2,436 research outputs found
Spin-Forster transfer in optically excited quantum dots
The mechanisms of energy and spin transfer in quantum dot pairs coupled via
the Coulomb interaction are studied. Exciton transfer can be resonant or
phonon-assisted. In both cases, the transfer rates strongly depend on the
resonance conditions. The spin selection rules in the transfer process come
from the exchange and spin-orbit interactions. The character of energy
dissipation in spin transfer is different than that in the traditional spin
currents. The spin-dependent photon cross-correlation functions reflect the
exciton transfer process. In addition, a mathematical method to calculate
F\"orster transfer in crystalline nanostructures beyond the dipole-dipole
approximation is described.Comment: 22 pages, 10 figures, Phys. Rev. B, in pres
Nonmonotonic energy harvesting efficiency in biased exciton chains
We theoretically study the efficiency of energy harvesting in linear exciton
chains with an energy bias, where the initial excitation is taking place at the
high-energy end of the chain and the energy is harvested (trapped) at the other
end. The efficiency is characterized by means of the average time for the
exciton to be trapped after the initial excitation. The exciton transport is
treated as the intraband energy relaxation over the states obtained by
numerically diagonalizing the Frenkel Hamiltonian that corresponds to the
biased chain. The relevant intraband scattering rates are obtained from a
linear exciton-phonon interaction. Numerical solution of the Pauli master
equation that describes the relaxation and trapping processes, reveals a
complicated interplay of factors that determine the overall harvesting
efficiency. Specifically, if the trapping step is slower than or comparable to
the intraband relaxation, this efficiency shows a nonmonotonic dependence on
the bias: it first increases when introducing a bias, reaches a maximum at an
optimal bias value, and then decreases again because of dynamic (Bloch)
localization of the exciton states. Effects of on-site (diagonal) disorder,
leading to Anderson localization, are addressed as well.Comment: 9 pages, 6 figures, to appear in Journal of Chemical Physic
Optical properties of coupled metal-semiconductor and metal-molecule nanocrystal complexes: the role of multipole effects
We investigate theoretically the effects of interaction between an optical
dipole (semiconductor quantum dot or molecule) and metal nanoparticles. The
calculated absorption spectra of hybrid structures demonstrate strong effects
of interference coming from the exciton-plasmon coupling. In particular, the
absorption spectra acquire characteristic asymmetric lineshapes and strong
anti-resonances. We present here an exact solution of the problem beyond the
dipole approximation and find that the multipole treatment of the interaction
is crucial for the understanding of strongly-interacting exciton-plasmon
nano-systems. Interestingly, the visibility of the exciton resonance becomes
greatly enhanced for small inter-particle distances due to the interference
phenomenon, multipole effects, and electromagnetic enhancement. We find that
the destructive interference is particularly strong. Using our exact theory, we
show that the interference effects can be observed experimentally even in the
exciting systems at room temperature.Comment: 9 page
Recommended from our members
Artifacts from manganese reduction in rock samples prepared by focused ion beam (FIB) slicing for X-ray microspectroscopic analysis
Abstract. Manganese (Mn)-rich natural rock coatings, so-called rock varnishes, are discussed controversially regarding their genesis. Biogenic and abiogenic mechanisms, as well as a combination of both, have been proposed to be responsible for the Mn oxidation and deposition process. We conducted scanning transmission X-ray microscopy - near edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS) measurements to examine the abundance and spatial distribution of the different oxidation states of Mn within these nano- to micrometer thick crusts. Such microanalytical measurements of thin and hard rock crusts require sample preparation with minimal contamination risk. Focused ion beam (FIB) slicing, a well-established technique in geosciences, was used in this study to obtain 100â200ânm thin slices of the samples for X-ray transmission spectroscopy. However, even though this preparation is suitable to investigate element distributions and structures in rock samples, we observed that, using standard parameters, modifications of the Mn oxidation states occur in the surfaces of the FIB slices. Based on our results, the preparation technique likely causes the reduction of Mn4+ to Mn2+/3+. We draw attention to this issue, since FIB slicing, SEM imaging, and other preparation and visualization techniques operating in the keV range are well-established in geosciences, but researchers are often unaware of the potential for reduction of Mn and possibly other elements in the samplesâ surface layers
Biosensing platform combining label-free and labelled analysis using Bloch surface waves
Bloch surface waves (BSW) propagating at the boundary of truncated photonic crystals (1D-PC) have emerged as an attractive approach for label-free sensing in plasmon-like sensor configurations. Due to the very low losses in such dielectric thin film stacks, BSW feature very low angular resonance widths compared to the surface plasmon resonance
(SPR) case. Besides label-free operation, the large field enhancement and the absence of quenching allow utilizing BSW coupled fluorescence detection to additionally sense the presence of fluorescent labels. This approach can be adapted to the case of angularly resolved resonance detection, thus giving rise to a combined label-free / labelled biosensor
platform. It features a parallel analysis of multiple spots arranged as a one-dimensional array inside a microfluidic channel of a disposable chip. Application of such a combined biosensing approach to the detection of the Angiopoietin-2 cancer biomarker in buffer solutions is reported
Magnetic structure in a U(Ru<sub>0.92</sub>Rh<sub>0.08</sub>)<sub>2</sub>Si<sub>2</sub> single crystal studied by neutron diffraction in static magnetic fields up to 24 T
We report the high-field induced magnetic phase in single crystal of
U(Ru0.92Rh0.08)2Si2. Our neutron study combined with high-field magnetization,
shows that the magnetic phase above the first metamagnetic transition at Hc1 =
21.6 T has an uncompensated commensurate antiferromagnetic structure with
propagation vector Q2 = ( 2/3 0 0) possessing two single-Q domains. U moments
of 1.45 (9) muB directed along the c axis are arranged in an up-up-down
sequence propagating along the a axis, in agreement with bulk measurements. The
U magnetic form factor at high fields is consistent with both the U3+ and U4+
type. The low field short-range order that emerges from the pure URu2Si2 due to
Rh-doping is initially strengthened by the field but disappears in the
field-induced phase. The tetragonal symmetry is preserved across the transition
but the a axis lattice parameter increases already at low fields. Our results
are in agreement with itinerant electron model with 5f states forming bands
pinned in the vicinity of the Fermi surface that is significantly reconstructed
by the applied magnetic field.Comment: 5 pages, 4 figures, accepted as Rapid Communication, Physical Review
B (2017
Fermi-surface topology of the iron pnictide LaFeP
We report on a comprehensive de Haas--van Alphen (dHvA) study of the iron
pnictide LaFeP. Our extensive density-functional band-structure
calculations can well explain the measured angular-dependent dHvA frequencies.
As salient feature, we observe only one quasi-two-dimensional Fermi-surface
sheet, i.e., a hole-like Fermi-surface cylinder around , essential for
pairing, is missing. In spite of considerable mass enhancements due to
many-body effects, LaFeP shows no superconductivity. This is likely
caused by the absence of any nesting between electron and hole bands.Comment: 5 pages, 4 figure
Extremely high magnetoresistance and conductivity in the type-II Weyl semimetals WP2 and MoP2
The peculiar band structure of semimetals exhibiting Dirac and Weyl crossings
can lead to spectacular electronic properties such as large mobilities
accompanied by extremely high magnetoresistance. In particular, two closely
neighbouring Weyl points of the same chirality are protected from annihilation
by structural distortions or defects, thereby significantly reducing the
scattering probability between them. Here we present the electronic properties
of the transition metal diphosphides, WP2 and MoP2, that are type-II Weyl
semimetals with robust Weyl points. We present transport and angle resolved
photoemission spectroscopy measurements, and first principles calculations. Our
single crystals of WP2 display an extremely low residual low-temperature
resistivity of 3 nohm-cm accompanied by an enormous and highly anisotropic
magnetoresistance above 200 million % at 63 T and 2.5 K. These properties are
likely a consequence of the novel Weyl fermions expressed in this compound. We
observe a large suppression of charge carrier backscattering in WP2 from
transport measurements.Comment: Appeared in Nature Communication
How Many Cooks Spoil the Soup?
In this work, we study the following basic question: "How much parallelism
does a distributed task permit?" Our definition of parallelism (or symmetry)
here is not in terms of speed, but in terms of identical roles that processes
have at the same time in the execution. We initiate this study in population
protocols, a very simple model that not only allows for a straightforward
definition of what a role is, but also encloses the challenge of isolating the
properties that are due to the protocol from those that are due to the
adversary scheduler, who controls the interactions between the processes. We
(i) give a partial characterization of the set of predicates on input
assignments that can be stably computed with maximum symmetry, i.e.,
, where is the minimum multiplicity of a state in
the initial configuration, and (ii) we turn our attention to the remaining
predicates and prove a strong impossibility result for the parity predicate:
the inherent symmetry of any protocol that stably computes it is upper bounded
by a constant that depends on the size of the protocol.Comment: 19 page
- âŠ