Study of the 2-d CP(N-1) models at \theta=0 and \pi

We present numerical results for 2-d CP(N-1) models at \theta=0 and \pi obtained in the D-theory formulation. In this formulation we construct an efficient cluster algorithm and we show numerical evidence for a first order transition for CP(N-1\geq 2) models at \theta = \pi. By a finite size scaling analysis, we also discuss the equivalence in the continuum limit of the D-theory formulation of the 2-d CP(N-1) models and the usual lattice definition.Comment: 3 pages, 2 figures. Talk presented at Lattice2004(spin), Fermilab, June 21-26, 200

Green's Functions from Quantum Cluster Algorithms

We show that cluster algorithms for quantum models have a meaning independent of the basis chosen to construct them. Using this idea, we propose a new method for measuring with little effort a whole class of Green's functions, once a cluster algorithm for the partition function has been constructed. To explain the idea, we consider the quantum XY model and compute its two point Green's function in various ways, showing that all of them are equivalent. We also provide numerical evidence confirming the analytic arguments. Similar techniques are applicable to other models. In particular, in the recently constructed quantum link models, the new technique allows us to construct improved estimators for Wilson loops and may lead to a very precise determination of the glueball spectrum.Comment: 15 pages, LaTeX, with four figures. Added preprint numbe

Interaction effects between impurities in low dimensional spin-1/2 antiferromagnets

We are considering the interplay between several non-magnetic impurities in the spin-1/2 Heisenberg antiferromagnet in chains, ladders and planes by introducing static vacancies in numerical quantum Monte Carlo simulations. The effective potential between two and more impurities is accurately determined, which gives a direct measure of the quantum correlations in the systems. Large effective interaction potentials are an indication of strong quantum correlations in the system and reflect the detailed nature of the valence bond ground states. In two-dimensions (2D) the interactions are smaller, but can still be analyzed in terms of valence bonds.Comment: 8 pages, 6 figures, accepted by Europhys. Lett. The latest pdf file is available at http://www.physik.uni-kl.de/eggert/papers/interact2d.pd

Normas Internacionales de Auditoria (NIA) : NIA 500 la evaluación de auditoria como elemento fundamental para emitir una opinión sobre los estados financieros de la compañia Sander"S Rosales,S.A. año 2016

Las normas internacionales de auditoria son directrices generales que ayudan a los auditores a cumplir con sus responsabilidades profesionales en la auditoría de estados financieros. Ello incluye la consideración de capacidades profesionales como lo son la competencia y la independencia, los requisitos de informes y la evidencia. Los objetivos de las normas internacionales de auditoria es proporcionar un mayor nivel de aseguramiento en lo que respecta a la uniformidad de la práctica de auditoría en todo el mundo. Nuestro trabajo se base principalmente en la evidencia de auditoria como elemento fundamental en la opinión de un auditor. La evidencia es cualquier tipo de datos que utiliza el auditor para determinar si la información que está auditando ha sido declarada de acuerdo con el criterio establecido. La importancia de la evidencia de auditoría se basa en la calidad de la información que soporta de cada una de las transacciones de los registros contables, adquirida por el auditor a través de las técnicas y procedimientos de auditoría ejecutados en la revisión. La evidencia es tan importante debido que sobre ella recae el fundamento del dictamen de la opinión del auditor. Posterior procedemos a conceptualizar los procedimientos de auditoria, son mecanismos o métodos básicos disponibles, aplicados o utilizados por el auditor durante el curso de su trabajo, para obtener la evidencia necesaria a fin de formar su juicio profesional. La fuente de los procedimientos de auditoría, son los diferentes sistemas de la organización, el control, la contabilidad y en general los detalles de operación del negocio, lo que hacen imposible establecer sistemas rígidos de prueba para el examen de los estados financieros. Por esta razón, el auditor deberá diseñar y desempeñar los procedimientos adicionales de auditoría cuya naturaleza, oportunidad y extensión, se basen en, y respondan a, los riesgos evaluados de representación errónea de importancia relativa a nivel aseveración. Posteriormente conceptualizaremos los papeles de trabajos que soportan la evidencia de auditoria, los papeles de trabajo son el medio para acumular toda la evidencia que necesita el auditor para emitir una opinión profesional, la documentación de auditoría es el recurso más importante para demostrar por medio de documentos que una auditoría fue realizada de forma adecuada. Finalmente se desarrolla un caso práctico, Compañía Sanders Rosales, S.A donde la actividad principal de la compañía es la prestación de servicios de construcción de obras verticales, horizontales (Residenciales) y alquiler de equipos, a través de contratos suscritos con clientes, en este caso delimitaremos descripción del perfil del negocio, el alcance de auditoria, evidencias de obtenidas y el dictamen de auditoría. Se concluye que la evidencia de auditoría es elemental y fundamental al momento que el auditor se encuentra en el proceso de revisión, lo que conlleva a que se realice una buena planeación de la auditoría así como establecer que procedimientos a desarrollar por la cuentas con importación relativa en los Estados Financieros y dar una opinión sustentada en las evidencias obtenidas a través del proceso de revisión

Charge ordering in extended Hubbard models: Variational cluster approach

We present a generalization of the recently proposed variational cluster perturbation theory to extended Hubbard models at half filling with repulsive nearest neighbor interaction. The method takes into account short-range correlations correctly by the exact diagonalisation of clusters of finite size, whereas long-range order beyond the size of the clusters is treated on a mean-field level. For one dimension, we show that quantum Monte Carlo and density-matrix renormalization-group results can be reproduced with very good accuracy. Moreover we apply the method to the two-dimensional extended Hubbard model on a square lattice. In contrast to the one-dimensional case, a first order phase transition between spin density wave phase and charge density wave phase is found as function of the nearest-neighbor interaction at onsite interactions U>=3t. The single-particle spectral function is calculated for both the one-dimensional and the two-dimensional system.Comment: 15 pages, 12 figure

Meron-Cluster Approach to Systems of Strongly Correlated Electrons

Numerical simulations of strongly correlated electron systems suffer from the notorious fermion sign problem which has prevented progress in understanding if systems like the Hubbard model display high-temperature superconductivity. Here we show how the fermion sign problem can be solved completely with meron-cluster methods in a large class of models of strongly correlated electron systems, some of which are in the extended Hubbard model family and show s-wave superconductivity. In these models we also find that on-site repulsion can even coexist with a weak chemical potential without introducing sign problems. We argue that since these models can be simulated efficiently using cluster algorithms they are ideal for studying many of the interesting phenomena in strongly correlated electron systems.Comment: 36 Pages, 13 figures, plain Late

Effects of Nonmagnetic Impurity Doping on Spin Ladder System

Effects of nonmagnetic impurity doping on an AF spin-1/2 Heisenberg ladder system are studied by the QMC method. A single nonmagnetic impurity induces a localized spin-1/2 moment accompanied by "static" and enhanced AF correlations around it. Small and finite concentration of impurities induces a remarkable change of magnetic and thermodynamic properties with gapless excitations. It also shows rather sharp but continuous crossover around the concentration of about 4%. Above the crossover concentration, all the spins are strongly coupled participating in the enhanced and rather uniform power-law decay of the antiferromagnetic correlation. Below the crossover, each impurity forms an antiferromagnetic cluster only weakly coupled each other. For random distribution of impurities, large Curie-like susceptibility accompanied with small residual entropy is obtained at low temperatures in agreement with recent experimental observation in Zn-doped $SrCu_{2}O_{3}$. Temperature dependence of AF susceptibility shows power-law-like but weaker divergence than the single chain AFH in the temperature range studied.Comment: 4 pages, LaTeX+epsf.sty, submitted to J.Phys.Soc.Jpn. New results of AF susceptibility are adde

From Spin Ladders to the 2-d O(3) Model at Non-Zero Density

The numerical simulation of various field theories at non-zero chemical potential suffers from severe complex action problems. In particular, QCD at non-zero quark density can presently not be simulated for that reason. A similar complex action problem arises in the 2-d O(3) model -- a toy model for QCD. Here we construct the 2-d O(3) model at non-zero density via dimensional reduction of an antiferromagnetic quantum spin ladder in a magnetic field. The complex action problem of the 2-d O(3) model manifests itself as a sign problem of the ladder system. This sign problem is solved completely with a meron-cluster algorithm.Comment: Based on a talk by U.-J. Wiese, 6 pages, 12 figures, to be published in computer physics communication

Comment on "Quantum Monte Carlo Evidence for Superconductivity in the Three-Band Hubbard Model in Two Dimensions"

In a recent Letter, Kuroki and Aoki [Phys. Rev. Lett. 76, 440 (1996)] presented quantum Monte-Carlo (QMC) results for pairing correlations in the three-band Hubbard model, which describes the Cu-d_{x^2-y^2} and O-p_{x,y} orbitals present in the CuO_2 planes of high-T_c materials. In this comment we argue that (i) the used parameter set is not appropriate for the description of high-T_c materials since it does not satisfy the minimal requirement of a charge-transfer gap at half-filling, and (ii) the observed increase in the d_{x^2-y^2} channel is dominantly produced by the pair-field correlations without the vertex part. Hence, the claim of evidence of ODLRO is not justified.Comment: 1 page latex and 2 eps-figures, uses epsfig, submitted to PR

Efficient Cluster Algorithm for CP(N-1) Models

Despite several attempts, no efficient cluster algorithm has been constructed for CP(N-1) models in the standard Wilson formulation of lattice field theory. In fact, there is a no-go theorem that prevents the construction of an efficient Wolff-type embedding algorithm. In this paper, we construct an efficient cluster algorithm for ferromagnetic SU(N)-symmetric quantum spin systems. Such systems provide a regularization for CP(N-1) models in the framework of D-theory. We present detailed studies of the autocorrelations and find a dynamical critical exponent that is consistent with z = 0.Comment: 14 pages, 3 figure