4,751 research outputs found
On vanishing theorems for Higgs bundles
We introduce the notion of Hermitian Higgs bundle as a natural generalization
of the notion of Hermitian vector bundle and we study some vanishing theorems
concerning Hermitian Higgs bundles when the base manifold is a compact complex
manifold. We show that a first vanishing result, proved for these objects when
the base manifold was K\"ahler, also holds when the manifold is compact
complex. From this fact and some basic properties of Hermitian Higgs bundles,
we conclude several results. In particular we show that, in analogy to the
classical case, there are vanishing theorems for invariant sections of tensor
products of Higgs bundles. Then, we prove that a Higgs bundle admits no nonzero
invariant sections if there is a condition of negativity on the greatest
eigenvalue of the Hitchin-Simpson mean curvature. Finally, we prove that
invariant sections of certain tensor products of a weak Hermitian-Yang-Mills
Higgs bundle are all parallel in the classical sense.Comment: 10 Pages, some typos corrected and minor change
Nitrate uptake across biomes and the influence of elemental stoichiometry: A new look at LINX II
Considering recent increases in anthropogenic N loading, it is essential to identify the controls on N removal and retention in aquatic ecosystems because the fate of N has consequences for water quality in streams and downstream ecosystems. Biological uptake of nitrate (NO3â) is a major pathway by which N is removed from these ecosystems. Here we used data from the second Lotic Intersite Nitrogen eXperiment (LINX II) in a multivariate analysis to identify the primary drivers of variation in NO3â uptake velocity among biomes. Across 69 study watersheds in North America, dissolved organic carbon:NO3â ratios and photosynthetically active radiation were identified as the two most important predictor variables in explaining NO3â uptake velocity. However, within a specific biome the predictor variables of NO3â uptake velocity varied and included various physical, chemical, and biological attributes. Our analysis demonstrates the broad control of elemental stoichiometry on NO3â uptake velocity as well as the importance of biome-specific predictors. Understanding this spatial variation has important implications for biome-specific watershed management and the downstream export of NO3â, as well as for development of spatially explicit global models that describe N dynamics in streams and rivers
Approximate Hermitian-Yang-Mills structures and semistability for Higgs bundles. II: Higgs sheaves and admissible structures
We study the basic properties of Higgs sheaves over compact K\"ahler
manifolds and we establish some results concerning the notion of semistability;
in particular, we show that any extension of semistable Higgs sheaves with
equal slopes is semistable. Then, we use the flattening theorem to construct a
regularization of any torsion-free Higgs sheaf and we show that it is in fact a
Higgs bundle. Using this, we prove that any Hermitian metric on a
regularization of a torsion-free Higgs sheaf induces an admissible structure on
the Higgs sheaf. Finally, using admissible structures we proved some properties
of semistable Higgs sheaves.Comment: 18 pages; some typos correcte
Electroreflectance spectroscopy in self-assembled quantum dots: lens symmetry
Modulated electroreflectance spectroscopy of semiconductor
self-assembled quantum dots is investigated. The structure is modeled as dots
with lens shape geometry and circular cross section. A microscopic description
of the electroreflectance spectrum and optical response in terms of an external
electric field () and lens geometry have been considered. The field
and lens symmetry dependence of all experimental parameters involved in the
spectrum have been considered. Using the effective mass formalism
the energies and the electronic states as a function of and dot
parameters are calculated. Also, in the framework of the strongly confined
regime general expressions for the excitonic binding energies are reported.
Optical selection rules are derived in the cases of the light wave vector
perpendicular and parallel to . Detailed calculation of the Seraphin
coefficients and electroreflectance spectrum are performed for the InAs and
CdSe nanostructures. Calculations show good agreement with measurements
recently performed on CdSe/ZnSe when statistical distribution on size is
considered, explaining the main observed characteristic in the
electroreflectance spectra
Chemical Raman Enhancement of Organic Adsorbates on Metal Surfaces
Using a combination of first-principles theory and experiments, we provide a
quantitative explanation for chemical contributions to surface-enhanced Raman
spectroscopy for a well-studied organic molecule, benzene thiol, chemisorbed on
planar Au(111) surfaces. With density functional theory calculations of the
static Raman tensor, we demonstrate and quantify a strong mode-dependent
modification of benzene thiol Raman spectra by Au substrates. Raman active
modes with the largest enhancements result from stronger contributions from Au
to their electron-vibron coupling, as quantified through a deformation
potential, a well-defined property of each vibrational mode. A straightforward
and general analysis is introduced that allows extraction of chemical
enhancement from experiments for specific vibrational modes; measured values
are in excellent agreement with our calculations.Comment: 5 pages, 4 figures and Supplementary material included as ancillary
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Electron-phonon renormalization of the absorption edge of the cuprous halides
Compared to most tetrahedral semiconductors, the temperature dependence of
the absorption edges of the cuprous halides (CuCl, CuBr, CuI) is very small.
CuCl and CuBr show a small increase of the gap with increasing
temperature, with a change in the slope of vs. at around 150 K: above
this temperature, the variation of with becomes even smaller. This
unusual behavior has been clarified for CuCl by measurements of the low
temperature gap vs. the isotopic masses of both constituents, yielding an
anomalous negative shift with increasing copper mass. Here we report the
isotope effects of Cu and Br on the gap of CuBr, and that of Cu on the gap of
CuI. The measured isotope effects allow us to understand the corresponding
temperature dependences, which we also report, to our knowledge for the first
time, in the case of CuI. These results enable us to develop a more
quantitative understanding of the phenomena mentioned for the three halides,
and to interpret other anomalies reported for the temperature dependence of the
absorption gap in copper and silver chalcogenides; similarities to the behavior
observed for the copper chalcopyrites are also pointed out.Comment: 14 pages, 5 figures, submitted to Phys. Rev.
Electronic, vibrational, and thermodynamic properties of ZnS (zincblende and rocksalt structure)
We have measured the specific heat of zincblende ZnS for several isotopic
compositions and over a broad temperature range (3 to 1100 K). We have compared
these results with calculations based on ab initio electronic band structures,
performed using both LDA and GGA exchange- correlation functionals. We have
compared the lattice dynamics obtained in this manner with experimental data
and have calculated the one-phonon and two-phonon densities of states. We have
also calculated mode Grueneisen parameters at a number of high symmetry points
of the Brillouin zone. The electronic part of our calculations has been used to
investigate the effect of the 3d core electrons of zinc on the spin-orbit
splitting of the top valence bands. The effect of these core electrons on the
band structure of the rock salt modification of ZnS is also discussed.Comment: 33pages, 16 Figures, submitted to Phys. Rev.
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