379 research outputs found
Resonant Raman scattering of quantum wire in strong magnetic field
The resonant Raman scattering of a quantum wire in a strong magnetic field is
studied, focused on the effect of long range Coulomb interaction and the
spin-charge separation. The energy-momentum dispersions of charge and spin
excitation obtained from Raman cross-section show the characteristc cross-over
behaviour induced by inter-edge Coulomb interaction. The "SPE" peak near
resonance in polarized spectra becomes broad due to the momentum dependence of
charge velocity. The broad peak in the depolarized spectra is shown to
originate from the disparity between charge and spin excitation velocity.Comment: RevTex file, 6 pages, no figure: To appear in Int. Jour. Mod. Phys.
Cadaveric Renal transplantation with Cyclosporine: Experiences in 148 patients at a single institution.
Estimativas populacionais de Dendrobates tinctorius (Anura: Dendrobatidae) em três áreas da Guiana Francesa e primeiro relato de quitridiomicose.
A regiĂŁo Neotropical abriga o maior nĂşmero de espĂ©cies de anuros da Terra e Ă© tambĂ©m uma das regiões em que os anfĂbios estĂŁo mais ameaçados. No entanto, poucos dados estĂŁo atualmente disponĂveis para avaliar a situação daspopulações de anfĂbios neotropicais. Estudamos trĂŞs populações de Dendrobates tinctorius na Guiana Francesa (Tresor, Favard e Nouragues) usando o modelo de Captura-Marcação-Recaptura (CMR) para a realização de estimativas robustas da densidade da espĂ©cie nesses trĂŞs locais. AlĂ©m disso, avaliamos a prevalĂŞncia do fungo patogĂŞnico Batrachochytrium dendrobatidis (Bd) em duaspopulações (Favard and Nouragues). O uso do modelo CMR revelou uma densidade de 4,67 indivĂduos/100 m² para Tresor, 8,43 indivĂduos/100 m² para Favard e 4,28 indivĂduos/100 m² para Nouragues, fornecendo estimativas das densidades populacionais de D. tinctorius na Guiana Francesa com as quais estimativas populacionais futuras poderĂŁo ser comparadas. Constatamos que 25 ocasiões de encontro podem ser suficientes para estimativas de uma população estável se as capturas forem concentradas no tempo. Bd foi detectado em ambas as áreas (Favard 7/152, Nouragues 3/18). Propomos, portanto, o inĂcio de um acompanhamento de longa-duração dessa espĂ©cie em vários locais (dentro e fora de áreas protegidas) da Guiana Francesa, combinando estudos de CMR com ocasiões de encontros concentrados em um curto perĂodo de tempo e o monitoramento de Bd.The Neotropics shelter the highest number of frog species on Earth and is also one of the regions where anurans aremost threatened. Nonetheless, few data are available to assess the population status of Neotropical anurans. We studied three populations (Tresor, Favard, and Nouragues) of the poison frog, Dendrobates tinctorius, in French Guiana and used Capture-Mark-Recapture (CMR) to make robust estimations of the species’ density at these three sites. In addition, we assessed the prevalence of the pathogen fungal Batrachochytrium dendrobatidis (Bd) in two populations (Favard and Nouragues). Based on the CMR protocol, the densities of frogs was 8.43 individuals/100 m² at Favard, 4.28 individuals/100 m² at Nouragues and from 2.30 to 4.67 individuals/100 m² at Tresor (depending on the CMR model used); these data provide a baseline for population densities of D. tinctorius in French Guiana, against which future population estimates can be compared. We found that 25 encounter events may be sufficient for stable population estimates, if the captures are concentrated in time. Bd was detected at both sites (Favard 7/152; Nouragues 3/18)
Theory of Resonant Raman Scattering in One Dimensional Electronic systems
A theory of resonant Raman scattering spectroscopy of one dimensional
electronic systems is developed on the assumptions that (i) the excitations of
the one dimensional electronic system are described by the Luttinger Liquid
model, (ii) Raman processes involve virtual excitations from a filled valence
band to an empty state of the one dimensional electronic system and (iii)
excitonic interactions between the valence and conduction bands may be
neglected. Closed form analytic expressions are obtained for the Raman
scattering cross sections, and are evaluated analytically and numerically for
scattering in the polarized channel, revealing a "double-peak" structure with
the lower peak involving multispinon excitations with total spin S=0 and the
higher peak being the conventional plasmon. A key feature of our results is a
nontrivial power law dependence, involving the Luttinger Liquid exponents, of
the dependence of the Raman cross sections on the difference of the laser
frequency from resonance. We find that near resonance the calculated ratio of
intensity in the lower energy feature to the intensity in the higher energy
feature saturates at a value of the order of unity (times a factor of the ratio
of the velocities of the two modes). We explicate the differences between the
'Luttinger liquid' and 'Fermi liquid' calculations of RRS spectra and argue
that excitonic effects, neglected in all treatments so far, are essential for
explaining the intensity ratios observed in quantum wires. We also discuss
other Luttinger liquid features which may be observed in future RRS
experiments
Shape Deformation driven Structural Transitions in Quantum Hall Skyrmions
The Quantum Hall ground state away from can be described by a
collection of interacting skyrmions. We show within the context of a nonlinear
sigma model, that the classical ground state away from is a skyrmion
crystal with a generalized N\'eel order. We show that as a function of filling
, the skyrmion crystal undergoes a triangle to square to triangle
transition at zero temperature. We argue that this structural transition,
driven by a change in the shape of the individual skyrmions, is stable to
thermal and quantum fluctuations and may be probed experimentally.Comment: 4 pages (REVTEX) and 4 .eps figure
Setting temporal baselines for biodiversity : the limits of available monitoring data for capturing the full impact of anthropogenic pressures
Temporal baselines are needed for biodiversity, in order for the change in biodiversity to be measured over time, the targets for biodiversity conservation to be defined and conservation progress to be evaluated. Limited biodiversity information is widely recognized as a major barrier for identifying temporal baselines, although a comprehensive quantitative assessment of this is lacking. Here, we report on the temporal baselines that could be drawn from biodiversity monitoring schemes in Europe and compare those with the rise of important anthropogenic pressures. Most biodiversity monitoring schemes were initiated late in the 20th century, well after anthropogenic pressures had already reached half of their current magnitude. Setting temporal baselines from biodiversity monitoring data would therefore underestimate the full range of impacts of major anthropogenic pressures. In addition, biases among taxa and organization levels provide a truncated picture of biodiversity over time. These limitations need to be explicitly acknowledged when designing management strategies and policies as they seriously constrain our ability to identify relevant conservation targets aimed at restoring or reversing biodiversity losses. We discuss the need for additional research efforts beyond standard biodiversity monitoring to reconstruct the impacts of major anthropogenic pressures and to identify meaningful temporal baselines for biodiversity
Bag Formation in Quantum Hall Ferromagnets
Charged skyrmions or spin-textures in the quantum Hall ferromagnet at filling
factor nu=1 are reinvestigated using the Hartree-Fock method in the lowest
Landau level approximation. It is shown that the single Slater determinant with
the minimum energy in the unit charge sector is always of the hedgehog form. It
is observed that the magnetization vector's length deviates locally from unity,
i.e. a bag is formed which accommodates the excess charge. In terms of a
gradient expansion for extended spin-textures a novel O(3) type of effective
action is presented, which takes bag formation into account.Comment: 13 pages, 3 figure
Reconstruction of the Quantum Hall Edge
The sharp \nu=1 quantum Hall edge present for hard confinement is shown to
have two modes that go soft as the confining potential softens. This signals a
second order transition to a reconstructed edge that is either a depolarized
spin-texture edge or a polarized charge density wave edge.Comment: 6 pages, 4 figures, to be published in the proceedings of the
workshop on ``Novel Physics in Low-Dimensional Electron Systems'' held in
Dresden, Physica
Resonant Raman scattering by collective modes of the one-dimensional electron gas
We show that the low-energy peak in the polarized resonant Raman spectra of
quantum wires, which is commonly associated with ``single particle
excitations'', can be interpreted as signature of intra-band collective spin
excitations. A broad maximum in the resonant depolarized spectra is predicted
to exist above the frequency of the spin density excitation, due to
simultaneous but independent propagation of spin- and charge-density modes.Comment: 4 pages, accepted for publication in Phys. Rev. Let
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