29,966 research outputs found
Inelastic Effects in Low-Energy Electron Reflectivity of Two-dimensional Materials
A simple method is proposed for inclusion of inelastic effects (electron
absorption) in computations of low-energy electron reflectivity (LEER) spectra.
The theoretical spectra are formulated by matching of electron wavefunctions
obtained from first-principles computations in a repeated vacuum-slab-vacuum
geometry. Inelastic effects are included by allowing these states to decay in
time in accordance with an imaginary term in the potential of the slab, and by
mixing of the slab states in accordance with the same type of distribution as
occurs in a free-electron model. LEER spectra are computed for various
two-dimensional materials, including free-standing multilayer graphene,
graphene on copper substrates, and hexagonal boron nitride (h-BN) on cobalt
substrates.Comment: 21 pages, 7 figure
The Mira-based distance to the Galactic centre
Mira variables are useful distance indicators, due to their high luminosities
and well-defined period-luminosity relation. We select 1863 Miras from SAAO and
MACHO observations to examine their use as distance estimators in the Milky
Way. We measure a distance to the Galactic centre of kpc,
which is in good agreement with other literature values. The uncertainty has
two components of 0.2 kpc each: the first is from our analysis and
predominantly due to interstellar extinction, the second is due to zero-point
uncertainties extrinsic to our investigation, such as the distance to the Large
Magellanic Cloud (LMC). In an attempt to improve existing period-luminosity
calibrations, we use theoretical models of Miras to determine the dependence of
the period-luminosity relation on age, metallicity, and helium abundance, under
the assumption that Miras trace the bulk stellar population. We find that at a
fixed period of , changes in the predicted magnitudes can
be approximated by , and these coefficients are
nearly independent of period. The expected overestimate in the Galactic centre
distance from using an LMC-calibrated relation is 0.3 kpc. This
prediction is not validated by our analysis; a few possible reasons are
discussed. We separately show that while the predicted color-color diagrams of
solar-neighbourhood Miras work well in the near-infrared, though there are
offsets from the model predictions in the optical and mid-infrared.Comment: Accepted for publication in The Astrophysical Journal. 16 pages, 8
figures, 6 table
On complex surfaces diffeomorphic to rational surfaces
In this paper we prove that no complex surface of general type is
diffeomorphic to a rational surface, thereby completing the smooth
classification of rational surfaces and the proof of the Van de Ven conjecture
on the smooth invariance of Kodaira dimension.Comment: 34 pages, AMS-Te
Morphology and Orientation Selection of Non-Metallic Inclusions in Electrified Molten Metal
The effect of electric current on morphology and orientation selection of non-metallic inclusions in molten metal has been investigated using theoretical modelling and numerical calculation. Two geometric factors, namely the circularity (fc) and alignment ratio (fe) were introduced to describe the inclusions shape and configuration. Electric current free energy was calculated and the values were used to determine the thermodynamic preference between different microstructures. Electric current promotes the development of inclusion along the current direction by either expatiating directional growth or enhancing directional agglomeration. Reconfiguration of the inclusions to reduce the system electric resistance drives the phenomena. The morphology and orientation selection follows the routine to reduce electric free energy. The numerical results are in agreement with our experimental observations
Checkerboard charge density wave and pseudogap in high- cuprates
We consider the scenario where a 4-lattice constant, rotationally symmetric
charge density wave (CDW) is present in the underdoped cuprates. We prove a
theorem that puts strong constraint on the possible form factor of such a CDW.
We demonstrate, within mean-field theory, that a particular form factor within
the allowed class describes the angle-resolved photoemission and scan tunneling
spectroscopy well. We conjecture that the ``large pseudogap'' in cuprates is
the consequence of this type of charge density wave.Comment: We add a new section II on the symmetry property of the checkerboard
CD
Effect of in-plane line defects on field-tuned superconductor-insulator transition behavior in homogeneous thin film
Field-tuned superconductor-insulator transition (FSIT) behavior in 2D
isotropic and homogeneous thin films is usually accompanied by a nonvanishing
critical resistance at low . It is shown that, in a 2D film including line
defects paralle to each other but with random positions perpendicular to them,
the (apparent) critical resistance in low limit vanishes, as in the 1D
quantum superconducting (SC) transition, under a current parallel to the line
defects. This 1D-like critical resistive behavior is more clearly seen in
systems with weaker point disorder and may be useful in clarifying whether the
true origin of FSIT behavior in the parent superconductor is the glass
fluctuation or the quantum SC fluctuation. As a by-product of the present
calculation, it is also pointed out that, in 2D films with line-like defects
with a long but {\it finite} correlation length parallel to the lines, a
quantum metallic behavior intervening the insulating and SC ones appears in the
resistivity curves.Comment: 16 pages, 14 figure
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