Vertex routing models

A class of models describing the flow of information within networks via routing processes is proposed and investigated, concentrating on the effects of memory traces on the global properties. The long-term flow of information is governed by cyclic attractors, allowing to define a measure for the information centrality of a vertex given by the number of attractors passing through this vertex. We find the number of vertices having a non-zero information centrality to be extensive/sub-extensive for models with/without a memory trace in the thermodynamic limit. We evaluate the distribution of the number of cycles, of the cycle length and of the maximal basins of attraction, finding a complete scaling collapse in the thermodynamic limit for the latter. Possible implications of our results on the information flow in social networks are discussed.Comment: 12 pages, 6 figure

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

Na2V3O7, a frustrated nanotubular system with spin-1/2 diamond rings

Following the recent discussion on the puzzling nature of the interactions in the nanotubular system Na2V3O7, we present a detailed ab-initio microscopic analysis of its electronic and magnetic properties. By means of a non-trivial downfolding study we propose an effective model in terms of tubes of nine-site rings with the geometry of a spin-diamond necklace with frustrated inter-ring interactions. We show that this model provides a quantitative account of the observed magnetic behavior.Comment: 5 pages, 5 figures. Phys. Rev. Lett. (in press

Projected Wavefunctions and High Temperature Superconductivity

We study the Hubbard model with parameters relevant to the cuprates, using variational Monte Carlo with projected d-wave states. For doping 0 < x < 0.35 we obtain a superconductor whose order parameter tracks the observed nonmonotonic Tc(x). The variational parameter Delta_{var}(x) scales with the (pi,0) ``hump'' and T* seen in photoemission. Projection leads to incoherence in the spectral function, and from the singular behavior of its moments we obtain the nodal quasiparticle weight Z which vanishes linearly in x, though the Fermi velocity remains finite as x approaches zero. The Drude weight D_{low} and superfluid density are consistent with experiments, and D_{low} scales with Z.Comment: 4 pages, 5 EPS figures. (1) Many improvements including discussion of n(k) and superfluid density. (2) Added comparison with slave boson mean field theory. (3) Added new reference

Characterization of glycospingolipids in Neurospora crassa

Characterization of glycospingolipids in Neurospora crass

Quantum phase transition in the dioptase magnetic lattice

The study of quantum phase transitions, which are zero-temperature phase transitions between distinct states of matter, is of current interest in research since it allows for a description of low-temperature properties based on universal relations. Here we show that the crystal green dioptase Cu_6Si_6O_18 . 6H_2O, known to the ancient Roman as the gem of Venus, has a magnetic crystal structure, formed by the Cu(II) ions, which allows for a quantum phase transition between an antiferromagnetically ordered state and a quantum spin liquid.Comment: 6 pages, 5 figures, EPL, in pres

Gossamer Superconductor, Mott Insulator, and Resonating Valence Bond State in Correlated Electron Systems

Gutzwiller variational method is applied to an effective two-dimensional Hubbard model to examine the recently proposed gossamer superconductor by Laughlin. The ground state at half filled electron density is a gossamer superconductor for smaller intra-site Coulomb repulsion U and a Mott insulator for larger U. The gossamer superconducting state is similar to the resonant valence bond superconducting state, except that the chemical potential is approximately pinned at the mid of the two Hubbard bands away from the half filled

The phase diagram of the square lattice bilayer Hubbard model: a variational Monte Carlo study

We investigate the phase diagram of the square lattice bilayer Hubbard model at half-filling with the variational Monte Carlo method for both the magnetic and the paramagnetic case as a function of the interlayer hopping t\u3c4 and on-site Coulomb repulsion U. With this study we resolve some discrepancies in previous calculations based on the dynamical mean-field theory, and we are able to determine the nature of the phase transitions between metal, Mott insulator and band insulator. In the magnetic case we find only two phases: an antiferromagnetic Mott insulator at small t\u3c4 for any value of U and a band insulator at large t\u3c4 . At large U values we approach the Heisenberg limit. The paramagnetic phase diagram shows at small t\u3c4 a metal to Mott insulator transition at moderate U values and a Mott to band insulator transition at larger U values. We also observe a re-entrant Mott insulator to metal transition and metal to band insulator transition for increasing t\u3c4 in the range of 5.5t < U < 7.5t. Finally, we discuss the phase diagrams obtained in relation to findings from previous studies based on different many-body approaches.\ua9 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft

Variational state based on the Bethe ansatz solution and a correlated singlet liquid state in the one-dimensional t-J model

The one-dimensional t-J model is investigated by the variational Monte Carlo method. A variational wave function based on the Bethe ansatz solution is newly proposed, where the spin-charge separation is realized, and a long-range correlation factor of Jastrow-type is included. In most regions of the phase diagram, this wave function provides an excellent description of the ground-state properties characterized as a Tomonaga-Luttinger liquid; Both of the amplitude and exponent of correlation functions are correctly reproduced. For the spin-gap phase, another trial state of correlated singlet pairs with a Jastrow factor is introduced. This wave function shows generalized Luther-Emery liquid behavior, exhibiting enhanced superconducting correlations and exponential decay of the spin correlation function. Using these two variational wave functions, the whole phase diagram is determined. In addition, relations between the correlation exponent and variational parameters in the trial functions are derived.Comment: REVTeX 3.0, 27 pages. 7 figures available upon request ([email protected]). To be published in Phys. Rev. B 5