Ground-state phase diagram of a half-filled one-dimensional extended Hubbard model

The density-matrix renormalization group is used to study the phase diagram of the one-dimensional half-filled Hubbard model with on-site (U) and nearest-neighbor (V) repulsion, and hopping t. A critical line V_c(U) approximately equal to U/2 separates a Mott insulating phase from a charge-density-wave phase. The formation of bound charge excitations for V > 2t changes the phase transition from continuous to first order at a tricritical point U_t = 3.7t, V_t=2t. A frustrating effective antiferromagnetic spin coupling induces a bond-order-wave phase on the critical line V_c(U) for U_t < U < 7-8 t.Comment: 4 pages (REVTEX 4), 3 EPS figures, shorter abstract, text and references modifie

Teaching Design for Environment in Product Design Classes

The paper presents an approach to teaching design for environment (DFE) in the context of a product design and development course. The teaching method has been applied in our classes for graduate engineering, business, and design students. Our approach includes a step-by-step DFE process and utilizes a recent Herman Miller chair as a case study to illustrate the successful application of each step in the process. The DFE process steps are based on our research at Herman Miller and on several published studies that investigated the integration of DFE into the product development process and which we assembled into the step-by-step DFE process. Furthermore, the teaching method includes a new approach to life cycle thinking by relating the product life cycle to the natural life cycle in order to from a closed-loop system

Comment on "Density Functional Simulation of a Breaking Nanowire"

In a recent Letter, Nakamura et al. [Phys. Rev. Lett. 82, 1538 (1999)] described first principles calculations for a breaking Na nanocontact. Their system consists of a periodic one-dimensional array of supercells, each of which contains 39 Na atoms, originally forming a straight, crystalline wire with a length of 6 atoms. The system is elongated by increasing the length of the unit cell. At each step, the atomic configuration is relaxed to a new local equilibrium, and the tensile force is evaluated from the change of the total energy with elongation. Aside from a discontinuity of the force occuring at the transition from a crytalline to an amorphous configuration during the early stages of elongation, they were unable to identify any simple correlations between the force and the number of electronic modes transmitted through the contact. An important question is whether their model is realistic, i.e., whether it can be compared to experimental results obtained for a single nanocontact between two macroscopic pieces of metal. In this Comment, we demonstrate that with such a small unit cell, the interference effects between neighboring contacts are of the same size as the force oscillations in a single nanocontact.Comment: 1 pag

The Impact of Flow on the Nuclear Translocation of NF-kB

In this dissertation, the impact of flow on the nuclear translocation of NF-kB in vascular endothelial cells was investigated. NF-kB is a key promoter of inflammatory responses and its mis-regulation is related to the development of vascular diseases. The aim was to establish a link between hemodynamic forces and NF-kB to gain insight in cardiovascular disease mechanisms such as aneurysms. Human umbilical vein endothelial cells (HUVEC) were transfected with two plasmids: H2B-mCherry and GFP-RelA. The nuclear translocation of NF-kB within the transfected primary cells was verified with TNF-a stimulation and compared to immunohistochemistry of TNF-a stimulated non-transfected cells. In the first part of the thesis, transfected HUVECs were exposed to different flow environments, including uniform low shear stress, uniform high shear stress and a shear stress gradient, and imaged live for 6 hours. Computer vision techniques were applied to track each individual cell and each nuclear NF-kB concentration was evaluated as a function of time. In each experiment, more than 1000 single cells were tracked and analysed. TNF-a stimulation caused a synchronised population response with a nuclear NF-kB peak at 30 minutes. The population mean of cells under static conditions remained constant, while spontaneous nuclear translocation of NF-kB in individual cells was observed. Uniform low shear stress stimulation increased translocating activity after 5 hours of flow. Alternatively, uniform high shear stress promoted increased nuclear translocation, directly after onset of flow and after 5 hours. Small differences of nuclear translocation of NF-kB at different shear stress magnitudes within a shear stress gradient were observed. The percentage of cells experiencing early nuclear translocation increased with increased shear stress. It is believed that high shear stress induces nuclear translocation early, while for low shear stresses, responses are delayed. In the second part of the thesis, a numerical model was developed to predict cell population responses of the NF-kB pathway. The model is deterministic but includes extrinsic noise to mimic stimulus dependent cell-to-cell variability. Population responses to different TNF-a concentrations were predicted in close agreement with live-cell measurements. The model was extended with a shear dependent activation module to predict small variabilities observed in nuclear translocation of NF-kB under different shear conditions. The close agreement between nuclear translocation of NF-kB in a shear stress gradient and the measurements allowed prediction of inflammatory responses in different flow environments such as a backward facing step channel. This work provides a first insight in the temporal dynamics of nuclear translocation of NF-kB in a large population of endothelial cells exposed to different flow environments. Although the effects were much weaker than with TNF-a stimulation, differences between static and flow conditions were observed, which indicate that hemodynamic forces affect intracellular signalling

Many-body theory of electronic transport in single-molecule heterojunctions

A many-body theory of molecular junction transport based on nonequilibrium Green's functions is developed, which treats coherent quantum effects and Coulomb interactions on an equal footing. The central quantity of the many-body theory is the Coulomb self-energy matrix $\Sigma_{\rm C}$ of the junction. $\Sigma_{\rm C}$ is evaluated exactly in the sequential tunneling limit, and the correction due to finite tunneling width is evaluated self-consistently using a conserving approximation based on diagrammatic perturbation theory on the Keldysh contour. Our approach reproduces the key features of both the Coulomb blockade and coherent transport regimes simultaneously in a single unified transport theory. As a first application of our theory, we have calculated the thermoelectric power and differential conductance spectrum of a benzenedithiol-gold junction using a semi-empirical $\pi$-electron Hamiltonian that accurately describes the full spectrum of electronic excitations of the molecule up to 8--10eV.Comment: 13 pages, 7 figure

A simplified picture for Pi electrons in conjugated polymers : from PPP Hamiltonian to an effective molecular crystal approach

An excitonic method proper to study conjugated oligomers and polymers is described and its applicability tested on the ground state and first excited states of trans-polyacetylene, taken as a model. From the Pariser-Parr-Pople Hamiltonian, we derive an effective Hamiltonian based on a local description of the polymer in term of monomers; the relevant electronic configurations are build on a small number of pertinent local excitations. The intuitive and simple microscopic physical picture given by our model supplement recent results, such as the Rice and Garstein ones. Depending of the parameters, the linear absorption appears dominated by an intense excitonic peak.Comment: 41 Pages, 6 postscript figure

Pressure-induced phase transitions of halogen-bridged binuclear metal complexes R_4[Pt_2(P_2O_5H_2)_4X]nH_2O

Recent contrasting observations for halogen (X)-bridged binuclear platinum complexes R_4[Pt_2(P_2O_5H_2)_4X]nH_2O, that is, pressure-induced Peierls and reverse Peierls instabilities, are explained by finite-temperature Hartree-Fock calculations. It is demonstrated that increasing pressure transforms the initial charge-polarization state into a charge-density-wave state at high temperatures, whereas the charge-density-wave state oppositely declines with increasing pressure at low temperatures. We further predict that higher-pressure experiments should reveal successive phase transitions around room temperature.Comment: 5 pages, 4 figures embedded, to be published in Phys. Rev. B 64, September 1 (2001) Rapid Commu

Electronic Excitations and Insulator-Metal Transition in Poly(3-hexylthiophene) Organic Field-Effect Transistors

We carry out a comprehensive theoretical and experimental study of charge injection in Poly(3-hexylthiophene) (P3HT) to determine the most likely scenario for metal-insulator transition in this system. We calculate the optical absorption frequencies corresponding to a polaron and a bipolaron lattice in P3HT. We also analyze the electronic excitations for three possible scenarios under which a first-- or a second--order metal--insulator transition can occur in doped P3HT. These theoretical scenarios are compared with data from infrared absorption spectroscopy on P3HT thin film field-effect transistors (FET). Our measurements and theoretical predictions suggest that charge-induced localized states in P3HT FETs are bipolarons and that the highest doping level achieved in our experiments approaches that required for a first-order metal--insulator transition.Comment: 9 pages, 4 figures. Phys. Rev. B, in pres

Quantum global vortex strings in a background field

We consider quantum global vortex string correlation functions, within the Kalb-Ramond framework, in the presence of a background field-strength tensor and investigate the conditions under which this yields a nontrivial contribution to those correlation functions. We show that a background field must be supplemented to the Kalb-Ramond theory, in order to correctly describe the quantum properties of the vortex strings. The explicit form of this background field and the associated quantum vortex string correlation function are derived. The complete expression for the quantum vortex creation operator is explicitly obtained. We discuss the potential applicability of our results in the physics of superfluids and rotating Bose-Einstein condensates.Comment: To appear in Journal of Physics A: Mathematical and Genera