Rigorous results on superconducting ground states for attractive extended Hubbard models

We show that the exact ground state for a class of extended Hubbard models including bond-charge, exchange, and pair-hopping terms, is the Yang "eta-paired" state for any non-vanishing value of the pair-hopping amplitude, at least when the on-site Coulomb interaction is attractive enough and the remaining physical parameters satisfy a single constraint. The ground state is thus rigorously superconducting. Our result holds on a bipartite lattice in any dimension, at any band filling, and for arbitrary electron hopping.Comment: 12 page

η\eta-pairing as a mechanism of superconductivity in models of strongly correlated electrons

We consider extended versions of the Hubbard model which contain additional interactions between nearest neighbours. In this letter we show that a large class of these models has a superconducting ground state in arbitrary dimensions. In some special cases we are able to find the complete phase diagram. The superconducting phase exist even for moderate repulsive values of the Hubbard interaction $U$.Comment: 9 pages, RevTex, ITP-SB-94-18, 1 PS figure appende

Physicochemical Characterization And Antioxidant Capacity Of Pitanga Fruits (eugenia Uniflora L.) [caracterização Fisico-química E Capacidade Antioxidante De Pitangas (eugenia Uniflora L.)]

This study was carried out to obtain more information about the physicochemical properties, composition, and antioxidant activity of pitanga fruits (Eugenia uniflora L.), particularly fruits from the State of Rio Grande do Sul, Brazil. Pitanga with different flesh colors (purple, red, and orange) from tree selections cultivated at Embrapa Clima Temperado (RS-Brazil) were analyzed. Only slight differences were observed in the quality parameters and in the proximate and fatty acid compositions among the fruits studied. The extracts from purple-fleshed pitanga had the highest total phenolic and anthocyanin contents along with the highest antioxidant capacity. The antioxidant capacity (DPPH and FRAP assays) of methanolic pitanga extracts was highly correlated with the total phenolic content, but in ethanolic extracts, the anthocyanin content was correlated only with the FRAP antioxidant capacity. Orange fleshed pitanga had higher β-cryptoxanthin and β-carotene levels than those of the red fruit, which had higher lycopene content. The results indicate that the purple-fleshed pitanga, cultivated in Rio Grande do Sul, is a rich source of phenolic compounds and has high antioxidant capacity. The red and orange-fleshed pitanga, on the other hand, are rich sources of carotenoids.311147154Abidille, M.D.H., Antioxidant activity of the extracts from Dillenia indica fruits (2005) Food Chemistry, 90 (4), pp. 891-896Adebajo, A.C., Oloki, K.J., Aladesanmi, A., Antimicrobial activity of the leaf extract of Eugenia uniflora (1989) Journal of Phytotherapy Resource, 3 (6), pp. 258-259Aherne, S.A., O'Brien, N.M., Dietary flavonols: Chemistry, food content, and metabolism (2002) Nutrition, 18 (1), pp. 75-81(1995) Official methods of analysis of the Association of the Official Analytical Chemists, , ASSOCIATION OF OFFICIAL ANALYTICAL CHEMISTS-AOAC, 16th ed. Arlington, Virginia: AOACAzevedo-Meleiro, C.H., Rodriguez-Amaya, D.B., Confirmation of the identity of the carotenoids of tropical fruits by HPLC-DAD and HPLC-MS (2004) Journal of Food Composition and Analysis, 17 (3-4), pp. 385-396Bagetti, M., Antioxidant capacity and composition of pitanga seeds (2009) Ciência Rural, 39 (8), pp. 204-2510Beekwilder, J., Antioxidant in raspberry: On-line analysis links antioxidant activity to a diversity of individual metabolites (2005) Journal of Agricultural and Food Chemistry, 53 (9), pp. 3313-3320Benzie, F.F.I., Strain, J.J., The ferric reducing ability of plasma (FRAP) as a measure of "antioxidant power": The FRAP assay (1996) Analytical Biochemistry, 239 (1), pp. 70-76Bligh, E.G., Dyer, W.J., A rapid method of total lipid extraction and purification (1959) Journal of Biochemistry and Physiology, 37 (8), pp. 911-917Block, G., Patterson, B., Subar, A., Fruits, vegetables and cancer prevention: A review of the epidemiological evidence (1992) Nutrition and Cancer, 18 (1), pp. 1-29Bors, W., Flavonoids as antioxidants: Determination of radical scavenging efficiencies (1990) Methods in Enzymology, 186, pp. 343-355Brand-Williams, W., Cuvelier, M.E., Berset, C., Use of a free radical method to evaluated antioxidant activity (1995) Lebensmittel-Wissenschaft und-Technologie, 28 (1), pp. 25-30Regulamento técnico geral para fixação dos padrões de identidade e qualidade para polpa de fruta (2000) Diário Oficial da República Federativa do Brasil, p. 54. , http://extranet.agricultura.gov.br/sislegisconsulta/consultarLegislacao. do?operacao=visualizar&id=7777, BRASIL. Instrução Normativa, no 1, de 7 de janeiro de 2000, Brasília, DF, 10 jan, Seção 1, Disponível em:, Acesso em: 18 dez. 2008Cavalcante, M.L., Rodriguez-Amaya, D.B., Carotenoid composition of the tropical fruits Eugenia uniflora and Malpighia glabra (1992) Food Science and Human Nutrition, pp. 643-650. , In: CHARALAMBOUS, G. (Ed.), Amsterdam: Elsevier Science PublishersClinton, S.K., Lycopene: Chemistry, biology, and implications for human health and disease (1998) Nutrition Reviews, 56 (2), pp. 35-51Consolini, A.E., Sarubbio, M., Pharmacological effects of Eugenia uniflora L. (Myrtaceae) aqueous extract on rat's heart (2002) Journal of Ethnopharmacology, 81 (1), pp. 57-63di Mascio, P., Kaiser, S., Sies, H., Lycopene as the most efficient biological carotenoid singlet oxygen quencher (1989) Archives of Biochemistry and Biophysics, 274 (2), pp. 532-538Dillard, C.J., German, J.B., Phytochemicals: Neutraceuticals and human health (2000) Journal of the Science of Food and Agriculture, 80 (12), pp. 1744-1756Diplock, A.T., Functional food sciences and defense against reactive oxidative species (1998) British Journal of Nutrition, 80 (1), pp. 77-112Escarpa, A., Gonzalez, M.C., Approach to the content of total extractable phenolic compounds from different food samples by comparison of chromatographic and spectrophotometric methods (2001) Analytica Chimica Acta, 427 (1), pp. 119-127Gemtchüjnicov, I.D., (1976) Manual de taxonomia vegetal: Plantas de interesse econômico, agrícola, ornamentais e medicinais, p. 368. , São Paulo: CeresGenovese, M.I., Bioactive compounds and antioxidant capacity of exotic fruits commercial frozen pulps from Brazil (2008) Food Science and Technology International, 4 (3), pp. 207-214Hartman, L., Lago, B.C., A rapid preparation of fatty methyl esters from lipids (1973) Laboratory Practice, 22 (6), pp. 475-477Hassimotto, N.M.A., Genovese, M.I., Lajolo, F.M., Antioxidant activity of dietary fruits, vegetables, and commercial frozen fruit pulps (2005) Journal of Agricultural and Food Chemistry, 53 (8), pp. 2928-2935Kaur, C., Kapoor, H., Antioxidants in fruits and vegetables-the millennium's health (2001) International Journal of Food Science and Technology, 36 (7), pp. 703-725Kimura, M., Rodriguez-Amaya, D.B., Yokoyama, S.M., Cultivar differences and geographic effects on the carotenoid composition and vitamin A value of papaya (1991) Lebensmittel-Wissenschaft und-Technologie, 24 (5), pp. 415-418Krinsky, N.I., Johnson, E.J., Carotenoid actions and their relation to health and disease (2005) Molecular Aspects of Medicine, 26 (6), pp. 459-516Kris-Etherton, P.M., Bioactive compounds in foods: Their role in the prevention of cardiovascular disease and cancer (2002) American Journal of Medicine, 113 (9), pp. 71-88Kuskoski, M.E., Frutas tropicais silvestres e polpas de frutas congeladas: Atividade antioxidante, polifenóis e antocianinas (2006) Ciência Rural, 36 (4), pp. 1283-1287Lees, D.H., Francis, F.J., Standardization of pigment analyses in cranberries (1972) Hortscience, 7 (1), pp. 83-84Lima, V.L.A.G., Mélo, E.A., Lima, D.E.S., Fenólicos e carotenóides totais em pitanga (2002) Scientia Agricola, 59 (3), pp. 447-450Niizu, P.Y., Rodriguez-Amaya, D.B., A melancia como fonte de licopeno (2003) Revista do Instituto Adolfo Lutz, 62 (3), pp. 195-199Oliveira, A.L., Volatile compounds from pitanga fruit (Eugenia uniflora L.) (2006) Food Chemistry, 99 (1), pp. 1-5Padula, M., Rodriguez-Amaya, D.B., Characterization of the carotenoids and assessment of the vitamin A value of Brazilian guavas (Psidium guajava L.) (1986) Food Chemistry, 20 (1), pp. 11-19Pellegrini, N., Evaluation of antioxidant capacity of some fruit and vegetable foods: Efficiency of extraction of a sequence of solvents (2007) Journal of the Science of Food and Agriculture, 87 (1), pp. 103-111Pietta, P.G., Flavonoids as antioxidants (2000) Journal of Natural Products, 63 (7), pp. 1035-1042Pinto, M.S., Lajolo, F.M., Genovese, M.I., Bioactive compounds and quantification of total ellagic acid in strawberries (Fragaria × ananassa Duch.) (2007) Food Chemistry, 107 (4), pp. 1629-1635Porcu, O.M., Rodriguez-Amaya, D.B., Variation in the carotenoid composition of the lycopene-rich Brazilian fruit Eugenia uniflora L (2008) Plant Foods for Human Nutrition, 63 (4), pp. 195-199Prior, R.L., Cao, G., Antioxidant phytochemicals in fruits and vegetables: Diet and health implications (2000) Horticulture Science, 35 (4), pp. 588-592Rahman, I., Adcock, I.M., Oxidative stress and redox regulation of lung inflammation in COPD (2006) European Respiratory Journal, 28 (1), pp. 219-242Reynerston, K.A., Quantitative analysis of antiradical phenolic constituents from fourteen edible Myrtaceae fruits (2008) Food Chemistry, 109 (4), pp. 883-890Robards, K., Antolovich, M., Analytical chemistry of fruit bioflavonoids (1997) Analyst, 122, pp. 11R-34RRodriguez-Amaya, D.B., (1999) A guide to carotenoid analysis in foods, , Washington, D.C.: ILSI PressSalgado, S.M., Guerra, N.B., Melo Filho, A.B., Frozen fruit pulps: Effects of the processing on dietary fiber contents (1999) Brazilian Journal of Nutrition, 12 (3), pp. 303-308Scalzo, J., Plant genotype affects total antioxidant capacity and phenolic contents in fruit (2005) Nutrition, 21 (2), pp. 207-213Singleton, V.L., Rossi Jr., J.A., Colorimetry of total phenolic with phosphomolybdic-phosphotungstic acid reagents (1965) American Journal of Enology and Viticulture, 16 (3), pp. 144-158Stahl, W., Sies, H., Bioactivity and protective effects of natural carotenoids (2005) Biochimica et Biophysica Acta, 1740 (2), pp. 101-107Tapiero, H., Townsend, D.M., Tew, K.D., The role of carotenoids in the prevention of human pathologies (2004) Biomedicine and Pharmacotherapy, 58 (2), pp. 100-110(2006) Tabela brasileira de composição de alimentos-TACO, p. 113. , UNIVERSIDADE DE CAMPINAS-UNICAMP, 2. ed. Version II. Campinas: NEPA/UNICAMPVison, J.A., Phenol antioxidant quantity and quality in foods: Vegetables (1998) Journal of Agricultural and Food Chemistry, 46 (9), pp. 4113-4117Weyerstahl, P., Volatile constituents of Eugenia uniflora leaf oil (1988) Planta Médica, 54 (6), pp. 546-54

Evaluation of the BCS Approximation for the Attractive Hubbard Model in One Dimension

The ground state energy and energy gap to the first excited state are calculated for the attractive Hubbard model in one dimension using both the Bethe Ansatz equations and the variational BCS wavefunction. Comparisons are provided as a function of coupling strength and electron density. While the ground state energies are always in very good agreement, the BCS energy gap is sometimes incorrect by an order of magnitude, particularly at half-filling. Finite size effects are also briefly discussed for cases where an exact solution in the thermodynamic limit is not possible. In general, the BCS result for the energy gap is poor compared to the exact result.Comment: 25 pages, 5 Postscript figure

Topological aspects of geometrical signatures of phase transitions

Certain geometric properties of submanifolds of configuration space are numerically investigated for classical lattice phi^4 models in one and two dimensions. Peculiar behaviors of the computed geometric quantities are found only in the two-dimensional case, when a phase transition is present. The observed phenomenology strongly supports, though in an indirect way, a recently proposed topological conjecture about a topology change of the configuration space submanifolds as counterpart of a phase transition.Comment: REVTEX file, 4 pages, 5 figure

Monte Carlo Simulations with Indefinite and Complex-Valued Measures

A method is presented to tackle the sign problem in the simulations of systems having indefinite or complex-valued measures. In general, this new approach is shown to yield statistical errors smaller than the crude Monte Carlo using absolute values of the original measures. Exactly solvable, one-dimensional Ising models with complex temperature and complex activity illustrate the considerable improvements and the workability of the new method even when the crude one fails.Comment: 10 A4 pages, postscript (140K), UM-P-93-7

Spin-polaron model: transport properties of EuB6_6

To understand anomalous transport properties of EuB$_6$, we have studied the spin-polaron Hamiltonian incorporating the electron-phonon interaction. Assuming a strong exchange interaction between the carriers and the localized spins, the electrical conductivity is calculated. The temperature and magnetic field dependence of the resistivity of EuB$_6$ are well explained. At low temperature, magnons dominate the conduction process, whereas the lattice contribution becomes significant at very high temperature due to the scattering with the phonons. Large negative magnetoresistance near the ferromagnetic transition is also reproduced as observed in EuB$_6$.Comment: 4 pages, 3 figures, accepted in Phys. Rev.

Microstructure and Velocity of Field-Driven SOS Interfaces: Analytic Approximations and Numerical Results

The local structure of a solid-on-solid (SOS) interface in a two-dimensional kinetic Ising ferromagnet with single-spin-flip Glauber dynamics, which is driven far from equilibrium by an applied field, is studied by an analytic mean-field, nonlinear-response theory [P.A. Rikvold and M. Kolesik, J. Stat. Phys. 100, 377 (2000)] and by dynamic Monte Carlo simulations. The probability density of the height of an individual step in the surface is obtained, both analytically and by simulation. The width of the probability density is found to increase dramatically with the magnitude of the applied field, with close agreement between the theoretical predictions and the simulation results. Excellent agreement between theory and simulations is also found for the field-dependence and anisotropy of the interface velocity. The joint distribution of nearest-neighbor step heights is obtained by simulation. It shows increasing correlations with increasing field, similar to the skewness observed in other examples of growing surfaces.Comment: 18 pages RevTex4 with imbedded figure

Nonadiabatic approach to dimerization gap and optical absorption coefficient of the Su-Schrieffer-Heeger model

An analytical nonadiabatic approach has been developed to study the dimerization gap and the optical absorption coefficient of the Su-Schrieffer-Heeger model where the electrons interact with dispersive quantum phonons. By investigating quantitatively the effects of quantum phonon fluctuations on the gap order and the optical responses in this system, we show that the dimerization gap is much more reduced by the quantum lattice fluctuations than the optical absorption coefficient is. The calculated optical absorption coefficient and the density of states do not have the inverse-square-root singularity, but have a peak above the gap edge and there exist a significant tail below the peak. The peak of optical absorption spectrum is not directly corresponding to the dimerized gap. Our results of the optical absorption coefficient agree well with those of the experiments in both the shape and the peak position of the optical absorption spectrum.Comment: 14 pages, 7 figures. to be published in PR

Electronic transport in EuB6_6

EuB$_6$ is a magnetic semiconductor in which defects introduce charge carriers into the conduction band with the Fermi energy varying with temperature and magnetic field. We present experimental and theoretical work on the electronic magnetotransport in single-crystalline EuB$_6$. Magnetization, magnetoresistance and Hall effect data were recorded at temperatures between 2 and 300 K and in magnetic fields up to 5.5 T. The negative magnetoresistance is well reproduced by a model in which the spin disorder scattering is reduced by the applied magnetic field. The Hall effect can be separated into an ordinary and an anomalous part. At 20 K the latter accounts for half of the observed Hall voltage, and its importance decreases rapidly with increasing temperature. As for Gd and its compounds, where the rare-earth ion adopts the same Hund's rule ground state as Eu$^{2+}$ in EuB$_{6}$, the standard antisymmetric scattering mechanisms underestimate the $size$ of this contribution by several orders of magnitude, while reproducing its $shape$ almost perfectly. Well below the bulk ferromagnetic ordering at $T_C$ = 12.5 K, a two-band model successfully describes the magnetotransport. Our description is consistent with published de Haas van Alphen, optical reflectivity, angular-resolved photoemission, and soft X-ray emission as well as absorption data, but requires a new interpretation for the gap feature deduced from the latter two experiments.Comment: 35 pages, 12 figures, submitted to PR