The directional contact distance of two ellipsoids: coarse-grained potentials for anisotropic interactions

We obtain the distance of closest approach of the surfaces of two arbitrary ellipsoids valid at any orientation and separation measured along their intercenter vector. This directional distance is derived from the elliptic contact function. The geometric meaning behind this approach is clarified. An elliptic pair potential for modeling arbitrary mixtures of elliptic particles, whether hard or soft, is proposed based on this distance. Comparisons with Gay-Berne potentials are discussed. Analytic expressions for the forces and torques acting on the elliptic particles are given.     

Ferrite-induced breakdown. Mechanism of induction by thermal surface explosion

The conditions are investigated for gas breakdown above an Ni–Zn (M4OONN) ferrite surface. The assumption that the discharge is initiated by a thermal explosion of a surface layer of the ferrite modified by current flowing through it is verified experimentally and theoretically. The electric and thermophysical properties of the (ferrite+annealed layer) system are determined. The experimentally observed temporal characteristics of the prebreakdown phase of the discharge and the dependences of the breakdown voltage on the discharge-circuit parameters are described within the framework of a single model.Translated from FIAN Preprint No. 120, Lebedev Physics Institute, Moscow, 1990.     

Vibrationally state-selective photoassociation by infrared sub-picosecond laserr pulses: model simulations for O + H → OH(ν)

The quantum dynamics of a photoassociation reaction in the electronic ground state controlled by an infrared picosecond laser pulse is investigated. The association reaction O + H → OH(ν) is simulated by representative wavepackets. The OH molecule to be formed is modeled as a non-rotating Morse oscillator. It is shown that the initial free continuum state of O + H can be transferred selectively into a specified vibrational bound state by interaction with an infrared laser pulse. Optimal design of the laser control field leads to high association probability with very high vibrational state selectivity.     

Kinetics of Radiation-Chemical Reactions in a Nonuniform Colloid Particle

The decomposition of ionol (2,6-di-tert-butyl-4-methylphenol) by the action of -radiation in nitrous oxide-saturated aqueous solutions of lecithin liposomes was studied. The ionol located in the nonpolar liposome layer was shown to be consumed upon irradiation, the consumption yield growing with increasing ionol concentration. This finding indicates that some portion of OH radicals penetrate across the polar to the nonpolar layer. The ratio of rate constants for reactions of OH radicals with the hydrocarbon moiety of long-chain fatty acids containing 17 carbon atoms and ionol was found to be 0.11 ± 0.03 as determined by the steady-state treatment. An equation for the flux of radicals passing through the reactive membrane (polar liposome layer) from the bulk of dispersion medium was obtained using the diffusion kinetic model. This equation describes the experimental data with the ratio of the diffusion coefficient of OH radical in the polar zone to that in the nonpolar zone equal to 6.5 ± 2.5.     

Self-assembly of multidomain peptides: balancing molecular frustration controls conformation and nanostructure

A series of nine, frustrated, multidomain peptides is described in which forces favoring self-assembly into a nanofiber versus those favoring disassembly could be easily modified. The peptides are organized into an ABA block motif in which the central B block is composed of alternating hydrophilic and hydrophobic amino acids (glutamine and leucine, respectively). This alternation allows the amino acid side chains to segregate on opposite sides of the peptide backbone when it is in a fully extended beta-sheet conformation. In water, packing between two such peptides stabilizes the extended conformation by satisfying the desire of the leucine side chains to exclude themselves from the aqueous environment. Once in this conformation intermolecular backbone hydrogen bonding can readily take place between additional peptides eventually growing into high aspect ratio fibers. B block assembly may continue infinitely or until monomeric peptides are depleted from solution which results in an insoluble precipitate. Block A consists of a variable number of positively charged lysine residues whose electrostatic repulsion at pH 7 works against the desire of the B block to assemble. Here we show that balancing the forces of block A against B allows the formation of controlled length, individually dispersed, and fully soluble nanofibers with a width of 6 +/- 1 nm and length of 120 +/- 30 nm. Analysis by infrared, circular dichroism, and vitreous ice cryo-transmission electron microscopy reveals that the relative sizes of blocks A and B dictate the peptide secondary structure which in turn controls the resulting nanostructure. The system described epitomizes the use of molecular frustration in the design of finite self-assembled structures. These materials, and ones based on their architecture, may find applications where nanostructured control over fiber architecture and chemical functionality is required.     

The sub-micrometer thickness n-InSb/i-GaAs epilayers for magnetoresistor applications at room temperatures of operation

Magnetotransport at fields up to 500 mT and LF-noise characteristics are reported for miniature magnetoresistors with ferrite concentrators based on Sn-doped n-InSb/i-GaAs heterostructures grown by MBE. The thickness of the InSb epilayers lie in the range 0.55–1.5 μm giving room temperature mobilities of 2.5–5.5 m² V⁻¹ s⁻¹ with carrier densities of (0.5–1.5)×10¹⁷ cm⁻³. The room temperature magnetoresistance (MR) for our two terminal devices could be as high as 115% at 50 mT which is comparable to the extraordinary MR (ExMR) recently reported in microscopic composite van der Pauw disks four terminal devices [Science 289 (2000) 1530]. In addition, a high signal-to-noise ratio and a good temperature stability of R(B)/R₀=0.5–0.83% K⁻¹ was observed for B<60 mT (below the saturation field B_sat for ferrite). Device resistance stability R₀(T) was equal to 0.27–0.66% K⁻¹ in zero field with a nominal device resistance R₀=197–224 Ω for DC currents in the range I=0.01–1.0 mA. The minimum detectable magnetic field is estimated from the reduced differential MR (∂R/∂B)/R=2000% T⁻¹ at B=31 mT and normalised 1/f current noise power spectral density measured at the same field. The resolution limit B_min=2.6 nT at 10² Hz and B_min=0.82 nT at 10³ Hz. These resolution limits are seven times better than those recently reported for the same material n-InSb/i-GaAs and ferrite fabricated Hall sensors [Magnetotransport and Raman characterization of n-InSb/i-GaAs epilayers, for Hall sensors applications over extremely wide ranges of temperature and magnetic field, Proceedings NGS 10, IPAP Conference Series 2, IPAP, Tokyo, 2001, pp. 151–154].