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 $\omega_1$ and $\omega_2$ are well away from the cyclotron frequency, $\omega_c$, our simple classical considerations demonstrate that the system becomes parametrically unstable with respect to fluctuations with frequency $(\omega_1+\omega_2)/2$. As an additional effect of nonparabolicity, this parametric instability can manifest itself in the dc properties of the system. This happens when $\omega_1$, $\omega_2$ and $\omega_c$ are related as 3:1:2, respectively. Even for weak detuning between $\omega_1$ and $\omega_2$, 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

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, $\delta G \propto T^4$. In addition, $\delta G$ 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 $^4$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 $\epsilon_d$ at large bias voltages $V$. 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 $V$, centered at $eV=\pm 2\epsilon_d/3$. 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