Bosons in a Lattice: Exciton-Phonon Condensate in Cu2O

We explore a nonlinear field model to describe the interplay between the ability of excitons to be Bose-condensed and their interaction with other modes of a crystal. We apply our consideration to the long-living para-excitons in Cu2O. Taking into account the exciton-phonon interaction and introducing a coherent phonon part of the moving condensate, we derive the dynamic equations for the exciton-phonon condensate. These equations can support localized solutions, and we discuss the conditions for the moving inhomogeneous condensate to appear in the crystal. We calculate the condensate wave function and energy, and a collective excitation spectrum in the semiclassical approximation; the inside-excitations were found to follow the asymptotic behavior of the macroscopic wave function exactly. The stability conditions of the moving condensate are analyzed by use of Landau arguments, and Landau critical parameters appear in the theory. Finally, we apply our model to describe the recently observed interference and strong nonlinear interaction between two coherent exciton-phonon packets in Cu2O.Comment: 34 pages, LaTeX, four figures (.ps) are incorporated by epsf. Submitted to Phys. Rev.

Spin relaxation in low-dimensional systems

We review some of the newest findings on the spin dynamics of carriers and excitons in GaAs/GaAlAs quantum wells. In intrinsic wells, where the optical properties are dominated by excitonic effects, we show that exciton-exciton interaction produces a breaking of the spin degeneracy in two-dimensional semiconductors. In doped wells, the two spin components of an optically created two-dimensional electron gas are well described by Fermi-Dirac distributions with a common temperature but different chemical potentials. The rate of the spin depolarization of the electron gas is found to be independent of the mean electron kinetic energy but accelerated by thermal spreading of the carriers.Comment: 1 PDF file, 13 eps figures, Proceedings of the 1998 International Workshop on Nanophysics and Electronics (NPE-98)- Lecce (Italy

Dispersal and population structure at different spatial scales in the subterranean rodent Ctenomys australis

<p>Abstract</p> <p><b>Background</b></p> <p>The population genetic structure of subterranean rodent species is strongly affected by demographic (e.g. rates of dispersal and social structure) and stochastic factors (e.g. random genetic drift among subpopulations and habitat fragmentation). In particular, gene flow estimates at different spatial scales are essential to understand genetic differentiation among populations of a species living in a highly fragmented landscape. <it>Ctenomys australis </it>(the sand dune tuco-tuco) is a territorial subterranean rodent that inhabits a relatively secure, permanently sealed burrow system, occurring in sand dune habitats on the coastal landscape in the south-east of Buenos Aires province, Argentina. Currently, this habitat is threatened by urban development and forestry and, therefore, the survival of this endemic species is at risk. Here, we assess population genetic structure and patterns of dispersal among individuals of this species at different spatial scales using 8 polymorphic microsatellite loci. Furthermore, we evaluate the relative importance of sex and habitat configuration in modulating the dispersal patterns at these geographical scales.</p> <p>Results</p> <p>Our results show that dispersal in <it>C. australis </it>is not restricted at regional spatial scales (~ 4 km). Assignment tests revealed significant population substructure within the study area, providing support for the presence of two subpopulations from three original sampling sites. Finally, male-biased dispersal was found in the Western side of our study area, but in the Eastern side no apparent philopatric pattern was found, suggesting that in a more continuous habitat males might move longer distances than females.</p> <p>Conclusions</p> <p>Overall, the assignment-based approaches were able to detect population substructure at fine geographical scales. Additionally, the maintenance of a significant genetic structure at regional (~ 4 km) and small (less than 1 km) spatial scales despite apparently moderate to high levels of gene flow between local sampling sites could not be explained simply by the linear distance among them. On the whole, our results support the hypothesis that males disperse more frequently than females; however they do not provide support for strict philopatry within females.</p

The conservation status of the tuco-tucos, genus Ctenomys (Rodentia: Ctenomyidae), in southern Brazil

The goal of conservation biology should be related to the preservation of species and also to the evolutionary and ecological processes that were responsible to form them and that are still acting. We review the conservation status of the species of tuco-tuco (Ctenomys torquatus, C. lami, C. minutus, and C. flamarioni) from southern Brazil, and relate these data to the geological history of a particular area in that region, the Coastal Plain of the States of Rio Grande do Sul and Santa Catarina. The implications of the data on these species from the Southeastern Brazil are also discussed in relation to the evolution and risk of extinction of these subterranean rodents

Global endemics-area relationships of vascular plants

Endemics–Area Relationships (EARs) are fundamental in theoretical and applied biogeography for understanding distribution patterns and promoting biodiversity conservation. However, calculating EARs for vascular plant species from existing data is problematic because of biased knowledge of endemic species distributions and differences between taxonomies. We aimed to overcome these challenges by developing a new standardized global dataset based on expert knowledge to produce a set of global EARs. We developed a nested circle design, with grain sizes of 104, 105, 106, 107, and 108 km2, respectively, and a global distribution of plots based on a stratified random scheme. The number of vascular plant species endemic to each circle was then estimated independently by five experts randomly chosen from a pool of 23, as both a minimum and a maximum value (lower and upper bounds of the estimation), taking into account the limitations of current knowledge and varied species concepts in existing taxonomies. This procedure resulted in a dataset of 3000 expert estimates. Based on the data, we produced three global EARs for endemic species richness using minimum, maximum and average estimates. As a validation, we used all three models to extrapolate to the entire world, producing estimates of 284,493 (minimum), 398,364 (maximum) and 312,243 (average) vascular plant species. These figures conform to the range of taxonomists’ estimates. From the models, we calculated the average area needed to harbour a single endemic species as 12,875 km2 (range 9675–20,529). The global vascular plant EARs we calculated represent the first standardized, quantitative expectations of plant endemism at any given scale (sampling unit size). These EARs allow us to provide a clear answer to a long-standing but elusive biogeographical question: how to assess whether any area on the surface of the Earth is rich or poor in endemics relative to the average

Nuclear structure of light exotic nuclei from break-up reactions

4 pages, 4 figures.-- Printed version published Dec 27, 2004.-- Issue title: "Proceedings of the Sixth International Conference on Radioactive Nuclear Beams (RNB6)" (Argonne, Illinois, US, Sep 22-26, 2003).One-nucleon removal reactions at relativistic energies have been used as a spectroscopic tool to characterise the ground state properties of several neutron-rich isotopes in the sd-shell. Using the FRS at GSI, the longitudinal momentum distributions of the emerging fragments after one-nucleon removal were measured. The relative contributions of the remaining fragments in their ground and excited states have been determined from measurements of γ rays in coincidence with the longitudinal momentum distributions. In particular the breakup of O-23 has been investigated. The interpretation of our measurements, in the framework of a simple theoretical model, favours a spin and parity assignment of 1/2+ for the O-23 ground state in agreement with shell model predictions.Peer reviewe

One-neutron knockout of O-23

4 pages, 4 figures, 1 table.-- PACS nrs.: 25.60.Gc, 25.60.Dz, 27.20.+n.Breakup reactions were used to study the ground-state configuration of the neutron-rich isotope O-23. The O-22 fragments produced in one-nucleon removal from O-23 at 938 MeV/nucleon in a carbon target were detected in coincidence with de-exciting γ rays, allowing to discern between 220 ground-state and excited-states contributions. From the comparison of exclusive experimental momentum distributions for the one-neutron removal channel to theoretical momentum distributions calculated in an Eikonal model for the knockout process, and spin and parity assignment of I-π = 1/2(+) was deduced for the 230 ground state. This result solved the existent experimental discrepancy.Peer reviewe

Experimental evidence for the B-8 ground state configuration

6 pages, 1 table, 3 figures.-- PACS nrs.: 25.60.Dz; 25.60.Gc; 27.20.+n.-- Printed version published March 14, 2002.The Be-7 fragments produced in one-proton removal from relativistic B-8 nuclei of 936 MeV/nucleon in a carbon target have been detected in coincidence with gamma rays emitted by these fragments at the reaction target. It is found that 13 ± 3% of the Be-7 fragments are released in the 429 keV excited state, Be-7*, which gives direct experimental information on the ground state configuration of B-8. The longitudinal momentum distributions (p||) of the Be-7 fragments have been measured, both inclusively and in coincidence with de-excitation γ-rays from Be-7*. The widths and cross sections of the respective p|| distributions are compared with calculations in a three-body cluster model.This work was supported by EU under contract ERBFMGECT95 0083, and by Internationales Büro Osteuropa-Verbindungsbüro des BMBF bei den DLR under contract WTZ Bonn SLA-002-96, BMBF under contract number 06DA915I and NFR, Sweden, contract F5102-1484/2001.Peer reviewe

Nuclear and Coulomb breakup of B-8

17 pages, 4 tables, 9 figures.-- PACS nrs.: 25.60.Dz; 25.60.Gc; 27.20.+n.-- Printed version published Jun 2, 2003.One-proton removal reactions of 936 MeV/u B-8 have been studied with carbon and lead as breakup targets. Using the FRS at GSI, the longitudinal momentum distributions of the Be-7 fragments and the one-proton removal cross section of B-8 were measured. The relative contributions of the breakup channels leading to Be-7 in its ground state and excited state at 429 keV (Be-7*) have been determined from measurements of gamma rays in coincidence with the fragments from the one-proton removal reaction. The experimental breakup data are discussed in the context of a theoretical (α + He-3 + p) three-body model. The combined data from the two targets allow a determination of the weight of the Be-7* component in the B-8 ground state wave function as (13.3 ± 2.2%).This work was supported by EU under contract ERBFMGECT95 0083, and by Internationales Büro Osteuropa-Verbindungsbüro des BMBF bei der DLR under contract WTZ Bonn SLA-002-96-,BMBF under contract number 06DA915I,VR, Sweden,contract F5102 - 1484/2001.Peer reviewe