Peierls-like transition induced by frustration in a two-dimensional antiferromagnet

We show that the introduction of frustration into the spin-1/2 two-dimensional (2D) antiferromagnetic Heisenberg model on a square lattice via a next-nearest neighbor exchange interaction can lead to a Peierls-like transition, from a tetragonal to an orthorhombic phase, when the spins are coupled to adiabatic phonons. The two different orthorhombic ground states define an Ising order parameter, which is expected to lead to a finite temperature transition. Implications for ${\rm Li_2VOSiO_4}$, the first realization of that model, will be discussed.Comment: 4 pages, to be published on Physical Review Letter

Optical conductivity of the Hubbard model at finite temperature

The optical conductivity, $\sigma(\omega)$, of the two dimensional one-band Hubbard model is calculated at finite temperature using exact diagonalization techniques on finite clusters. The in-plane d.c. resistivity, $\rho_{ab}$, is also evaluated. We find that at large U/t and temperature T, $\rho_{ab}$ is approximately linear with temperature, in reasonable agreement with experiments on high-T$_c$ superconductors. Moreover, we note that $\sigma(\omega)$ displays charge excitations, a mid-infrared (MIR) band and a Drude peak, also as observed experimentally. The combination of the Drude peak and the MIR oscillator strengths leads to a conductivity that decays slower than $1/\omega^2$ at energies smaller than the insulator gap near half-filling.Comment: 12 pages, 3 figures appended, Revtex version 2.0, preprin

Twisting gauged non-linear sigma-models

We consider gauged sigma-models from a Riemann surface into a Kaehler and hamiltonian G-manifold X. The supersymmetric N=2 theory can always be twisted to produce a gauged A-model. This model localizes to the moduli space of solutions of the vortex equations and computes the Hamiltonian Gromov-Witten invariants. When the target is equivariantly Calabi-Yau, i.e. when its first G-equivariant Chern class vanishes, the supersymmetric theory can also be twisted into a gauged B-model. This model localizes to the Kaehler quotient X//G.Comment: 33 pages; v2: small additions, published versio

Induction Chemotherapy Followed by Concurrent Chemoradiation Therapy Versus Concurrent Chemoradiation Therapy Upfront in Locally Advanced Oral Cavity Cancer: Systematic Review and Meta-Analysis of Individual Data

Univ Sao Paulo, Hosp Siriolibanes, Dept Radiat Oncol, Sao Paulo, BrazilUniv Sao Paulo, Fac Med, Inst Canc Estado Sao Paulo, Sao Paulo, BrazilUniv Fed Sao Paulo, Escola Paulista Med UNIFESP EPM, Brazilian Cochrane Ctr, Sao Paulo, BrazilUniv Fed Sao Paulo, Escola Paulista Med UNIFESP EPM, Discipline Emergency Med & Evidence Based Med, Sao Paulo, BrazilHarvard Univ, Sch Med, Dana Farber Canc Inst, Dept Med Oncol, 44 Binney St, Boston, MA 02115 USABrigham & Womens Hosp, Dept Med, 75 Francis St, Boston, MA 02115 USAUniv Calif San Diego, La Jolla, CA 92093 USAHarvard Univ, Sch Med, Dana Farber Canc Inst, Head & Neck Oncol Program, 44 Binney St, Boston, MA 02115 USAAC Camargo Canc Ctr, Dept Head & Neck Surg & Otorhinolaryngol, Sao Paulo, BrazilUniv Fed Sao Paulo, Escola Paulista Med UNIFESP EPM, Brazilian Cochrane Ctr, Sao Paulo, BrazilUniv Fed Sao Paulo, Escola Paulista Med UNIFESP EPM, Discipline Emergency Med & Evidence Based Med, Sao Paulo, BrazilWeb of Scienc

Diagonalization in Reduced Hilbert Spaces using a Systematically Improved Basis: Application to Spin Dynamics in Lightly Doped Ladders

A method is proposed to improve the accuracy of approximate techniques for strongly correlated electrons that use reduced Hilbert spaces. As a first step, the method involves a change of basis that incorporates exactly part of the short distance interactions. The Hamiltonian is rewritten in new variables that better represent the physics of the problem under study. A Hilbert space expansion performed in the new basis follows. The method is successfully tested using both the Heisenberg model and the $t-J$ model with holes on 2-leg ladders and chains, including estimations for ground state energies, static correlations, and spectra of excited states. An important feature of this technique is its ability to calculate dynamical responses on clusters larger than those that can be studied using Exact Diagonalization. The method is applied to the analysis of the dynamical spin structure factor $S(q,\omega)$ on clusters with $2 \times 16$ sites and 0 and 2 holes. Our results confirm previous studies (M. Troyer, H. Tsunetsugu, and T. M. Rice, Phys. Rev. $B 53$, 251 (1996)) which suggested that the state of the lowest energy in the spin-1 2-holes subspace corresponds to the bound state of a hole pair and a spin-triplet. Implications of this result for neutron scattering experiments both on ladders and planes are discussed.Comment: 9 pages, 8 figures, Revtex + psfig; changed conten

Poultry compost in the composition of substrates for flower seedlings production

Se evaluaron sustratos formulados con distintas proporciones (20, 50 y 80%) de tres tipos de compost de guano de gallina (CGG1, CGG2, CGG3) mezclados con compost de corteza de pino y un sustrato comercial. Los diez tratamientos fueron usados para el cultivo de plantines de Impatiens walleriana y Salvia splendens. Se caracterizó física y químicamente cada sustrato, y sobre los plantines se evaluó sobrevivencia, masa seca y calcio, magnesio, potasio y sodio en la hoja. Los sustratos con 20% y 50% de los tres CGG tuvieron la mayor capacidad de retención de agua, y con 80%, la mayor porosidad de aire. Los sustratos con 50% y 80% presentaron los más altos valores de pH (≥6,6), potasio y sodio. Para las mismas proporciones, CGG2 y CGG3 presentaron valores de CE >1,1 dS cm-1. La mayor masa seca de los plantines se logró para el sustrato comercial, los sustratos con 20% de los tres CGG y con 50% de CGG1. Estos sustratos exhibieron en las hojas una mayor concentración de calcio y magnesio, y menor de potasio. Las mejores características de sustratos para el desarrollo de los plantines evaluados se lograron en los sustratos con 20% de los tres CGG y con 50% del compost CGG1.Substrates formulated with different proportions (20, 50 and 80%) of three types of poultry compost (CGG1, CGG2, CGG3) mixed with pine bark compost and commercial substrate were evaluated. The ten treatments were used to cultivate seedlings of walleriana Impatiens and Salvia splendens. Each substrate was characterised physically and chemically, and the survival, dry mass and calcium, magnesium, potassium and sodium in the leaf were evaluated in seedlings. Substrates with 20% and 50% among the three CGG presented the highest water retention capacity and, with 80% the highest air porosity. The substrates with 50% and 80% showed the highest values of pH (≥ 6.6), potassium and sodium. For the same proportions, CGG2 and CGG3 presented CE values > 1.1 dS cm-1. The highest dry weight of the seedlings was achieved with the commercial substrate, the substrates with 20% of the three CGG and with 50% of CGG1. These substrates showed a higher concentration of calcium and magnesium, and a lower concentration of potassium in the leaves. The best characteristics of substrates for the development of the seedlings tested were achieved in the substrates with 20% of the three CGG and with 50% of compost CGG1.Fil: Bárbaro, L. A.. Instituto Nacional de Tecnología Agropecuaria. Centro Nacional de Investigaciones Agropecuarias. Centro de Investigación de Recursos Naturales. Instituto de Floricultura; ArgentinaFil: Karlanian, M. A.. Instituto Nacional de Tecnología Agropecuaria. Centro Nacional de Investigaciones Agropecuarias. Centro de Investigación de Recursos Naturales. Instituto de Floricultura; ArgentinaFil: Rizzo, P. F.. Instituto Nacional de Tecnología Agropecuaria. Centro Nacional de Investigaciones Agropecuarias. Centro de Investigación de Ciencias Veterinarias y Agronómicas. Instituto de Microbiología y Zoología Agrícola; ArgentinaFil: Riera, N. I.. Instituto Nacional de Tecnología Agropecuaria. Centro Nacional de Investigaciones Agropecuarias. Centro de Investigación de Ciencias Veterinarias y Agronómicas. Instituto de Microbiología y Zoología Agrícola; ArgentinaFil: Della Torre, V.. Instituto Nacional de Tecnología Agropecuaria. Centro Nacional de Investigaciones Agropecuarias. Centro de Investigación de Ciencias Veterinarias y Agronómicas. Instituto de Microbiología y Zoología Agrícola; ArgentinaFil: Beltrán, M.. Instituto Nacional de Tecnologia Agropecuaria. Centro Nacional de Inv. Agropecuarias. Centro de Invest.de Recursos Naturales. Instituto de Suelos; ArgentinaFil: Crespo, Diana. Instituto Nacional de Tecnología Agropecuaria. Centro Nacional de Investigaciones Agropecuarias. Centro de Investigación de Ciencias Veterinarias y Agronómicas. Instituto de Microbiología y Zoología Agrícola; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Subsurface carbon: a general feature of noble metals

Carbon moieties on late transition metals are regarded as poisoning agents in heterogeneous catalysis. Recent studies show the promoting catalytic role of subsurface C atoms in Pd surfaces and their existence in Ni and Pt surfaces. Here energetic and kinetic evidence obtained by accurate simulations on surface and nanoparticle models shows that such subsurface C species are a general issue to consider even in coinage noble-metal systems. Subsurface C is the most stable situation in densely packed (111) surfaces of Cu and Ag, with sinking barriers low enough to be overcome at catalytic working temperatures. Low-coordinated sites at nanoparticle edges and corners further stabilize them, even in Au, with negligible subsurface sinking barriers. The malleability of low-coordinated sites is key in the subsurface C accommodation. The incorporation of C species decreases the electron density of the surrounding metal atoms, thus affecting their chemical and catalytic activity

Intersecting Solitons, Amoeba and Tropical Geometry

We study generic intersection (or web) of vortices with instantons inside, which is a 1/4 BPS state in the Higgs phase of five-dimensional N=1 supersymmetric U(Nc) gauge theory on R_t \times (C^\ast)^2 \simeq R^{2,1} \times T^2 with Nf=Nc Higgs scalars in the fundamental representation. In the case of the Abelian-Higgs model (Nf=Nc=1), the intersecting vortex sheets can be beautifully understood in a mathematical framework of amoeba and tropical geometry, and we propose a dictionary relating solitons and gauge theory to amoeba and tropical geometry. A projective shape of vortex sheets is described by the amoeba. Vortex charge density is uniformly distributed among vortex sheets, and negative contribution to instanton charge density is understood as the complex Monge-Ampere measure with respect to a plurisubharmonic function on (C^\ast)^2. The Wilson loops in T^2 are related with derivatives of the Ronkin function. The general form of the Kahler potential and the asymptotic metric of the moduli space of a vortex loop are obtained as a by-product. Our discussion works generally in non-Abelian gauge theories, which suggests a non-Abelian generalization of the amoeba and tropical geometry.Comment: 39 pages, 11 figure

High frequency dielectric and magnetic anomaly at the phase transition in NaV2O5

We found anomalies in the temperature dependence of the dielectric and the magnetic susceptibiliy of NaV_2O_5 in the microwave and far infrared frequency ranges. The anomalies occur at the phase transition temperature T_c, at which the spin gap opens. The real parts of the dielectric constants epsilon_a and epsilon_c decrease below T_c. The decrease of epsilon_a (except for the narrow region close to T_c) is proportional to the intensity of the x-ray reflection appearing at T_c. The dielectric constant anomaly can be explained by the zigzag charge ordering in the ab-plane appearing below T_c. The anomaly of the microwave magnetic losses is probably related to the coupling between the spin and charge degrees of freedom in vanadium ladders.Comment: 3 PS-figures, LATEX-text, new experimental data added, typos correcte