1,112 research outputs found
Political compromise and endogenous formation of coalitions
A one-dimensional model of spatial political competition with
endogenous party formation is developed. It is proved that at equilibrium
there are only two parties. These parties propose alternatives in the extreme
positions of the policy space. The adopted policy, however, is a compromise
between these two extremesThe second author
wishes to acknowledge financial support from the Spanish Ministry of Education,
projects no. PB93-0940, PB94-1504 and D.G.I.C.Y.T. mobility grantThe first author is
supported by the German Academic Exchange Service (DAAD). The second author
wishes to acknowledge financial support from the Spanish Ministry of Education,
projects no. PB93-0940, PB94-1504 and D.G.I.C.Y.T. mobility grantPublicad
The citation merit of scientific publications
We propose a new method to assess the merit of any set of scientific papers in a given field based on the citations they receive. Given a citation indicator, such as the mean citation or the h-index, we identify the merit of a given set of n articles with the probability that a randomly drawn sample of n articles from a reference set of articles in that field presents a lower citation index. The method allows for comparisons between research units of different sizes and fields. Using a dataset acquired from Thomson Scientific that contains the articles published in the periodical literature in the period 1998-2007, we show that the novel approach yields rankings of research units different from those obtained by a direct application of the mean citation or the h-index.Citation analysis, Citation merit, Mean citation, h-index
Emergent Symmetry at the N\'eel to Valence-Bond-Solid Transition
We show numerically that the `deconfined' quantum critical point between the
N\'eel antiferromagnet and the columnar valence-bond-solid, for a square
lattice of spin-1/2s, has an emergent symmetry. This symmetry allows
the N\'eel vector and the valence-bond-solid order parameter to be rotated into
each other. It is a remarkable 2+1-dimensional analogue of the symmetry that appears in the scaling limit for the
spin-1/2 Heisenberg chain. The emergent is strong evidence that the
phase transition in the 2+1D system is truly continuous, despite the violations
of finite-size scaling observed previously in this problem. It also implies
surprising relations between correlation functions at the transition. The
symmetry enhancement is expected to apply generally to the critical
two-component Abelian Higgs model (non-compact model). The result
indicates that in three dimensions there is an -symmetric conformal
field theory which has no relevant singlet operators, so is radically different
to conventional Wilson-Fisher-type conformal field theories.Comment: 4+epsilon pages, 6 figure
Effects of many-electron jumps in relaxation and conductivity of Coulomb glasses
A numerical study of the energy relaxation and conductivity of the Coulomb
glass is presented. The role of many-electron transitions is studied by two
complementary methods: a kinetic Monte Carlo algorithm and a master equation in
configuration space method. A calculation of the transition rate for
two-electron transitions is presented, and the proper extension of this to
multi-electron transitions is discussed. It is shown that two-electron
transitions are important in bypassing energy barriers which effectively block
sequential one-electron transitions. The effect of two-electron transitions is
also discussed.Comment: 8 pages, 6 figure
Deconfined Quantum Criticality, Scaling Violations, and Classical Loop Models
Numerical studies of the N\'eel to valence-bond solid phase transition in 2D
quantum antiferromagnets give strong evidence for the remarkable scenario of
deconfined criticality, but display strong violations of finite-size scaling
that are not yet understood. We show how to realise the universal physics of
the Neel-VBS transition in a 3D classical loop model (this includes the
interference effect that suppresses N\'eel hedgehogs). We use this model to
simulate unprecedentedly large systems (of size ). Our results are
compatible with a direct continuous transition at which both order parameters
are critical, and we do not see conventional signs of first-order behaviour.
However, we find that the scaling violations are stronger than previously
realised and are incompatible with conventional finite-size scaling over the
size range studied, even if allowance is made for a weakly/marginally
irrelevant scaling variable. In particular, different determinations of the
anomalous dimensions and yield very
different results. The assumption of conventional finite-size scaling gives
estimates which drift to negative values at large , in violation of
unitarity bounds. In contrast, the behaviour of correlators on scales much
smaller than is consistent with large positive anomalous dimensions.
Barring an unexpected reversal in behaviour at still larger sizes, this implies
that the transition, if continuous, must show unconventional finite-size
scaling, e.g. from a dangerously irrelevant scaling variable. Another
possibility is an anomalously weak first-order transition. By analysing the
renormalisation group flows for the non-compact model (-component
Abelian Higgs model) between two and four dimensions, we give the simplest
scenario by which an anomalously weak first-order transition can arise without
fine-tuning of the Hamiltonian.Comment: 20 pages, 19 figure
The skewness of science in 219 sub-fields and a number of aggregates
This paper studies evidence from Thomson Scientific about the citation process of 3.7 million articles published in the period 1998-2002 in 219 Web of Science categories, or sub-fields. Reference and citation distributions have very different characteristics across sub-fields. However, when analyzed with the Characteristic Scores and Scales technique, which is replication and scale invariant, the shape of these distributions over three broad categories of articles appears strikingly similar. Reference distributions are mildly skewed, but citation distributions with a five-year citation window are highly skewed: the mean is twenty points above the median, while 9-10% of all articles in the upper tail account for about 44% of all citations. The aggregation of sub-fields into disciplines and fields according to several aggregation schemes preserve this feature of citation distributions. It should be noted that when we look into subsets of articles within the lower and upper tails of citation distributions the universality partially breaks down. On the other hand, for 140 of the 219 sub-fields the existence of a power law cannot be rejected. However, contrary to what is generally believed, at the sub-field level the scaling parameter is above 3.5 most of the time, and power laws are relatively small: on average, they represent 2% of all articles and account for 13.5% of all citations. The results of the aggregation into disciplines and fields reveal that power law algebra is a subtle phenomenon.
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