1,721,130 research outputs found
Critical behavior in colloid-polymer mixtures : theory and simulation
No full textWe extensively investigated the critical behavior of mixtures of colloids and polymers via the two-component Asakura-Oosawa model and its reduction to a one-component colloidal fluid using accurate theoretical and simulation techniques. In particular the theoretical approach, hierarchical reference theory [A. Parola and L. Reatto, Adv. Phys. 44, 211 (1995)], incorporates realistically the effects of long-range fluctuations on phase separation giving exponents which differ strongly from their mean-field values, and are in good agreement with those of the three-dimensional Ising model. Computer simulations combined with finite-size scaling analysis confirm the Ising universality and the accuracy of the theory, although some discrepancy in the location of the critical point between one-component and full-mixture description remains. To assess the limit of the pair-interaction description, we compare one-component and two-component results
Bose-Einstein condensation of incommensurate solid He-4
No full textIt is pointed out that the simulation computation of energy performed so far cannot be used to decide if the ground state of solid He-4 has the number of lattice sites equal to the number of atoms (commensurate state) or if it is different (incommensurate state). The best variational wave function, a shadow wave function, gives an incommensurate state, but the equilibrium concentration of vacancies remains to be determined. We have computed the one-body density matrix in solid He-4 for the incommensurate state by means of an exact ground state projector method in which incommensurability occurs spontaneously. We find a vacancy induced Bose-Einstein condensation of about 0.23 atoms per vacancy at a pressure of 54 bar. This means that bulk solid He-4 is supersolid at low enough temperature if the exact ground state is incommensurate
Microphase separation in two-dimensional systems with competing interactions
No full textThe formation of clusters in condition of thermodynamic equilibrium can be easily observed both in two and three dimensions. In two dimensions relevant cases include pattern formation in Langmuir monolayers and ferrofluids, while in three dimensions cluster phases have been observed in colloids and in protein solutions. We have analyzed the problem within the scenario of competing interactions: typically, a short-range attractive interaction against a long-range repulsive one. This simplified approach is suggested by the fact that the forces, governing self-organization, act on a length scale which is larger than the molecular size; as a consequence many specific details of the molecules of interest are not necessary for studying the general features of microphases. We have tackled the microphase formation by simulations in bidimensional fluids, exploiting the parallel tempering scheme. In particular, we have analyzed the density range in which the particles arrange in circular domains (droplets), and the temperature range in which the system goes from microphases to the homogeneous fluid phase. As the density increases, the droplet size increases as well, but above a certain density the morphology changes and stripes are formed. Moreover at low density, we observe the formation of a liquidlike phase of disordered droplets; at higher densities, instead, the droplets tend to arrange onto a triangular superlattice. Such a change affects the features of the static structure factor, which gives well defined signatures of the microphase morphology. In each case, the specific heat exhibits a peak close to the transition from microphases to the homogeneous fluid phase, which is due to the breaking up of the clusters. The saturation of the height of the specific heat peak, with the increasing of the system size, suggests the possibility of a Kosterlitz-Thouless transition
Recent developments of the Hierarchical Reference Theory of Fluids and its relation to the Renormalization Group
No full textThe Hierarchical Reference Theory (HRT) of fluids is a general framework for the description of phase transitions in microscopic models of classical and quantum statistical physics. The foundations of HRT are briefly reviewed in a self-consistent formulation which includes both the original sharp cut-off procedure and the smooth cut-off implementation, which has been recently investigated. The critical properties of HRT are summarized, together with the behaviour of the theory at first-order phase transitions. However, the emphasis of this presentation is on the close relationship between HRT and non-perturbative renormalization group methods, as well as on recent generalizations of HRT to microscopic models of interest in soft matter and quantum many body physics
A bidimensional fluid system with competing interactions: spontaneous and induced pattern formation
No full textIn this paper we present a study of pattern formation in bidimensional systems with competing short-range attractive and long-range repulsive interactions. The interaction parameters are chosen in such a way as to allow us to analyse two different situations: the spontaneous pattern formation due to the presence of strong competing interactions on different length scales and the pattern formation as a response to an external modulating potential when the system is close to its Lifshitz point. We compare different Monte Carlo techniques showing that the parallel tempering technique represents a promising approach for the study of such systems and we present detailed results for the specific heat and the structural properties. We also present random phase approximation predictions concerning spontaneous pattern formation (or microphase separation), as well as linear response theory predictions concerning the induced pattern formation due to the presence of an external modulating field. In particular we observe that the response of our systems to external fields is much stronger than the response of a Lennard-Jones fluid
Disorder Phenomena in Quantum Solids with Vacancies
No full textWe have investigated disorder phenomena in solid 4He near megting in presence of vacancies by means of Quantum Monte Carlo (MC) simulations using the Shadow Wave Function variational technique and a new exact projector MC algorithm, the Shadow Path Integral Ground State. With these methods the Bose symmetry is manifestly maintained so that defects can be delocalized and relaxation around them is allowed. We have obtained the first microscopic ab-initio calculation of the excitation spectrum of a vacancy. We have studied different crystal lattices (hcp, fcc, bcc) and the vacancy effective mass ranges from 0.3 to 0.39 4He masses. The band width is rather large, it ranges from 6 K to 19 K depending on the crystal lattice and on the particular direction in the lattice. We have also computed the excitation spectrum of longitudinal phonons and we find good agreement with experimental data both for hcp and bcc
The Shadow Path Integral Ground State Method: Study of Confined Solid 4He
No full textWe have studied the zero temperature phase diagram of confined He-4 by means of a new exact projector Quantum Monte Carlo method: the Shadow Path Integral Ground State (SPIGS). This method is able to compute exact ground state expectation values without extrapolations and with a Shadow Wave Function (SWF) as trial variational state for the importance sampling. Thanks to the ability of SWF in describing the solid phase via spontaneously broken translational symmetry, this is an important extension which opens the possibility to study disorder phenomena in the solid phase by an exact method at zero temperature. We have applied this technique to study He-4 confined in a lattice of big static objects with a purely repulsive interaction with He-4 atoms. We have studied the equation of state of this system finding that a well defined liquid-solid transition is still present but with an increased freezing pressure. In the case we have studied this pressure goes from 25.2 atm for pure bulk system to about 38 atm. This is similar to what is experimentally found in He-4 confined in porous material like vycor. We have found also that the disorder due to the mismatch between the crystal structure and the spherical geometry of the object induces delocalization of He-4 atoms
Microphase morphology in 2D fluids under lateral confinement
No full textWe study the effects of confinement between two parallel walls on a two dimensional fluid with competing
interactions which lead to the formation of particle microdomains at the thermodynamic equilibrium microphases
or microseparation . The possibility to induce structural changes of the morphology of the microdomains
is explored, under different confinement conditions and temperatures. In the presence of neutral walls,
a switch from stripes of particles to circular clusters droplets occurs as the temperature decreases, which does
not happen in bulk. While the passage from droplets to stripes, as the density increases, is a well-known
phenomenon, the change of the stripes into droplets as an effect of temperature is rather unexpected. Depending
on the wall separation and on the wall-fluid interaction parameters, the stripes can switch from parallel to
perpendicular to the walls and also a mixed morphology can be stable
- …
