1,144 research outputs found

    Nonsteady end effects in Hele-Shaw cells

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    Origin of charge density at LaAlO3-on-SrTiO3 hetero-interfaces; possibility of intrinsic doping

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    As discovered by Ohtomo et al., a large sheet charge density with high mobility exists at the interface between SrTiO3 and LaAlO3. Based on transport, spectroscopic and oxygen-annealing experiments, we conclude that extrinsic defects in the form of oxygen vacancies introduced by the pulsed laser deposition process used by all researchers to date to make these samples is the source of the large carrier densities. Annealing experiments show a limiting carrier density. We also present a model that explains the high mobility based on carrier redistribution due to an increased dielectric constant.Comment: 14 pages, 3 figures, 1 table; accepted for publication in Phys. Rev. Lett

    Location of sugars in multilamellar membranes at low hydration

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    Severe dehydration is lethal for most biological species. However, there are a number of organisms which have evolved mechanisms to avoid damage during dehydration. One of these mechanisms is the accumulation of small solutes (e.g. sugars), which have been shown to preserve membranes by inhibiting deleterious phase changes at low hydration. Specifically, sugars reduce the gel to fluid phase transition temperatures of model lipid/water mixtures. However, there is a debate about the precise mechanism, the resolution of which hinges on the location of the sugars. In excess water, it has been observed using contrast variation SANS that the sugar concentration in the excess phase is higher than in the interlamellar region [Deme and Zemb, J. Appl. Crystallog. 33 (2000) 569]. This raises two questions regarding the location of the sugars at low hydrations: first, does the system phase separate to give a sugar/water phase in equilibrium with a lipid/water/sugar lamellar region (with different sugar concentrations); and second, is the sugar in the interlamellar region uniformly distributed, or does it concentrate preferentially either in close proximity to the lipids, or towards the center of the interbilayer region. In this paper we present the preliminary results of measurements using contrast variation SANS to determine the location of sugars in lipid/water mixtures

    Invariant expansion for the trigonal band structure of graphene

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    We present a symmetry analysis of the trigonal band structure in graphene, elucidating the transformational properties of the underlying basis functions and the crucial role of time-reversal invariance. Group theory is used to derive an invariant expansion of the Hamiltonian for electron states near the K points of the graphene Brillouin zone. Besides yielding the characteristic k-linear dispersion and higher-order corrections to it, this approach enables the systematic incorporation of all terms arising from external electric and magnetic fields, strain, and spin-orbit coupling up to any desired order. Several new contributions are found, in addition to reproducing results obtained previously within tight-binding calculations. Physical ramifications of these new terms are discussed.Comment: 10 pages, 1 figure; expanded version with more details and additional result

    Optical-Model Description of Time-Reversal Violation

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    A time-reversal-violating spin-correlation coefficient in the total cross section for polarized neutrons incident on a tensor rank-2 polarized target is calculated by assuming a time-reversal-noninvariant, parity-conserving ``five-fold" interaction in the neutron-nucleus optical potential. Results are presented for the system n+165Hon + {^{165}{\rm Ho}} for neutron incident energies covering the range 1--20 MeV. From existing experimental bounds, a strength of 2±102 \pm 10 keV is deduced for the real and imaginary parts of the five-fold term, which implies an upper bound of order 10410^{-4} on the relative TT-odd strength when compared to the central real optical potential.Comment: 11 pages (Revtex

    Generalized "Quasi-classical" Ground State for an Interacting Two Level System

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    We treat a system (a molecule or a solid) in which electrons are coupled linearly to any number and type of harmonic oscillators and which is further subject to external forces of arbitrary symmetry. With the treatment restricted to the lowest pair of electronic states, approximate "vibronic" (vibration-electronic) ground state wave functions are constructed having the form of simple, closed expressions. The basis of the method is to regard electronic density operators as classical variables. It extends an earlier "guessed solution", devised for the dynamical Jahn-Teller effect in cubic symmetry, to situations having lower (e.g., dihedral) symmetry or without any symmetry at all. While the proposed solution is expected to be quite close to the exact one, its formal simplicity allows straightforward calculations of several interesting quantities, like energies and vibronic reduction (or Ham) factors. We calculate for dihedral symmetry two different qq-factors ("qzq_z" and "qxq_x") and a pp-factor. In simplified situations we obtain p=qz+qx1p=q_z +q_x -1. The formalism enables quantitative estimates to be made for the dynamical narrowing of hyperfine lines in the observed ESR spectrum of the dihedral cyclobutane radical cation.Comment: 28 pages, 4 figure

    Quadrupole Susceptibility and Elastic Softening due to a Vacancy in Silicon Crystal

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    We investigate the electronic states around a single vacancy in silicon crystal by using the Green's function approach. The triply degenerate vacancy states within the band gap are found to be extended over a large distance 20A˚\sim20 {\rm \AA} from the vacancy site and contribute to the reciprocal temperature dependence of the quadrupole susceptibility resulting in the elastic softening at low temperture. The Curie constant of the quadrupole susceptibility for the trigonal mode (Oyz,Ozx,OxyO_{yz},O_{zx},O_{xy}) is largely enhanced as compared to that for the tetragonal mode (O20,O22O_{2}^{0},O_{2}^{2}). The obtained results are consistent with the recent ultrasonic experiments in silicon crystal down to 20 mK. We also calculate the dipole and octupole susceptibilities and find that the octupole susceptibilities are extremely enhannced for a specific mode.Comment: 6 pages, with 5 figure
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