of dot size, shape, and occupancy.
Explicitly, in Coulomb blockaded QDs the transmission phase increased
monotonically by pi throughout each conductance peak, thereafter, in the
conductance valleys the phase returned sharply to its base value. Expected
mesoscopic features in the phase, related to spin degeneracy or to exchange
effects, were never observed. Presently, there is no satisfactory full
explanation for the observed phase universality. Unfortunately, the phase in a
few-electron QDs, where it can be better understood was never measured. Here we
report on such measurements on a small QD that occupy only 1-20 electrons. Such
dot was embedded in one arm of a two path electron interferometer, with an
electron counter near the dot. Unlike the repetitive behavior found in larger
dots we found now mesoscopic features for dot occupation of less than some 10
electrons. An unexpected feature in this regime is a clear observation of the
occupation of two different orbital states by the first two electrons -
contrary to the recent publications. As the occupation increased the phase
evolved and turned universal like for some 14 electrons and higher. The present
measurements allowed us to determine level occupancy and parity. More
importantly, they suggest that QDs go through a phase transition, from
mesoscopic to universal like behavior, as the occupancy increases. These
measurements help in singling out potential few theoretical models among the
many proposed.Comment: 12 pages, 6 figure
Nonequilibrium transport in quantum impurity models: Exact path integral simulations
We simulate the nonequilibrium dynamics of two generic many-body quantum
impurity models by employing the recently developed iterative
influence-functional path integral method [Phys. Rev. B {\bf 82}, 205323
(2010)]. This general approach is presented here in the context of quantum
transport in molecular electronic junctions. Models of particular interest
include the single impurity Anderson model and the related spinless two-state
Anderson dot. In both cases we study the time evolution of the dot occupation
and the current characteristics at finite temperature. A comparison to
mean-field results is presented, when applicable
Dynamical stabilization and time in open quantum systems
The meaning of time in an open quantum system is considered under the
assumption that both, system and environment, are quantum mechanical objects.
The Hamilton operator of the system is non-Hermitian. Its imaginary part is the
time operator. As a rule, time and energy vary continuously when controlled by
a parameter. At high level density, where many states avoid crossing, a
dynamical phase transition takes place in the system under the influence of the
environment. It causes a dynamical stabilization of the system what can be seen
in many different experimental data. Due to this effect, time is bounded from
below: the decay widths (inverse proportional to the lifetimes of the states)
do not increase limitless. The dynamical stabilization is an irreversible
process.Comment: Contribution to the Special Issue "Quantum Physics with Non-Hermitian
Operators: Theory and Experiment", Fortschritte der Physik - Progress of
Physic
Values in middle childhood: Social and genetic contributions
Theories of value development often identify adolescence as the period for value formation, and cultural and familial factors as the sources for value priorities. However, recent research suggests that value priorities can be observed as early as in middle childhood, and several studies, including one on preadolescents (Knafo & Spinath, 2011), have suggested a genetic contribution to individual differences in values. In the current study, 174 pairs of monozygotic and dizygotic 7-year old Israeli twins completed the Picture-Based Value Survey for Children (PBVS–C; Döring et al., 2010). We replicated basic patterns of relations between value priorities and variables of socialisation – gender, religiosity, and socioeconomic status– that have been found in studies with adults. Most important, values of Self-transcendence, Self-enhancement, and Conservation, were found to be significantly affected by genetic factors (29%, 47% and 31% respectively), as well as non-shared environment (71%, 53% and 69% respectively). Openness to change values, in contrast, were found to be unaffected by genetic factors at this age and were influenced by shared (19%) and non-shared (81%) environment. These findings support the recent view that values are formed at earlier ages than had been assumed previously, and they further our understanding of the genetic and environmental factors involved in value formation at young ages
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