Operating policies in a model with terminal scrapping

We draw on three strands of literature dealing with utilization, maintenance, and scrapping in order to analyze the properties of the respective policies and their interac-tions. We do so by focusing on the last period of the received multi-period service life model and extending it in three directions: first, by associating the physical deteriora-tion of equipment to the intensity of its utilization and maintenance; second, by ex-panding on the range of explainable operating policies to allow for idling, mothballing, capacity depleting, capacity preserving, full capacity, upgrading, and downgrading; and, third, by linking the operating policies to the capital policy of scrapping. Owing to these enhancements, the analysis leads to several important findings. One among them is that optimal operating policies behave usually in opposite directions, proceed-ing in time from harder to softer or vice versa, depending on the net revenue earning capability of the equipment under consideration. Another is that profit (loss) making equipment is scrappable iff on the average the operating capital deteriorates faster (slower), or equivalently improves slower (faster), than the scrapping capital. And still an-other result is that operating policies are determined jointly with scrapping policy capi-tal policies, thus suggesting that empirical investigations of their determinants should allow for this simultaneity.Utilization, maintenance, idling, mothballing, capacity depleting, capacity pre-serving, upgrading, downgrading, scrapping.

A Rehabilitation of Economic Replacement Theory

Our objective in this paper is to shed light on the economic forces and the specific way in which they combine to determine the service life, and hence the replacement demand for durables, in the short run and in the long run. For this purpose the received multiperiod economic replacement model is extended in the light of more recent theoretical developments and solved for the number and duration of replacements. Owing mainly to the intuition that the latter decisions are inexplicably related to the owner s profit horizon, aside from steady state replacement, the model is shown to yield a range of transitional and limiting replacement policies that have been largely ignored in the literature. In addition, the results indicate that : a) the optimal service life is consistently determined by such conventional forces of short-term variation as utilization, maintenance, operating safety, interest rate, uncertainty due to technological breakthroughs, the price of new and used durables, etc., b) switching among replacement policies produces bursts or slumps in replacement investment much like the spikes discovered recently at the plant level, and c) in non- stationary economic environments the error from applying steady state replacement, instead of the more appropriate transitory replacement policies reported in this paper, may be substantial.service life, replacement, and scrapping

The Transmission Property of the Discrete Heisenberg Ferromagnetic Spin Chain

We present a mechanism for displaying the transmission property of the discrete Heisenberg ferromagnetic spin chain (DHF) via a geometric approach. By the aid of a discrete nonlinear Schr\"odinger-like equation which is the discrete gauge equivalent to the DHF, we show that the determination of transmitting coefficients in the transmission problem is always bistable. Thus a definite algorithm and general stochastic algorithms are presented. A new invariant periodic phenomenon of the non-transmitting behavior for the DHF, with a large probability, is revealed by an adoption of various stochastic algorithms.Comment: 16 pages, 7 figure

Nonlinear optics of III-V semiconductors in the terahertz regime: an ab-initio study

We compute from first principles the infrared dispersion of the nonlinear susceptibility $\chi^{(2)}$ in zincblende semiconductors. At terahertz frequencies the nonlinear susceptibility depends not only on the purely electronic response $\chi^{(2)}_{\infty}$, but also on three other parameters $C_1$, $C_2$ and $C_3$ describing the contributions from ionic motion. They relate to the TO Raman polarizability, the second-order displacement-induced dielectric polarization, and the third-order lattice potential. Contrary to previous theory, we find that mechanical anharmonicity ($C_3$) dominates over electrical anharmonicity ($C_2$), which is consistent with recent experiments on GaAs. We predict that the sharp minimum in the intensity of second-harmonic generation recently observed for GaAs between $\omega_{\rm TO}/2$ and $\omega_{\rm TO}$ does not occur for several other III-V compounds.Comment: 9 pages, 3 figures; updated bibliograph

From Single-Cell to Whole-Body: Developing a Molecular Neuroscience Toolkit

Throughout my Ph.D. I have worked on technology development, at first to answer basic scientific questions and eventually for therapeutic applications. This technology development applied to a variety of fields, from neuroscience to development to gene therapy, and acted upon biological systems in a wide range of scale, from the single-cell monitoring to organism-wide gene-transfer. My graduate research began with the engineering of microbial rhodopsin spectral properties and fluorescence. By making use of their ability to absorb light and emit fluorescence in a voltage-dependent manner, I aimed to interrogate neuronal activity during behavior at the single-cell level. That line of research ended with publication of the voltage-sensor Archer, which I used to track activity of a single cell in vivo in awake, behaving worms. I then shifted from tracking activity at the single cell level, to visualizing entire organisms, by developing clearing techniques that enable a high-resolution, three-dimensional analysis of a diverse range of tissues. I began by optimizing tissue-clearing parameters for various tissue types and a wide variety of experimental needs. I then took that knowledge and applied it to visualizing and tracking the developing neural crest in cleared, whole-mount chicken embryos, discovering some unexpected derivates. Finally, I became interested not only in visualizing entire organisms, but in developing technologies to facilitate gene transfer throughout the body. The rapidly growing field of gene therapy is in constant need of new tools that target specific tissues, avoiding off-target effects. The end of my Ph.D. has been spent engineering viruses that can be delivered body-wide, but target only specific areas of therapeutic interest, like the brain and lungs.</p

On the Optimal Lifetime of Real Assets

We show that the “abandonment” model emphasized by researchers in capital budgeting and the “steady state” replacement model emphasized by economic theorists constitute sub-cases of a more general class of transitory replacement models in which the horizon of reinvestments is determined endogenously along with the other decision variables. Moreover, comparisons between our model and that of steady state replacement revealed that there are considerable differences. In particular, we found that: i) the two models lead to different estimates concerning the profit horizon, the duration of replacements, the timing of abandonment or scrapping, and the impact of productive capacity and market structure on service lives, as these are determined by various parameters, ii) even though the steady state replacement policy may result in higher total profit, it does so at great expense in flexibility for the planner, because the replacements are built into the model from the beginning, and iii) the transitory replacement policy seems more realistic in that the replacements are undertaken only if forced on the planner by decreasing profits

Broad gene expression throughout the mouse and marmoset brain after intravenous delivery of engineered AAV capsids

Genetic intervention is increasingly explored as a therapeutic option for debilitating disorders of the central nervous system. The safety and efficacy of gene therapies relies upon expressing a transgene in affected cells while minimizing off-target expression. To achieve organ/cell-type specific targeting after intravenous delivery of viral vectors, we employed a Cre-transgenic-based screening platform for fast and efficient capsid selection, paired with sequential engineering of multiple surface-exposed loops. We identified capsid variants that are enriched in the brain and detargeted from the liver in mice. The improved enrichment in the brain extends to non-human primates, enabling robust, non-invasive gene delivery to the marmoset brain following IV administration. Importantly, the capsids identified display non-overlapping cell-type tropisms within the brain, with one exhibiting high specificity to neurons. The ability to cross the blood–brain barrier with cell-type specificity in rodents and non-human primates enables new avenues for basic research and potential therapeutic interventions unattainable with naturally occurring serotypes

Broad gene expression throughout the mouse and marmoset brain after intravenous delivery of engineered AAV capsids

Genetic intervention is increasingly explored as a therapeutic option for debilitating disorders of the central nervous system. The safety and efficacy of gene therapies relies upon expressing a transgene in affected cells while minimizing off-target expression. To achieve organ/cell-type specific targeting after intravenous delivery of viral vectors, we employed a Cre-transgenic-based screening platform for fast and efficient capsid selection, paired with sequential engineering of multiple surface-exposed loops. We identified capsid variants that are enriched in the brain and detargeted from the liver in mice. The improved enrichment in the brain extends to non-human primates, enabling robust, non-invasive gene delivery to the marmoset brain following IV administration. Importantly, the capsids identified display non-overlapping cell-type tropisms within the brain, with one exhibiting high specificity to neurons. The ability to cross the blood–brain barrier with cell-type specificity in rodents and non-human primates enables new avenues for basic research and potential therapeutic interventions unattainable with naturally occurring serotypes

Transition from ballistic to drift motion in high-field transport in GaAs

With strong THz pulses, we measure ultrafast transport of electrons, holes, and an electron-hole plasma in GaAs. The transition from ballistic to drift-like transport is strongly influenced by electron-hole scattering

Nonequilibrium Electron Interactions in Metal Films

Ultrafast relaxation dynamics of an athermal electron distribution is investigated in silver films using a femtosecond pump-probe technique with 18 fs pulses in off-resonant conditions. The results yield evidence for an increase with time of the electron-gas energy loss rate to the lattice and of the free electron damping during the early stages of the electron-gas thermalization. These effects are attributed to transient alterations of the electron average scattering processes due to the athermal nature of the electron gas, in agreement with numerical simulations