Strain Stiffening Induced by Molecular Motors in Active Crosslinked Biopolymer Networks

We have studied the elastic response of actin networks with both compliant and rigid crosslinks by modeling molecular motors as force dipoles. Our finite element simulations show that for compliant crosslinkers such as filamin A, the network can be stiffened by two orders of magnitude while stiffening achieved with incompliant linkers such as scruin is significantly smaller, typically a factor of two, in excellent agreement with recent experiments. We show that the differences arise from the fact that the motors are able to stretch the compliant crosslinks to the fullest possible extent, which in turn causes to the deformation of the filaments. With increasing applied strain, the filaments further deform leading to a stiffened elastic response. When the crosslinks are incompliant, the contractile forces due to motors do not alter the network morphology in a significant manner and hence only small stiffening is observed.Comment: 4 pages, 5 figure

Effect of doping on polarization profiles and switching in semiconducting ferroelectric thin films

This paper proposes a theory to describe the polarization and switching behavior of ferroelectrics that are also wide-gap semiconductors. The salient feature of our theory is that it does not make any a priori assumption about either the space charge distribution or the polarization profile. The theory is used to study a metal-ferroelectric-metal capacitor configuration, where the ferroelectric is n-type doped. The main result of our work is a phase diagram as a function of doping level and thickness that shows different phases, namely, films with polarization profiles that resemble that of undoped classical ferroelectrics, paraelectric, and a new head-to-tail domain structure. We have identified a critical doping level, which depends on the energy barrier in the Landau energy and the built-in potential, which is decided by the electronic structures of both the film and the electrodes. When the doping level is below this critical value, the behavior of the films is almost classical. We see a depleted region, which extends through the film when the film thickness is very small, but is confined to two boundary layers near the electrodes for large film thickness. When the doping level is higher than the critical value, the behavior is classical for only very thin films. Thicker films at this doping level are forced into a tail-to-tail configuration with three depletion layers, lose their ferroelectricity, and may thus be described as nonlinear dielectric or paraelectric. For films which are doped below the critical level, we show that the field required for switching starts out at the classical coercive field for very thin films, but gradually decreases

Depletion Layers and Domain Walls in Semiconducting Ferroelectric Thin Films

Commonly used ferroelectric perovskites are also wide-band-gap semiconductors. In such materials, the polarization and the space-charge distribution are intimately coupled, and this Letter studies them simultaneously with no a priori ansatz on either. In particular, we study the structure of domain walls and the depletion layers that form at the metal-ferroelectric interfaces. We find the coupling between polarization and space charges leads to the formation of charge double layers at the 90° domain walls, which, like the depletion layers, are also decorated by defects like oxygen vacancies. In contrast, the 180° domain walls do not interact with the defects or space charges. Implications of these results to domain switching and fatigue in ferroelectric devices are discussed

Losing the World's Best and Brightest

Presents findings from a survey of Indian, Chinese, and European students at U.S. colleges and universities on their decisions to stay or return home after graduation and the factors behind the decisions, such as where they see the best opportunities

Inventory Management under Product Mis-identification/Shipment Errors

“Wrong-product” delivery - the delivery of a product different from that desired - is a significant, but as yet unexplored problem in supply-chain management research. There are basically two reasons for wrong-product delivery: either the wrong product is mistakenly ordered or the right product is ordered but the wrong product is picked/shipped. This paper defines and analyzes the “wrong-product delivery” problem using a 2-product newsvendor model. Two non-substitutable products may be ordered at the beginning of each time period. However, whenever product i is ordered, then with known probability i, product j is delivered; i, j = 1, 2(i 6= j). First, we analyze the “no-recourse scenario”, where management correctly stores whatever was received, but takes no other action. We establish the form of the optimal policy and conduct sensitivity analysis. Although our modeling framework is simple, our results are unexpected and non-intuitive. For example, it is well known that in the single-product newsvendor model, increasing the uncertainty of demand or supply will yield an increase in the corresponding target basestocks and safety stocks. However, increasing the risk of a wrong-product error yields a decrease in the corresponding basestocks and safety stocks. Further, although target basestocks in the single-product newsvendor model are invariant to increases in on-hand inventory, we show that the target basestock for either product is non-decreasing as its inventory increases. We also demonstrate that the cost impact of wrong-product uncertainty is comparable, if not larger than, the cost impact of demand uncertainty. Next, we analyze the “recourse scenario” where management is able to correct errors but only by incurring a fixed cost of $K. We show that it is optimal to take recourse when the wrong-product uncertainty is sufficiently small, but not take recourse when the wrong-product uncertainty is high. In strategic terms, our analysis provides insight into the cost impact of wrong-product errors, and, hence, the importance of reducing them.Supply chain management, Inventory management, Shipment errors, Ordering errors, Yield management, Unreliable supply

Natural Regulation of Energy Flow in a Green Quantum Photocell

Manipulating the flow of energy in nanoscale and molecular photonic devices is of both fundamental interest and central importance for applications in light harvesting optoelectronics. Under erratic solar irradiance conditions, unregulated power fluctuations in a light harvesting photocell lead to inefficient energy storage in conventional solar cells and potentially fatal oxidative damage in photosynthesis. Here, we show that regulation against these fluctuations arises naturally within a two-channel quantum heat engine photocell, thus enabling the efficient conversion of varying incident solar spectrum at Earth's surface. Remarkably, absorption in the green portion of the spectrum is avoided, as it provides no inherent regulatory benefit. Our findings illuminate a quantum structural origin of regulation, provide a novel optoelectronic design strategy, and may elucidate the link between photoprotection in photosynthesis and the predominance of green plants on Earth.Comment: 17 pages, 4 figure