Microcanonical finite-size scaling in specific heat diverging 2nd order phase transitions

A Microcanonical Finite Site Ansatz in terms of quantities measurable in a Finite Lattice allows to extend phenomenological renormalization (the so called quotients method) to the microcanonical ensemble. The Ansatz is tested numerically in two models where the canonical specific-heat diverges at criticality, thus implying Fisher-renormalization of the critical exponents: the 3D ferromagnetic Ising model and the 2D four-states Potts model (where large logarithmic corrections are known to occur in the canonical ensemble). A recently proposed microcanonical cluster method allows to simulate systems as large as L=1024 (Potts) or L=128 (Ising). The quotients method provides extremely accurate determinations of the anomalous dimension and of the (Fisher-renormalized) thermal $\nu$ exponent. While in the Ising model the numerical agreement with our theoretical expectations is impressive, in the Potts case we need to carefully incorporate logarithmic corrections to the microcanonical Ansatz in order to rationalize our data.Comment: 13 pages, 8 figure

Self-Averaging in the Three Dimensional Site Diluted Heisenberg Model at the critical point

We study the self-averaging properties of the three dimensional site diluted Heisenberg model. The Harris criterion \cite{critharris} states that disorder is irrelevant since the specific heat critical exponent of the pure model is negative. According with some analytical approaches \cite{harris}, this implies that the susceptibility should be self-averaging at the critical temperature ($R_\chi=0$). We have checked this theoretical prediction for a large range of dilution (including strong dilution) at critically and we have found that the introduction of scaling corrections is crucial in order to obtain self-averageness in this model. Finally we have computed critical exponents and cumulants which compare very well with those of the pure model supporting the Universality predicted by the Harris criterion.Comment: 11 pages, 11 figures, 14 tables. New analysis (scaling corrections in the g2=0 scenario) and new numerical simulations. Title and conclusions chang

Path-integral calculation of the two-dimensional 4He phase diagram

Path-integral Monte Carlo simulations have been used to determine the phase diagram of a two-dimensional 4He film in a range of temperatures and coverages where it undergoes solidification, superfluidity, and a liquid-gas transition (0.25K<~T<~1.5K¿0<¿<0.094¿Å-2). We determine the phase-transition densities and give the coefficients for a functional form of the free energy in the liquid, solid, and gas phases. The phase diagram is similar to the one determined from experimental measurements of a second layer of helium on graphite.Universidad Pablo de Olavide. Departamento de Sistemas Físicos, Químicos y NaturalesVersión del edito

Superfluidity in H2 films

Path integral Monte Carlo calculations were used to study a 2D system of H2 molecules. The ground state on a flat substrate is a triangular lattice, a localized solid. We find that the introduction of some weakly attractive impurities (specifically alkali metal atoms) in the right concentration (~0.01¿¿2) stabilizes a liquid hydrogen phase of concentration 0.04¿¿2 which undergoes a Kosterlitz-Thouless superfluid transition below approximately 1.2 K.Universidad Pablo de Olavide. Departamento de Sistemas Físicos, Químicos y NaturalesVersión del edito

Effects of the presence of Cs impurities on the two-dimensional 4He phase diagram

The changes in the two-dimensional (2D) 4He phase diagram due to the introduction of impurities (Cs atoms) are estimated using path-integral Monte Carlo simulations. Our results indicate that when we increase the concentration of Cs atoms, the liquid-gas coexistence zone decreases in size and eventually disappears for ¿Cs~0.005Å-2, so that 2D 4He gases will be stable at low temperatures.Universidad Pablo de Olavide. Departamento de Sistemas Físicos, Químicos y NaturalesVersión del edito

Two-dimensional H2 clusters : a path-integral Monte Carlo study

We performed path-integral Monte Carlo simulations of two-dimensional H2 clusters of different sizes. The smaller of these clusters are superfluid at low temperatures. On increasing the size we observe the formation of a solidlike inner core and a liquidlike outer shell.Universidad Pablo de Olavide. Departamento de Sistemas Físicos, Químicos y NaturalesVersión del edito

A new device for the generation of microbubbles

In this paper we present a new method for the production of bubble-liquid suspensions (from now on BLS) composed of micron-sized bubbles and with gas to liquid volume ratios larger than unity. We show that the BLS gas fraction ʎ=Qg and Q1 , being Qg and Q1 the flow rates of gas and liquid, respectively, is controlled by a dimensionless parameter which accounts for the ratio of the gas pressure inside the device to the liquid viscous pressure drop from the orifices where the liquid is injected to the exit, where the BLS is obtained. This parameter permits the correct scaling of the BLS gas volume fraction of all the experiments presented

Preparation of ethylcellulose/methylcellulose blends by supercritical antisolvent precipitation

The supercritical antisolvent (SAS) techniquewas used to prepare ethyl cellulose/methyl cellulose blends, two biocompatible polymers commonly used as drug carriers in controlled delivery systems. Ethyl cellulose is widely used as a drug carrier. The drug release of the delivery devices can be controlled to some extent by addition of a water-soluble or water swellable polymer, such as methyl cellulose. This leads to the solubility enhancement of poorly water-soluble molecules. SAS experiments were carried out at different operational conditions and microspheres with mean diameters ranging from 5 to 30 m were obtained. The effect of CO2 and liquid flow, temperature and pressure on particle size and particle size distribution was evaluated. The microspheres were precipitated from a mixture of dichloromethane (DCM) and dimethylsulfoxide (DMSO) (4:1 ratio). The best process conditions for this mixture were according to our study 40 ◦C and 80 bar

The effect of disorder on the critical temperature of a dilute hard sphere gas

We have performed Path Integral Monte Carlo (PIMC) calculations to determine the effect of quenched disorder on the superfluid density of a dilute 3D hard sphere gas. The disorder was introduced by locating set of hard cylinders randomly inside the simulation cell. Our results indicate that the disorder leaves the superfluid critical temperature basically unchanged. Comparison to experiments of helium in Vycor is made.Comment: 4 pages, 4 figure

On the possibility to supercool molecular hydrogen down to superfluid transition

Recent calculations by Vorobev and Malyshenko (JETP Letters, 71, 39, 2000) show that molecular hydrogen may stay liquid and superfluid in strong electric fields of the order of $4\times 10^7 V/cm$. I demonstrate that strong local electric fields of similar magnitude exist beneath a two-dimensional layer of electrons localized in the image potential above the surface of solid hydrogen. Even stronger local fields exist around charged particles (ions or electrons) if surface or bulk of a solid hydrogen crystal is statically charged. Measurements of the frequency shift of the $1 \to 2$ photoresonance transition in the spectrum of two-dimensional layer of electrons above positively or negatively charged solid hydrogen surface performed in the temperature range 7 - 13.8 K support the prediction of electric field induced surface melting. The range of surface charge density necessary to stabilize the liquid phase of molecular hydrogen at the temperature of superfluid transition is estimated.Comment: 5 pages, 2 figure