717 research outputs found
Evidence of breakdown of the spin symmetry in diluted 2D electron gases
Recent claims of an experimental demonstration of spontaneous spin
polarisation in dilute electron gases \cite{young99} revived long standing
theoretical discussions \cite{ceper99,bloch}. In two dimensions, the
stabilisation of a ferromagnetic fluid might be hindered by the occurrence of
the metal-insulator transition at low densities \cite{abra79}. To circumvent
localisation in the two-dimensional electron gas (2DEG) we investigated the low
populated second electron subband, where the disorder potential is mainly
screened by the high density of the first subband. This letter reports on the
breakdown of the spin symmetry in a 2DEG, revealed by the abrupt enhancement of
the exchange and correlation terms of the Coulomb interaction, as determined
from the energies of the collective charge and spin excitations. Inelastic
light scattering experiments and calculations within the time-dependent local
spin-density approximation give strong evidence for the existence of a
ferromagnetic ground state in the diluted regime.Comment: 4 pages, 4 figures, Revte
Trophic interactions among grouper (Epinephelus marginatus), octopus (Octopus vulgaris) and red lobster (Palinurus elephas) in the Western Mediterranean
In the present paper a first evaluation of the trophic interactions between grouper, octopus and spiny red lobster in the western Mediterranean is presented. The efficiency of octopus preying on lobsters is also estimated from trap catches. Grouper and octopus base their diet on the same major taxonomic groups and this is reflected in the values of diet overlap. Trophic diversity of grouper and octopus is also very similar. The estimation of the octopus efficiency preying on lobsters shows that 23.5% of the attacks are successfulPublicado
Hall-effect and resistivity measurements in CdTe and ZnTe at high pressure: Electronic structure of impurities in the zincblende phase and the semi-metallic or metallic character of the high-pressure phases
We carried out high-pressure resistivity and Hall-effect measurements in
single crystals of CdTe and ZnTe up to 12 GPa. Slight changes of transport
parameters in the zincblende phase of CdTe are consitent with the shallow
character of donor impurities. Drastic changes in all the transport parameters
of CdTe were found around 4 GPa, i.e. close to the onset of the cinnabar to
rock-salt transition. In particular, the carrier concentration increases by
more than five orders of magnitude. Additionally, an abrupt decrease of the
resistivity was detected around 10 GPa. These results are discussed in
comparison with optical, thermoelectric, and x-ray diffraction experiments. The
metallic character of the Cmcm phase of CdTe is confirmed and a semi-metallic
character is determined for the rock-salt phase. In zincblende ZnTe, the
increase of the hole concentration by more than two orders of magnitude is
proposed to be due to a deep-to-shallow transformation of the acceptor levels.
Between 9 and 11 GPa, transport parameters are consistent with the
semiconducting character of cinnabar ZnTe. A two orders of magnitude decrease
of the resistivity and a carrier-type inversion occurs at 11 GPa, in agreement
with the onset of the transition to the Cmcm phase of ZnTe. A metallic
character for this phase is deduced.Comment: 20 pages, 4 figure
Far-infrared absorption in parallel quantum wires with weak tunneling
We study collective and single-particle intersubband excitations in a system
of quantum wires coupled via weak tunneling. For an isolated wire with
parabolic confinement, the Kohn's theorem guarantees that the absorption
spectrum represents a single sharp peak centered at the frequency given by the
bare confining potential. We show that the effect of weak tunneling between two
parabolic quantum wires is twofold: (i) additional peaks corresponding to
single-particle excitations appear in the absorption spectrum, and (ii) the
main absorption peak acquires a depolarization shift. We also show that the
interplay between tunneling and weak perpendicular magnetic field drastically
enhances the dispersion of single-particle excitations. The latter leads to a
strong damping of the intersubband plasmon for magnetic fields exceeding a
critical value.Comment: 18 pages + 6 postcript figure
Enhanced suppresion of localization in a continuous Random-Dimer Model
We consider a one-dimensional continuous (Kronig-Penney) extension of the
(tight-binding) Random Dimer model of Dunlap et al. [Phys. Rev. Lett. 65, 88
(1990)]. We predict that the continuous model has infinitely many resonances
(zeroes of the reflection coefficient) giving rise to extended states instead
of the one resonance arising in the discrete version. We present exact,
transfer-matrix numerical calculations supporting, both realizationwise and on
the average, the conclusion that the model has a very large number of extended
states.Comment: 10 pages, 3 Figures available on request, REVTeX 3.0, MA/UC3M/1/9
Localized thinning for strain concentration in suspended germanium membranes and optical method for precise thickness measurement
We deposited Ge layers on (001) Si substrates by molecular beam epitaxy and used them to fabricate suspended membranes with high uniaxial tensile strain. We demonstrate a CMOS-compatible fabrication strategy to increase strain concentration and to eliminate the Ge buffer layer near the Ge/Si hetero-interface deposited at low temperature. This is achieved by a two-steps patterning and selective etching process. First, a bridge and neck shape is patterned in the Ge membrane, then the neck is thinned from both top and bottom sides. Uniaxial tensile strain values higher than 3% were measured by Raman scattering in a Ge membrane of 76 nm thickness. For the challenging thickness measurement on micrometer-size membranes suspended far away from the substrate a characterization method based on pump-and-probe reflectivity measurements was applied, using an asynchronous optical sampling technique.EC/FP7/628197/EU/Heat Propagation and Thermal Conductivity in Nanomaterials for Nanoscale Energy Management/HEATPRONAN
Coastal meroplanktonic larval stages of peninsula de Llevant natural reserve determined with light traps
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