1,489 research outputs found
Mass Renormalization in the Su-Schrieffer-Heeger Model
This study of the one dimensional Su-Schrieffer-Heeger model in a weak
coupling perturbative regime points out the effective mass behavior as a
function of the adiabatic parameter , is the
zone boundary phonon energy and is the electron band hopping integral.
Computation of low order diagrams shows that two phonons scattering processes
become appreciable in the intermediate regime in which zone boundary phonons
energetically compete with band electrons. Consistently, in the intermediate
(and also moderately antiadiabatic) range the relevant mass renormalization
signals the onset of a polaronic crossover whereas the electrons are
essentially undressed in the fully adiabatic and antiadiabatic systems. The
effective mass is roughly twice as much the bare band value in the intermediate
regime while an abrupt increase (mainly related to the peculiar 1D dispersion
relations) is obtained at .Comment: To be published in Phys.Rev.B - 3 figure
Passive phloem loading and long-distance transport in a synthetic tree-on-a-chip
Vascular plants rely on differences of osmotic pressure to export sugars from
regions of synthesis (mature leaves) to sugar sinks (roots, fruits). In this
process, known as M\"unch pressure flow, the loading of sugars from
photosynthetic cells to the export conduit (the phloem) is crucial, as it sets
the pressure head necessary to power long-distance transport. Whereas most
herbaceous plants use active mechanisms to increase phloem concentration above
that of the photosynthetic cells, in most tree species, for which transport
distances are largest, loading seems to occur via passive symplastic diffusion
from the mesophyll to the phloem. Here, we use a synthetic microfluidic model
of a passive loader to explore the nonlinear dynamics that arise during export
and determine the ability of passive loading to drive long-distance transport.
We first demonstrate that in our device, phloem concentration is set by the
balance between the resistances to diffusive loading from the source and
convective export through the phloem. Convection-limited export corresponds to
classical models of M\"unch transport, where phloem concentration is close to
that of the source; in contrast, diffusion-limited export leads to small phloem
concentrations and weak scaling of flow rates with the hydraulic resistance. We
then show that the effective regime of convection-limited export is predominant
in plants with large transport resistances and low xylem pressures. Moreover,
hydrostatic pressures developed in our synthetic passive loader can reach
botanically relevant values as high as 10 bars. We conclude that passive
loading is sufficient to drive long-distance transport in large plants, and
that trees are well suited to take full advantage of passive phloem loading
strategies
Polarons and bipolarons in strongly interacting electron-phonon systems
The Holstein Hubbard and Holstein t--J models are studied for a wide range of
phonon frequencies, electron--electron and electron--phonon interaction
strengths on finite lattices with up to ten sites by means of direct Lanczos
diagonalization. Previously the necessary truncation of the phononic Hilbert
space caused serious limitations to either very small systems (four or even two
sites) or to weak electron--phonon coupling, in particular in the adiabatic
regime. Using parallel computers we were able to investigate the transition
from `large' to `small' polarons in detail. By resolving the low--lying
eigenstates of the Hamiltonian and by calculating the spectral function we can
identify a polaron band in the strong--coupling case, whose dispersion deviates
from the free--particle dispersion at low and intermediate phonon frequencies.
For two electrons (holes) we establish the existence of bipolaronic states and
discuss the formation of a bipolaron band. For the 2D Holstein t--J model we
demonstrate that the formation of hole--polarons is favoured by strong Coulomb
correlations. Analyzing the hole--hole correlation functions we find that hole
binding is enhanced as a dynamical effect of the electron--phonon interaction.Comment: 23 pages (Revtex) with 13 figures (ps, uuencoded
Evolutionary history and species delimitations: a case study of the hazel dormouse, Muscardinus avellanarius
Robust identification of species and significant evolutionary units (ESUs) is essential to implement appropriate conservation strategies for endangered species. However, definitions of species or ESUs are numerous and
sometimes controversial, which might lead to biased conclusions, with serious consequences for the management of
endangered species. The hazel dormouse, an arboreal rodent of conservation concern throughout Europe is an
ideal model species to investigate the relevance of species identification for conservation purposes. This species is a
member of the Gliridae family, which is protected in Europe and seriously threatened in the northern part of its
range. We assessed the extent of genetic subdivision in the hazel dormouse by sequencing one mitochondrial gene
(cytb) and two nuclear genes (BFIBR, APOB) and genotyping 10 autosomal microsatellites. These data were analysed using a combination of phylogenetic analyses and species delimitation methods. Multilocus analyses revealed
the presence of two genetically distinct lineages (approximately 11 % cytb genetic divergence, no nuclear alleles
shared) for the hazel dormouse in Europe, which presumably diverged during the Late Miocene. The phylogenetic
patterns suggests that Muscardinus avellanarius populations could be split into two cryptic species respectively
distributed in western and central-eastern Europe and Anatolia. However, the comparison of several species
definitions and methods estimated the number of species between 1 and 10. Our results revealed the difficulty in
choosing and applying an appropriate criterion and markers to identify species and highlight the fact that consensus
guidelines are essential for species delimitation in the future. In addition, this study contributes to a better
knowledge about the evolutionary history of the species
Sense of place in the changing process of house form: Case studies from Ankara, Turkey
This paper aims to investigate the impact of typomorphological changes of residential environments on residents’ sense of place’. Seven housing developments representing different types introduced in Ankara, Turkey since the late 19th-century are selected as case studies. Their morphological characters at the building, street and neighbourhood scales are examined, and typological transformations among the cases in terms of the degrees of continuity are identified. The paper proposes a conceptual model consisting of ten indicators to assess sense of place at the building, street and neighbourhood scales of the residents of the seven cases. The scores of sense of place are generated through structured interviews with the residents and analysed in SPSS. The results show that sense of place is negatively affected by typomorphological changes over time, particularly when mutational changes occur. Continuity in typomorphological transformation helps to maintain sense of place at a desirable level. Furthermore, physical changes at the street and neighbourhood scales have larger impact on sense of place than that at the building scale. The research thus suggests that planning and design should be responsive to traditional types in residential development, particularly at the street and neighbourhood scales to maintain residents’ sense of place
Results from a blind and a non-blind randomised trial run in parallel: experience from the Estonian Postmenopausal Hormone Therapy (EPHT) Trial
<p>Abstract</p> <p>Background</p> <p>The Estonian Postmenopausal Hormone Therapy (EPHT) Trial assigned 4170 potential participants prior to recruitment to blind or non-blind hormone therapy (HT), with placebo or non-treatment the respective alternatives. Before having to decide on participation, women were told whether they had been randomised to the blind or non-blind trial. Eligible women who were still willing to join the trial were recruited. After recruitment participants in the non-blind trial (N = 1001) received open-label HT or no treatment, participants in the blind trial (N = 777) remained blinded until the end of the trial. The aim of this paper is to analyse the effect of blinding on internal and external validity of trial outcomes.</p> <p>Methods</p> <p>Effect of blinding was calculated as the hazard ratio of selected chronic diseases, total mortality and all outcomes. For analysing the effect of blinding on external validity, the hazard ratios from women recruited to the placebo arm and to the non-treatment arm were compared with those not recruited; for analysing the effect of blinding on internal validity, the hazard ratios from the blind trial were compared with those from the non-blind trial.</p> <p>Results</p> <p>The women recruited to the placebo arm had less cerebrovascular disease events (HR 0.43; 95% CI: 0.26-0.71) and all outcomes combined (HR 0.76; 95% CI: 0.63-0.91) than those who were not recruited. Among women recruited or not recruited to the non-treatment arm, no differences were observed for any of the outcomes studied.</p> <p>Among women recruited to the trial, the risk for coronary heart disease events (HR 0.77; 95% CI: 0.64-0.93), cerebrovascular disease events (HR 0.66; 95%CI: 0.47-0.92), and all outcomes combined (HR 0.82; 95% CI: 0.72-0.94) was smaller among participants in the blind trial than in the non-blind trial. There was no difference between the blind and the non-blind trial for total cancer (HR 0.95; 95% CI: 0.64-1.42), bone fractures (0.93; 95% CI: 0.74-1.16), and total mortality (HR 1.03; 95% CI: 0.53-1.98).</p> <p>Conclusions</p> <p>The results from blind and non-blind trials may differ, even if the target population is the same. Blinding may influence both internal and external validity. The effect of blinding may vary for different outcome events.</p> <p>Trial registration</p> <p>[<a href="http://www.controlled-trials.com/ISRCTN35338757">ISRCTN35338757</a>]</p
Conformal field theory approach to gapless 1D fermion systems and application to the edge excitations of nu = 1/(2p+1) quantum Hall sequences
We present a comprehensive study of the effective Conformal Field Theory
(CFT) describing the low energy excitations of a gas of spinless interacting
fermions on a circle in the gapless regime (Luttinger liquid). Functional
techniques and modular transformation properties are used to compute all
correlation functions in a finite size and at finite temperature. Forward
scattering disorder is treated exactly. Laughlin experiments on charge
transport in a Quantum Hall Fluid on a cylinder are reviewed within this CFT
framework. Edge excitations above a given bulk excitation are described by a
twisted version of the Luttinger effective theory. Luttinger CFTs corresponding
to the nu =1/(2p+1) filling fractions appear to be rational CFTs (RCFT).
Generators of the extended symmetry algebra are identified as edge fermions
creators and annihilators, thus giving a physical meaning to the RCFT point of
view on edge excitations of these sequences.Comment: 69 pages, 1 figure, LaTeX2e + amstex and graphicx packages needed,
fullpage.sty used (not compulsory
Surface chemistry of a Cu(I) beta-diketonate precursor and the atomic layer deposition of Cu 2
Hot Phonons in an Electrically Biased Graphene Constriction
Phonon carrier interactions can have significant impact on device
performance. They can be probed by measuring the phonon lifetime, which
reflects the interaction strength of a phonon with other quasi-particles in
particular charge carriers as well as its companion phonons. The carrier phonon
and phonon-phonon contributions to the phonon lifetime can be disentangled from
temperature dependent studies. Here, we address the importance of phonon
carrier interactions in Joule-heated graphene constrictions in order to
contribute to the understanding of energy dissipation in graphene based
electronic devices. We demonstrate that gapless graphene grants electron phonon
interactions uncommon significance in particular at low carrier density. In
conventional semiconductors, the bandgap usually prevents the decay of phonons
through electron-hole generation and also in metals or other semimetals the
Fermi temperature is excessively large to enter the regime where electron
phonon coupling plays such a dominant role as in graphene in the investigated
phonon temperature regime from 300 to 1600 K.Comment: Nano Letters (Web publication on 30th Dec. 2009,
DOI:10.1021/nl903167f
UV continuum emission and diagnostics of hydrogen-containing non-equilibrium plasmas
For the first time the emission of the radiative dissociation continuum of
the hydrogen molecule ( electronic
transition) is proposed to be used as a source of information for the
spectroscopic diagnostics of non-equilibrium plasmas. The detailed analysis of
excitation-deactivation kinetics, rate constants of various collisional and
radiative transitions and fitting procedures made it possible to develop two
new methods of diagnostics of: (1) the ground state
vibrational temperature from the relative intensity
distribution, and (2) the rate of electron impact dissociation
(d[\mbox{H_{2}}]/dt)_{\text{diss}} from the absolute intensity of the
continuum. A known method of determination of from relative
intensities of Fulcher- bands was seriously corrected and simplified
due to the revision of transition probabilities and cross sections of
electron impact excitation. General considerations are illustrated
with examples of experiments in pure hydrogen capillary-arc and H+Ar
microwave discharges.Comment: REVTeX, 25 pages + 12 figures + 9 tables. Phys. Rev. E, eprint
replaced because of resubmission to journal after referee's 2nd repor
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