Nonlinear screening of charged macromolecules

We present several aspects of the screening of charged macromolecules in an electrolyte. After a review of the basic mean field approach, based on the linear Debye-Huckel theory, we consider the case of highly charged macromolecules, where the linear approximation breaks down and the system is described by full nonlinear Poisson-Boltzmann equation. Some analytical results for this nonlinear equation give some interesting insight on physical phenomena like the charge renormalization and the Manning counterion condensation

Stabilization of charge ordering in La_(1/3)Sr_(2/3)FeO_(3-d) by magnetic exchange

The magnetic exchange energies in charge ordered La_(1/3)Sr_(2/3)FeO_(3-d) (LSFO) and its parent compound LaFeO_(3) (LFO) have been determined by inelastic neutron scattering. In LSFO, the measured ratio of ferromagnetic exchange between Fe3+ - Fe5+ pairs (J_F) and antiferromagnetic exchange between Fe3+ - Fe3+ pairs (J_AF) fulfills the criterion for charge ordering driven by magnetic interactions (|J_F/J_AF| > 1). The 30% reduction of J_AF as compared to LFO indicates that doped holes are delocalized, and charge ordering occurs without a dominant influence from Coulomb interactions.Comment: 18 pages, 4 color figure

Electroneutrality and Phase Behavior of Colloidal Suspensions

Several statistical mechanical theories predict that colloidal suspensions of highly charged macroions and monovalent microions can exhibit unusual thermodynamic phase behavior when strongly deionized. Density-functional, extended Debye-H\"uckel, and response theories, within mean-field and linearization approximations, predict a spinodal phase instability of charged colloids below a critical salt concentration. Poisson-Boltzmann cell model studies of suspensions in Donnan equilibrium with a salt reservoir demonstrate that effective interactions and osmotic pressures predicted by such theories can be sensitive to the choice of reference system, e.g., whether the microion density profiles are expanded about the average potential of the suspension or about the reservoir potential. By unifying Poisson-Boltzmann and response theories within a common perturbative framework, it is shown here that the choice of reference system is dictated by the constraint of global electroneutrality. On this basis, bulk suspensions are best modeled by density-dependent effective interactions derived from a closed reference system in which the counterions are confined to the same volume as the macroions. Linearized theories then predict bulk phase separation of deionized suspensions only when expanded about a physically consistent (closed) reference system. Lower-dimensional systems (e.g., monolayers, small clusters), depending on the strength of macroion-counterion correlations, may be governed instead by density-independent effective interactions tied to an open reference system with counterions dispersed throughout the reservoir, possibly explaining observed structural crossover in colloidal monolayers and anomalous metastability of colloidal crystallites.Comment: 12 pages, 5 figures. Discussion clarified, references adde

Charge and orbital order in Fe_3O_4

Charge and orbital ordering in the low-temperature monoclinic structure of magnetite (Fe_3O_4) is investigated using LSDA+U. While the difference between t_{2g} minority occupancies of Fe^{2+}_B and Fe^{3+}_B cations is large and gives direct evidence for charge ordering, the screening is so effective that the total 3d charge disproportion is rather small. The charge order has a pronounced [001] modulation, which is incompatible with the Anderson criterion. The orbital order agrees with the Kugel-Khomskii theory.Comment: 4 pages, 2 figure

Long range polarization attraction between two different likely charged macroions

It is known that in a water solution with multivalent counterions (Z-ions), two likely charged macroions can attract each other due to correlations of Z-ions adsorbed on their surfaces. This "correlation" attraction is short-ranged and decays exponentially with increasing distance between macroions at characteristic distance A/2\pi, where A is the average distance between Z-ions on the surfaces of macroions. In this work, we show that an additional long range "polarization" attraction exists when the bare surface charge densities of the two macroions have the same sign, but are different in absolute values. The key idea is that with adsorbed Z-ions, two insulating macroions can be considered as conductors with fixed but different electric potentials. Each potential is determined by the difference between the entropic bulk chemical potential of a Z-ion and its correlation chemical potential at the surface of the macroion determined by its bare surface charge density. When the two macroions are close enough, they get polarized in such a way that their adjacent spots form a charged capacitor, which leads to attraction. In a salt free solution this polarization attractive force is long ranged: it decays as a power of the distance between the surfaces of two macroions, d. The polarization force decays slower than the van der Waals attraction and therefore is much larger than it in a large range of distances. In the presence of large amount of monovalent salt, when A/2\pi<< d<< r_s (r_s is the Debye-H\"{u}ckel screening radius), this force is still much stronger than the van der Waals attraction and the correlation attraction mentioned above.Comment: 12 pages, 7 figures. Small change in the text, no change in result

The Trainer Project: A New Simulator-Based Driver Training Curriculum

The purpose of the EU funded TRAINER project is to develop a new cost-effective Pan-European driver training curriculum, includingcomputer-based interactive multimedia and simulator technology. Thecurriculum will pay significant attention to higher order skills including riskawareness. For this purpose a number of scenarios were developed thataddresses the most important needs of learner drivers. These scenarios are usedin a PC-based interactive multimedia tool as well as in a driving simulator. Theinteractive multimedia tool allows training and assessment of higher cognitiveskills (i.e., strategic and manoeuvring tasks), familiarisation of novice driverswith the basic principles of driving, and contributing to a better understandingof (potential) risks. A low cost stationary driving simulator is used for acquiringskills in vehicle handling and negotiating common traffic situations (i.e.,manoeuvring and control tasks). In addition, a mean cost semi-dynamic drivingsimulator is developed for supporting the needs of specific driver cohorts, suchas novice drivers with enhanced knowledge problems and drivers in high-riskgroups. Application of such an advanced computer-based curriculum alsoimplies development of criteria to allow driving instructors to determinetraining progress. These criteria are based on a database of normative driverbehaviour. This paper mainly focuses on the description of the technical (softandhardware) requirements for both low-cost and mean-cost simulators

The mobility of Na+, Li+, and K+ ions in thermally grown SiO2 films

Na+, Li+, and K+ ions have been implanted in thermally grown oxides (Tox =1000 °C) of metal‐oxide‐silicon structures. The mobilities of the Na+ and Li+ ions have been determined by means of the isothermal transient ionic current method in the temperature range 100–300 °C. The K+ mobility has been obtained by means of triangular voltage sweep measurements in the range 350–450 °C. The results show that the Na+ and the Li+ mobilities are not significantly different. The activation energies of the three measured mobilities are shown to agree with a quantitative model which has been developed by Anderson and Stuart [J. Am. Ceram. Soc. 37, 573 (1954)]

Vanishing Fe 3d orbital moments in single-crystalline magnetite

We show detailed magnetic absorption spectroscopy results of an in situ cleaved high quality single crystal of magnetite. In addition the experimental setup was carefully optimized to reduce drift, self absorption, and offset phenomena as far as possible. In strong contradiction to recently published data, our observed orbital moments are nearly vanishing and the spin moments are quite close to the integer values proposed by theory. This very important issue supports the half metallic full spin polarized picture of magnetite.Comment: 7 pages, 4 figure

Uniaxial and biaxial soft deformations of nematic elastomers

We give a geometric interpretation of the soft elastic deformation modes of nematic elastomers, with explicit examples, for both uniaxial and biaxial nematic order. We show the importance of body rotations in this non-classical elasticity and how the invariance under rotations of the reference and target states gives soft elasticity (the Golubovic and Lubensky theorem). The role of rotations makes the Polar Decomposition Theorem vital for decomposing general deformations into body rotations and symmetric strains. The role of the square roots of tensors is discussed in this context and that of finding explicit forms for soft deformations (the approach of Olmsted).Comment: 10 pages, 10 figures, RevTex, AmsTe