A.A. Caparica

Polarizability of a donor impurity in a GaAsAlGaAs quantum wire

Abstract The polarizability and the ground state binding energy of a donor impurity located in a semiconductor quantum well wire of rectangular cross-section with finite barrier potential is calculated through the variational method. We assume that the donor is located at the center of the wire and that the electric field is applied in the direction perpendicular to the wire length ( z -direction), making an angle θ with the x -axis. We present our results for the ground state binding energy as a function of the size of the wire for several values of the electric field strength and of the angle θ. It is found that the polarizabilities decrease as the wire dimensions ( L x and L y ) decrease, until they reach a minimum at a certain value of L x ( L y fixed) and then increase as the width becomes smaller. The binding energies and the polarizabilities, as expected, have a strong dependence on the geometrical form of the wire and on the angle θ.

Two-dimensional lattice polymers: Adaptive windows simulations

Abstract We report a numerical study of self-avoiding polymers on the square lattice, including an attractive potential between nonconsecutive monomers occupying neighboring lattice sites. Using Wang–Landau sampling (WLS) with adaptive windows, we obtain the density of states for chains of up to N = 300 monomers and associated thermodynamic quantities. Finite size scaling analysis yields a transition temperature of Θ = 1.505 ( 18 ) . WLS with adaptive windows enables one to simulate accurately the low-temperature regime, which is virtually inaccessible using traditional methods. Instead of defining fixed energy windows, as in usual WLS, this method uses windows with boundaries that depend on the set of energy values on which the histogram is flat at a given stage of the simulation. Shifting the windows each time the modification factor f is reduced, we eliminate border effects that arise in simulations using fixed windows.

Thermal and elastic properties of solid sodium based on an effective interionic potential

Basing on the Schiff effective interionic potential which has oscillatory character we have calculated a complete set of equilibrium thermodynamic properties of solid sodium at normal pressure above the martensitic transition temperature. To take into account the anharmonicity which is strong at high temperatures we use the correlative method of unsymmetrized self-consistent field elaborated previously. Our results are in agreement with measured values and with available data of computer simulations. We discuss the thermodynamic stability of the b.c.c. lattice of sodium and the mechanism of its loss. The metastability of a hypothetical f.c.c. lattice with Schiff potential is discussed.

Isotherms, thermodynamic properties and stability of the FCC phase of the C60 fullerite: A theoretical study

We are pursuing statistical-mechanical investigations of thermodynamic properties of the high-temperature modification of the C60 fullerite taking into account the intramolecular degrees of freedom and the strong anharmonicity of lattice vibrations. In our theoretical calculations we employed the correlative method of an unsymmetrized self-consistent field for strongly anharmonic crystals using the available experimental information about the vibrational spectrum of this molecule. The Girifalco potential and the Yakub approximation are used. In the present work we have calculated isotherms of this fullerite and a complete set of its equilibrium properties, including the components of the elastic tensors. Each isotherm has two branches V1(P) 0, while at the right one, BT(P, V2) < 0; the latter represents the absolute instability of the states V2(P). We have also investigated the thermodynamic stability of this fullerite and the mechanism of loss of stability depending on the temperature. Our results for thermal and elastic properties are in good agreement with experimental data available at low pressures and temperatures.

Static critical behavior of the q-states Potts model: High-resolution entropic study

Here we report a precise computer simulation study of the static critical properties of the two-dimensional q-states Potts model using very accurate data obtained from a modified Wang–Landau (WL) scheme proposed by Caparica and Cunha-Netto (2012). This algorithm is an extension of the conventional WL sampling, but the authors changed the criterion to update the density of states during the random walk and established a new procedure to windup the simulation run. These few changes have allowed a more precise microcanonical averaging which is essential to a reliable finite-size scaling analysis. In this work we used this new technique to determine the static critical exponents β, γ, and ν, in an unambiguous fashion. The static critical exponents were determined as β=0.10811(77), γ=1.4459(31), and ν=0.8197(17), for the q=3 case, and β=0.0877(37), γ=1.3161(69), and ν=0.7076(10), for the q=4 Potts model. A comparison of the present results with conjectured values and with those obtained from other well established approaches strengthens this new way of performing WL simulations.

Finite-size Scaling Considerations on the Ground State Microcanonical Temperature in Entropic Sampling Simulations

In this work, we discuss the behavior of the microcanonical temperature ∂S(E)∂E

On thermodynamic properties of crystals in the metastable region

\frac {\partial S(E)}{\partial E}

Critical Behavior of the Spin-1/2 Baxter-Wu Model: Entropic Sampling Simulations

obtained by means of numerical entropic sampling studies. It is observed that in almost all cases, the slope of the logarithm of the density of states S(E) is not infinite in the ground state, since as expected it should be directly related to the inverse temperature 1T

Constructing ground state configurations of lattice polymers

\frac {1}{T}

O isocronismo dos triângulos retângulos

. Here, we show that these finite slopes are in fact due to finite-size effects and we propose an analytic expression aln(bL) for the behavior of ΔSΔE