2,007 research outputs found
Parametric resonance of a two-dimensional electron gas under bichromatic irradiation
In an ultrahigh mobility 2D electron gas, even a weak nonparabolicity of the
electron dispersion, by violating Kohn's theorem, can have a drastic effect on
dc magnetotransport under ac drive. We study theoretically the manifestation of
this effect in the dc response to the combined action of two driving ac-fields
(bichromatic irradiation). Compared to the case of monochromatic irradiation,
which is currently intensively studied both experimentally and theoretically,
the presence of a second microwave source provides additional insight into the
properties of an ac-driven 2D electron gas. In particular, we find that
nonparabolicity, being the simplest cause for a violation of Kohn's theorem,
gives rise to new qualitative effects specific to bichromatic irradiation.
Namely, when the frequencies and are well away from the
cyclotron frequency, , our simple classical considerations
demonstrate that the system becomes parametrically unstable with respect to
fluctuations with frequency . As an additional effect of
nonparabolicity, this parametric instability can manifest itself in the dc
properties of the system. This happens when , and
are related as 3:1:2, respectively. Even for weak detuning between
and , the effect of the bichromatic irradiation on the dc
response in the presence of nonparabolicity can differ dramatically from the
monochromatic case. In particular, the equations of motion can acquire
multistable solutions. As a result, the diagonal dc-conductivity can assume
several stable negative values at the same magnetic field.Comment: 11 pages, 10 figure
The Improved Sun Basket
In recent years and months prices for fuel and light have rapidly increased in India, so that housewives and others interested in saving fuel have shown renewed interest in alternative sources of energy for household use. This is especially true for regions of India where, on the one hand, low rainfall and limited vegetative growth restrict the availability of firewood so that prices are high, but, on the other hand, sunshine is relatively abundant so that solar powered devices such as box cookers or sun baskets do hold a potential.
The Sun Basket was proposed for cooking with solar energy several years ago. This was a low cost papier machee made parabolic mirror in a bamboo frame equipped with a simple automatic tracking device, synchronizing the adjustment of the parabolic mirror with the sun's movement during the day. This tracking device operates on a simple principle: a weight floating on a gradually descending water level pulls a string, which turns the basket. However, because of a number of drawbacks at that time the original low cost Sun Basket found only limited acceptance
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Bedeutungsvolle Orte. Eine kultursoziologische Annäherung an kreative Handlungsressourcen in Städten
Die Großstadt mit ihrer heterogenen Bevölkerungszusammensetzung galt schon immer als der Ort, an dem das Neue in die Welt kommt. Im Zuge der gegenwärtigen Diskussion um die kreative Stadt sind neue Konzepte zum Zusammenhang von Stadtentwicklung und Kreativität entstanden. Gleichwohl bringen sie nach wie vor Größe und Heterogenität als materielle urbane Ressourcen in Anschlag, um die dynamische Entwicklung der Kultur- und Kreativwirtschaft in Städten zu erklären. In dem vorliegenden Beitrag wird demgegenüber argumentiert, dass eine kultursoziologisch erweiterte Untersuchung von städtischen Räumen zu differenzierteren Ergebnissen kommt: die Stadt mit ihren vielfältigen Angeboten bildet zwar den materiellen Kontext von Kreativität und Innovation; mindestens ebenso bedeutsam ist jedoch die Frage, inwieweit eine Stadt oder ein Stadtteil zugleich als bedeutungsvoller Ort für kreative Arbeit und kreatives Leben konstituiert und gewertet wird. Auf der Grundlage von Befunden aus zwei Projekten, die in Berlin und Offenbach zwischen 2007 und 2011 durchgeführt wurden, werden Diskurse und Praktiken von kreativ Tätigen rekonstruiert, die sich um Deutungen des Städtischen ranken. Dabei haben sich drei solcher geteilter Bedeutungsdimensionen herauskristallisiert, die aus Sicht der kreativ Tätigen die Attraktivität ihrer Stadt prägen, die sich als Offenheit, Beweglichkeit und Glaubwürdigkeit charakterisieren lassen. Diese Konstrukte tauchen in beiden Städten mit ähnlichen Konnotationen auf, sie stehen in engem Zusammenhang miteinander; und sie liegen quer zu den Dimensionen Größe und Heterogenität. (Autorenreferat) The relationship between creative processes and urban spaces is a rather old question in urban sociology, since the urban has long been seen as the locus for modernity and societal progress. The current creative city discourse in urban development has given rise to new urban growth theories based on creativity. Explanations focus on spatial-structural dimensions such as size and heterogeneity to emphasize urban spaces as a specific material context for creative processes. This paper, however, focuses on the social practices of creative practitioners in relation to cities, their ways of acting and talking, and how they attach meanings and ascribe qualities to particular places to constitute them as meaningful places for creative work and life. The aim of this paper is to present a cultural sociological view on creativity in the city. It is not just the urban context that breeds creativity, but culturally (re)produced perceptions of particular places that creative people value as meaningful for creative work. In a qualitative approach, discourses and practices of creative practitioners are reconstructed from the findings of two research projects undertaken between 2007 and 2011 in Berlin and Offenbach. The argument is that heterogeneity and size must be regarded as negotiable qualities rather than as fixed categories. Furthermore, three dimensions emerge from observations of these collective meaning-making processes that are important for urban spaces as places for creative work: openness, agility, and authenticity
Pair tunneling through single molecules
By a polaronic energy shift, the effective charging energy of molecules can
become negative, favoring ground states with even numbers of electrons. Here,
we show that charge transport through such molecules near ground-state
degeneracies is dominated by tunneling of electron pairs which coexists with
(featureless) single-electron cotunneling. Due to the restricted phase space
for pair tunneling, the current-voltage characteristics exhibits striking
differences from the conventional Coulomb blockade. In asymmetric junctions,
pair tunneling can be used for gate-controlled current rectification and
switching.Comment: 4+ pages, 4 figures; minor changes, version published in Phys. Rev.
Let
Inelastic electron backscattering in a generic helical edge channel
We evaluate the low-temperature conductance of a weakly interacting
one-dimensional helical liquid without axial spin symmetry. The lack of that
symmetry allows for inelastic backscattering of a single electron, accompanied
by forward-scattering of another. This joint effect of weak interactions and
potential scattering off impurities results in a temperature-dependent
deviation from the quantized conductance, . In addition,
is sensitive to the position of the Fermi level. We determine
numerically the parameters entering our generic model for the
Bernevig-Hughes-Zhang Hamiltonian of a HgTe/CdTe quantum well in the presence
of Rashba spin-orbit coupling.Comment: 4+ pages, 3 figures, published versio
Collective excitations in quantum Hall liquid crystals: Single-mode approximation calculations
A variety of recent experiments probing the low-temperature transport
properties of quantum Hall systems have suggested an interpretation in terms of
liquid crystalline mesophases dubbed {\em quantum Hall liquid crystals}. The
single mode approximation (SMA) has been a useful tool for the determination of
the excitation spectra of various systems such as phonons in He and in the
fractional quantum Hall effect. In this paper we calculate (via the SMA) the
spectrum of collective excitations in a quantum Hall liquid crystal by
considering {\em nematic}, {\em tetratic}, and {\em hexatic} generalizations of
Laughlin's trial wave function having two-, four- and six-fold broken
rotational symmetry, respectively. In the limit of zero wavevector \qq the
dispersion of these modes is singular, with a gap that is dependent on the
direction along which \qq=0 is approached for {\em nematic} and {\em
tetratic} liquid crystalline states, but remains regular in the {\em hexatic}
state, as permitted by the fourth order wavevector dependence of the
(projected) oscillator strength and static structure factor.Comment: 6 pages, 5 eps figures include
Scattering theory of current-induced forces in mesoscopic systems
We develop a scattering theory of current-induced forces exerted by the
conduction electrons of a general mesoscopic conductor on slow "mechanical"
degrees of freedom. Our theory describes the current-induced forces both in and
out of equilibrium in terms of the scattering matrix of the phase-coherent
conductor. Under general nonequilibrium conditions, the resulting mechanical
Langevin dynamics is subject to both non-conservative and velocity-dependent
Lorentz-like forces, in addition to (possibly negative) friction. We illustrate
our results with a two-mode model inspired by hydrogen molecules in a break
junction which exhibits limit-cycle dynamics of the mechanical modes.Comment: 4+ pages, 1 figure; v2: minor modification
Localization Properties of Two Interacting Electrons in a Disordered Quasi One-Dimensional Potential
We study the transport properties of two electrons in a quasi one-dimensional
disordered wire. The electrons are subject to both, a disorder potential and a
short range two-body interaction. Using the approach developed by Iida et al. [
Ann. Phys. (N.Y.) 200 (1990) 219 ], the supersymmetry technique, and a suitable
truncation of Hilbert space, we work out the two-point correlation function in
the framework of a non-linear sigma model. We study the loop corrections to
arbitrary order. We obtain a remarkably simple and physically transparent
expression for the change of the localization length caused by the two-body
interaction.Comment: 10 page
Electron Pair Resonance in the Coulomb Blockade
We study many-body corrections to the cotunneling current via a localized
state with energy at large bias voltages . We show that the
transfer of {\em electron pairs}, enabled by the Coulomb repulsion in the
localized level, results in ionization resonance peaks in the third derivative
of the current with respect to , centered at . Our
results predict the existence of previously unnoticed structure within
Coulomb-blockade diamonds.Comment: 5 pages, 4 figure
Electron-vibron coupling in suspended carbon nanotube quantum dots
Motivated by recent experiments, we investigate the electron-vibron coupling
in suspended carbon nanotube quantum dots, starting with the electron-phonon
coupling of the underlying graphene layer. We show that the coupling strength
depends sensitively on the type of vibron and is strongly sample dependent. The
coupling strength becomes particularly strong when inhomogeneity-induced
electronic quantum dots are located near regions where the vibronic mode is
associated with large strain. Specifically, we find that the longitudinal
stretching mode and the radial breathing mode are coupled via the strong
deformation potential, while twist modes couple more weakly via a mechanism
involving modulation of the electronic hopping amplitudes between carbon sites.
A special case are bending modes: for symmetry reasons, their coupling is only
quadratic in the vibron coordinate. Our results can explain recent experiments
on suspended carbon nanotube quantum dots which exibit vibrational sidebands
accompanied by the Franck-Condon blockade with strong electron-vibron coupling
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