6,804 research outputs found

    Solving the puzzle of an unconventional phase transition for a 2d dimerized quantum Heisenberg model

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    Motivated by the indication of a new critical theory for the spin-1/2 Heisenberg model with a spatially staggered anisotropy on the square lattice as suggested in \cite{Wenzel08}, we re-investigate the phase transition of this model induced by dimerization using first principle Monte Carlo simulations. We focus on studying the finite-size scaling of ρs12L\rho_{s1} 2L and ρs22L\rho_{s2} 2L, where LL stands for the spatial box size used in the simulations and ρsi\rho_{si} with i{1,2}i \in \{1,2\} is the spin-stiffness in the ii-direction. Remarkably, while we do observe a large correction to scaling for the observable ρs12L\rho_{s1}2L as proposed in \cite{Fritz11}, the data for ρs22L\rho_{s2}2L exhibit a good scaling behavior without any indication of a large correction. As a consequence, we are able to obtain a numerical value for the critical exponent ν\nu which is consistent with the known O(3) result with moderate computational effort. Specifically, the numerical value of ν\nu we determine by fitting the data points of ρs22L\rho_{s2}2L to their expected scaling form is given by ν=0.7120(16)\nu=0.7120(16), which agrees quantitatively with the most accurate known Monte Carlo O(3) result ν=0.7112(5)\nu = 0.7112(5). Finally, while we can also obtain a result of ν\nu from the observable second Binder ratio Q2Q_2 which is consistent with ν=0.7112(5)\nu=0.7112(5), the uncertainty of ν\nu calculated from Q2Q_2 is more than twice as large as that of ν\nu determined from ρs22L\rho_{s2}2L.Comment: 7 figures, 1 table; brief repor

    Selection of medicines in Chilean public hospitals: an exploratory study

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    Background There is a growing interest in high income countries to control expenditure on medicines by improving the rationale for their selection. However, in middle income countries with differing priorities and needs, little attention has been paid to this issue. In this paper we explore the policies and processes for the selection and use of medicines in a group of hospitals in Chile, a middle income country which has recently joined the OECD. Methods A combination of qualitative and quantitative methods was used. A national survey questionnaire was distributed to investigate the role and operation of PTCs (Pharmacy and Therapeutics Committees). Interviews were conducted with key actors in the selection of medicines in large urban public hospitals. Results The national survey had an overall response rate of 42% (83 out of 196), whilst 7 out of 14 hospitals participated in the qualitative study. High complexity hospitals are large urban hospitals; all of which claim to have a working PTC. The pharmacy offices are mainly involved in dispensing medicines with little involvement in clinical duties. The interviews conducted suggest that the formulary of all the hospitals visited is no more than a stock list. PTCs are unable to influence the prescribing practices of doctors. Members do not feel prepared to challenge the opinions of specialists requesting a certain drug, and decisions are based primarily on costs. The inclusion of medicines in the clinical practice of hospitals is as a result of doctors bypassing the PTC and requesting the purchase of exceptional items, some of which are included in the formulary if they are widely used. Conclusions There is an urgent need to develop medicine policies in hospitals in Chile. The procedures used to purchase medicines need to be revised. Central guidance for PTCs could help ensure a more rational use of medicines. PTCs need to be empowered to design formularies which cover all the clinical needs of doctors, training members in the analysis of scientific evidence beyond their own specialities. An influential PTC can take the appropriate measures and design workable policies to enforce a cost effective-use of resources

    Specific heat of the simple-cubic Ising model

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    We provide an expression quantitatively describing the specific heat of the Ising model on the simple-cubic lattice in the critical region. This expression is based on finite-size scaling of numerical results obtained by means of a Monte Carlo method. It agrees satisfactorily with series expansions and with a set of experimental results. Our results include a determination of the universal amplitude ratio of the specific-heat divergences at both sides of the critical point.Comment: 20 pages, 3 figure

    Finite size effects in nonequilibrium wetting

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    Models with a nonequilibrium wetting transition display a transition also in finite systems. This is different from nonequilibrium phase transitions into an absorbing state, where the stationary state is the absorbing one for any value of the control parameter in a finite system. In this paper, we study what kind of transition takes place in finite systems of nonequilibrium wetting models. By solving exactly a microscopic model with three and four sites and performing numerical simulations we show that the phase transition taking place in a finite system is characterized by the average interface height performing a random walk at criticality and does not discriminate between the bounded-KPZ classes and the bounded-EW class. We also study the finite size scaling of the bKPZ universality classes, showing that it presents peculiar features in comparison with other universality classes of nonequilibrium phase transitions.Comment: 14 pages, 6figures, major change

    On the finite-size behavior of systems with asymptotically large critical shift

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    Exact results of the finite-size behavior of the susceptibility in three-dimensional mean spherical model films under Dirichlet-Dirichlet, Dirichlet-Neumann and Neumann-Neumann boundary conditions are presented. The corresponding scaling functions are explicitly derived and their asymptotics close to, above and below the bulk critical temperature TcT_c are obtained. The results can be incorporated in the framework of the finite-size scaling theory where the exponent λ\lambda characterizing the shift of the finite-size critical temperature with respect to TcT_c is smaller than 1/ν1/\nu, with ν\nu being the critical exponent of the bulk correlation length.Comment: 24 pages, late

    Weak first order transition in the three-dimensional site-diluted Ising antiferromagnet in a magnetic field

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    We perform intensive numerical simulations of the three-dimensional site-diluted Ising antiferromagnet in a magnetic field at high values of the external applied field. Even if data for small lattice sizes are compatible with second-order criticality, the critical behavior of the system shows a crossover from second-order to first-order behavior for large system sizes, where signals of latent heat appear. We propose "apparent" critical exponents for the dependence of some observables with the lattice size for a generic (disordered) first-order phase transition.Comment: Final version, accepted for publicatio

    The anisotropic XY model on the inhomogeneous periodic chain

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    The static and dynamic properties of the anisotropic XY-model (s=1/2)(s=1/2) on the inhomogeneous periodic chain, composed of NN cells with nn different exchange interactions and magnetic moments, in a transverse field h,h, are determined exactly at arbitrary temperatures. The properties are obtained by introducing the Jordan-Wigner fermionization and by reducing the problem to a diagonalization of a finite matrix of nthnth order. The quantum transitions are determined exactly by analyzing, as a function of the field, the induced magnetization 1/n\sum_{m=1}^{n}\mu_{m}\left (jj denotes the cell, mm the site within the cell, μm\mu_{m} the magnetic moment at site mm within the cell) and the spontaneous magnetization 1/nm=1n<Sj,mx,>1/n\sum_{m=1}^{n}\left< S_{j,m}^{x},\right> which is obtained from the correlations <Sj,mxSj+r,mx>\left< S_{j,m}^{x}S_{j+r,m}^{x}\right> for large spin separations. These results, which are obtained for infinite chains, correspond to an extension of the ones obtained by Tong and Zhong(\textit{Physica B} \textbf{304,}91 (2001)). The dynamic correlations, <Sj,mz(t)Sj,mz(0)>\left< S_{j,m}^{z}(t)S_{j^{\prime},m^{\prime}}^{z}(0)\right>, and the dynamic susceptibility, χqzz(ω),\chi_{q}^{zz}(\omega), are also obtained at arbitrary temperatures. Explicit results are presented in the limit T=0, where the critical behaviour occurs, for the static susceptibility χqzz(0)\chi_{q}^{zz}(0) as a function of the transverse field hh, and for the frequency dependency of dynamic susceptibility χqzz(ω)\chi_{q}^{zz}(\omega).Comment: 33 pages, 13 figures, 01 table. Revised version (minor corrections) accepted for publiction in Phys. Rev.

    Modelling thermal flow in a transition regime using a lattice Boltzmann approach

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    Lattice Boltzmann models are already able to capture important rarefied flow phenomena, such as velocity-slip and temperature jump, provided the effects of the Knudsen layer are minimal. However, both conventional hydrodynamics, as exemplified by the Navier-Stokes-Fourier equations, and the lattice Boltzmann method fail to predict the nonlinear velocity and temperature variations in the Knudsen layer that have been observed in kinetic theory. In the present paper, we propose an extension to the lattice Boltzmann method that will enable the simulation of thermal flows in the transition regime where Knudsen layer effects are significant. A correction function is introduced that accounts for the reduction in the mean free path near a wall. This new approach is compared with direct simulation Monte Carlo data for Fourier flow and good qualitative agreement is obtained for Knudsen numbers up to 1.58
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