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© 2009 The American Physical Society. We have been partly supported through Research Contracts No. FIS2006-08533-C03, No. FIS2007-60977 MICINN, Spain, and No. GR58/08, 910383 Banco de Santander-UCM. The simulations for this work were performed at BIFI., Ministerio de Ciencia e Innovación (MICINN), Banco de Santander-UCM, Depto. de Física Teórica, Fac. de Ciencias Físicas, TRUE, pub

© 2008 American Physical Society. This work has been partially supported by MEC through Contracts No. FIS2004-01399, No. FIS2006-08533-C03, No. FIS2007-60977 and by CAM and BSCH. Computer time was obtained at BIFI, UCM, UEX, and, mainly, in the Mare Nostrum. The authors thankfully acknowledge the computer resources and technical expertise provided by the Barcelona Supercomputing Center., We present the first detailed numerical study in three dimensions of a first-order phase transition that remains first order in the presence of quenched disorder (specifically, the ferromagnetic-paramagnetic transition of the site-diluted four states Potts model). A tricritical point, which lies surprisingly near the pure-system limit and is studied by means of finite-size scaling, separates the first-order and second-order parts of the critical line. This investigation has been made possible by a new definition of the disorder average that avoids the diverging-variance probability distributions that plague the standard approach. Entropy, rather than free energy, is the basic object in this approach that exploits a recently introduced microcanonical Monte Carlo method., MEC (Spain), CAM (Spain), BSCH, Depto. de Física Teórica, Fac. de Ciencias Físicas, TRUE, pub

© 2005 The American Physical Society. It is a pleasure to thank F. Guinea for useful conversation at the beginning of this work. We have maintained interesting discussions with A. Pelissetto regarding the large-N approximation. This work has been partially supported through research contracts FPA2001-1813, FPA2000-0956, BFM2001-0718, BFM2003-8532, PB98-0842 sMCyTd and HPRN-CT-2002-00307 sEUd. We have used the PentiumIV cluster RTN3 at the Universidad de Zaragoza for the simulations. S.J. was supported by DGA., The phase diagram of the simplest approximation to double-exchange systems, the bosonic double-exchange model with antiferromagnetic (AFM) superexchange coupling, is fully worked out by means of Monte Carlo simulations, large-N expansions, and variational mean-field calculations. We find a rich phase diagram, with no first-order phase transitions. The most surprising finding is the existence of a segmentlike ordered phase at low temperature for intermediate AFM coupling which cannot be detected in neutron-scattering experiments. This is signaled by a maximum (a cusp) in the specific heat. Below the phase transition, only short-range ordering would be found in neutron scattering. Researchers looking for a quantum critical point in manganites should be wary of this possibility. Finite-size scaling estimates of critical exponents are presented, although large scaling corrections are present in the reachable lattice sizes., DGA, Depto. de Física Teórica, Fac. de Ciencias Físicas, TRUE, pub

© 2003 American Physical Society. We acknowledge partial financial support from CICyT (AEN97-1680, FPA2000-0956, FPA2001-1813 and PB98- 0842) and DGA (P46/97). We also want to acknowledge the contribution of Prof. Domingo González to the development of Spanish physics, and in particular his help to our group, which has been essential to build our dedicated computers. We finally thank Giorgio Parisi for interesting comments. S. J. is a DGA fellow., We study the fluctuation-dissipation relations for a three dimensional Ising spin glass in a magnetic field both in the high temperature phase as well as in the low temperature one. In the region of times simulated we have found that our results support a picture of the low temperature phase with broken replica symmetry, but a droplet behavior cannot be completely excluded., CICyT , Spain, DGA, Spain, Depto. de Física Teórica, Fac. de Ciencias Físicas, TRUE, pub

© 2000 The American Physical Society. We gratefully acknowledge discussions with D. Belanger, J. Cardy and H. Rieger. We are grateful for partial financial support from CICyT (AEN97-1708 and AEN97-1693). The computations have been carried out using the RTNN machines (Universidad de Zaragoza and Universidad Complutense de Madrid) and the ORIGIN2000 at the Centro de Supercomputación Complutense (CSC)., We have studied numerically the effect of quenched site dilution on a weak first-order phase transition in three dimensions. We have simulated the site diluted three-states Potts model studying in detail the secondorder region of its phase diagram. We have found that the n exponent is compatible with the one of the three-dimensional diluted Ising model, whereas the h exponent is definitely different., CICyT, Spain, Depto. de Física Teórica, Fac. de Ciencias Físicas, TRUE, pub

© 1999 IOP Publishing Ltd. We acknowledge interesting discussions with D Stauffer and R Ziff. We thank CICyT (AEN97-1708 and AEN97-1693) for partial financial support. The computations have been carried out using the RTNN machines at Universidad de Zaragoza and Universidad Complutense de Madrid., Using Finite-Size Scaling techniques we obtain accurate results for critical quantities of the Ising model and the site percolation, in three dimensions. We pay special attention in parameterizing the corrections-to-scaling, what is necessary to put the systematic errors below the statistical ones., CICyT, Spain, Depto. de Física Teórica, Fac. de Ciencias Físicas, TRUE, pub

© 1998 Elsevier Science B.V. International Symposium on Lattice Field Theory (Lattice 97) (15th. 1997. Edinburgh, Scothland). Partially supported by CICyT (AEN94-0218, AEN96-1634). JJRL is granted by EC HMC (ERBFMBICT950429). Presented by HGB., We study the four dimensional site-diluted Ising model using finite-size scaling techniques. We explore the whole parameter space (density-coupling) in order to determine the Universality Class of the transition line. Our data are compatible with Mean Field behavior plus logarithmic corrections., CICyT, Spain, EC HMC, Depto. de Física Teórica, Fac. de Ciencias Físicas, TRUE, pub

© 1998 Elsevier Science B.V. We thank the CICyT (contracts AEN93-0604, AEN96-1634) for partial financial support, especially for the use of dedicated Pentium Pro machines on which we have carried out the simulations. JJRL is granted by EC HMC (ERBFMBICT950429). We also acknowledge to E. Luijten for pointing out the right scaling of the Binder cumulant in the pure case., Using finite-size scaling techniques, we study the critical properties of the site-diluted Ising model in four dimensions. We carry out a high-statistics Monte Carlo simulation for several values of the dilution. The results support the perturbative scenario: there is only the Ising fixed point with large logarithmic scaling corrections. We obtain, using the Perturbative Renormalization Group, functional forms for the scaling of several observables that are in agreement with the numerical data., CICyT, Spain, EC HMC, Depto. de Física Teórica, Fac. de Ciencias Físicas, TRUE, pub

We study the four dimensional site-diluted Ising model using finite-size scaling techniques. We explore the whole parameter space (density-coupling) in order to determine the Universality Class of the transition line. Our data are compatible with Mean Field behavior plus logarithmic corrections., Comment: Contribution to LATTICE 97

© 1995 Elsevier Science B.V. International Symposium on Lattice Field Theory (LATTICE 94) (12. 1994. Bielefeld, Germany). We acknowledge CICyT for partial financial support with the projects AEN93-0604, AEN93-0776 and AEN94-218. One of us (JJRL) is granted by MEC (Spain)., We propose a family of renormalization group transformations characterized by free parameters that may be tuned in order to reduce the truncation effects. As a check we test them in the three dimensional XY model. The Schwinger-Dyson equations are used to study the renormalization group flow., CICyT, Spain, MEC, Spain, Depto. de Física Teórica, Fac. de Ciencias Físicas, TRUE, pub

© 1995 Published by Elsevier B.V.We thank G. Parisi for useful suggestions concerning this work and for many discussions and G.C. Rossi for a careful reading of the manuscript. We acknowledge interesting discussions with E. Marinari. We would like to thank F. Marzano, J. Pech, F. Rapuano for encouragements and support. L.A.F., A.M.S., J.J.R.L. and A.T. acknowledge CICyT (Spain) for partial financial support. J.J.R.L. is also supported by a grant of MEC (Spain)., Polyakov type loops are responsible for the difference between quenched and unquenched finite size effects on the QCD mass spectrum. With a numerical simulation, using appropriate sea quark spatial boundary conditions, we show that we can align the phases of spatial Polyakov loops in a predelined direction. Starting from these results, we propose a procedure to minimize fluctuations due to these effects in meson propagators., CICyT (Spain), MEC (Spain), Depto. de Física Teórica, Fac. de Ciencias Físicas, TRUE, pub