219 research outputs found
Screened Interaction and Self-Energy in an Infinitesimally Polarized Electron Gas via the Kukkonen-Overhauser Method
The screened electron-electron interaction and the
electron self-energy in an infinitesimally polarized electron gas are derived
by extending the approach of Kukkonen and Overhauser. Various quantities in the
expression for are identified in terms of the relevant
response functions of the electron gas. The self-energy is obtained from
by making use of the GW method which in this case
represents a consistent approximation. Contact with previous calculations is
made.Comment: 7 page
Applicability valuation for evaluation of surface deflection in automotive outer panels
Upon unloading in a forming process there is elastic recovery, which is the release of the elastic strains and the redistribution of the residual stresses through the thickness direction, thus producing surface deflection. It causes changes in shape and dimensions that can create major problem in the external appearance of outer panels. Thus surface deflection prediction is an important issue in sheet metal forming industry. Many factors could affect surface deflection in the process, such as material variations in mechanical properties, sheet thickness, tool geometry, processing parameters and lubricant condition. The shape and dimension problem in press forming is defined as a trouble mainly caused by the elastic recovery of materials during the forming. The use of high strength steel sheets in the manufacturing of automobile outer panels has increased in the automotive industry over the years because of its lightweight and fuel-efficient improvement. But one of the major concerns of stamping is surface deflection in the formed outer panels. Hence, to be cost effective, accurate prediction must be made of its formability. The automotive industry places rigi
Characterization of the Atherosclerotic Plaque Tissue
Cardiovascular diseases (CVD) are the leading causes of morbidity and mortality globally. Atherosclerosis is a chronic inflammatory CVD associated with the accumulation of plaque activated by the complex interactions between systemic, hemodynamic and biological factors. Thus, identification of plaque vulnerability is essential for the prevention of acute events and treatment of the disease. Despite, advanced imaging technologies, patient-specific computational simulations and availability of experimental data, there are still challenges in developing accurate risk stratification techniques. Therefore, this study aims to characterize the carotid plaque components structurally (histological analysis and immunostaining), mechanically (Nanoindentation tests) and chemically (Fourier Transform Infrared (FT-IR) micro-spectroscopy). The preliminary results showed that arterial remodelling is a dynamic interaction between mechanical forces and plaque progression. The biological content and composition of human atherosclerotic plaque tissue have been shown to significantly influence the mechanical response of samples. This data represents a step towards an enhanced understanding of the behaviour of human atherosclerotic plaque. Future large-scale experimental studies with more cross-sections along the length of the plaque could be used to develop a risk stratification technique
Many-Polaron Effects in the Holstein Model
We derive an effective polaronic interaction Hamiltonian, {\it exact to
second order in perturbation}, for the spinless one-dimensional Holstein model.
The small parameter is given by the ratio of the hopping term () to the
polaronic energy () in all the region of validity for our
perturbation; however, the exception being the regime of extreme
anti-adiabaticity () and small electron-phonon coupling () where the small parameter is . We map our polaronic
Hamiltonian onto a next-to-nearest-neighbor interaction anisotropic Heisenberg
spin model. By studying the mass gap and the power-law exponent of the
spin-spin correlation function for our Heisenberg spin model, we analyze the
Luttinger liquid to charge-density-wave transition at half-filling in the
effective polaronic Hamiltonian. We calculate the structure factor at all
fillings and find that the spin-spin correlation length decreases as one
deviates from half-filling. We also extend our derivation of polaronic
Hamiltonian to -dimensions.Comment: Content changed. Accepted in Phys. Rev.
Integrated nonlinear structural simulation of composite buildings in fire
The collapse of several tall composite buildings over the last two decades has shown that the performance of tall, composite and complex buildings in fire is a necessary design consideration that ought to go beyond simple code compliance. To this end, several advancements in the field of numerical simulation of both the fire and the thermomechanical response of structures have been made. In isolation, the practical benefit of these advancements is limited, and their true potential is only unlocked when the results of those numerical simulations are integrated. This paper starts by showcasing recent developments in the thermal and thermomechanical analysis of structures using OpenSees. Integration of these developments into a unified simulation environment combining fire simulation, heat transfer, and mechanical analysis is then introduced. Finally, a demonstration example based on the large compartment Cardington test is used to showcase the necessity and efficiency of the developed simulation environment for thermomechanical simulation of composite structures in fire
Phase transition and phase diagram at a general filling in the spinless one-dimensional Holstein Model
Among the mechanisms for lattice structural deformation, the electron-phonon
interaction mediated Peierls charge-density-wave (CDW) instability in single
band low-dimensional systems is perhaps the most ubiquitous. The standard
mean-field picture predicts that the CDW transition occurs at all fillings and
all values of the electron-phonon coupling and the adiabaticity parameter
. Here, we correct the mean-field expression for the Peierls
instability condition by showing that the non-interacting static
susceptibility, at twice the Fermi momentum, should be replaced by the dynamic
one. We derive the Luttinger liquid (LL) to CDW transition condition, {\it
exact to second order in a novel blocked perturbative approach}, for the
spinless one-dimensional Holstein model in the adiabatic regime. The small
parameter is the ratio . We present the phase diagram at
non-half-filling by obtaining the surprising result that the CDW occurs in a
more restrictive region of a two parameter ( and )
space than at half-filling.Comment: Made changes in the appendices and also in notatio
Jahn-Teller polarons and their superconductivity in a molecular conductor
We present a theoretical study of a possibility of superconductivity in a
three dimensional molecular conductor in which the interaction between
electrons in doubly degenerate molecular orbitals and an {\em intra}molecular
vibration mode is large enough to lead to the formation of
Jahn-Teller small polarons. We argue that the effective polaron-polaron
interaction can be attractive for material parameters realizable in molecular
conductors. This interaction is the source of superconductivity in our model.
On analyzing superconducting instability in the weak and strong coupling
regimes of this attractive interaction, we find that superconducting transition
temperatures up to 100 K are achievable in molecular conductors within this
mechanism. We also find, for two particles per molecular site, a novel Mott
insulating state in which a polaron singlet occupies one of the doubly
degenerate orbitals on each site. Relevance of this study in the search for new
molecular superconductors is pointed out.Comment: Submitted to Phys. Rev.
Peierls to superfluid crossover in the one-dimensional, quarter-filled Holstein model
We use continuous-time quantum Monte Carlo simulations to study retardation
effects in the metallic, quarter-filled Holstein model in one dimension. Based
on results which include the one- and two-particle spectral functions as well
as the optical conductivity, we conclude that with increasing phonon frequency
the ground state evolves from one with dominant diagonal order---2k_F charge
correlations---to one with dominant off-diagonal fluctuations, namely s-wave
pairing correlations. In the parameter range of this crossover, our numerical
results support the existence of a spin gap for all phonon frequencies. The
crossover can hence be interpreted in terms of preformed pairs corresponding to
bipolarons, which are essentially localised in the Peierls phase, and
"condense" with increasing phonon frequency to generate dominant pairing
correlations.Comment: 11 pages, 5 figure
Interaction-Induced Enhancement of Spin-Orbit Coupling in Two-Dimensional Electronic System
We study theoretically the renormalization of the spin-orbit coupling
constant of two-dimensional electrons by electron-electron interactions. We
demonstrate that, similarly to the factor, the renormalization corresponds
to the enhancement, although the magnitude of the enhancement is weaker than
that for the factor. For high electron concentrations (small interaction
parameter ) the enhancement factor is evaluated analytically within the
static random phase approximation. For large we use an approximate
expression for effective electron-electron interaction, which takes into
account the local field factor, and calculate the enhancement numerically. We
also study the interplay between the interaction-enhanced Zeeman splitting and
interaction-enhanced spin-orbit coupling.Comment: 18 pages, 2 figures, REVTe
Correlated singlet phase in the one-dimensional Hubbard-Holstein model
We show that a nearest-neighbor singlet phase results (from an effective
Hamiltonian) for the one-dimensional Hubbard-Holstein model in the regime of
strong electron-electron and electron-phonon interactions and under
non-adiabatic conditions (). By mapping the system of
nearest-neighbor singlets at a filling onto a hard-core-boson (HCB)
- model at a filling , we demonstrate explicitly that
superfluidity and charge-density-wave (CDW) occur mutually exclusively with the
diagonal long range order manifesting itself only at one-third filling.
Furthermore, we also show that the Bose-Einstein condensate (BEC) occupation
number for the singlet phase, similar to the for a HCB tight
binding model, scales as ; however, the coefficient of in
the for the interacting singlet phase is numerically demonstrated to be
smaller.Comment: Corrected a few reference
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