840 research outputs found
Luttinger Liquid at the Edge of a Graphene Vacuum
We demonstrate that an undoped two-dimensional carbon plane (graphene) whose
bulk is in the integer quantum Hall regime supports a non-chiral Luttinger
liquid at an armchair edge. This behavior arises due to the unusual dispersion
of the non-interacting edges states, causing a crossing of bands with different
valley and spin indices at the edge. We demonstrate that this stabilizes a
domain wall structure with a spontaneously ordered phase degree of freedom.
This coherent domain wall supports gapless charged excitations, and has a power
law tunneling with a non-integral exponent. In proximity to a bulk lead,
the edge may undergo a quantum phase transition between the Luttinger liquid
phase and a metallic state when the edge confinement is sufficiently strong
relative to the interaction energy scale.Comment: 4 pages, 3 figure
Collective charge density fluctuations in superconducting layered systems with bilayer unit cells
Collective modes of bilayered superconducting superlattices (e.g., YBCO) are
investigated within the conserving gauge-invariant ladder diagram approximation
including both the nearest interlayer single electron tunneling and the
Josephson-type Cooper pair tunneling. By calculating the density-density
response function including Coulomb and pairing interactions, we examine the
two collective mode branches corresponding to the in-phase and out-of-phase
charge fluctuations between the two layers in the unit cell. The out-of-phase
collective mode develops a long wavelength plasmon gap whose magnitude depends
on the tunneling strength with the mode dispersions being insensitive to the
specific tunneling mechanism (i.e., single electron or Josephson). We also show
that in the presence of tunneling the oscillator strength of the out-of-phase
mode overwhelms that of the in-phase-mode at and finite ,
where and are respectively the mode wave vectors perpendicular
and along the layer. We discuss the possible experimental observability of the
phase fluctuation modes in the context of our theoretical results for the mode
dispersion and spectral weight.Comment: 9 pages, 3 figure
Theory of Phonon Shakeup Effects on Photoluminescence from the Wigner Crystal in a Strong Magnetic Field
We develop a method to compute shakeup effects on photoluminescence from a
strong magnetic field induced two-dimensional Wigner crystal. Only localized
holes are considered. Our method treats the lattice electrons and the tunneling
electron on an equal footing, and uses a quantum-mechanical calculation of the
collective modes that does not depend in any way on a harmonic approximation.
We find that shakeup produces a series of sidebands that may be identified with
maxima in the collective mode density of states, and definitively distinguishes
the crystal state from a liquid state in the absence of electron-hole
interaction. In the presence of electron-hole interaction, sidebands also
appear in the liquid state coming from short-range density fluctuations around
the hole. However, the sidebands in the liquid state and the crystal state have
different qualitative behaviors. We also find a shift in the main luminescence
peak, that is associated with lattice relaxation in the vicinity of a vacancy.
The relationship of the shakeup spectrum with previous mean-field calculations
is discussed.Comment: 14 pages, uuencoded postscript file for entire paper, also available
at (click phd14) http://rainbow.uchicago.edu/~ldz/paper/paper.htm
Floquet Spectrum and Transport Through an Irradiated Graphene Ribbon
Graphene subject to a spatially uniform, circularly-polarized electric field
supports a Floquet spectrum with properties akin to those of a topological
insulator, including non-vanishing Chern numbers associated with bulk bands and
current-carrying edge states. Transport properties of this system however are
complicated by the non-equilibrium occupations of the Floquet states. We
address this by considering transport in a two-terminal ribbon geometry for
which the leads have well-defined chemical potentials, with an irradiated
central scattering region. We demonstrate the presence of edge states, which
for infinite mass boundary conditions may be associated with only one of the
two valleys. At low frequencies, the bulk DC conductivity near zero energy is
shown to be dominated by a series of states with very narrow anticrossings,
leading to super-diffusive behavior. For very long ribbons, a ballistic regime
emerges in which edge state transport dominates.Comment: 4.2 pages, 3 figure
Robots Refurbish Space Shuttle Hardware USBI
USBI, a division of United Technologies Corporation (UTC ), was awarded a contract by the National Aeronautics and Space Administration (NASA) to refurbish the non-motor sections (nosecap, frustum, forward skirt, and aft skirt) of the Solid Rocket Boosters (SRB) used in the space shuttle program. As NASA\u27s major contractor for processing and refurbishment of the SRB\u27s, our company has successfully refurbished all the SRB\u27s flight hardware to date. The planned insulation refurbishment process of the SRB\u27s consists of thermal protection system (TPS) removal; surface preparation; reapplication of Marshall Sprayable Ablative (MSA-2), a thermal ablative material developed at Marshall Space Flight Center; and topcoat spraying of Hypalon paint to seal the ablative material.
In the past, refurbishment of the SRB\u27s was done by hand at the Vehicle Assembly Building (VAB). This process was slow and tedious, and introduced personnel to potentially hazardous working environments. A means of reducing costs and providing enhanced quality assurance was desired. Therefore, the need arose to implement a degree of automation to the refurbishment process. The introduction of a new automated process system would reduce the manual labor element, limiting the need for personnel to come in direct contact with hazardous materials. And because of the reliability and accuracy of robotics, NASA would be assured of receiving the highest possible quality product , at the lowest cost
Diluted Graphene Antiferromagnet
We study RKKY interactions between local magnetic moments for both doped and
undoped graphene. We find in both cases that the interactions are primarily
ferromagnetic for moments on the same sublattice, and antiferromagnetic for
moments on opposite sublattices. This suggests that at sufficiently low
temperatures dilute magnetic moments embedded in graphene can order into a
state analogous to that of a dilute antiferromagnet. We find that in the
undoped case one expects no net magnetic moment, and demonstrate numerically
that this effect generalizes to ribbons where the magnetic response is
strongest at the edge, suggesting the possibility of an unusual spin-transfer
device. For doped graphene we find that moments at definite lattice sites
interact over longer distances than those placed in interstitial sites of the
lattice ( vs. ) because the former support a Kohn anomaly that is
suppressed in the latter due to the absence of backscattering.Comment: 5 pages, two figures include
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