79 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
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
Contribution of soft-bodied meiofaunal taxa to Italian marine biodiversity
Meiofauna includes an astonishing diversity of organisms, whose census is far from being complete. Most classic
ecological studies have focused on hard-bodied Ecdysozoan taxa (notably Copepoda and Nematoda), whose cuticle
allows determination at species-level after fixation, rather than soft-bodied, Spiralian taxa, which most often lose any
diagnostic feature in fixed samples. Yet, metabarcoding studies have recently revealed a species-richness of softbodied taxa comparable, and in cases superior, to that of Copepoda and Nematoda together. However, given
objective difficulties inherent to their study, which necessarily has to be performed on living individuals, and their
limited utilisation for ecological and applicative research, taxonomic expertise on soft-bodied organisms has declined
over the years, and diversity of these phyla in most areas of the world is presently completely unknown. Here we
present an expert-based survey of current knowledge on the composition and distribution of soft-bodied meiofaunal
taxa in Italy, with special references to the predominantly or exclusively meiobenthic phyla Gastrotricha,
Gnathostomulida, Platyhelminthes, Rotifera, Xenacoelomorpha, and macrofaunal taxa with conspicuous meiofaunal
representatives (Annelida, Mollusca and Nemertea). A total of 638 described species have been reported from
Italian coasts; furthermore, the existence of a large number of undescribed species is mentioned. Knowledge of
Annelida, Gastrotricha, and Rotifera appears particularly detailed, placing Italy among the best-known country
worldwide. In contrast, knowledge of Platyhelminthes and Xenacoelomorpha appears patchy, and limited to few
areas. Sampling effort has been uneven, with most species recorded from the Tyrrhenian Sea, while large sections of
the Adriatic and Ionian seas have been poorly explored. Results highlight the role that Marine Biological Stations,
notably the Zoological Station “Anton Dohrn” in Naples, have had in promoting the study of soft-bodied taxa in
Ital
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
The Yellow Excitonic Series of Cu2O Revisited by Lyman Spectroscopy
We report on the observation of the yellow exciton Lyman series up to the
fourth term in Cu2O by time-resolved mid-infrared spectroscopy. The dependence
of the oscillator strength on the principal quantum number n can be well
reproduced using the hydrogenic model including an AC dielectric constant, and
precise information on the electronic structure of the 1s exciton state can be
obtained. A Bohr radius a_{1s}=7.9 A and a 1s-2p transition dipole moment
\mu_{1s-2p}= 4.2 eA were found
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
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
Diversity and abundance of pteropods and heteropods along a latitudinal gradient across the Atlantic Ocean
AbstractShelled pteropods and heteropods are two independent groups of holoplanktonic gastropods that are potentially good indicators of the effects of ocean acidification. Although insight into their ecology and biogeography is important for predicting species-specific sensitivities to ocean change, the species abundances and biogeographical distributions of pteropods and heteropods are still poorly known. Here, we examined abundance and distribution patterns of pteropods (euthecosomes, pseudothecosomes, gymnosomes) and heteropods at 31 stations along a transect from 46°N to 46°S across the open waters of the Atlantic Ocean (Atlantic Meridional Transect cruise AMT24). We collected a total of 7312 pteropod specimens belonging to at least 31 species. Pteropod abundances were low north of 40°N with <15 individuals per 1000m3, varied between 100 and 2000ind./1000m3 between 30°N and 40°S, and reached >4000ind./1000m3 just south of 40°S. This accounted for an estimated biomass of 3.2mgm−3 south of 40°S and an average of 0.49mgm−3 along the entire transect. Species richness of pteropods was highest in the stratified (sub)tropical waters between 30°N and 30°S, with a maximum of 15 species per station. The biogeographical distribution of pteropod assemblages inferred by cluster analysis was largely congruent with the distribution of Longhurst’s biogeochemical provinces. Some pteropod species distributions were limited to particular oceanographic provinces, for example, subtropical gyres (e.g. Styliola subula) or warm equatorial waters (e.g. Creseis virgula). Other species showed much broader distributions between ∼35°N and ∼35°S (e.g. Limacina bulimoides and Heliconoides inflatus). We collected 1812 heteropod specimens belonging to 18 species. Highest heteropod abundances and species richness were found between 30°N and 20°S, with up to ∼700ind./1000m3 and a maximum of 14 species per station. Heteropods were not restricted to tropical and subtropical waters, however, as some taxa were also relatively abundant in subantarctic waters. Given the variation in distribution patterns among pteropod and heteropod species, it is likely that species will differ in their response to ocean changes
Positive selection on panpulmonate mitogenomes provide new clues on adaptations to terrestrial life
- …