984 research outputs found
Critical mass and the dependency of research quality on group size
Academic research groups are treated as complex systems and their cooperative
behaviour is analysed from a mathematical and statistical viewpoint. Contrary
to the naive expectation that the quality of a research group is simply given
by the mean calibre of its individual scientists, we show that intra-group
interactions play a dominant role. Our model manifests phenomena akin to phase
transitions which are brought about by these interactions, and which facilitate
the quantification of the notion of critical mass for research groups. We
present these critical masses for many academic areas. A consequence of our
analysis is that overall research performance of a given discipline is improved
by supporting medium-sized groups over large ones, while small groups must
strive to achieve critical mass.Comment: 16 pages, 6 figures consisting of 16 panels. Presentation and
reference list improved for version
The Weakly Coupled Gross-Neveu Model with Wilson Fermions
The nature of the phase transition in the lattice Gross-Neveu model with
Wilson fermions is investigated using a new analytical technique. This involves
a new type of weak coupling expansion which focuses on the partition function
zeroes of the model. Its application to the single flavour Gross-Neveu model
yields a phase diagram whose structure is consistent with that predicted from a
saddle point approach. The existence of an Aoki phase is confirmed and its
width in the weakly coupled region is determined. Parity, rather than chiral
symmetry breaking naturally emerges as the driving mechanism for the phase
transition.Comment: 15 pages including 1 figur
Thermal flight performance reveals impact of warming on bumblebee foraging potential
1. The effects of environmental temperature on components of insect flight determine life history traits, fitness, adaptability, and ultimately, organism ecosystem functional roles. Despite the crucial role of flying insects across landscapes, our understanding of how temperature affects insect flight performance remains limited. 2. Many insect pollinators are considered under threat from climatic warming. Quantifying the relationship between temperature and behavioural performance traits allows us to understand where species are operating in respect to their thermal limits, helping predict responses to projected temperature increases and/or erratic weather events. 3. Using a tethered flight mill, we quantify how flight performance of a widespread bumblebee, Bombus terrestris, varies over a temperature range (12-30oC). Given that body mass constrains insect mobility and behaviour, bumblebees represent a useful system to study temperature-mediated size-dependence of flight performance owing to the large intra-colony variation in worker body size they exhibit.. 4. Workers struggled to fly over a few hundred metres at the lowest tested temperature of 12oC, however flight endurance increased as temperatures rose, peaking around 25oC after which it declined. Our findings further revealed variation in flight capacity across the workforce, with larger workers flying further, longer, and faster than their smaller nestmates. Body mass was also positively related with the likelihood of flight, although importantly this relationship became stronger as temperatures cooled, such that at 12oC only the largest workers were successful fliers. Our study thus highlights that colony foraging success under variable thermal environments can be dependent on the body mass distribution of constituent workers, and more broadly suggests smaller-bodied insects may benefit disproportionately more from warming than larger-bodied ones in terms of flight performance. 5. By incorporating both flight endurance and likelihood of flight, we calculated a simple metric termed ‘temperature-mediated foraging potential’ to gain a clearer understanding of how temperature may constrain colony foraging. Of our tested temperatures, 27oC supported the highest potential, indicating that for much of the range of this species, higher mean daily temperatures as forecasted under climate warming will push colonies closer to their thermal optimum for flight. Subsequently, warming may have positive implications for bumblebee foraging returns and pollination provision
Scaling Relations for Logarithmic Corrections
Multiplicative logarithmic corrections to scaling are frequently encountered
in the critical behavior of certain statistical-mechanical systems. Here, a
Lee-Yang zero approach is used to systematically analyse the exponents of such
logarithms and to propose scaling relations between them. These proposed
relations are then confronted with a variety of results from the literature.Comment: 4 page
The dimer model on the triangular lattice
We analyze the partition function of the dimer model on an triangular lattice wrapped on torus obtained by Fendley,
Moessner and Sondhi [Phys. Rev. B \textbf{66}, 214513 (2002)]. From a
finite-size analysis we have found that the dimer model on such a lattice can
be described by conformal field theory having central charge . The shift
exponent for the specific heat is found to depend on the parity of the number
of lattice sites along a given lattice axis: e.g., for odd
we obtain the shift exponent , while for even
it is infinite (). In the former case, therefore,
the finite-size specific-heat pseudocritical point is size dependent, while in
the latter case, it coincides with the critical point of the thermodynamic
limit.Comment: 15 pages, 4 figure
Fermion loop simulation of the lattice Gross-Neveu model
We present a numerical simulation of the Gross-Neveu model on the lattice
using a new representation in terms of fermion loops. In the loop
representation all signs due to Pauli statistics are eliminated completely and
the partition function is a sum over closed loops with only positive weights.
We demonstrate that the new formulation allows to simulate volumes which are
two orders of magnitude larger than those accessible with standard methods
Testing fixed points in the 2D O(3) non-linear sigma model
Using high statistic numerical results we investigate the properties of the
O(3) non-linear 2D sigma-model. Our main concern is the detection of an
hypothetical Kosterlitz-Thouless-like (KT) phase transition which would
contradict the asymptotic freedom scenario. Our results do not support such a
KT-like phase transition.Comment: Latex, 7 pgs, 4 eps-figures. Added more analysis on the
KT-transition. 4-loop beta function contains corrections from D.-S.Shin
(hep-lat/9810025). In a note-added we comment on the consequences of these
corrections on our previous reference [16
Variability in bacteriophage and antibiotic sensitivity in serial Pseudomonas aeruginosa isolates from cystic fibrosis airway cultures over 12 months
Antibiotic treatment for Pseudomonas aeruginosa (Pa) in cystic fibrosis is limited in efficacy and may lead to multi-drug resistance (MDR). Alternatives such as bacteriophages are being explored but well designed, and controlled trials are crucial. The rational selection of patients with bacteriophage susceptible infections is required for both safety and efficacy monitoring. We questioned whether bacteriophage susceptibility profiles were constant or variable over time, variability having been reported with antibiotics. Serial Pa isolates (n = 102) from 24 chronically infected cystic fibrosis (CF) patients over one year were investigated with plaque and antibiotic disc diffusion assays. Variable number tandem repeat (VNTR) analysis identified those patients with >1 isolate. A median (range) of 4 (3–6) isolates/patient were studied. Twenty-one (87.5%) individuals had a single VNTR type; three (12.5%) had two VNTR types at different times. Seventy-five percent of isolates were sensitive to bacteriophage at ≥ 1 concentration; 50% of isolates were antibiotic multidrug resistant. Serial isolates, even when representing a single VNTR type, varied in sensitivity to both bacteriophages and antibiotics. The rates of sensitivity to bacteriophage supports the development of this therapy; however, the variability in response has implications for the selection of patients in future trials which must be on the basis of current, not past, isolate testing
Scaling Analysis of the Site-Diluted Ising Model in Two Dimensions
A combination of recent numerical and theoretical advances are applied to
analyze the scaling behaviour of the site-diluted Ising model in two
dimensions, paying special attention to the implications for multiplicative
logarithmic corrections. The analysis focuses primarily on the odd sector of
the model (i.e., that associated with magnetic exponents), and in particular on
its Lee-Yang zeros, which are determined to high accuracy. Scaling relations
are used to connect to the even (thermal) sector, and a first analysis of the
density of zeros yields information on the specific heat and its corrections.
The analysis is fully supportive of the strong scaling hypothesis and of the
scaling relations for logarithmic corrections.Comment: 15 pages, 3 figures. Published versio
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