29 research outputs found
Allocation in Practice
How do we allocate scarcere sources? How do we fairly allocate costs? These
are two pressing challenges facing society today. I discuss two recent projects
at NICTA concerning resource and cost allocation. In the first, we have been
working with FoodBank Local, a social startup working in collaboration with
food bank charities around the world to optimise the logistics of collecting
and distributing donated food. Before we can distribute this food, we must
decide how to allocate it to different charities and food kitchens. This gives
rise to a fair division problem with several new dimensions, rarely considered
in the literature. In the second, we have been looking at cost allocation
within the distribution network of a large multinational company. This also has
several new dimensions rarely considered in the literature.Comment: To appear in Proc. of 37th edition of the German Conference on
Artificial Intelligence (KI 2014), Springer LNC
Interaction potential between dynamic dipoles: polarized excitons in strong magnetic fields
The interaction potential of a two-dimensional system of excitons with
spatially separated electron-hole layers is considered in the strong magnetic
field limit. The excitons are assumed to have free dynamics in the -
plane, while being constrained or `polarized' in the direction. The model
simulates semiconductor double layer systems under strong magnetic field normal
to the layers. The {\em residual} interaction between excitons exhibits
interesting features, arising from the coupling of the center-of-mass and
internal degrees of freedom of the exciton in the magnetic field. This coupling
induces a dynamical dipole moment proportional to the center-of-mass magnetic
moment of the exciton. We show the explicit dependence of the inter-exciton
potential matrix elements, and discuss the underlying physics. The unusual
features of the interaction potential would be reflected in the collective
response and non-equilibrium properties of such system.Comment: REVTEX - 11 pages - 1 fi
Electric field and exciton structure in CdSe nanocrystals
Quantum Stark effect in semiconductor nanocrystals is theoretically
investigated, using the effective mass formalism within a
Baldereschi-Lipari Hamiltonian model for the hole states. General expressions
are reported for the hole eigenfunctions at zero electric field. Electron and
hole single particle energies as functions of the electric field
() are reported. Stark shift and binding energy of the
excitonic levels are obtained by full diagonalization of the correlated
electron-hole Hamiltonian in presence of the external field. Particularly, the
structure of the lower excitonic states and their symmetry properties in CdSe
nanocrystals are studied. It is found that the dependence of the exciton
binding energy upon the applied field is strongly reduced for small quantum dot
radius. Optical selection rules for absorption and luminescence are obtained.
The electric-field induced quenching of the optical spectra as a function of
is studied in terms of the exciton dipole matrix element. It
is predicted that photoluminescence spectra present anomalous field dependence
of the emission lines. These results agree in magnitude with experimental
observation and with the main features of photoluminescence experiments in
nanostructures.Comment: 9 pages, 7 figures, 1 tabl
Central Amygdala Prepronociceptin-Expressing Neurons Mediate Palatable Food Consumption and Reward
Food palatability is one of many factors that drives food consumption, and the hedonic drive to feed is a key contributor to obesity and binge eating. In this study, we identified a population of prepronociceptin-expressing cells in the central amygdala (PnocCeA)that are activated by palatable food consumption. Ablation or chemogenetic inhibition of these cells reduces palatable food consumption. Additionally, ablation of PnocCeA cells reduces high-fat-diet-driven increases in bodyweight and adiposity. PnocCeA neurons project to the ventral bed nucleus of the stria terminalis (vBNST), parabrachial nucleus (PBN), and nucleus of the solitary tract (NTS), and activation of cell bodies in the central amygdala (CeA)or axons in the vBNST, PBN, and NTS produces reward behavior but did not promote feeding of palatable food. These data suggest that the PnocCeA network is necessary for promoting the reinforcing and rewarding properties of palatable food, but activation of this network itself is not sufficient to promote feeding
Ultrafast carrier relaxation and vertical-transport phenomena in semiconductor superlattices: A Monte Carlo analysis
The ultrafast dynamics of photoexcited carriers in semiconductor superlattices is studied theoretically on the basis of a Monte Carlo solution of the coupled Boltzmann transport equations for electrons and holes. The approach allows a kinetic description of the relevant interaction mechanisms such as intra- miniband and interminiband carrier-phonon scattering processes. The energy relaxation of photoexcited carriers, as well as their vertical transport, is investigated in detail. The effects of the multiminiband nature of the superlattice spectrum on the energy relaxation process are discussed with particular emphasis on the presence of Bloch oscillations induced by an external electric field. The analysis is performed for different superlattice structures and excitation conditions. It shows the dominant role of carrier-polar-optical-phonon interaction in determining the nature of the carrier dynamics in the low-density limit. In particular, the miniband width, compared to the phonon energy, turns out to be a relevant quantity in predicting the existence of Bloch oscillations
Resonant and off-resonant transients in electromagnetically induced transparency: Turn-on and turn-off dynamics
Published versio
Luminescence spectra and kinetics of disordered solid solutions
We have studied both theoretically and experimentally the luminescence spectra and kinetics of crystalline, disordered solid solutions after pulsed excitation. First, we present the model calculations of the steady-state luminescence band shape caused by recombination of excitons localized in the wells of random potential induced by disorder. Classification of optically active tail states of the main exciton band into two groups is proposed. The majority of the states responsible for the optical absorption corresponds to the group of extended states belonging to the percolation cluster, whereas only a relatively small group of âradiativeâ states forms the steady-state luminescence band. The continuum percolation theory is applied to distinguish the âradiativeâ localized states, which are isolated in space and have no ways for nonradiative transitions along the tail states. It is found that the analysis of the exciton-phonon interaction gives the information about the character of the localization of excitons. We have shown that the model used describes quite well the experimental cw spectra of CdS(1âc)Sec and ZnSe(1âc)Tec solid solutions. Further, the experimental results are presented for the temporal evolution of the luminescence band. It is shown that the changes of band shape with time come from the interplay of population dynamics of extended states and spatially isolated âradiativeâ states. Finally, the measurements of the decay of the spectrally integrated luminescence intensity at long delay times are presented. It is shown that the observed temporal behavior can be described in terms of relaxation of separated pairs followed by subsequent exciton formation and radiative recombination. Electron tunneling processes are supposed to be responsible for the luminescence in the long-time limit at excitation below the exciton mobility edge. At excitation by photons with higher energies the diffusion of electrons can account for the observed behavior of the luminescence
BOUND EXCITON TRANSFER IN WEAKLY DISORDERED SYSTEMS : III-V ALLOYS
Les mécanismes de transfert des excitons liés à l'azote dans les alliages GaxIn1-xP : N et GaAs1-xPx : N sont observés en fonction de la température par des mesures de luminescence résolue en temps (laser picoseconde). Les resultats (saut direct entre des états localisés et piégeage multiple via les excitons libres) sont différents de ceux obtenus pour le binaire GaP : N. Nous montrons ainsi l'importance du desordre d'alliage sur la dynamique des excitons.Transfer mechanisms of N-bound excitons in GaxIn1-xP : N and GaAs1-xPx : N alloys are studied as a function of temperature using picosecond time resolved photoluminescence. The results, direct exciton tunneling between localized states and multiple trapping via free excitons, are different from those seen in the binary GaP : N. Thus, we demonstrate the importance of alloy disorder on the exciton dynamics