20 research outputs found
Excitonic properties of strained wurtzite and zinc-blende GaN/Al(x)Ga(1-x)N quantum dots
We investigate exciton states theoretically in strained GaN/AlN quantum dots
with wurtzite (WZ) and zinc-blende (ZB) crystal structures, as well as strained
WZ GaN/AlGaN quantum dots. We show that the strain field significantly modifies
the conduction and valence band edges of GaN quantum dots. The piezoelectric
field is found to govern excitonic properties of WZ GaN/AlN quantum dots, while
it has a smaller effect on WZ GaN/AlGaN, and very little effect on ZB GaN/AlN
quantum dots. As a result, the exciton ground state energy in WZ GaN/AlN
quantum dots, with heights larger than 3 nm, exhibits a red shift with respect
to the bulk WZ GaN energy gap. The radiative decay time of the red-shifted
transitions is large and increases almost exponentially from 6.6 ns for quantum
dots with height 3 nm to 1100 ns for the quantum dots with height 4.5 nm. In WZ
GaN/AlGaN quantum dots, both the radiative decay time and its increase with
quantum dot height are smaller than those in WZ GaN/AlN quantum dots. On the
other hand, the radiative decay time in ZB GaN/AlN quantum dots is of the order
of 0.3 ns, and is almost independent of the quantum dot height. Our results are
in good agreement with available experimental data and can be used to optimize
GaN quantum dot parameters for proposed optoelectronic applications.Comment: 18 pages, accepted for publication in the Journal of Applied Physic
Photoluminescence of tetrahedral quantum-dot quantum wells
Taking into account the tetrahedral shape of a quantum dot quantum well
(QDQW) when describing excitonic states, phonon modes and the exciton-phonon
interaction in the structure, we obtain within a non-adiabatic approach a
quantitative interpretation of the photoluminescence spectrum of a single
CdS/HgS/CdS QDQW. We find that the exciton ground state in a tetrahedral QDQW
is bright, in contrast to the dark ground state for a spherical QDQW. The
position of the phonon peaks in the photoluminescence spectrum is attributed to
interface optical phonons. We also show that the experimental value of the
Huang-Rhys parameter can be obtained only within the nonadiabatic theory of
phonon-assisted transitions.Comment: 4 pages, 4 figures, E-mail addresses: [email protected],
[email protected], [email protected], [email protected],
to be published in Phys. Rev. Letter
Spectral Complexity of Directed Graphs and Application to Structural Decomposition
We introduce a new measure of complexity (called spectral complexity) for
directed graphs. We start with splitting of the directed graph into its
recurrent and non-recurrent parts. We define the spectral complexity metric in
terms of the spectrum of the recurrence matrix (associated with the reccurent
part of the graph) and the Wasserstein distance. We show that the total
complexity of the graph can then be defined in terms of the spectral
complexity, complexities of individual components and edge weights. The
essential property of the spectral complexity metric is that it accounts for
directed cycles in the graph. In engineered and software systems, such cycles
give rise to sub-system interdependencies and increase risk for unintended
consequences through positive feedback loops, instabilities, and infinite
execution loops in software. In addition, we present a structural decomposition
technique that identifies such cycles using a spectral technique. We show that
this decomposition complements the well-known spectral decomposition analysis
based on the Fiedler vector. We provide several examples of computation of
spectral and total complexities, including the demonstration that the
complexity increases monotonically with the average degree of a random graph.
We also provide an example of spectral complexity computation for the
architecture of a realistic fixed wing aircraft system.Comment: We added new theoretical results in Section 2 and introduced a new
section 2.2 devoted to intuitive and physical explanations of the concepts
from the pape
Polar optical phonons in wurtzite spheroidal quantum dots: Theory and application to ZnO and ZnO/MgZnO nanostructures
Polar optical-phonon modes are derived analytically for spheroidal quantum
dots with wurtzite crystal structure. The developed theory is applied to a
freestanding spheroidal ZnO quantum dot and to a spheroidal ZnO quantum dot
embedded into a MgZnO crystal. The wurtzite (anisotropic) quantum dots are
shown to have strongly different polar optical-phonon modes in comparison with
zincblende (isotropic) quantum dots. The obtained results allow one to explain
and accurately predict phonon peaks in the Raman spectra of wurtzite
nanocrystals, nanorods (prolate spheroids), and epitaxial quantum dots (oblate
spheroids).Comment: 11 page
Scalable Approach to Uncertainty Quantification and Robust Design of Interconnected Dynamical Systems
Development of robust dynamical systems and networks such as autonomous
aircraft systems capable of accomplishing complex missions faces challenges due
to the dynamically evolving uncertainties coming from model uncertainties,
necessity to operate in a hostile cluttered urban environment, and the
distributed and dynamic nature of the communication and computation resources.
Model-based robust design is difficult because of the complexity of the hybrid
dynamic models including continuous vehicle dynamics, the discrete models of
computations and communications, and the size of the problem. We will overview
recent advances in methodology and tools to model, analyze, and design robust
autonomous aerospace systems operating in uncertain environment, with stress on
efficient uncertainty quantification and robust design using the case studies
of the mission including model-based target tracking and search, and trajectory
planning in uncertain urban environment. To show that the methodology is
generally applicable to uncertain dynamical systems, we will also show examples
of application of the new methods to efficient uncertainty quantification of
energy usage in buildings, and stability assessment of interconnected power
networks
The Redistribution of Power: Neurocardiac Signaling, Alcohol and Gender
Human adaptability involves interconnected biological and psychological control processes that determine how successful we are in meeting internal and environmental challenges. Heart rate variability (HRV), the variability in consecutive R-wave to R-wave intervals (RRI) of the electrocardiogram, captures synergy between the brain and cardiovascular control systems that modulate adaptive responding. Here we introduce a qualitatively new dimension of adaptive change in HRV quantified as a redistribution of spectral power by applying the Wasserstein distance with exponent 1 metric (W1) to RRI spectral data. We further derived a new index, D, to specify the direction of spectral redistribution and clarify physiological interpretation. We examined gender differences in real time RRI spectral power response to alcohol, placebo and visual cue challenges. Adaptive changes were observed as changes in power of the various spectral frequency bands (i.e., standard frequency domain HRV indices) and, during both placebo and alcohol intoxication challenges, as changes in the structure (shape) of the RRI spectrum, with a redistribution towards lower frequency oscillations. The overall conclusions from the present study are that the RRI spectrum is capable of a fluid and highly flexible response, even when oscillations (and thus activity at the sinoatrial node) are pharmacologically suppressed, and that low frequency oscillations serve a crucial but less studied role in physical and mental health
Confined Optical Phonon Modes in Aligned Nanorod Arrays Detected by Resonant Inelastic Light Scattering
n/
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Radiative lifetime of excitons in ZnO nanocrystals: The dead-layer effect
We theoretically investigate exciton states of colloidal nearly spherical ZnO nanocrystals with diameters from 2 nm to 6 nm. The sizes of considered ZnO nanocrystals are chosen to be slightly larger than the exciton Bohr radius of bulk ZnO. A number of characteristic features of excitons are revealed in this intermediate quantum confinement regime. The exciton center of mass is found to be prolate along the c axis of wurtzite ZnO and squeezed to the center of the ZnO nanocrystal, thus forming a dead layer near the nanocrystal surface. The thickness of the exciton dead layer is found to increase with the nanocrystal size reaching the value of about 1.6 nm for the nanocrystal with diameter of 6 nm. Based on our calculations we proposed an analytical approximation for the exciton radiative-lifetime dependence on radius R in ZnO nanocrystal written as tau_0/[1+(R/R_0)^3] with tau_0 = 73.4 ps and R_0 = 2.55 nm. Presented results and proposed analytical approximation can be used for interpretation of experimental data, and optimization of ZnO quantum dot structures for optoelectronic applications. (C) 2004 American Physical Society
Spectral Complexity of Directed Graphs and Application to Structural Decomposition
We introduce a new measure of complexity (called spectral complexity) for directed graphs. We start with splitting of the directed graph into its recurrent and nonrecurrent parts. We define the spectral complexity metric in terms of the spectrum of the recurrence matrix (associated with the reccurent part of the graph) and the Wasserstein distance. We show that the total complexity of the graph can then be defined in terms of the spectral complexity, complexities of individual components, and edge weights. The essential property of the spectral complexity metric is that it accounts for directed cycles in the graph. In engineered and software systems, such cycles give rise to subsystem interdependencies and increase risk for unintended consequences through positive feedback loops, instabilities, and infinite execution loops in software. In addition, we present a structural decomposition technique that identifies such cycles using a spectral technique. We show that this decomposition complements the well-known spectral decomposition analysis based on the Fiedler vector. We provide several examples of computation of spectral and total complexities, including the demonstration that the complexity increases monotonically with the average degree of a random graph. We also provide an example of spectral complexity computation for the architecture of a realistic fixed wing aircraft system