quasi-1D Model Research Articles

Adiabatic self-trapped states in zigzag nanotubes

We study the polaron (soliton) states of a quasiparticle (electron, hole, exciton) in aquasi-one-dimensional (quasi-1D) model which describes a carbon-type zigzag nanotubestructure. In the Hamiltonian of the system we include the electron–phonon interactionthat arises from the dependence of both the on-site and the hopping interaction energies onthe lattice deformation. We derive, in the adiabatic approximation, the equations for theself-trapped states of a quasiparticle in a zigzag nanotube. We show that the ground stateof such a system depends on the strength of the electron–phonon coupling and we findpolaron-type solutions with different symmetries. Namely, at a relatively weakcoupling a quasiparticle is self-trapped in a quasi-1D polaron state which has anazimuthal symmetry. When the coupling constant exceeds some critical value, theazimuthal symmetry breaks down and the quasiparticle state can be described as atwo-dimensional small polaron on the nanotube surface. In the crossover regionbetween the two solutions there is a range of intermediate couplings, in whichthe two structures, the quasi-1D polaron and the strongly localized 2D polaron,coexist as their energies are very close together. We note that the results of thisanalytical study are in quantitative agreement with what has recently been observednumerically.

Bose-Condensed Gases in a 1D Optical Lattice at Finite Temperatures

We study equilibrium properties of Bose-Condensed gases in a one-dimensional (1D) optical lattice at finite temperatures. We assume that an additional harmonic confinement is highly anisotropic, in which the confinement in the radial directions is much tighter than in the axial direction. We derive a quasi-1D model of the Gross-Pitaeavkill equation and the Bogoliubov equations, and numerically solve these equations to obtain the condensate fraction as a function of the temperature.

Theoretical and numerical analysis of temperature gradient focusing via Joule heating

We present a detailed theoretical and numerical analysis of temperature gradient focusing (TGF) via Joule heating-an analytical species concentration and separation technique relying upon the dependence of an analyte's velocity on temperature due to the temperature dependence of a buffer's ionic strength and viscosity. The governing transport equations are presented, analyzed, and implemented into a quasi-1D numerical model to predict the resulting temperature, velocity, and concentration profiles along a microchannel of varying width under an applied electric field. Numerical results show good agreement with experimental trials presented in previous work. The model is used to analyze the effects of varying certain geometrical and experimental parameters on the focusing performance of the device. Simulations also help depict the separation capability of the device, as well as the effectiveness of different buffer systems used in the technique. The analysis provides rule-of-thumb methodology for implementation of TGF into analytical systems, as well as a fundamental model applicable to any lab-on-a-chip system in which Joule heating and temperature-dependent electrokinetic transport are to be analyzed.

Multi-Resolution PEM Fuel Cell Model Validation and Accuracy Analysis

A multi-resolution simulation method was developed for the polymer electrolyte membrane (PEM) fuel cell simulation: a full 3D model was employed for the membrane and diffusion layer; a 1D+2D model was applied to the catalyst layer, that is, at each location of the fuel cell plate, the governing equations were integrated only in the direction perpendicular to the fuel cell plate; and a quasi-1D model with high numerical efficiency and reasonable accuracy was employed for the flow channels. The simulation accuracy was assessed in terms of the fuel cell polarization curves and membrane Ohmic overpotential. Overall, good agreements between the simulated results and the experimental data were obtained. However, at large current densities, with high relative humidity reactant inputs, the simulation under-predicted the fuel cell performance due to the single-phase assumption; the simulation slightly over-predicted the fuel cell performance for a dry cathode input, possibly due to the nonlinearity of the membrane properties in dehydration case. Further, a parameter study was performed under both fully humidified and relatively dry conditions for the parameters related to the cathode catalyst layer and the gas diffusion layer (GDL). It is found that the effects of liquid water in both the GDL and catalyst layer on the cell performance, and the accurate identification of the cathode catalyst layer parameters such as the cathodic transfer coefficient should be focused for future studies in order to further improve the model accuracy.

Magnetoelastic polarons in the hole-doped quasi-one-dimensional model systemY2−xCaxBaNiO5

Charge transport in the hole-doped quasi-1D model system Y$_{2-x}$Ca$_x$BaNiO$_5$ (x $ leq$ 0.15) is investigated in the 50-300 K temperature range. The resistivity temperature dependence is characterized by a constant activation energy $E_{a}/k_{B} sim$ 1830 K at room temperature while $E_{a}$ decreases upon cooling. We suggest that $E_{a}$ measures the binding energy of the doped holes which form magneto-acoustic polarons when polarizing the neighboring Ni spins. A semi-classical model is proposed which allows to relate the electrical measurements and the bulk magnetic susceptibility. This model gives a picture of the spin-charge-lattice relation in this inhomogeneously doped quasi-1D system and explains its unusual one-particle charge excitation spectrum close to the Fermi level.

A quasi-1D model for fast contraction flows of polymer melts

This paper develops an isothermal quasi-1D model for fast (high Deborah number) contraction flows of polymers from a reservoir to a die of circular or rectangular cross-sections. Two components of composite flow models, inhomogeneous elongation and a modified unsteady shearing, employed in different regions of the full flow domain are connected in succession with the use of asymptotic matching boundary conditions. The model, based on a stable and descriptive set of viscoelastic constitutive equations, demonstrates a reasonable good agreement with experimental and direct numerical data with no adjustable parameters. It employs easy PC numerical calculations. The predictions of the model are improved for contraction flows with higher values of Deborah/Weissenberg number and contraction ratio.

A quasi-1D model for fast contraction flows of polymer melts

This paper develops an isothermal quasi-1D model for fast (high Deborah number) contraction flows of polymers from a reservoir to a die of circular or rectangular cross-sections. Two components of composite flow models, inhomogeneous elongation and a modified unsteady shearing, employed in different regions of the full flow domain are connected in succession with the use of asymptotic matching boundary conditions. The model, based on a stable and descriptive set of viscoelastic constitutive equations, demonstrates a reasonable good agreement with experimental and direct numerical data with no adjustable parameters. It employs easy PC numerical calculations. The predictions of the model are improved for contraction flows with higher values of Deborah/Weissenberg number and contraction ratio.

Modeling of a high-power thruster with anode layer

Among Hall thruster technologies, the thruster with anode layer (TAL) has much wider technical capabilities, especially in the high-power regime of operation. In this paper, various aspects of the plasma flow in a high-power thruster with anode layer are studied. Based on a 2D hydrodynamic model, the formation of a space-charge sheath near the acceleration channel wall and the sheath expansion in the acceleration channel are calculated. It is found that the high-voltage sheath near the channel wall expands significantly and the quasineutral plasma region is confined in the middle of the channel. For instance, in the case of a 3 kV discharge voltage, the sheath thickness is about 1 cm, which is a significant portion of the channel width (which is typically a few cm). In addition, a simplified quasi-1D model is developed to study the anode acceleration layer, which is confined by channel walls. It is found that near-wall sheath expansion leads to an increase in current density along the channel, and this in turn causes decrease of the acceleration region length. This is an important finding as it has implications for high-power TAL behavior, in which contact of the plasma with acceleration channel walls can be limited.

On laterally spreading of space–charge-region in planar metal-semiconductor-metal structures

Lateral spreading along the surface of space–charge-region (SCR) of planar metal-semiconductor-metal (MSM) structures has been investigated. Planar MSM structures have been attracted much attention as viable optical sensor structures. For this purpose, the SCR of such a structure plays a key role in generating photocurrent and thus, in dc (static) and/or low frequency scheme, the wider SCR along the active surface is the better from the efficiency point of view. To study the spreading properties of the SCR along the surface of MSM structures, we prepared planar MSM structures leaving the undepleted region between the electrodes. We examined their SCR spreading through the photocurrent–bias voltage characteristics. The experimental results were compared with the numerical simulation using a quasi-1D (one-dimensional) model of such a planar structure. It was found that the change in the photocurrents is proportional to the square root of the bias applied, implying that the current varies with the lateral spreading of the SCR. Further, the value of the surface spreading of the SCR might be roughly equal to the results predicted from the numerical simulation. These results also suggest that the effective photocurrent generation is occurring principally at the very surface of the SCR.

A mathematical model of flow in a liquid-filled visco-elastic tube.

In biofluid mechanics the fluid-solid interaction is important. To this aim the propagation of waves in a distensible tube filled with a viscous fluid was studied numerically. Based on the assumption of long wavelength and small amplitude of pressure waves, a quasi-1D differential model was adopted. The model accounted for vessel wall visco-elasticity and included the wall deformations in both radial and axial directions. The non-linear problem was solved in non-dimensional form by a finite difference method on a staggered grid. The boundary conditions were for two relevant cases: natural oscillations in a deformable tube fixed at the ends and persistent oscillations due to a periodical forcing pressure. The natural frequency St* was found to vary as the square root of the elasticity coefficient K, with 0 < or = K < or = 6000, and was not affected by the viscosity. These results highlight a strong influence of both wall visco-elasticity and blood viscosity. The natural oscillations are damped in a few time units and the damping time was found to be inversely proportional to the wall viscosity coefficient and the fluid viscosity provided an even larger damping factor.

Metal–insulator transition in the acentric Hubbard ladders at quarter filling: a self-doping mechanism

We examine the acentric Hubbard ladder model at quarter filling, using the mean-field approximation, the weak-coupling theory, the Bethe-ansatz method, and the DMRG technique. This model is the simplest quasi-1D model where the metal–insulator transition is realized by a self-doping mechanism. We clarify the ground-state properties of the model and find that the reentrant insulator–metal–insulator transition occurs with increasing U/ t ∥.

Axial Anomaly Effect in Chiralp-Wave Superconductor

We analyze the chiral p-wave superconductor in the low temperature region. The superconductor has a epsilon_{x} p_{x} + i epsilon_{y} p_{y}-wave gap in two dimensional space (2D). Near the second superconducting transition point, the system could be described by a quasi-1D chiral p-wave model in 2D. The axial anomaly occurs in such a model and causes an accumulation of the quasiparticle in an inhomogeneous magnetic field. The effect is related to the winding number of the gap.

00/01172 World energy prospects to 2020

Increasing energy-demand and environmental concerns are leading to a growing interest in alternative and sustainable energy solutions. Low-grade waste heat recovery through ORC systems is considered to be one of the most feasible eco-friendly energy conversion technologies.In this paper, a comprehensive model of an organic Rankine cycle (ORC) system for micro-CHP applications is presented in order to compare the behavior under part-load conditions of two types of positive displacement expanders, a single-screw expander and a scroll expander. The ORC model is implemented in the AMESim® environment, which allows system modeling both for steady-state and transient analysis. Typical residential electrical and heating loads are considered to study the performances of a suitable small-scale system.By means of a detailed quasi-1D model for a single-screw expander and a Reverse Engineering (RE)-Computational Fluid Dynamics (CFD) approach for a scroll expander, the performance characteristic maps of both expansion machines, in terms of isentropic efficiency as function of expansion ratio and rotational speed, have been determined and implemented in the AMESim® model.A comparison between the selected expanders is presented to address the influence of different positive displacement-type expanders on the overall ORC performances at off-design conditions. The capability of the system to meet the requested loads is also discussed.

NMR evidence for heterogeneous disorder and quasi-1D metallic state in polyaniline CSA

Spin dynamics has been investigated by NMR techniques in highly conducting CSA doped polyaniline (PANI-CSA). Proton nuclear relaxation times have been measured as a function of frequency and temperature. Effective intra and interchain diffusion rates are determined in the frame of a quasi-1D diffusion model. The results are analyzed in the frame of an heterogeneous disorder with metallic regions (crystalline or well-ordered parts), linked by poorly conducting electronically isolated chains (amorphous parts). Spin carriers execute a strongly anisotropic quasi-1D motion in both types of region. Hopping motion in disordered regions appears as the limiting mechanism in the conduction process.

Direct comparison of cross-sectional scanning capacitance microscope dopant profile and vertical secondary ion-mass spectroscopy profile

The scanning capacitance microscope (SCM) has been shown to be useful for quantitative 2D dopant profiling near the surface of silicon. An atomic force microscope is used to position a nanometer scale tip at a silicon surface, and local capacitance change is measured as a function of sample bias. A new feedback method has been recently demonstrated in which the magnitude of the ac bias voltage applied to the sample is adjusted to maintain a constant capacitance change as the tip is scanned across the sample surface. The applied ac bias voltage as a function of position is then input into an inversion algorithm to extract the dopant density profile. The new feedback approach allows for the use of a quasi-1D model in the inversion algorithm. Since there are no alternative 2D dopant profiling techniques which are well established at present, evaluation of the quantitative character of 2D SCM measured profiles has been a challenge. To avoid this obstacle, we have developed sample preparation methods which allow direct comparison of lateral SCM measured profiles on cleaved wafers (cross-sectional plane) with vertical secondary ion-mass spectroscopy (SIMS) profiles. The direct comparison of inverted SCM data and SIMS profiles indicates that quantitative 2D dopant profiling can be achieved by the SCM on a nanometer scale.

Quantitative two-dimensional dopant profile measurement and inverse modeling by scanning capacitance microscopy

Quantitative dopant profile measurements are performed on a nanometer scale by scanning capacitance microscopy (SCM). An atomic force microscope is used to position a nanometer scale tip at a semiconductor surface, and local capacitance change is measured as a function of sample bias. A new feedback method has been demonstrated in which the magnitude of the ac bias voltage applied to the sample is adjusted to maintain a constant capacitance change as the tip is scanned across the sample surface. A quasi-1D model is used to extract dopant density profiles from the SCM measurements. The inverted SCM dopant profiles are compared with profiles obtained by process simulation and secondary ion mass spectroscopy measurement. Good agreement was found between the SCM measured profile and the lateral profile predicted by SUPREM 4 over the concentration range from 1017 to 1020 cm−3.

Non-stationary excitonic-insulator states in photoexcited semiconductors

Time dependent solutions of the semiconductor Bloch equations (SBE) are presented for zero external field which can be identified with a possible dynamics of the so called excitonic-insulator state (EIS) of a semiconductor after the laser pulse has switched off. The collective oscillation of the macroscopic polarization locks into a definite frequency depending on the energy absorbed during the interaction time with the ultra short laser pulse. This dynamical state is different from a stationary EIS proposed previously to be the final state for detunings above the exciton instability and will be demonstrated to appear already for off-resonant excitation. Numerical results are presented for a quasi 1d-model with long-range Coulomb interaction.