9,936 research outputs found
Semiconductor-metal nanoparticle molecules: hybrid excitons and non-linear Fano effect
Modern nanotechnology opens the possibility of combining nanocrystals of
various materials with very different characteristics in one superstructure.
The resultant superstructure may provide new physical properties not
encountered in homogeneous systems. Here we study theoretically the optical
properties of hybrid molecules composed of semiconductor and metal
nanoparticles. Excitons and plasmons in such a hybrid molecule become strongly
coupled and demonstrate novel properties. At low incident light intensity, the
exciton peak in the absorption spectrum is broadened and shifted due to
incoherent and coherent interactions between metal and semiconductor
nanoparticles. At high light intensity, the absorption spectrum demonstrates a
surprising, strongly asymmetric shape. This shape originates from the coherent
inter-nanoparticle Coulomb interaction and can be viewed as a non-linear Fano
effect which is quite different from the usual linear Fano resonance.Comment: 5 pages, 5 figures, submitted to Phys. Rev. Let
Multiband theory of quantum-dot quantum wells: Dark excitons, bright excitons, and charge separation in heteronanostructures
Electron, hole, and exciton states of multishell CdS/HgS/CdS quantum-dot
quantum well nanocrystals are determined by use of a multiband theory that
includes valence-band mixing, modeled with a 6-band Luttinger-Kohn Hamiltonian,
and nonparabolicity of the conduction band. The multiband theory correctly
describes the recently observed dark-exciton ground state and the lowest,
optically active, bright-exciton states. Charge separation in pair states is
identified. Previous single-band theories could not describe these states or
account for charge separation.Comment: 10 pages of ReVTex, 6 ps figures, submitted to Phys. Rev.
Calibrated Sub-Bundles in Non-Compact Manifolds of Special Holonomy
This paper is a continuation of math.DG/0408005. We first construct special
Lagrangian submanifolds of the Ricci-flat Stenzel metric (of holonomy SU(n)) on
the cotangent bundle of S^n by looking at the conormal bundle of appropriate
submanifolds of S^n. We find that the condition for the conormal bundle to be
special Lagrangian is the same as that discovered by Harvey-Lawson for
submanifolds in R^n in their pioneering paper. We also construct calibrated
submanifolds in complete metrics with special holonomy G_2 and Spin(7)
discovered by Bryant and Salamon on the total spaces of appropriate bundles
over self-dual Einstein four manifolds. The submanifolds are constructed as
certain subbundles over immersed surfaces. We show that this construction
requires the surface to be minimal in the associative and Cayley cases, and to
be (properly oriented) real isotropic in the coassociative case. We also make
some remarks about using these constructions as a possible local model for the
intersection of compact calibrated submanifolds in a compact manifold with
special holonomy.Comment: 20 pages; for Revised Version: Minor cosmetic changes, some
paragraphs rewritten for improved clarit
Laser stripping of hydrogen atoms by direct ionization
Direct ionization of hydrogen atoms by laser irradiation is investigated as a potential new scheme to generate proton beams without stripping foils. The time-dependent Schrödinger equation describing the atom-radiation interaction is numerically solved obtaining accurate ionization cross-sections for a broad range of laser wavelengths, durations and energies. Parameters are identified where the Doppler frequency up-shift of radiation colliding with relativistic particles can lead to efficient ionization over large volumes and broad bandwidths using currently available lasers
Metering and Calibration in LoanSTAR Buildings
End-use metering in commercial buildings often
requires installation of a large variety of transducers and data
loggers. The metering installation group in the LoanSTAR
monitoring program has the primary responsibility for the
installation and maintenance of the metering hardware. This
paper provides an overview of the responsibilities and first
year experiences of the metering installation group of the
LoanSTAR monitoring program. In addition, the calibration
laboratory is also described
PP-waves with torsion and metric-affine gravity
A classical pp-wave is a 4-dimensional Lorentzian spacetime which admits a
nonvanishing parallel spinor field; here the connection is assumed to be
Levi-Civita. We generalise this definition to metric compatible spacetimes with
torsion and describe basic properties of such spacetimes. We use our
generalised pp-waves for constructing new explicit vacuum solutions of
quadratic metric-affine gravity.Comment: 17 pages, LaTeX2
Predicion of charge separation in GaAs/AlAs cylindrical Russian Doll nanostructures
We have contrasted the quantum confinement of (i) multiple quantum wells of
flat GaAs and AlAs layers, i.e. (\GaAs)_{m}/(\AlAs)_n/(\GaAs)_p/(\AlAs)_q,
with (ii) ``cylindrical Russian Dolls'' -- an equivalent sequence of wells and
barriers arranged as concentric wires. Using a pseudopotential plane-wave
calculation, we identified theoretically a set of numbers ( and )
such that charge separation can exist in ``cylindrical Russian Dolls'': the CBM
is localized in the inner GaAs layer, while the VBM is localized in the outer
GaAs layer.Comment: latex, 8 page
PHOTOCHEMISTRY OF PHYCOBILIPROTEINS
Native PEC from the cyanobacterium, Mastigocladus laminosus, and its isolated α-subunit show photoreversibly photochromic reactions with difference-maxima around 502 and 570 nm in the spectral region of the α-84 phycoviolobilin chromophore. (b) Native PEC and its ÎČ-subunit show little if any reversible photochemistry in the 600â620 nm region, where the phycocyanobilin chromophores on the ÎČ-subunit absorb maximally, (c) Reversible photochemistry is retained in ureadenatured PEC at pH = 7.0 or pH †3. The difference maxima are shifted to 510 and 600 nm, and the amplitudes are decreased. An irreversible absorbance increase occurs around 670 nm (pH †3). (d) The amplitude of the reversible photoreaction difference spectrum is maximum in the presence of 4â5 M urea or 1 M KSCN, conditions known to dissociate phycobiliprotein aggregates into monomers. At the same time, the phycocyanobilin chromophore(s) are bleached irreversibly, (e) The amplitude becomes very small in high aggregates, e.g. in phycobilisomes. (f) In a reciprocal manner, the phototransformation of native PEC leads to a reversible shift of its aggregation equilibrium between trimer and monomer. The latter is favored by orange, the former by green light, (g) It is concluded that the phycoviolobilin chromophore of PEC is responsible for reversible photochemistry in PEC, and that there is not only an influence of aggregation state on photochemistry, but also vice versa an effect of the status of the chromophore on aggregation state. This could constitute a primary signal in the putative function as sensory pigment, either directly, or indirectly via the release of other polypeptides, via photodynamic effects, or the like
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Collaborative development of diffraction-limited beamline optical systems at US DOE light sources
An ongoing collaboration among four US Department of Energy (DOE) National Laboratories has demonstrated key technology prototypes and software modeling tools required for new high-coherent flux beamline optical systems. New free electron laser (FEL) and diffraction-limited storage ring (DLSR) light sources demand wavefront preservation from source to sample to achieve and maintain optimal performance. Fine wavefront control was achieved using a novel, roomtemperature cooled mirror system called REAL (resistive element adjustable length) that combines cooling with applied, spatially variable auxiliary heating. Single-grating shearing interferometry (also called Talbot interferometry) and Hartmann wavefront sensors were developed and used for optical characterization and alignment on several beamlines, across a range of photon energies. Demonstrations of non-invasive hard x-ray wavefront sensing were performed using a thin diamond single-crystal as a beamsplitter
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