Theory for Magnetism and Triplet Superconductivity in LiFeAs

Superconducting pnictides are widely found to feature spin-singlet pairing in the vicinity of an antiferromagnetic phase, for which nesting between electron and hole Fermi surfaces is crucial. LiFeAs differs from the other pnictides by (i) poor nesting properties and (ii) unusually shallow hole pockets. Investigating magnetic and pairing instabilities in an electronic model that incorporates these differences, we find antiferromagnetic order to be absent. Instead we observe almost ferromagnetic fluctuations which drive an instability toward spin-triplet p-wave superconductivity.Comment: Published versio

Internal slackening scoring methods

We deal with the ranking problem of the nodes in a directed graph. The bilateral relationships specifed by a directed graph may reect the outcomes of a sport competi- tion, the mutual reference structure between websites, or a group preference structure over alternatives. We introduce a class of scoring methods for directed graphs, indexed by a single nonnegative parameter. This parameter reects the internal slackening of a node within an underlying iterative process. The class of so-called internal slackening scoring methods, denoted by ??, consists of the limits of these processes. It is seen that ??0 extends the invariant scoring method, while ??1 extends the fair bets scoring method. Method ??1corresponds with the existing ??-scoring method of Borm et al. (2002) and can be seen as a compromise between ??0 and ??1. In particular, an explicit proportionality relation between ?? and ??1 is derived. Moreover, the internal slackening scoring methods are applied to the setting of social choice situations where they give rise to a class of social choice correspondences that refine both the top cycle correspondence and the uncovered set correspondence

Unraveling Orbital Correlations via Magnetic Resonant Inelastic X-ray Scattering

Although orbital degrees of freedom are a factor of fundamental importance in strongly correlated transition metal compounds, orbital correlations and dynamics remain very difficult to access, in particular by neutron scattering. Via a direct calculation of scattering amplitudes we show that instead magnetic resonant inelastic x-ray scattering (RIXS) does reveal orbital correlations. In contrast to neutron scattering, the intensity of the magnetic excitations in RIXS depends very sensitively on both the symmetry of the orbitals that spins occupy, and on photon polarizations. We show in detail how this effect allows magnetic RIXS to distinguish between alternating orbital ordered and ferro-orbital (or orbital liquid) states.Comment: 7 pages, 4 figures. Supplemental material adde

Electronic Correlations in Oligo-acene and -thiophene Organic Molecular Crystals

From first principles calculations we determine the Coulomb interaction between two holes on oligo-acene and -thiophene molecules in a crystal, as a function of the oligomer length. The relaxation of the molecular geometry in the presence of holes is found to be small. In contrast, the electronic polarization of the molecules that surround the charged oligomer, reduces the bare Coulomb repulsion between the holes by approximately a factor of two. In all cases the effective hole-hole repulsion is much larger than the calculated valence bandwidth, which implies that at high doping levels the properties of these organic semiconductors are determined by electron-electron correlations.Comment: 5 pages, 3 figure

The use of the API 20 NE bacteria classification procedure to identify Pasteurellaceae strains in rodents and rabbits

Forty growth-factor independent Pasteurellaceae strains representing most known taxa from rodents and  rabbits were API 20 NE profiled by four laboratory animal diagnostic laboratories using their routine  methodology. Significant differences were found in the number of bacterial strains classified with the  Pasteurellaceae. The results of the four laboratories taken together showed that 136 (85 %) of the 160 tests  carried out with the 40 strains led to classification with the family. The 23 Pasteurellaceae strains from  species (Pasteurella aerogenes, P. multocida, P. pneumotropica) present in the API 20 NE data base (API  taxa strains) and the 17 strains from taxa not included (non - API taxa strains) were classified with the family  with similar frequency.Pasteurella species designation differed significantly between the laboratories and full agreement in speciation  was found with only 9 (22.5 %) of the 40 strains. Of the tests carried out with P. multocida and P.  pneumotropica, 42 and 52 % respectively led to misclassification. Conversely 38 % of the profiles  obtained with non- API taxa strains led to identification as P. multocida or P. pneumotropica.  We conclude that identification of Pasteurellaceae to the species level by the API 20 NE system is unreliable,  but the system is useful to classify various Pasteurellaceae taxa from rodents and rabbits with the  bacterial family.

On Two New Social Choice Correspondences

Preferences of a set of n individuals over a set of alternatives can be represented by a preference profile being an n-tuple of preference relations over these alternatives.A social choice correspondence assigns to every preference profile a subset of alternatives that can be viewed as the `most prefered' alternatives by the society consisting of all individuals.Two new social choice correspondences are introduced and analyzed.Both are Pareto optimal and are refinements of the well known Top cycle correspondence in case the corresponding simple majority win digraph is a tournament.One of them even is such a refinement for arbitrary preference profiles.

Inter-site Coulomb interaction and Heisenberg exchange

Based on exact diagonalization results for small clusters we discuss the effect of inter-site Coulomb repulsion in Mott-Hubbard or charge transfer insulators. Whereas the exchange constant J for direct exchange is substantially enhanced by inter-site Coulomb interaction, that for superexchange is suppressed. The enhancement of J in the single-band models holds up to the critical value for the charge density wave (CDW) instability, thus opening the way for large values of J. Single-band Hubbard models with sufficiently strong inter-site repulsion to be near a CDW instability thus may provide `physical' realizations of t-J like models with the `unphysical' parameter ratio J/t=1.Comment: Revtex file, 4 PRB pages, with 5 embedded ps-files. To appear in PRB, rapid communications. Hardcopies of figures or the entire manuscript may also be obtained by e-mail request to: [email protected]

Positional Power in Hierarchies

Power is a core concept in the analysis and design of organisations. In this paper we consider positional power in hierarchies. One of the problems with the extant literature on positional power in hierarchies is that it is mainly restricted to the analysis of power in terms of the bare positions of the actors. While such an analysis informs us about the authority structure within an organisation, it ignores the decision-making mechanisms completely. The few studies which take into account the decision-making mechanisms make all use of adaptations of well-established approaches for the analysis of power in non-hierarchical organisations such as the Banzhaf measure; and thus they are all based on the structure of a simple game, i.e. they are ‘membershipbased’. We demonstrate that such an approach is in general inappropriate for characterizing power in hierarchies as it cannot be extended to a class of decision-making mechanisms which allow certain actors to terminate a decision before all other members have been involved. As this kind of sequential decision-making mechanism turns out to be particularly relevant for hierarchies, we suggest an action-b! ased approach - represented by an extensive game form - which can take the features of such mechanisms into account. Based on this approach we introduce a power score and measure that can be applied to ascribe positional power to actors in sequential decision making mechanisms

Completeness and properness of refinement operators in inductive logic programming

AbstractWithin Inductive Logic Programming, refinement operators compute a set of specializations or generalizations of a clause. They are applied in model inference algorithms to search in a quasi-ordered set for clauses of a logical theory that consistently describes an unknown concept. Ideally, a refinement operator is locally finite, complete, and proper. In this article we show that if an element in a quasi-ordered set 〈S, ≥〉 has an infinite or incomplete cover set, then an ideal refinement operator for 〈S, ≥〉 does not exist. We translate the nonexistence conditions to a specific kind of infinite ascending and descending chains and show that these chains exist in unrestricted sets of clauses that are ordered by θ-subsumption. Next we discuss how the restriction to a finite ordered subset can enable the construction of ideal refinement operators. Finally, we define an ideal refinement operator for restricted θ-subsumption ordered sets of clauses