140 research outputs found
Optimal control of non-stationary differential linear repetitive processes
Differential repetitive processes are a distinct class of continuousdiscrete 2D linear systems of both systems theoretic and applications interest. The feature which makes them distinct from other classes of such systems is the fact that information propagation in one of the two independent directions only occurs over a finite interval. Applications areas include iterative learning control and iterative solution algorithms for classes of dynamic nonlinear optimal control problems based on the maximum principle, and the modelling of numerous industrial processes such as metal rolling, and long-wall cutting etc. The new results in is paper solve a general optimal problem in the presence of non-stationary dynamics
Viscosity Limits for Zeroth-Order Pseudodifferential Operators
Motivated by the work of Colin de Verdière and Saint-Raymond on spectral theory for zeroth-order pseudodifferential operators on tori, we consider viscosity limits in which zeroth-order operators, P, are replaced by P + iν Δ, ν > 0. By adapting the Helffer–Sjöstrand theory of scattering resonances, we show that, in a complex neighbourhood of the continuous spectrum, eigenvalues of P + iν Δ have limits as the viscosity ν goes to 0. In the simplified setting of tori, this justifies claims made in the physics literature. © 2021 The Authors. Communications on Pure and Applied Mathematics published by Wiley Periodicals LLC
Constrained optimal control theory for differential linear repetitive processes
Differential repetitive processes are a distinct class of continuous-discrete two-dimensional linear systems of both systems theoretic and applications interest. These processes complete a series of sweeps termed passes through a set of dynamics defined over a finite duration known as the pass length, and once the end is reached the process is reset to its starting position before the next pass begins. Moreover the output or pass profile produced on each pass explicitly contributes to the dynamics of the next one. Applications areas include iterative learning control and iterative solution algorithms, for classes of dynamic nonlinear optimal control problems based on the maximum principle, and the modeling of numerous industrial processes such as metal rolling, long-wall cutting, etc. In this paper we develop substantial new results on optimal control of these processes in the presence of constraints where the cost function and constraints are motivated by practical application of iterative learning control to robotic manipulators and other electromechanical systems. The analysis is based on generalizing the well-known maximum and -maximum principles to the
On the connection between discrete linear repetitive processes and 2-D discrete linear systems
A direct method is developed that reduces a polynomial system matrix describinga discrete linear repetitive process to a 2-D singular state-space form such that all the relevant properties, including the zero structure of the system matrix, are retained. It is shown that the transformation linking the original polynomial system matrix with its associated 2-D singular form is zero coprime system equivalence. The exact nature of the resulting system matrix in singular form and the transformation involved are established
Revealing large-scale homogeneity and trace impurity sensitivity of GaAs nanoscale membranes
III-V nanostructures have the potential to revolutionize optoelectronics and
energy harvesting. For this to become a reality, critical issues such as
reproducibility and sensitivity to defects should be resolved. By discussing
the optical properties of MBE grown GaAs nanomembranes we highlight several
features that bring them closer to large scale applications. Uncapped membranes
exhibit a very high optical quality, expressed by extremely narrow neutral
exciton emission, allowing the resolution of the more complex excitonic
structure for the first time. Capping of the membranes with an AlGaAs shell
results in a strong increase of emission intensity but also to a shift and
broadening of the exciton peak. This is attributed to the existence of
impurities in the shell, beyond MBE-grade quality, showing the high sensitivity
of these structures to the presence of impurities. Finally, emission properties
are identical at the sub-micron and sub-millimeter scale, demonstrating the
potential of these structures for large scale applications.Comment: just accepted in Nano Letters,
http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.7b0025
Isotopic composition of daily precipitation along the southern foothills of the Himalayas: impact of marine and continental sources of atmospheric moisture
The flow
of the Himalayan rivers, a key source of fresh water for more than a billion
people primarily depends upon the strength, behaviour and duration of the
Indian summer monsoon (ISM) and the western disturbances (WD), two
contrasting circulation regimes of the regional atmosphere. An analysis of
the 2H and 18O isotope composition of daily precipitation
collected along the southern foothills of the Himalayas, combined with
extensive backward trajectory modelling, was used to gain deeper insight into
the mechanisms controlling the isotopic composition of precipitation and the
origin of atmospheric moisture and precipitation during ISM and WD periods.
Daily precipitation samples were collected during the period from September
2008 to December 2011 at six stations, extending from Srinagar in the west
(Kashmir state) to Dibrugarh in the east (Assam state). In total, 548 daily
precipitation samples were collected and analysed for their stable isotope
composition. It is suggested that the gradual reduction in the 2H
and 18O content of precipitation in the study region, progressing
from δ18O values close to zero down to ca. −10 ‰ in
the course of ISM evolution, stems from regional, large-scale recycling of
moisture-driven monsoonal circulation. Superimposed on this general trend are
short-term fluctuations of the isotopic composition of rainfall, which might
have stem from local effects such as enhanced convective activity and the
associated higher degree of rainout of moist air masses (local amount
effect), the partial evaporation of raindrops, or the impact of isotopically
heavy moisture generated in evapotranspiration processes taking place in the
vicinity of rainfall sampling sites. Seasonal footprint maps constructed for
three stations representing the western, central and eastern portions of the
Himalayan region indicate that the influence of monsoonal circulation reaches
the western edges of the Himalayan region. While the characteristic imprint
of monsoonal air masses (increase of monthly rainfall amount) can be
completely absent in the western Himalayas, the onset of the ISM period in
this region is still clearly visible in the isotopic composition of daily
precipitation. A characteristic feature of daily precipitation collected
during the WD period is the gradual increase of 2H and
18O content, reaching positive δ2H and
δ18O values towards the end of the period. This trend can be
explained by the growing importance of moisture of continental origin as a
source of daily precipitation. High deuterium-excess (d-excess) values of
daily rainfall recorded at the monitoring stations (38 cases in total, range
from 20.6 to 44.0 ‰) are attributed to moisture of continental
origin released into the atmosphere during the evaporation of surface water
bodies and/or soil water evaporation
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Excitonic Properties of Low-Band-Gap Lead-Tin Halide Perovskites
The MAPb1–xSnxI3 (x = 0–1) (MA = methylammonium)
perovskite family comprises a range of ideal absorber band gaps for
single- and multijunction perovskite solar cells. Here, we use spectroscopic
measurements to reveal a range of hitherto unknown fundamental properties
of this materials family. Temperature-dependent transmission results
show that the temperature of the tetragonal to orthorhombic structural
transition decreases with increasing tin content. Through low-temperature
magnetospectroscopy, we show that the exciton binding energy is lower
than 16 meV, revealing that the dominant photogenerated species at
typical operational conditions of optoelectronic devices are free
charges rather than excitons. The reduced mass increases approximately
proportionally to the band gap, and the mass values (0.075–0.090me) can be described with a two-band k·p
perturbation model extended across the broad band gap range of 1.2–2.4
eV. Our findings can be generalized to predict values for the effective
mass and binding energy for other members of this family of materials
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