Quadratic response theory for spin-orbit coupling in semiconductor heterostructures

This paper examines the properties of the self-energy operator in lattice-matched semiconductor heterostructures, focusing on nonanalytic behavior at small values of the crystal momentum, which gives rise to long-range Coulomb potentials. A nonlinear response theory is developed for nonlocal spin-dependent perturbing potentials. The ionic pseudopotential of the heterostructure is treated as a perturbation of a bulk reference crystal, and the self-energy is derived to second order in the perturbation. If spin-orbit coupling is neglected outside the atomic cores, the problem can be analyzed as if the perturbation were a local spin scalar, since the nonlocal spin-dependent part of the pseudopotential merely renormalizes the results obtained from a local perturbation. The spin-dependent terms in the self-energy therefore fall into two classes: short-range potentials that are analytic in momentum space, and long-range nonanalytic terms that arise from the screened Coulomb potential multiplied by a spin-dependent vertex function. For an insulator at zero temperature, it is shown that the electronic charge induced by a given perturbation is exactly linearly proportional to the charge of the perturbing potential. These results are used in a subsequent paper to develop a first-principles effective-mass theory with generalized Rashba spin-orbit coupling.Comment: 20 pages, no figures, RevTeX4; v2: final published versio

Hyaluronic acid modulates growth, morphology and cytoskeleton in embryonic chick skin fibroblasts

The action that hyaluronic acid (HA) exerts on cell proliferation was investigated in embryonic chick skin fibroblasts at different ages (7-14 days) and in different cell-cycle stages evaluated by flow cytometry (cells maintained with and without serum). Proliferation was estimated by 3H- thymidine incorporation and cell counting. The results demonstrated hyaluronic acid inhibits cell multiplication in all different environmental conditions examined. The inhibitory effect of HA is more evident in 14-day than 7-day old fibroblasts. The ability of HA to modulate 3H-thymidine incorporation did not involve a change in the time required for cells entering the S phase of the replicating cycle, but is due to a smaller number of cells entering in this phase. As the relationships between components of the extracellular matrix (ECM) and the cytoskeleton are known, parallel studies were carried out on some cytoskeleton proteins. Furthermore, by modifying the capacity of cells to adhere to the substrate, HA induced alterations in cell shape and in cytoskeleton components involved in these processes. We may hypothesize that HA, binding specific membrane receptors, affects cell adhesion and morphology inducing less receptivity of fibroblasts to mitogenic stimuli by transmembrane interactions with cytoskeleton

Some Remarks on the Question of Charge Densities in Stationary-Current-Carrying Conductors

Recently, some discussions arose as to the definition of charge and the value of the density of charge in stationary-current-carrying conductors. We stress that the problem of charge definition comes from a misunderstanding of the usual definition. We provide some theoretical elements which suggest that positive and negative charge densities are equal in the frame of the positive ions.Comment: 14 pages, TeX, macro newsym.tex include

Glycosaminoglycan profile in macrophages exposed to Candida albicans and interleukins

Glycosaminoglycans (GAG), are extracellular matrix macromolecules that affect the phagocytic properties of macrophages. In order to assess whether the interaction between macrophages and Candida albicans (iCa) provokes changes in the phenotype, we analyzed the GAG profiles in two macrophage lines, ANA-1 (from murine bone-marrow) and BV-2 (from murine brain). We also investigated GAG modulation by interleukin-1alpha (IL-1alpha) and interleukin-6 (IL-6). During iCa treatment and even after the addition of ILs, ANA-1 accumulated less total GAG compared to controls. IL-1 treatment, combined with iCa exposure, induced a decrease in heparan sulfate and chondroitin sulfate chains, and an increase in the hyaluronic acid percentage. IL-6 treatment, with or without iCa, decreased the hyaluronic acid/sulfated GAG ratio. The GAG pattern in BV-2 appears to be different to ANA-1 and iCa exposure does not induce any difference in total GAG. The inhibitory effect induced by ILs on GAG synthesis is less than that observed in ANA-1 and the GAG elution profile is modulated to a lesser extent by treatment with ILs and/or iCa compared to the ANA-1. We suggest that the observed changes in the expression of the individual GAG classes may be responsible for the macrophage functional heterogeneity

Ab initio studies of phonon softening and high pressure phase transitions of alpha-quartz SiO2

Density functional perturbation theory calculations of alpha-quartz using extended norm conserving pseudopotentials have been used to study the elastic properties and phonon dispersion relations along various high symmetry directions as a function of bulk, uniaxial and non-hydrostatic pressure. The computed equation of state, elastic constants and phonon frequencies are found to be in good agreement with available experimental data. A zone boundary (1/3, 1/3, 0) K-point phonon mode becomes soft for pressures above P=32 GPa. Around the same pressure, studies of the Born stability criteria reveal that the structure is mechanically unstable. The phonon and elastic softening are related to the high pressure phase transitions and amorphization of quartz and these studies suggest that the mean transition pressure is lowered under non-hydrostatic conditions. Application of uniaxial pressure, results in a post-quartz crystalline monoclinic C2 structural transition in the vicinity of the K-point instability. This structure, intermediate between quartz and stishovite has two-thirds of the silicon atoms in octahedral coordination while the remaining silicon atoms remain tetrahedrally coordinated. This novel monoclinic C2 polymorph of silica, which is found to be metastable under ambient conditions, is possibly one of the several competing dense forms of silica containing octahedrally coordinated silicon. The possible role of high pressure ferroelastic phases in causing pressure induced amorphization in silica are discussed.Comment: 17 pages, 8 figs., 8 Table

First-principles envelope-function theory for lattice-matched semiconductor heterostructures

In this paper a multi-band envelope-function Hamiltonian for lattice-matched semiconductor heterostructures is derived from first-principles norm-conserving pseudopotentials. The theory is applicable to isovalent or heterovalent heterostructures with macroscopically neutral interfaces and no spontaneous bulk polarization. The key assumption -- proved in earlier numerical studies -- is that the heterostructure can be treated as a weak perturbation with respect to some periodic reference crystal, with the nonlinear response small in comparison to the linear response. Quadratic response theory is then used in conjunction with k.p perturbation theory to develop a multi-band effective-mass Hamiltonian (for slowly varying envelope functions) in which all interface band-mixing effects are determined by the linear response. To within terms of the same order as the position dependence of the effective mass, the quadratic response contributes only a bulk band offset term and an interface dipole term, both of which are diagonal in the effective-mass Hamiltonian. Long-range multipole Coulomb fields arise in quantum wires or dots, but have no qualitative effect in two-dimensional systems beyond a dipole contribution to the band offsets.Comment: 25 pages, no figures, RevTeX4; v3: final published versio

Lung regions differently modulate bronchial branching development and extracellular matrix plays a role in regulating the development of chick embryo whole lung.

Normal branching development is dependent on the correlation between cells and extracellular matrix. In this interaction glycosaminoglycans, cytokines and growth factors play a fundamental role. In order to verify the distribution and influence of extracellular matrix and related enzymes on chick embryo lung development, 6 day-old whole lungs were maintained in vitro with testicular hyaluronidase, beta-N-acetyl-D-glucosaminidase and chondrotinase ABC or in linkage with apical, medial and caudal lung regions of 6-day development before and after enzyme treatment. In a separate lung region beta-N-acetyl-D-glucosaminidase and hyaluronidase were determined. Our data show that the whole lung cultures increase bronchial branching development when the medial region is admixed separately, while the separate apical or caudal regions or apical combined with caudal region do not affect bronchial branching development. The enzyme treatment of medial region prevents the branching development in associated whole lung. The bronchial branching development of whole lung cultured in medium containing the enzymes related to glycosaminoglycans turnover is significantly altered. In conclusion, these data show that the different influence of separate apical, medial, caudal lung regions on bronchial branching development is related to the extracellular matrix composition

Hole dynamics in a quantum antiferromagnet beyond the retraceable path approximation

The one-hole spectral weight for two chains and two dimensional lattices is studied numerically using a new method of analysis of the spectral function within the Lanczos iteration scheme: the Lanczos spectra decoding method. This technique is applied to the $t-J_z$ model for $J_z \to 0$, directly in the infinite size lattice. By a careful investigation of the first 13 Lanczos steps and the first 26 ones for the two dimensional and the two chain cases respectively, we get several new features of the one-hole spectral weight. A sharp incoherent peak with a clear momentum dispersion is identified, together with a second broad peak at higher energy. The spectral weight is finite up to the Nagaoka energy where it vanishes in a non-analytic way. Thus the lowest energy of one hole in a quantum antiferromagnet is degenerate with the Nagaoka energy in the thermodynamic limit.Comment: RevTeX 3.0, SISSA preprint 156/93/CM/MB, 10 pages + postscript file appended, contains more accurate calculations in Fig.

Enhancement of piezoelectricity in a mixed ferroelectric

We use first-principles density-functional total energy and polarization calculations to calculate the piezoelectric tensor at zero temperature for both cubic and simple tetragonal ordered supercells of Pb_3GeTe_4. The largest piezoelectric coefficient for the tetragonal configuration is enhanced by a factor of about three with respect to that of the cubic configuration. This can be attributed to both the larger strain-induced motion of cations relative to anions and higher Born effective charges in the tetragonal case. A normal mode decomposition shows that both cation ordering and local relaxation weaken the ferroelectric instability, enhancing piezoelectricity.Comment: 5 pages, revtex, 2 eps figure

O(N) methods in electronic structure calculations

Linear scaling methods, or O(N) methods, have computational and memory requirements which scale linearly with the number of atoms in the system, N, in contrast to standard approaches which scale with the cube of the number of atoms. These methods, which rely on the short-ranged nature of electronic structure, will allow accurate, ab initio simulations of systems of unprecedented size. The theory behind the locality of electronic structure is described and related to physical properties of systems to be modelled, along with a survey of recent developments in real-space methods which are important for efficient use of high performance computers. The linear scaling methods proposed to date can be divided into seven different areas, and the applicability, efficiency and advantages of the methods proposed in these areas is then discussed. The applications of linear scaling methods, as well as the implementations available as computer programs, are considered. Finally, the prospects for and the challenges facing linear scaling methods are discussed.Comment: 85 pages, 15 figures, 488 references. Resubmitted to Rep. Prog. Phys (small changes