International Journal of Modern Physics C

Niobium carbide nanocrystals of ~1:1 stoichiometry have recently been observed for particle sizes ranging from Nb₄C₄ to Nb₅₀C₅₀. Infrared (IR) spectroscopic measurements show that a new band of IR vibrational modes appears with increasing particle size at Nb₉C₉. Using density-functional theory, we show that the vibrational modes in the new band involve structural features present only in nanocrystals with three or more atomic layers in every direction. The Nb₉C₉ nanocrystal is right at this structural threshold.

The use of graphics processing units (GPUs) in scientific computing has gathered considerable momentum in the past five years. While GPUs in general promise high performance and excellent performance per Watt ratios, not every class of problems is equally well suitable for exploiting the massively parallel architecture they provide. Lattice spin models appear to be prototypic examples of problems suitable for this architecture, at least as long as local update algorithms are employed. In this review, I summarize our recent experience with the simulation of a wide range of spin models on GPU employing an equally wide range of update algorithms, ranging from Metropolis and heat bath updates, over cluster algorithms to generalized ensemble simulations.

The existence of a vortex in the ground state of magnetic nano-disks has open a wide range of possibilities for constructing new ultra-compact devices. In this work we study the dynamical behavior of a vortex in a magnetic nano-particle. First, we introduce magnetic impurities in the system. It is observed that depending on the strength of the interaction the impurities can behave both as a pinning (attractive) or scattering (repulsive). By using the known values of the parameters for Permalloy-79 we have calculated the interaction energy of the vortex core with a single defect. We estimated the interaction range as approximately 10 nm. Both results agree quite well with experimental measurements. As a second point we discuss how the vortex dynamics in nano-disks can be used for building a nano-spin valve.

Conformational and transition behavior of finite, semiflexible homopolymers is studied using an extension of the Wang–Landau algorithm. Generation of a flat distribution in the sampling parameters energy and stiffness allows for efficient investigation of transitions between various conformational phases. Of particular importance is the ability to predict behavior for a given stiffness value, where three classes of minimum energy conformations are expected: Solid-globular, rod-like and toroidal. We present first results highlighting the behavior of a single N = 20 length chain.

We investigate the ejection dynamics of a flexible polymer chain out of confined environment by means of scaling considerations and Monte Carlo simulations. Situations of this kind arise in different physical contexts, including a flexible synthetic polymer partially confined in a nanopore and a viral genome partially ejected from its capsid. In the case of cylindric confinement the entropic driving force which pulls the chain out of the pore is argued to be constant once a few persistent lengths are out of the pore. We demonstrate that in this case the ejection dynamics follows a -law with elapsed time t. The mean ejection time τ depends nonmonotonically on chain length N. However, if the geometric constraints comprise a wider capsid chamber attached to a narrow exit tube, the mechanism of ejection changes and involves the surmounting of an activation barrier. The driving force then varies in time. One finds good agreement of theory and computer simulation with recent experiments with DNA.

A set of single domain particles has been studied through Monte Carlo simulations on three different lattices. A simple cubic lattice, a face centered cubic lattice and a liquid-like structure. The particles are coupled by long-range dipolar forces and present a single ion uniaxial anisotropy, whose magnitude is chosen from a Gaussian distribution, and whose easy magnetization axes are oriented randomly in the three-dimensional space. We determined the blocking temperature and the hysteresis curves as a function of the ratio between the magnitude of dipolar coupling and uniaxial anisotropy. We show that the remanence and coercive field depend strongly on the nature of the magnetic arrangement. These results are compared with those found for a system of noninteracting particles.

We describe a systematic method for complete enumeration of configuration classes (CCs) of the spin-1/2 Ising model in the energy-magnetization plane. This technique is applied to the antiferromagnetic (AF) Ising model in an external magnetic field on the square lattice, which is simulated using the tomographic entropic sampling (TES) algorithm. We estimate the number of configurations, Ω(E,m,L), and related microcanonical averages, for all allowed energies E and magnetizations m for L = 10 to 30, with ΔL = 2. With prior knowledge of the CCs, we can be sure that all allowed classes are sampled during the simulation. Complete enumeration of CCs also enables us to use the final estimate of Ω(E,m,L) to obtain good initial estimates, Ω₀(E,m,L′), for successive system sizes (L′ > L) through a two-dimensional interpolation. Using these results we calculate canonical averages of the thermodynamic quantities of interest as continuous functions of temperature T and external field h. In addition, we determine the critical line in the h-T plane using finite-size scaling analysis, and compare these results with several approximate theoretical expressions.

The hydrophobic-polar (HP) model has emerged as one of the standard approaches for simulating protein folding. In this work, we used this model together with Wang–Landau (WL) sampling and appropriate Monte Carlo trial moves to determine the density of states and thermodynamics for two cases: Protein adsorption and protein confinement, in the vicinity of attractive surfaces. The influence on the adsorption behavior of surface attractive strength in the adsorption case and volumetric spaces in the confinement case will be discussed.

We study the nature of collective excitations in classical anharmonic lattices with aperiodic and pseudo-random harmonic spring constants. The aperiodicity was introduced in the harmonic potential by using a sinusoidal function whose phase varies as a power-law, ϕ ∝ n^ν, where n labels the positions along the chain. In the absence of anharmonicity, we numerically demonstrate the existence of extended states and energy propagation for a sufficiently large degree of aperiodicity. Calculations were done by using the transfer matrix formalism (TMF), exact diagonalization and numerical solution of the Hamilton's equations. When nonlinearity is switched on, we numerically obtain a rich framework involving stable and unstable solitons.

We introduce a variant of the Bethe lattice, the fully connected Bethe lattice (FCBL); with every site in each shell of the Bethe lattice connected to every other site in the shell. A recently introduced decimation renormalization group (RG) method is utilized to obtain the quantum transmission through the Bethe lattice and FCBL. Although classically the extra intra-shell connections would not change the transmission, they have a large effect on the quantum transport and the wave functions.

Colloidal systems are often modeled as fluids of hard particles (possibly with an additional soft attraction, e.g. caused by polymers also contained in the suspension). In simulations of such systems, the virial theorem cannot be straightforwardly applied to obtain the components of the pressure tensor. In systems confined by walls, it is hence also not straightforward to extract the excess energy due to the wall (the “wall tension”) from the pressure tensor anisotropy. A comparative evaluation of several methods to circumvent this problem is presented, using as examples fluids of hard spheres and the Asakura–Oosawa model of colloid-polymer mixtures with a size ratio q = 0.15 (for which the effect of the polymers can be integrated out to yield an effective attractive potential between the colloids). Factors limiting the accuracy of the various methods are carefully discussed, and controlling these factors very good mutual agreement between the various methods is found.

In this work we apply a refined Wang–Landau simulation to a simple polymer model which has an exact solution both in the microcanonical and the canonical formalisms. We investigate the behavior of the microcanonical and canonical averages during the Wang–Landau simulation. The simulations were carried out using conventional Wang–Landau sampling (WLS) and the 1/t scheme. Our results show that updating the density of states only after every N monomer moves leads to a much better precision. During the simulations the canonical averages such as the location of the maximum of the specific heat calculated from independent runs tend asymptotically to values around the correct value obtained from the exact calculations of the density of states and remain unchanged for some final modification factor. Since this f_final is found for the model analyzed, one has a criterion to stop the simulations. We compare our results with the exact value and with those of the 1/t scheme.

An important feature in the design of superhydrophobic surfaces is their robustness against collapse from the Cassie–Baxter configuration to the Wenzel state. Upon such a transition a surface loses its properties of low adhesion and friction. We describe how to adapt the Surface Evolver algorithm to predict the parameters and mechanism of the collapse transition on posts of arbitrary shape. In particular, contributions to the free energy evaluated over the solid–liquid surface are reduced to line integrals to give good convergence. The algorithm is validated for straight, vertical and inclined, posts. Numerical results for curved posts with a horizontal section at their ends show that these are more efficient in stabilizing the Cassie state than straight posts, and identify whether the interface first depins from the post sides or the post tips.

Over the last 25 years a lot of work has been undertaken on constructing continuum models for segregation of particles of different sizes. We focus on one model that is designed to predict segregation and remixing of two differently sized particle species. This model contains two dimensionless parameters: S_r, a measure of the segregation rate, and D_r, a measure of the strength of diffusion. These, in general, depend on both flow and particle properties and one of the weaknesses of the model is that these dependencies are not predicted. They have to be determined by either experiments or simulations.

We present steady-state, periodic, chute-flow simulations using the discrete particle method (DPM) for several bi-disperse systems with different size ratios. The aim is to determine one parameter in the continuum model, i.e. the segregation Péclet number (ratio of the segregation rate to diffusion, S_r/D_r) as a function of the particle size ratio.

Reasonable agreement is found; but, also measurable discrepancies are reported; mainly, in the simulations a thick pure phase of large particles is formed at the top of the flow. Additionally, it was found that the Péclet number increases linearly with the size ratio for low values, but saturates to a value of approximately 7.7.

A disordered binary Ising model, with only nearest-neighbor spin exchange interactions J > 0 on the square lattice, is studied through Monte Carlo simulations. The system consists of two different particles with spin-1/2 and spin-1, randomly distributed on the lattice. We found the critical temperatures for several values of the concentration x of spin-1/2 particles, and also the corresponding critical exponents.

The time evolution of an order parameter towards equilibrium can be described by nonlinear Ginzburg–Landau (GL) type of equations, also known as time-dependent nonlinear Schrödinger equations. Environmental effects of random nature are usually taken into account by noise sources, turning the GL equations into stochastic equations. Noise sources give rise to lattice-spacing dependence of the solutions of the stochastic equations. We present a systematic method to renormalize the equations on a spatial lattice to obtain lattice-spacing independent solutions. We illustrate the method in approximation schemes designed to treat nonlinear and nonlocal GL equations that appear in real time thermal field theory and stochastic quantization.