3,570 research outputs found
Polarization anisotropy in the optical properties of silicon ellipsoids
A new real space quantum mechanical approach with local field effects
included is applied to the calculation of the optical properties of silicon
nanocrystals. Silicon ellipsoids are studied and the role of surface
polarization is discussed in details. In particular, surface polarization is
shown to be responsible for a strong optical anisotropy in silicon ellipsoids,
much more pronounced with respect to the case in which only quantum confinement
effects are considered. The static dielectric constant and the absorption
spectra are calculated, showing that the perpendicular and parallel components
have a very different dependence on the ellipsoid aspect ratio. Then, a
comparison with the classical dielectric model is performed, showing that the
model only works for large and regular structures, but it fails for thin
elongated ellipsoids.Comment: 5 pages, 4 figures, International Conference on NANO-Structures
Self-Assemblin
Trade in secured debt, adjustment in haircuts and international portfolios
I study the composition of international portfolios under collateral constraints and the implied cross-border transmission of shocks. I develop an international portfolio model with these features, in which leveraged investors seek diversification in both assets and secured liabilities and in which the pledgeable portion of assets adjusts to the state of the economy, reflecting borrowers’ credit risk. The new analytical results are as follows. First, agents choose endogenously how much to borrow from each country. Second, the collateral constraint, being a contractual link between secured and unsecured financial instruments, permits to compute portfolios without an arbitrage condition between those classes of assets. Finally, haircuts adjust endogenously through the change in the collateral values. After estimating the parameters governing this adjustment, I find that both portfolios and international transmission mechanism are quite sensitive to leveraged investors’ funding. As for portfolios, secured bonds have particularly effective hedging properties in managing the terms of trade risk. As for the international transmission, tightening haircuts affect the economic slowdown: initially severe contractions are followed by quick reversions to the long-term equilibrium. On a cumulative basis, these two effects compensate if haircuts adjust precisely to the economic state. But in case of uncertainty about this adjustment, collateral constraints are a source of risk which cannot be internationally diversified.Financial flows, borrowing limits, creditworthiness, risk premia, international business cycle, macroeconomic interdipendence.
Tight binding formulation of the dielectric response in semiconductor nanocrystals
We report on a theoretical derivation of the electronic dielectric response
of semiconductor nanocrystals using a tight-binding framework. Extending to the
nanoscale the Hanke and Sham approach [Phys. Rev. B 12, 4501 (1975)] developed
for bulk semiconductors, we show how local field effects can be included in the
study of confined systems. A great advantage of this scheme is that of being
formulated in terms of localized orbitals and thus it requires very few
computational resources and times. Applications to the optical and screening
properties of semiconductor nanocrystals are presented here and discussed.
Results concerning the absorption cross section, the static polarizability and
the screening function of InAs (direct gap) and Si (indirect gap) nanocrystals
compare well to both first principles results and experimental data. We also
show that the present scheme allows us to easily go beyond the continuum
dielectric model, based on the Clausius-Mossotti equation, which is frequently
used to include the nanocrystal surface polarization. Our calculations indicate
that the continuum dielectric model, used in conjunction with a size dependent
dielectric constant, underestimates the nanocrystal polarizability, leading to
exceedingly strong surface polarization fields.Comment: 9 pages, 5 figures; corrected typos, added reference
Sharp estimates for the first -Laplacian eigenvalue and for the -torsional rigidity on convex sets with holes
We study, in dimension , the eigenvalue problem and the torsional
rigidity for the -Laplacian on convex sets with holes, with external Robin
boundary conditions and internal Neumann boundary conditions. We prove that the
annulus maximizes the first eigenvalue and minimizes the torsional rigidity
when the measure and the external perimeter are fixed.Comment: 17 page
The impact of metallicity and dynamics on the evolution of young star clusters
The early evolution of a dense young star cluster (YSC) depends on the
intricate connection between stellar evolution and dynamical processes. Thus,
N-body simulations of YSCs must account for both aspects. We discuss N-body
simulations of YSCs with three different metallicities (Z=0.01, 0.1 and 1
Zsun), including metallicity-dependent stellar evolution recipes and
metallicity-dependent prescriptions for stellar winds and remnant formation. We
show that mass-loss by stellar winds influences the reversal of core collapse.
In particular, the post-collapse expansion of the core is faster in metal-rich
YSCs than in metal-poor YSCs, because the former lose more mass (through
stellar winds) than the latter. As a consequence, the half-mass radius expands
more in metal-poor YSCs. We also discuss how these findings depend on the total
mass and on the virial radius of the YSC. These results give us a clue to
understand the early evolution of YSCs with different metallicity.Comment: to appear in "Massive Young Star Clusters Near and Far: From the
Milky Way to Reionization", 2013 Guillermo Haro Conference, Eds. Y. D. Mayya,
D. Rosa-Gonzalez & E. Terlevich, INAOE and AMC. 4 pages, 2 figure
Forming circumnuclear disks and rings in galactic nuclei: a competition between supermassive black hole and nuclear star cluster
We investigate the formation of circumnuclear gas structures from the tidal
disruption of molecular clouds in galactic nuclei, by means of smoothed
particle hydrodynamics simulations. We model galactic nuclei as composed of a
supermassive black hole (SMBH) and a nuclear star cluster (NSC) and consider
different mass ratios between the two components. We find that the relative
masses of the SMBH and the NSC have a deep impact on the morphology of the
circumnuclear gas. Extended disks form only inside the sphere of influence of
the SMBH. In contrast, compact rings naturally form outside the SMBH's sphere
of influence, where the gravity is dominated by the NSC. This result is in
agreement with the properties of the Milky Way's circumnuclear ring, which
orbits outside the SMBH sphere of influence. Our results indicate that compact
circumnuclear rings can naturally form outside the SMBH sphere of influence.Comment: Accepted for publication in ApJ. 12 pages, 6 figures, 3 tables.
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