83 research outputs found
Frictional Drag Between Coupled 2D Hole Gases in GaAs/AlGaAs Heterostructures
We report on the first measurements of the drag effect between coupled
2D-hole gases. We investigate the coupling by changing the carrier densities in
the quantum wells, the widths of the barriers between the gases and the
perpendicular magnetic field. From the data we are able to attribute the
frictional drag to phonon coupling, because the non-parabolicity allows to tune
the Fermi wavevector and the Fermi velocity separately and, thereby, to
distinguish between phonon- and plasmon-dominated coupling.Comment: 10 pages, 5 figure
Exchange and correlation effects on the plasmon dispersions and the Coulomb drag in low-density electron bilayers
We investigate the effect of exchange and correlation (xc) on the plasmon
spectrum and the Coulomb drag between spatially separated low-density
two-dimensional electron layers. We adopt a new approach, which employs dynamic
xc kernels in the calculation of the bi-layer plasmon spectra and of the
plasmon-mediated drag, and static many-body local field factors in the
calculation of the particle-hole contribution to the drag. The spectrum of
bi-layer plasmons and the drag resistivity are calculated in a broad range of
temperatures taking into account both intra- and inter-layer correlation
effects. We observe that both plasmon modes are strongly affected by xc
corrections. After the inclusion of the complex dynamic xc kernels, a decrease
of the electron density induces shifts of the plasmon branches in opposite
directions. And this is in stark contrast to the tendency obtained within the
RPA that both optical and acoustical plasmons move away from the boundary of
the particle-hole continuum with a decrease in the electron density. We find
that the introduction of xc corrections results in a significant enhancement of
the transresistivity and qualitative changes in its temperature dependence. In
particular, the large high-temperature plasmon peak that is present in the
random phase approximation is found to disappear when the xc corrections are
included. Our numerical results at low temperatures are in good agreement with
the results of recent experiments by M. Kellogg {\it et al.}, Solid State
Commun. \textbf{123}, 515 (2002).Comment: 28 pages, 15 figure
Size quantization of Dirac fermions in graphene constrictions
Quantum point contacts (QPCs) are cornerstones of mesoscopic physics and
central building blocks for quantum electronics. Although the Fermi wave-length
in high-quality bulk graphene can be tuned up to hundreds of nanometers, the
observation of quantum confinement of Dirac electrons in nanostructured
graphene systems has proven surprisingly challenging. Here we show ballistic
transport and quantized conductance of size-confined Dirac fermions in
lithographically-defined graphene constrictions. At high charge carrier
densities, the observed conductance agrees excellently with the Landauer theory
of ballistic transport without any adjustable parameter. Experimental data and
simulations for the evolution of the conductance with magnetic field
unambiguously confirm the identification of size quantization in the
constriction. Close to the charge neutrality point, bias voltage spectroscopy
reveals a renormalized Fermi velocity () in
our graphene constrictions. Moreover, at low carrier density transport
measurements allow probing the density of localized states at edges, thus
offering a unique handle on edge physics in graphene devices.Comment: 24 pages including 20 figures and 1 table. Corrected typos. To appear
in Nature Communication
Frictional Drag between Two Dilute Two-Dimensional Hole Layers
We report drag measurements on dilute double layer two-dimensional hole
systems in the regime of r_s=19~39. We observed a strong enhancement of the
drag over the simple Boltzmann calculations of Coulomb interaction, and
deviations from the T^2 dependence which cannot be explained by
phonon-mediated, plasmon-enhanced, or disorder-related processes. We suggest
that this deviation results from interaction effects in the dilute regime.Comment: 4 pages, 3 figures, accepted in Phys. Rev. Lett. Added single layer
transport dat
Sign-reversal of drag in bilayer systems with in-plane periodic potential modulation
We develop a theory for describing frictional drag in bilayer systems with
in-plane periodic potential modulations, and use it to investigate the drag
between bilayer systems in which one of the layers is modulated in one
direction. At low temperatures, as the density of carriers in the modulated
layer is changed, we show that the transresistivity component in the direction
of modulation can change its sign. We also give a physical explanation for this
behavior.Comment: 4 pages, 4 figure
Frictional drag between non-equilibrium charged gases
The frictional drag force between separated but coupled two-dimensional
electron gases of different temperatures is studied using the non-equilibrium
Green function method based on the separation of center-of-mass and relative
dynamics of electrons. As the mechanisms of producing the frictional force we
include the direct Coulomb interaction, the interaction mediated via virtual
and real TA and LA phonons, optic phonons, plasmons, and TA and LA
phonon-electron collective modes. We found that, when the distance between the
two electron gases is large, and at intermediate temperature where plasmons and
collective modes play the most important role in the frictional drag, the
possibility of having a temperature difference between two subsystems modifies
greatly the transresistivity.Comment: 8figure
Surface electromyographic activity of trunk muscles during trunk control exercises for people after stroke; effect of a mobile and stable seat for rehabilitation
The aim of this study was to explore differences in trunk muscle activity on a stable and mobile seat for people after stroke and healthy participants. Trunk control exercises are known to have a beneficial effect on trunk control, balance, and mobility after stroke. The effect of such exercises could be enhanced by the use of a mobile seat to provide further training stimuli. However, little research on the musculoskeletal effects of trunk training on mobile seats has been carried out. On a stable and a mobile seat, thirteen people after stroke and fifteen healthy participants performed two selective trunk control exercises, which were lateral flexion initiated by the pelvis and the thorax. The maximal surface electromyography relative to static sitting of the muscles multifidus, erector spinae, and obliquus externus was recorded bilaterally. The effects of group, seat condition, trunk control exercise, and muscle side were investigated employing within-subject linear-mixed-models. Compared to the stable seat, the maximal muscle activity of people after stroke on the mobile seat was higher during the thorax-initiated exercise and lower during the pelvis-initiated exercise. Healthy participants showed opposite results with higher muscle activity on the mobile seat during the pelvis-initiated exercise. For trunk control training on a mobile seat with high muscle activation people after stroke should perform trunk control exercises initiated by the thorax, for training with lower muscle activity people after stroke should initiate selective trunk movements by the pelvis. The results can support the planning of progressive trunk control rehabilitation programs
Instability of the QHE induced by a nearby two-dimensional-electron system
We report on the observation of structures in the longitudinal resistance of a single two-dimensional electron gas (2DEG) in a perpendicular magnetic field at filling factor νcongruent with1±0.1. The observed structures can be switched off by depleting a second 2DEG, which is separated from the first by a 30 nm or a 60 nm AlGaAs-barrier, using a gate-electrode. Measurements as a function of temperature and drive current indicate that these structures are precursors of the breakdown of the quantum-Hall-effect (QHE) at ν=1. The QHE becomes unstable when another 2DEG is in the neighbourhood. This effect is most likely due to macroscopic effects like screening, because other coupling processes between these two 2DEGs, such as frictional drag, are weak
Exploiting vulnerabilities of SWI/SNF chromatin remodelling complexes for cancer therapy
Multi-subunit ATPase-dependent chromatin remodelling complexes SWI/SNF (switch/sucrose non-fermentable) are fundamental epigenetic regulators of gene transcription. Functional genomic studies revealed a remarkable mutation prevalence of SWI/SNF-encoding genes in 20–25% of all human cancers, frequently driving oncogenic programmes. Some SWI/SNF-mutant cancers are hypersensitive to perturbations in other SWI/SNF subunits, regulatory proteins and distinct biological pathways, often resulting in sustained anticancer effects and synthetic lethal interactions. Exploiting these vulnerabilities is a promising therapeutic strategy. Here, we review the importance of SWI/SNF chromatin remodellers in gene regulation as well as mechanisms leading to assembly defects and their role in cancer development. We will focus in particular on emerging strategies for the targeted therapy of SWI/SNF-deficient cancers using chemical probes, including proteolysis targeting chimeras, to induce synthetic lethality
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