2,154 research outputs found
Documenting The Experiences And Perceptions Of Youth Participants In The Shelton Leadership Challenge Program
This ethnographic case study (Flyvbjerg, 2011) drew on intensive participant observation, interviewing, and analysis of archival materials, to document the experiences and perspectives of high school and college age participants in the Shelton Challenge, a summer camp designed to inspire and scaffold values-based, transformative leadership. This study focused in particular on youth reports of the impact of the Shelton Challenge experience on their use of reflection, critical thinking, and values in decision-making
Environmental effects in the third moment of voltage fluctuations in a tunnel junction
We present the first measurements of the third moment of the voltage
fluctuations in a conductor. This technique can provide new and complementary
information on the electronic transport in conducting systems. The measurement
was performed on non-superconducting tunnel junctions as a function of voltage
bias, for various temperatures and bandwidths up to 1GHz. The data demonstrate
the significant effect of the electromagnetic environment of the sample.Comment: Major revision. More experimental results. New interpretation. 4
pages, 3 figure
Electron-magnon coupling and nonlinear tunneling transport in magnetic nanoparticles
We present a theory of single-electron tunneling transport through a
ferromagnetic nanoparticle in which particle-hole excitations are coupled to
spin collective modes. The model employed to describe the interaction between
quasiparticles and collective excitations captures the salient features of a
recent microscopic study. Our analysis of nonlinear quantum transport in the
regime of weak coupling to the external electrodes is based on a rate-equation
formalism for the nonequilibrium occupation probability of the nanoparticle
many-body states. For strong electron-boson coupling, we find that the
tunneling conductance as a function of bias voltage is characterized by a large
and dense set of resonances. Their magnetic field dependence in the large-field
regime is linear, with slopes of the same sign. Both features are in agreement
with recent tunneling experiments.Comment: 4 pages, 2 figure
Effect of the shot-noise on a Coulomb blockaded single Josephson junction
We have investigated how the Coulomb blockade of a mesoscopic Josephson
junction in a high-impedance environment is suppressed by shot noise from an
adjacent junction. The presented theoretical analysis is an extension of the
phase correlation theory for the case of a non-Gaussian noise. Asymmetry of the
non-Gaussian noise should result in the shift of the conductance minimum from
zero voltage and the ratchet effect (nonzero current at zero voltage), which
have been experimentally observed. The analysis demonstrates that a Coulomb
blockaded tunnel junction in a high impedance environment can be used as an
effective noise detector.Comment: 4 pages, 1 figure; figure and typos corrected, added reference
Superconductor-metal transition in an ultrasmall Josephson junction biased by a noisy voltage source
Shot noise in a voltage source changes the character of the quantum
(dissipative) phase transition in an ultrasmall Josephson junction: The
superconductor-insulator transition transforms into the superconductor-metal
transition. In the metallic phase the IV curve probes the voltage distribution
generated by shot noise, whereas in the superconducting phase it probes the
counting statistics of electrons traversing the noise junction.Comment: 4 pages, 3 figures. Corrected typos and style, added reference
Dynamical Coulomb blockade and spin-entangled electrons
We consider the production of mobile and nonlocal pairwise spin-entangled
electrons from tunneling of a BCS-superconductor (SC) to two normal Fermi
liquid leads. The necessary mechanism to separate the two electrons coming from
the same Cooper pair (spin-singlet) is achieved by coupling the SC to leads
with a finite resistance. The resulting dynamical Coulomb blockade effect,
which we describe phenomenologically in terms of an electromagnetic
environment, is shown to be enhanced for tunneling of two spin-entangled
electrons into the same lead compared to the process where the pair splits and
each electron tunnels into a different lead. On the other hand in the
pair-split process, the spatial correlation of a Cooper pair leads to a current
suppression as a function of distance between the two tunnel junctions which is
weaker for effectively lower dimensional SCs.Comment: 5 pages, 2 figure
Transport of flexible chiral objects in a uniform shear flow
The transport of slightly deformable chiral objects in a uniform shear flow
is investigated. Depending on the equilibrium configuration one finds up to
four different asymptotic states that can be distinguished by a lateral drift
velocity of their center of mass, a rotational motion about the center of mass
and deformations of the object. These deformations influence the magnitudes of
the principal axes of the second moment tensor of the considered object and
also modify a scalar index characterizing its chirality. Moreover, the
deformations induced by the shear flow are essential for the phenomenon of
dynamical symmetry breaking: Objects that are achiral under equilibrium
conditions may dynamically acquire chirality and consequently experience a
drift in the lateral direction.Comment: 25 pages, 16 figure
Giant current fluctuations in an overheated single electron transistor
Interplay of cotunneling and single-electron tunneling in a thermally
isolated single-electron transistor (SET) leads to peculiar overheating
effects. In particular, there is an interesting crossover interval where the
competition between cotunneling and single-electron tunneling changes to the
dominance of the latter. In this interval, the current exhibits anomalous
sensitivity to the effective electron temperature of the transistor island and
its fluctuations. We present a detailed study of the current and temperature
fluctuations at this interesting point. The methods implemented allow for a
complete characterization of the distribution of the fluctuating quantities,
well beyond the Gaussian approximation. We reveal and explore the parameter
range where, for sufficiently small transistor islands, the current
fluctuations become gigantic. In this regime, the optimal value of the current,
its expectation value, and its standard deviation differ from each other by
parametrically large factors. This situation is unique for transport in
nanostructures and for electron transport in general. The origin of this
spectacular effect is the exponential sensitivity of the current to the
fluctuating effective temperature.Comment: 10 pages, 11 figure
Tunneling of Cooper pairs across voltage biased asymmetric single-Cooper-pair transistors
We analyze tunneling of Cooper pairs across voltage biased asymmetric
single-Cooper-pair transistors. Also tunneling of Cooper pairs across two
capacitively coupled Cooper-pair boxes is considered, when the capacitive
coupling and Cooper pair tunneling are provided by a small Josephson junction
between the islands. The theoretical analysis is done at subgap voltages, where
the current-voltage characteristics depend strongly on the macroscopic
eigenstates of the island(s) and their coupling to the dissipative environment.
As the environment we use an impedance which satisfies Re[Z]<<R_Q and a few
LC-oscillators in series with Z. The numerically calculated I-V curves are
compared with experiments where the quantum states of mesoscopic SQUIDs are
probed with inelastic Cooper pair tunneling. The main features of the observed
I-V data are reproduced. Especially, we find traces of band structure in the
higher excited states of the Cooper-pair boxes as well as traces of multiphoton
processes between two Cooper-pair boxes in the regime of large Josephson
coupling.Comment: 9 pages, 9 figures, Revtex
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