Effect of fluctuations on the freezing of a colloidal suspension in an external periodic potential

We incorporate the effects of fluctuations in a density functional analysis of the freezing of a colloidal liquid in the presence of an external potential generated by interfering laser beams. A mean-field treatment, using a density functional theory, predicts that with the increase in the strength of the modulating potential, the freezing transition changes from a first order to a continuous one via a tricritical point for a suitable choice of the modulating wavevectors. We demonstrate here that the continuous nature of the freezing transition at large values of the external potential V-e survives the presence of fluctuations. We also show that fluctuations tend to stabilize the liquid phase in the large V-e regime.

Nonlinear Optical Properties of Stacked Conjugated Systems

We study linear and nonlinear optical properties of two push-pull polyenes stacked in head to head (HtH) and head to tail (HtT) configurations, at different stacking angles within the Pariser-Parr-Pople model using exact diagonalization method. By varying the stacking angle between the polyenes, we find that the optical gap varies marginally, but transition dipoles show large variations. We find that the dominant first-order hyperpolarizability component beta(XXX) for HtH arrangement and beta(YYY) for HtT arrangement strongly depend on the distance of separation between molecules, while the other smaller component beta(XYY) for HtH arrangement and beta(XXY) for HtT arrangement) does not show this variation with distance. We find that the beta(XXX) for HtH configuration shows a maximum at an angle away from 0, in contrast with the oriented gas model. This angle varies with distance between the polyenes, and at large distance it falls to 0. The ratio of all components of beta of a dimer to monomer is less than two for HtH configuration for all angles. But for HtT configurations the ratio of the dominant beta component is greater than two at large angles. Our ZINDO study on two monomers (4-hydroxy-4'-nitroazobenzene) connected in a nonconjugative fashion shows a linear increase in vertical bar(beta) over right arrow (av)vertical bar without much red shift in optical gap. There is a linear increase in vertical bar(beta) over right arrow (av)vertical bar with increase in number of monomers connected nonconjugatively without resulting in a red shift in optical gap.

A Macroscopic Model for First Return Stroke of Lightning

A lightning return stroke model for a downward flash is proposed. The model includes underlying physical phenomena governing return stroke evolution, namely, electric field due to charge distributed along the leader and cloud, transient enhancement of series channel conductance at the bridging regime, and the nonlinear variation of channel conductance, which supports the return stroke current evolution. Thermal effects of free burning arc at the stroke wave front and its impact on channel conductance are studied. A first-order arc model for determining the dynamic channel conductance along with a field-dependent conductivity for corona sheath is used in the model. The model predicts consistent current propagation along the channel with regard to current amplitude and return stroke velocity. The model is also capable of predicting the remote electromagnetic fields that are consistent with the experimental observations.

Reliability based design optimization of gravity retaining walls

In this paper the use of probability theory in reliability based optimum design of reinforced gravity retaining wall is described. The formulation for computing system reliability index is presented. A parametric study is conducted using advanced first order second moment method (AFOSM) developed by Hasofer-Lind and Rackwitz-Fiessler (HL-RF) to asses the effect of uncertainties in design parameters on the probability of failure of reinforced gravity retaining wall. Totally 8 modes of failure are considered, viz overturning, sliding, eccentricity, bearing capacity failure, shear and moment failure in the toe slab and heel slab. The analysis is performed by treating back fill soil properties, foundation soil properties, geometric properties of wall, reinforcement properties and concrete properties as random variables. These results are used to investigate optimum wall proportions for different coefficients of variation of φ (5% and 10%) and targeting system reliability index (βt) in the range of 3 – 3.2.

Robust Fixed Order Lateral H2 Controller for Micro Air Vehicle

This paper presents a robust fixed order H2controller design using strengthened discrete optimal projection equations, which approximate the first order necessary optimality condition. The novelty of this work is the application of the robust H2controller to a micro aerial vehicle named Sarika2 developed in house. The controller is designed in discrete domain for the lateral dynamics of Sarika2 in the presence of low frequency atmospheric turbulence (gust) and high frequency sensor noise. The design specification includes simultaneous stabilization, disturbance rejection and noise attenuation over the entire flight envelope of the vehicle. The resulting controller performance is comprehensively analyzed by means of simulation

Simulation of Influence of Bilayer Melting on Dynamics and Thermodynamics of Interfacial Water

We investigate the effect of bilayer melting transition on thermodynamics and dynamics of interfacial water using molecular dynamics simulation with the two-phase thermodynamic model. We show that the diffusivity of interface water depicts a dynamic crossover at the chain melting transition following an Arrhenius behavior until the transition temperature. The corresponding change in the diffusion coefficient from the bulk to the interface water is comparable with experimental observations found recently for water near 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) vesicles Phys. Chem. Chem. Phys. 13, 7732 (2011)]. The entropy and potential energy of interfacial water show distinct changes at the bilayer melting transition, indicating a strong correlation in the thermodynamic state of water and the accompanying first-order phase transition of the bilayer membrane. DOI: 10.1103/PhysRevLett.110.018303

Sensory-organ-like response determines the magnetism of zigzag-edged honeycomb nanoribbons

We present an analytical effective theory for the magnetic phase diagram for zigzag-edge terminated honeycomb nanoribbons described by a Hubbard model with an interaction parameter U. We show that the edge magnetic moment varies as ln U and uncover its dependence on the width W of the ribbon. The physics of this owes its origin to the sensory-organ-like response of the nanoribbons, demonstrating that considerations beyond the usual Stoner-Landau theory are necessary to understand the magnetism of these systems. A first-order magnetic transition from an antiparallel orientation of the moments on opposite edges to a parallel orientation occurs upon doping with holes or electrons. The critical doping for this transition is shown to depend inversely on the width of the ribbon. Using variational Monte Carlo calculations, we show that magnetism is robust to fluctuations. Additionally, we show that the magnetic phase diagram is generic to zigzag-edge terminated nanostructures such as nanodots. Furthermore, we perform first-principles modeling to show how such magnetic transitions can be realized in substituted graphene nanoribbons. DOI: 10.1103/PhysRevB.87.085412

Quantum phases and phase transitions of frustrated hard-core bosons on a triangular ladder

Kinetically frustrated bosons at half filling in the presence of a competing nearest-neighbor repulsion support a wide supersolid regime on the two-dimensional triangular lattice. We study this model on a two-leg ladder using the finite-size density-matrix renormalization-group method, obtaining a phase diagram which contains three phases: a uniform superfluid (SF), an insulating charge density wave (CDW) crystal, and a bond ordered insulator (BO). We show that the transitions from SF to CDW and SF to BO are continuous in nature, with critical exponents varying continuously along the phase boundaries, while the transition from CDW to BO is found to be first order. The phase diagram is also found to contain an exactly solvable Majumdar Ghosh point, and reentrant SF to CDW phase transitions.

Hard-core bosons in a zig-zag optical superlattice

We study a system of hard-core bosons at half-filling in a one-dimensional optical superlattice. The bosons are allowed to hop to nearest-and next-nearest-neighbor sites. We obtain the ground-state phase diagram as a function of microscopic parameters using the finite-size density-matrix renormalization-group method. Depending on the sign of the next-nearest-neighbor hopping and the strength of the superlattice potential the system exhibits three different phases, namely the bond-order (BO) solid, the superlattice induced Mott insulator (SLMI), and the superfluid (SF) phase. When the signs of both hopping amplitudes are the same (the unfrustratedase), the system undergoes a transition from the SF to the SLMI at a nonzero value of the superlattice potential. On the other hand, when the two amplitudes differ in sign (the frustrated case), the SF is unstable to switching on a superlattice potential and also exists only up to a finite value of the next-nearest-neighbor hopping. This part of the phase diagram is dominated by the BO phase which breaks translation symmetry spontaneously even in the absence of the superlattice potential and can thus be characterized by a bond-order parameter. The transition from BO to SLMI appears to be first order.

Supersolid in a one-dimensional model of hard-core bosons

We study a system of hard-core boson on a one-dimensional lattice with frustrated next-nearest-neighbor hopping and nearest-neighbor interaction. At half filling, for equal magnitude of nearest- and next-nearest-neighbor hopping, the ground state of this system exhibits a first-order phase transition from a bond-ordered solid to a charge-density-wave solid as a function of the nearest- neighbor interaction. Moving away from half filling we investigate the system at incommensurate densities, where we find a supersolid phase which has concurrent off-diagonal long-range order and density-wave order which is unusual in a system of hard-core bosons in one dimension. Using the finite-size density-matrix renormalization group method, we obtain the complete phase diagram for this model.

Structural and thermodynamic properties of Fe1.12Te with multiple phase transitions

The parent compound of iron chalcogenide superconductors, Fe1+yTe, with a range of excess Fe concentrations exhibits intriguing structural and magnetic properties. Here, the interplay of magnetic and structural properties of Fe1.12Te single crystals have been probed by low-temperature synchrotron X-ray powder diffraction, magnetization, and specific heat measurements. Thermodynamic measurements reveal two distinct phase transitions, considered unique to samples possessing excess Fe content in the range of 0.11 <= y <= 0.13. On cooling, an antiferromagnetic transition, T-N approximate to 57K is observed. A closer examination of powder diffraction data suggests that the transition at TN is not purely magnetic, but accompanied by the commencement of a structural phase transition from tetragonal to orthorhombic symmetry. This is followed by a second prominent first-order structural transition at T-S with T-S < T-N, where an onset of monoclinic distortion is observed. The results point to a strong magneto-structural coupling in this material. (C) 2014 AIP Publishing LLC.

Pressure-Induced Bond Rearrangement and Reversible Phase Transformation in a Metal-Organic Framework

Pressure-induced phase transformations (PIPTs) occur in a wide range of materials. In general, the bonding characteristics, before and after the PIPT, remain invariant in most materials, and the bond rearrangement is usually irreversible due to the strain induced under pressure. A reversible PIPT associated with a substantial bond rearrangement has been found in a metal-organic framework material, namely tmenH(2)]Er(HCOO)(4)](2) (tmenH(2)(2+) = N,N,N',N'-tetramethylethylenediammonium). The transition is first-order and is accompanied by a unit cell volume change of about 10%. High-pressure single-crystal X-ray diffraction studies reveal the complex bond rearrangement through the transition. The reversible nature of the transition is confirmed by means of independent nanoindentation measurements on single crystals.

Phonon anomalies, orbital-ordering and electronic raman scattering in iron-pnictide Ca(Fe0.97Co0.03)(2)As-2: temperature-dependent Raman study

We report inelastic light scattering studies on Ca(Fe0.97Co0.03)(2)As-2 in a wide spectral range of 120-5200 cm(-1) from 5 to 300 K, covering the tetragonal to orthorhombic structural transition as well as magnetic transition at T-sm similar to 160 K. The mode frequencies of two first-order Raman modes B-1g and E-g, both involving the displacement of Fe atoms, show a sharp increase below T-sm. Concomitantly, the linewidths of all the first-order Raman modes show anomalous broadening below T-sm, attributed to strong spin-phonon coupling. The high frequency modes observed between 400 and 1200 cm(-1) are attributed to electronic Raman scattering involving the crystal field levels of d-orbitals of Fe2+. The splitting between xz and yz d-orbital levels is shown to be similar to 25 meV, which increases as temperature decreases below T-sm. A broad Raman band observed at similar to 3200 cm(-1) is assigned to two-magnon excitation of the itinerant Fe 3d antiferromagnet.

Magnetization-steps in Y2CoMnO6 double perovskite: The role of antisite disorder

Antisite disorder is observed to have significant impact on the magnetic properties of the double perovskite Y2CoMnO6 which has been recently identified as a multiferroic. A paramagnetic-ferromagnetic phase transition occurs in this material at T-c approximate to 75 K. At 2K, it displays a strong ferromagnetic hysteresis with a significant coercive field of H-c approximate to 15 kOe. Sharp steps are observed in the hysteresis curves recorded below 8K. In the temperature range 2K <= T <= 5K, the hysteresis loops are anomalous as the virgin curve lies outside the main loop. The field-cooling conditions as well as the rate of field-sweep are found to influence the steps. Quantitative analysis of the neutron diffraction data shows that at room temperature, Y2CoMnO6 consists of 62% of monoclinic P2(1)/n with nearly 70% antisite disorder and 38% Pnma. The bond valence sums indicate the presence of other valence states for Co and Mn which arise from disorder. We explain the origin of steps by using a model for pinning of magnetization at the antiphase boundaries created by antisite disorder. The steps in magnetization closely resemble the martensitic transformations found in intermetallics and display first-order characteristics as revealed in the Arrott's plots. (C) 2014 AIP Publishing LLC.

Adaptive mesh generation for singularly perturbed fourth-order ordinary differential equations

In this paper, we consider a singularly perturbed boundary-value problem for fourth-order ordinary differential equation (ODE) whose highest-order derivative is multiplied by a small perturbation parameter. To solve this ODE, we transform the differential equation into a coupled system of two singularly perturbed ODEs. The classical central difference scheme is used to discretize the system of ODEs on a nonuniform mesh which is generated by equidistribution of a positive monitor function. We have shown that the proposed technique provides first-order accuracy independent of the perturbation parameter. Numerical experiments are provided to validate the theoretical results.