Liquid-like Interfaces Mediate Structural Phase Transitions in Lead Halide Perovskites

Microscopic pathways of structural phase transitions in metal halide perovskites are difficult to probe because they occur over disparate time and length scales and because electron-based microscopies typically used to directly probe nanoscale dynamics of phase transitions often damage metal halide perovskite materials. Using in situ nanoscale cathodoluminescence microscopy with low electron beam exposure, we visualize nucleation and growth in the thermally driven transition to the perovskite phase in hundreds of non-perovskite phase nanowires. In combination with molecular dynamics simulations, we reveal that the transformation does not follow a simple martensitic mechanism, but proceeds despite a substantial energy barrier via ion diffusion through a liquid-like interface between the two structures. While cations are disordered in this liquid-like region, the halide ions retain substantial spatial correlations. This detailed picture not only reveals how phase transitions between disparate structures can proceed, but also opens the possibility to control such processes

Great expectations: Russian defence conversion

Conversion of defence industries is neither simple in concept nor execution. While many wish to paint a rosy future and extol the costless benefits, pragma-tists stress the real and nominal disincentives to converting. Regarding the process in Russia, we outline what is involved and show that long-term obstacles remain. Conversion is not a state of nature, nor can it be imposed by domestic or international actors. Rather, it is a process, evolving from the new realities of the post-Cold War world.Defense conversion, Russia, Costs and benefits,

Server Assignment Policies for Maximizing the Steady-State Throughput of Finite Queueing Systems

For a system of finite queues, we study how servers should be assigned dynamically to stations in order to obtain optimal (or near-optimal) long-run average throughput. We assume that travel times between different service facilities are negligible, that each server can work on only one job at a time, and that several servers can work together on one job. We show that when the service rates depend only on either the server or the station (and not both), then all nonidling server assignment policies are optimal. Moreover, for a Markovian system with two stations in tandem and two servers, we show that the optimal policy assigns one server to each station unless that station is blocked or starved (in which case the server helps at the other station), and we specify the criterion used for assigning servers to stations. Finally, we propose a simple server assignment policy for tandem systems in which the number of stations equals the number of servers, and we present numerical results that show that our policy appears to yield near-optimal throughput under general conditions.Markov Decision Processes, Markovian Queueing Systems, Tandem Queues, Finite Buffers, Mobile and Cooperating Servers, Preemptive Service, Manufacturing Blocking