964 research outputs found
The mutualism of Melissotarsus ants and armoured scale insects in Africa and Magadascar: distribution, host plants and biology
Είδη μυρμηγκιών του γένους Melissotarsus Emery είναι διαδεδομένα στην Αφροτροπική περιοχή (τρία είδη M. beccarii Emery, M. emeryi Forel και M. weissi Santschi) καθώς και ένα είδος στην Μαγαδασκάρη (M. insularis Santschi). Τα μυρμήγκια όλων αυτών των ειδών δημιουργούν τις στοές τους σε ζωντανό ξύλο διαφόρων δικοτυλήδονων δένδρων, κοντά στο φλοιό. Τα μυρμήγκια διατηρούν εντός των στοών πληθυσμούς διαφόρων ειδών κοκκοειδών εντόμων τηςοικογένειας Diaspididae. Στην παρούσα εργασία παρουσιάζεται μια ανασκόπηση πάνω στη γεωγραφική κατανομή της παρατηρούμενης συμβίωσης των ειδών μυρμηγκιών του γένους Melissotarsus και των δέκα ειδών κοκκοειδών εντόμων της οικογένειας Diaspididae, καθώς και των φυτών ξενιστών όπου παρατηρείται η συμβίωση. Η οικολογία της παρατηρούμενης συμβίωσης συζητείται καθώς και τα πιθανά οφέλη που προκύπτουν για τα είδη που συμβιώνουν.Species of the ant genus Melissotarsus Emery are widespread in the Afrotropical region (three species, namely M. beccarii Emery, M. emeryi Forel and M. weissi Santschi) and in the Madagascar region (one species, namely M. insularis Santschi). The ants of all these species tunnel their galleries in live wood of various dicotyledonous trees, close to the bark surface. The ants maintain within these galleries populations of different species of armoured scale insects. A review is presented on the geographical distribution of mutualism, of the Melissotarsus species, the associated 10 species of armoured scale insects, and the host plants on which the mutualism takes place. The ecology of the mutualism is discussed also, together with suggestions on the benefits that the partners gain from the associations
Order Parameter Description of the Anderson-Mott Transition
An order parameter description of the Anderson-Mott transition (AMT) is
given. We first derive an order parameter field theory for the AMT, and then
present a mean-field solution. It is shown that the mean-field critical
exponents are exact above the upper critical dimension. Renormalization group
methods are then used to show that a random-field like term is generated under
renormalization. This leads to similarities between the AMT and random-field
magnets, and to an upper critical dimension for the AMT. For
, an expansion is used to calculate the critical
exponents. To first order in they are found to coincide with the
exponents for the random-field Ising model. We then discuss a general scaling
theory for the AMT. Some well established scaling relations, such as Wegner's
scaling law, are found to be modified due to random-field effects. New
experiments are proposed to test for random-field aspects of the AMT.Comment: 28pp., REVTeX, no figure
Effectiveness of CFD simulation for the performance prediction of phase change building boards in the thermal environment control of indoor spaces
This is the post-print version of the Article. The official published version can be accessed from the link below - Copyright @ 2013 ElsevierThis paper reports on a validation study of CFD models used to predict the effect of PCM clay boards on the control of indoor environments, in ventilated and non-ventilated situations. Unlike multi-zonal models, CFD is important in situations where localised properties are essential such as in buildings with complex and large geometries. The employed phase change model considers temperature/enthalpy hysteresis and varying enthalpy-temperature characteristics to more accurately simulate the phase change behaviour of the PCM boards compared to the standard default modelling approach in the commercial CFD codes. Successful validation was obtained with a mean error of 1.0 K relative to experimental data, and the results show that in addition to providing satisfactory quantitative results, CFD also provides qualitative results which are useful in the effective design of indoor thermal environment control systems utilising PCM. These results include: i) temperature and air flow distribution within the space resulting from the use of PCM boards and different night ventilation rates; ii) the fraction of PCM experiencing phase change and is effective in the control of the indoor thermal environment, enabling optimisation of the location of the boards; and iii) the energy impact of PCM boards and adequate ventilation configurations for effective night charging.This work was funded through sponsorship from the UK Engineering and Physical Sciences Research Council (EPSRC), Grant No: EP/H004181/1
Superconducting ``metals'' and ``insulators''
We propose a characterization of zero temperature phases in disordered
superconductors on the basis of the nature of quasiparticle transport. In three
dimensional systems, there are two distinct phases in close analogy to the
distinction between normal metals and insulators: the superconducting "metal"
with delocalized quasiparticle excitations and the superconducting "insulator"
with localized quasiparticles. We describe experimental realizations of either
phase, and study their general properties theoretically. We suggest experiments
where it should be possible to tune from one superconducting phase to the
other, thereby probing a novel "metal-insulator" transition inside a
superconductor. We point out various implications of our results for the phase
transitions where the superconductor is destroyed at zero temperature to form
either a normal metal or a normal insulator.Comment: 18 page
Parametric pumping at finite frequency
We report on a first principles theory for analyzing the parametric electron
pump at a finite frequency. The pump is controlled by two pumping parameters
with phase difference . In the zero frequency limit, our theory predicts
the well known result that the pumped current is proportional to .
For the more general situation of a finite frequency, our theory predicts a
non-vanishing pumped current even when the two driving forces are in phase, in
agreement with the recent experimental results. We present the physical
mechanism behind the nonzero pumped current at , which we found to be
due to photon-assisted processes
Properties of spin-triplet, even-parity superconductors
The physical consequences of the spin-triplet, even-parity pairing that has
been predicted to exist in disordered two-dimensional electron systems are
considered in detail. We show that the presence of an attractive interaction in
the particle-particle spin-triplet channel leads to an instability of the
normal metal that competes with the localizing effects of the disorder. The
instability is characterized by a diverging length scale, and has all of the
characteristics of a continuous phase transition. The transition and the
properties of the ordered phase are studied in mean-field theory, and by taking
into account Gaussian fluctuations. We find that the ordered phase is indeed a
superconductor with an ordinary Meissner effect and a free energy that is lower
than that of the normal metal. Various technical points that have given rise to
confusion in connection with this and other manifestations of odd-gap
superconductivity are also discussed.Comment: 15 pp., REVTeX, psfig, 2 ps figs, final version as publishe
Universality of the critical conductance distribution in various dimensions
We study numerically the metal - insulator transition in the Anderson model
on various lattices with dimension (bifractals and Euclidian
lattices). The critical exponent and the critical conductance
distribution are calculated. We confirm that depends only on the {\it
spectral} dimension. The other parameters - critical disorder, critical
conductance distribution and conductance cummulants - depend also on lattice
topology. Thus only qualitative comparison with theoretical formulae for
dimension dependence of the cummulants is possible
Short-Range Interactions and Scaling Near Integer Quantum Hall Transitions
We study the influence of short-range electron-electron interactions on
scaling behavior near the integer quantum Hall plateau transitions. Short-range
interactions are known to be irrelevant at the renormalization group fixed
point which represents the transition in the non-interacting system. We find,
nevertheless, that transport properties change discontinuously when
interactions are introduced. Most importantly, in the thermodynamic limit the
conductivity at finite temperature is zero without interactions, but non-zero
in the presence of arbitrarily weak interactions. In addition, scaling as a
function of frequency, , and temperature, , is determined by the
scaling variable (where is the exponent for the temperature
dependence of the inelastic scattering rate) and not by , as it would
be at a conventional quantum phase transition described by an interacting fixed
point. We express the inelastic exponent, , and the thermal exponent, ,
in terms of the scaling dimension, , of the interaction strength
and the dynamical exponent (which has the value ), obtaining
and .Comment: 9 pages, 4 figures, submitted to Physical Review
Random Mass Dirac Fermions in Doped Spin-Peierls and Spin-Ladder systems: One-Particle Properties and Boundary Effects
Quasi-one-dimensional spin-Peierls and spin-ladder systems are characterized
by a gap in the spin-excitation spectrum, which can be modeled at low energies
by that of Dirac fermions with a mass. In the presence of disorder these
systems can still be described by a Dirac fermion model, but with a random
mass. Some peculiar properties, like the Dyson singularity in the density of
states, are well known and attributed to creation of low-energy states due to
the disorder. We take one step further and study single-particle correlations
by means of Berezinskii's diagram technique. We find that, at low energy
, the single-particle Green function decays in real space like
. It follows that at these energies the
correlations in the disordered system are strong -- even stronger than in the
pure system without the gap. Additionally, we study the effects of boundaries
on the local density of states. We find that the latter is logarithmically (in
the energy) enhanced close to the boundary. This enhancement decays into the
bulk as and the density of states saturates to its bulk value on
the scale . This scale is different from
the Thouless localization length . We
also discuss some implications of these results for the spin systems and their
relation to the investigations based on real-space renormalization group
approach.Comment: 26 pages, LaTex, 9 PS figures include
- …