1,504 research outputs found
Qualitative and quantitative analysis of mixtures of compounds containing both hydrogen and deuterium
Method allows qualitative and quantitative analysis of mixtures of partially deuterated compounds. Nuclear magnetic resonance spectroscopy determines location and amount of deuterium in organic compounds but not fully deuterated compounds. Mass spectroscopy can detect fully deuterated species but not the location
Tuning Fermi-surface properties through quantum confinement in metallic meta-lattices: New metals from old atoms
We describe a new class of nanoscale structured metals wherein the effects of
quantum confinement are combined with dispersive metallic electronic states to
induce modifications to the fundamental low-energy microscopic properties of a
three-dimensional metal: the density of states, the distribution of Fermi
velocities, and the collective electronic response.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let
Doping effects on the electronic and structural properties of CoO2: An LSDA+U study
A systematic LSDA+U study of doping effects on the electronic and structural
properties of single layer CoO2 is presented. Undoped CoO2 is a charge transfer
insulator within LSDA+U and a metal with a high density of states (DOS) at the
Fermi level within LSDA. (CoO2), on the other hand, is a band
insulator with a gap of 2.2 eV. Systems with fractional doping are metals if no
charge orderings are present. Due to the strong interaction between the doped
electron and other correlated Co d electrons, the calculated electronic
structure of (CoO2) depends sensitively on the doping level x. Zone
center optical phonon energies are calculated under the frozen phonon
approximation and are in good agreement with measured values. Softening of the
phonon at doping x ~0.25 seems to indicate a strong electron-phonon
coupling in this system. Possible intemediate spin states of Co ions, Na
ordering, as well as magnetic and charge orderings in this system are also
discussed.Comment: 11 pages, 12 figure
Discrete transverse superconducting modes in nano-cylinders
Spatial variation in the superconducting order parameter becomes significant
when the system is confined at dimensions well below the typical
superconducting coherence length. Motivated by recent experimental success in
growing single-crystal metallic nanorods, we study quantum confinement effects
on superconductivity in a cylindrical nanowire in the clean limit. For large
diameters, where the transverse level spacing is smaller than superconducting
order parameter, the usual approximations of Ginzburg-Landau theory are
recovered. However, under external magnetic field the order parameter develops
a spatial variation much stronger than that predicted by Ginzburg-Landau
theory, and gapless superconductivity is obtained above a certain field
strength. At small diameters, the discrete nature of the transverse modes
produces significant spatial variations in the order parameter with increased
average magnitude and multiple shoulders in the magnetic response.Comment: 10 pages, 8 figure
Defect Engineering: Graphene Gets Designer Defects
An extended one-dimensional defect that has the potential to act as a
conducting wire has been embedded in another perfect graphene sheet.Comment: 2 pages, 1 figur
Magneto-optical Kerr Effect Studies of Square Artificial Spin Ice
We report a magneto-optical Kerr effect study of the collective magnetic
response of artificial square spin ice, a lithographically-defined array of
single-domain ferromagnetic islands. We find that the anisotropic inter-island
interactions lead to a non-monotonic angular dependence of the array coercive
field. Comparisons with micromagnetic simulations indicate that the two
perpendicular sublattices exhibit distinct responses to island edge roughness,
which clearly influence the magnetization reversal process. Furthermore, such
comparisons demonstrate that disorder associated with roughness in the island
edges plays a hitherto unrecognized but essential role in the collective
behavior of these systems.Comment: Physical Review B, Rapid Communications (in press
Catastrophic senescence and semelparity in the Penna aging model
The catastrophic senescence of the Pacific salmon is among the initial tests
used to validate the Penna aging model. Based on the mutation accumulation
theory, the sudden decrease in fitness following reproduction may be solely
attributed to the semelparity of the species. In this work, we report other
consequences of mutation accumulation. Contrary to earlier findings, such
dramatic manifestation of aging depends not only on the choice of breeding
strategy but also on the value of the reproduction age, R, and the mutation
threshold, T. Senescence is catastrophic when . As the organism's
tolerance for harmful genetic mutations increases, the aging process becomes
more gradual. We observe senescence that is threshold dependent whenever T>R.
That is, the sudden drop in survival rate occurs at age equal to the mutation
threshold value
Changes in Floquet state structure at avoided crossings: delocalization and harmonic generation
Avoided crossings are common in the quasienergy spectra of strongly driven
nonlinear quantum wells. In this paper we examine the sinusoidally driven
particle in a square potential well to show that avoided crossings can alter
the structure of Floquet states in this system. Two types of avoided crossings
are identified: on type leads only to temporary changes (as a function of
driving field strength) in Floquet state structure while the second type can
lead to permanent delocalization of the Floquet states. Radiation spectra from
these latter states show significant increase in high harmonic generation as
the system passes through the avoided crossing.Comment: 8 pages with 10 figures submitted to Physical Review
Topological Phases in Graphitic Cones
The electronic structure of graphitic cones exhibits distinctive topological
features associated with the apical disclinations. Aharonov-Bohm
magnetoconductance oscillations (period Phi_0) are completely absent in rings
fabricated from cones with a single pentagonal disclination. Close to the apex,
the local density of states changes qualitatively, either developing a cusp
which drops to zero at the Fermi energy, or forming a region of nonzero density
across the Fermi energy, a local metalization of graphene.Comment: 4 pages, RevTeX 4, 3 PostScript figure
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