On the Stacking Charge Order in NaV2O5

We propose a mechanism for the observed stacking charge order in the quarter-filled ladder compound NaV2O5. Via a standard mapping of the charge degrees of freedom onto Ising spins we explain the stacking order as a result of competition between couplings of the nearest and next-nearest planes with the 4-fold degenerate super-antiferroelectric in-plane order.Comment: 4 pages, 5 figure

‘Big Think’, Disjointed Incrementalism: Chinese Economic Success and Policy Lessons for Africa, or the Case for Pan-Africanism

Chinese economic success is not the product of free market accidental coincidence. Rather, it is orchestrated by the State through a mixture of nationalism (‘big think’) and pragmatic decisions (disjointed incrementalism) in agriculture, finance and industry. Furthermore, these decisions build upon existing institutions (e.g. the Household Responsibility System, Township Village Enterprises, etc), some dating back to pre-revolutionary China (e.g. Special Economic Zones), rather than imported ones from outside China. The article explores the utility (and lack thereof) of the Chinese model in the African context, as well as the possibilities of an Africa-centred ‘big think’ (Pan-Africanism) capable of mobilizing the continent for development

Attractor Metadynamics in Adapting Neural Networks

Slow adaption processes, like synaptic and intrinsic plasticity, abound in the brain and shape the landscape for the neural dynamics occurring on substantially faster timescales. At any given time the network is characterized by a set of internal parameters, which are adapting continuously, albeit slowly. This set of parameters defines the number and the location of the respective adiabatic attractors. The slow evolution of network parameters hence induces an evolving attractor landscape, a process which we term attractor metadynamics. We study the nature of the metadynamics of the attractor landscape for several continuous-time autonomous model networks. We find both first- and second-order changes in the location of adiabatic attractors and argue that the study of the continuously evolving attractor landscape constitutes a powerful tool for understanding the overall development of the neural dynamics

Covering classes and 1-tilting cotorsion pairs over commutative rings

We are interested in characterising the commutative rings for which a 1-tilting cotorsion pair provides for covers, that is when the class A is a covering class. We use Hrbekšfs bijective correspondence between the 1-tilting cotorsion pairs over a commutative ring R and the faithful finitely generated Gabriel topologies on R. Moreover, we use results of Bazzoni-Positselski, in particular a generalisation of Matlis equivalence and their characterisation of covering classes for 1-tilting cotorsion pairs arising from flat injective ring epimorphisms. Explicitly, if is the Gabriel topology associated to the 1-tilting cotorsion pair, and R is the ring of quotients with respect to, we show that if A is covering, then G is a perfect localisation (in Stenstromšfs sense [B. Stenstrom, Rings of Quotients, Springer, New York, 1975]) and the localisation R has projective dimension at most one as an R-module. Moreover, we show that is covering if and only if both the localisation RG and the quotient rings R/J are perfect rings for every J ∈. Rings satisfying the latter two conditions are called G-almost perfect

Exact Diagonalization Dynamical Mean Field Theory for Multi-Band Materials: Effect of Coulomb correlations on the Fermi surface of Na_0.3CoO_2

Dynamical mean field theory combined with finite-temperature exact diagonalization is shown to be a suitable method to study local Coulomb correlations in realistic multi-band materials. By making use of the sparseness of the impurity Hamiltonian, exact eigenstates can be evaluated for significantly larger clusters than in schemes based on full diagonalization. Since finite-size effects are greatly reduced this approach allows the study of three-band systems down to very low temperatures, for strong local Coulomb interactions and full Hund exchange. It is also shown that exact diagonalization yields smooth subband quasi-particle spectra and self-energies at real frequencies. As a first application the correlation induced charge transfer between t2g bands in Na_0.3CoO_2 is investigated. For both Hund and Ising exchange the small eg' Fermi surface hole pockets are found to be slightly enlarged compared to the non-interacting limit, in agreement with previous Quantum Monte Carlo dynamical mean field calculations for Ising exchange, but in conflict with photoemission data.Comment: 9 pages, 7 figure

The Spin-SAF transition in NaV2O5 induced by spin-pseudospin coupling

We present microscopic estimates for the spin-spin and spin-speudospin interactions of the quarter-filled ladder compound NaV2O5, obtained by exactly diagonalizing appropriate clusters of the underlying generalized Hubbard Hamiltonian. We present evidence for a substantial interladder spin-pseudospin interaction term which would allow simultaneously for the superantiferroelectric (SAF) charge (pseudospin) ordering and spin dimerization. We discuss the values of the coupling constants appropriate for NaV2O5 and deduce the absence of a soft antiferroelectric mode

Properties of the energetic particle distributions during the October 28, 2003 solar flare from INTEGRAL/SPI observations

Analysis of spectra obtained with the gamma-ray spectrometer SPI onboard INTEGRAL of the GOES X17-class flare on October 28, 2003 is presented. In the energy range 600 keV - 8 MeV three prominent narrow lines at 2.223, 4.4 and 6.1 MeV, resulting from nuclear interactions of accelerated ions within the solar atmosphere could be observed. Time profiles of the three lines and the underlying continuum indicate distinct phases with several emission peaks and varying continuum-to-line ratio for several minutes before a smoother decay phase sets in. Due to the high-resolution Ge detectors of SPI and the exceptional intensity of the flare, detailed studies of the 4.4 and 6.1 MeV line shapes was possible for the first time. Comparison with calculated line shapes using a thick target interaction model and several energetic particle angular distributions indicates that the nuclear interactions were induced by downward-directed particle beams with alpha-to-proton ratios of the order of 0.1. There are also indications that the 4.4 MeV to 6.1 MeV line fluence ratio changed between the beginning and the decay phase of the flare, possibly due to a temporal evolution of the energetic particle alpha-to-proton ratio.Comment: 24 pages, 10 figures, accepted for publication by A&