Anderson impurity in the one-dimensional Hubbard model on finite size systems

An Anderson impurity in a Hubbard model on chains with finite length is studied using the density-matrix renormalization group (DMRG) technique. In the first place, we analyzed how the reduction of electron density from half-filling to quarter-filling affects the Kondo resonance in the limit of Hubbard repulsion U=0. In general, a weak dependence with the electron density was found for the local density of states (LDOS) at the impurity except when the impurity, at half-filling, is close to a mixed valence regime. Next, in the central part of this paper, we studied the effects of finite Hubbard interaction on the chain at quarter-filling. Our main result is that this interaction drives the impurity into a more defined Kondo regime although accompanied in most cases by a reduction of the spectral weight of the impurity LDOS. Again, for the impurity in the mixed valence regime, we observed an interesting nonmonotonic behavior. We also concluded that the conductance, computed for a small finite bias applied to the leads, follows the behavior of the impurity LDOS, as in the case of non-interacting chains. Finally, we analyzed how the Hubbard interaction and the finite chain length affect the spin compensation cloud both at zero and at finite temperature, in this case using quantum Monte Carlo techniques.Comment: 9 pages, 9 figures, final version to be published in Phys. Rev.

Numerical study of finite size effects in the one-dimensional two-impurity Anderson model

We study the two-impurity Anderson model on finite chains using numerical techniques. We discuss the departure of magnetic correlations as a function of the interimpurity distance from a pure 2k_F oscillation due to open boundary conditions. We observe qualitatively different behaviors in the interimpurity spin correlations and in transport properties at different values of the impurity couplings. We relate these different behaviors to a change in the relative dominance between the Kondo effect and the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction. We also observe that when RKKY dominates there is a definite relation between interimpurity magnetic correlations and transport properties. In this case, there is a recovery of 2k_F periodicity when the on-site Coulomb repulsion on the chain is increased at quarter-filling. The present results could be relevant for electronic nanodevices implementing a non-local control between two quantum dots that could be located at variable distance along a wire.Comment: final version to appear in Phys. Rev.

Melting of Partially Fluorinated Graphene: From Detachment of Fluorine Atoms to Large Defects and Random Coils

The melting of fluorographene is very unusual and depends strongly on the degree of fluorination. For temperatures below 1000 K, fully fluorinated graphene (FFG) is thermo-mechanically more stable than graphene but at T$_m\approx$2800 K FFG transits to random coils which is almost twice lower than the melting temperature of graphene, i.e. 5300 K. For fluorinated graphene (PFG) up to 30 % ripples causes detachment of individual F-atoms around 2000 K while for 40-60 % fluorination, large defects are formed beyond 1500 K and beyond 60% of fluorination F-atoms remain bonded to graphene until melting. The results agree with recent experiments on the dependence of the reversibility of the fluorination process on the percentage of fluorination.Comment: 16 pages, 6 figure

Photocatalytic reactions of a nickel(II) annulene complex incorporated in polymeric structures

The photochemical reactions of the Ni(II) annulene complex, [NiII([5,7,12,14]-tetra methyl dibenzo[2,3- b:2,3-b,i][1,4,8,11]tetraaza[14]annulenate)], grafted into a poly(isobutylene-alt-maleate) backbone were investigated in aqueous media. The grafted Ni(II) complex becomes soluble in aqueous and organic solvents where the strands form aggregates with medium-dependent shapes. Irradiation of the polymer at 532 or 351 nm produce charge-separated macrocyclic pendants, CS, with a lifetime s 30 ns. CS reacts with electron donors and acceptors before it decays with a lifetime s 1 ms. In parallel to the decay of CS, an excited state-excited state annihilation process gives rise to luminescence whose spectrum spans wavelengths shorter than the wavelength of the irradiation, lex > 500 nm. Theoretical calculations were carried out with the aim of understanding the morphology and structures of strand aggregates, to confirm the nature of reaction products and to account for the spectroscopic and photochemical properties of the Ni(II) pendants. The endothermic reduction of CO2 to CO by S(IV) species was used as a test of the Ni(II) complex´s ability to photocatalyze the reaction. In the photoprocess, the Ni(II) complex fulfills the double role of antenna and catalyst.Fil: Estiu, G.. University Of Notre Dame-indiana; Estados UnidosFil: Ferraudi, G.. University Of Notre Dame-indiana; Estados UnidosFil: Lappin, A. G.. University Of Notre Dame-indiana; Estados UnidosFil: Ruiz, Gustavo Teodosio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; ArgentinaFil: Vericat, Carolina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; ArgentinaFil: Costamagna, J.. Universidad de Santiago de Chile; ChileFil: Villagrán, M.. Universidad de Santiago de Chile; Chil

Large Magnetoresistance in a Manganite Spin-Tunnel-Junction Using LaMnO3 as Insulating Barrier

A spin-tunnel-junction based on manganites, with La$_{1-x}$Sr$_x$MnO$_3$ (LSMO) as ferromagnetic metallic electrodes and the undoped parent compound LaMnO$_3$ (LMO) as insulating barrier, is here theoretically discussed using double exchange model Hamiltonians and numerical techniques. For an even number of LMO layers, the ground state is shown to have anti-parallel LSMO magnetic moments. This highly resistive, but fragile, state is easily destabilized by small magnetic fields, which orient the LSMO moments in the direction of the field. The magnetoresistance associated with this transition is very large, according to Monte Carlo and Density Matrix Renormalization Group studies. The influence of temperature, the case of an odd number of LMO layers, and the differences between LMO and SrTiO$_3$ as barriers are also addressed. General trends are discussed.Comment: 14 pages, 18 figure

COMPARISONS OF TWO SYMMETRIC DENSITY FUNCTION SOLUTIONS OF APHID POPULATION GROWTH MODELS

Aphids are among the world\u27s most devastating crop pests, and their population trajectories in field crops are characterized by rapid boom and bust, under the influence of bottom up (host plant) and top down (natural enemy) forces. Theoretical development in aphid growth trajectory modeling has recently advanced quite significantly, and the logistic and normal probability density functions have been found to provide analytical solutions to mechanistic models of the aphid population growth dynamics. The logistic or hyperbolic secant squared model captures a growth trajectory shaped by negative feedback of the aphid population on itself, due to the accumulation of adverse effect on its host plant and the coupling with natural enemies (bottom up as well as top down effect), while the normal model can be derived on the basis of a relationship between intrinsic growth rate and the host plant phenology. In this paper, we fit both models to a large number of observed aphid population trajectors and explore model properties. It is shown that, despite the diverging mechanistic underpinnings of the model, the generated growth curves, as fitted to the data, are very similar, as are characteristics, such as the height of the peak, the time of the peak and the accumulated area under the curve. Both models are useful workhorses for capturing aphid growth dynamics, but fitting one or either model cannot be used as evidence for the underpinning mechanisms, as different underpinning mechanisms result in similar population dynamics

Reinforced Concrete Building with IED Detonation: Test and Simulation

There is growing concern about the possibility of a suicide bomber being immolated when the army forces or the law enforcement agencies discover the place where they prepare their material or simply find themselves inside a building. To study the possible effects that these improvised explosive devices (IEDs) would have on the structures, eight tests were carried out with various configurations of IEDs with vest bombs inside a reinforced concrete (including walls and roof) building constructed ad hoc for these tests. These vests were made with different explosives (black powder, ANFO, AN/AL, PG2). For the characterization of these tests, a high-speed camera and pressure and acceleration sensors were used. The structure behaved surprisingly well, as it withstood all the first seven detonations without apparent structural damage. In the last detonation, located on the ground and with a significant explosive charge, the structural integrity of the roof and some of the walls was compromised. The simulation of the building was carried out with the LS-DYNA software with a Lagrangian formulation for the walls, using the LBE (based on CONWEP) module for the application of the charge. Despite the difficulty of this simulation, the results obtained, in terms of applied pressures and measured accelerations, are acceptable with differences of about 20%

Melting of Partially Fluorinated Graphene: From Detachment of Fluorine Atoms to Large Defects and Random Coils

The melting of fluorographene is very unusual and depends strongly on the degree of fluorination. For temperatures below 1000 K, fully fluorinated graphene (FFG) is thermo-mechanically more stable than graphene but at T m ≈ 2800 K FFG transits to random coils which is almost twice lower than the melting temperature of graphene, i.e. 5300 K. For fluorinated graphene (PFG) up to 30% ripples causes detachment of individual F-atoms around 2000 K while for 40-60% fluorination, large defects are formed beyond 1500 K and beyond 60% of fluorination F-atoms remain bonded to graphene until melting. The results agree with recent experiments on the dependence of the reversibility of the fluorination process on the percentage of fluorination.Fil: Singh, Sandeep Kumar. Universiteit Antwerpen. Department of Physics; BélgicaFil: Costamagna, Sebastian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensura; ArgentinaFil: Neek Amal, M.. Universiteit Antwerpen. Department of Physics; BélgicaFil: Peeters, F. M.. Universiteit Antwerpen. Department of Physics; Bélgic

Numerical Prediction of the Performance of Integrated Planar Solid-Oxide Fuel Cells, with Comparisons of Results from Several Codes

A numerical study of the thermal and electrochemical performance of a single-tube Integrated Planar Solid Oxide Fuel Cell (IP-SOFC) has been performed. Results obtained from two finite-volume computational fluid dynamics (CFD) codes FLUENT and SOHAB and from a two-dimensional inhouse developed finite-volume GENOA model are presented and compared. Each tool uses physical and geometric models of differing complexity and comparisons are made to assess their relative merits. Several single-tube simulations were run using each code over a range of operating conditions. The results include polarization curves, distributions of local current density, composition and temperature. Comparisons of these results are discussed, along with their relationship to the respective imbedded phenomenological models for activation losses, fluid flow and mass transport in porous media. In general, agreement between the codes was within 15% for overall parameters such as operating voltage and maximum temperature. The CFD results clearly show the effects of internal structure on the distributions of gas flows and related quantities within the electrochemical cells