Orbital magnetoelectric coupling in band insulators

Magnetoelectric responses are a fundamental characteristic of materials that break time-reversal and inversion symmetries (notably multiferroics) and, remarkably, of "topological insulators" in which those symmetries are unbroken. Previous work has shown how to compute spin and lattice contributions to the magnetoelectric tensor. Here we solve the problem of orbital contributions by computing the frozen-lattice electronic polarization induced by a magnetic field. One part of this response (the "Chern-Simons term") can appear even in time-reversal-symmetric materials and has been previously shown to be quantized in topological insulators. In general materials there are additional orbital contributions to all parts of the magnetoelectric tensor; these vanish in topological insulators by symmetry and also vanish in several simplified models without time-reversal and inversion those magnetoelectric couplings were studied before. We give two derivations of the response formula, one based on a uniform magnetic field and one based on extrapolation of a long-wavelength magnetic field, and discuss some of the consequences of this formula.Comment: 13 page

Shading and Smothering of Gamma Ray Bursts

The gamma ray burst (GRB) 980425 is distinctive in that it seems to be associated with supernova (SN) 1998bw, has no X-ray afterglow, and has a single peak light curve and a soft spectrum. The supernova is itself unusual in that its expansion velocity exceeds c/6. We suggest that many of these features can be accounted for with the hypothesis that we observe the GRB along a penumbral line of sight that contains mainly photons that have scattered off ejected baryons. The hypothesis suggests a baryon poor jet (BPJ) existing within a baryon rich outflow. The sharp distinction can be attributed to whether or not the magnetic field lines thread an event horizon. Such a configuration suggests that there will be some non-thermal acceleration of pick-up ex-neutrons within the BPJ. This scenario might produce observable spallation products and neutrinos.Comment: 7 pages, 2 figures, submitted to ApJ

A Political Economy of Access: Infrastructure, Networks, Cities, and Institutions

Why should you read another book about transport and land use? This book differs in that we won’t focus on empirical arguments – we present political arguments. We argue the political aspects of transport policy shouldn’t be assumed away or treated as a nuisance. Political choices are the core reasons our cities look and function the way they do. There is no original sin that we can undo that will lead to utopian visions of urban life. The book begins by introducing and expanding on the idea of Accessibility. Then we proceed through several major parts: Infrastructure Preservation, Network Expansion, Cities, and Institutions. Infrastructure preservation concerns the relatively short-run issues of how to maintain and operate the existing surface transport system (roads and transit). Network expansion in contrast is a long-run problem, how to enlarge the network, or rather, why enlarging the network is now so difficult. Cities examines how we organize, regulate, and expand our cities to address the failures of transport policy, and falls into the time-frame of the very long-run, as property rights and land uses are often stickier than the concrete of the network is durable. In the part on Institutions we consider things that might at first blush appear to be short-run and malleable, are in fact very long-run. Institutions seem to outlast the infrastructure they manage. Many of the transport and land use problems we want to solve already have technical solutions. What these problems don’t have, and what we hope to contribute, are political solutions. We expect the audience for this book to be practitioners, planners, engineers, advocates, urbanists, students of transport, and fellow academics

Estimating the Social Gap With a Game Theory Model of Lane Changing

Changing lanes is a commonly-used technique for drivers to either overtake slow-moving cars or enter/exit highway ramps. Optional lane changes may save drivers travel time but increase the risk of collision with others. Drivers make such decisions based on experience and emotion rather than analysis, and thus may fail to select the best solution while in a dynamic state of flux. Unlike human drivers, autonomous vehicles can systematically analyze their surroundings and make real-time decisions accordingly. This paper develops a game theory-based lane-changing model by comparing two types of optimization methods. To realize our expectations, we need to first investigate the payoff function of drivers in discretionary lane-changing maneuvers and then quantify it in an equation of costs that trades-off safety and time-saving. After the evaluation for each alternative strategy combination, the results show that there exists a social gap in the discretionary lane-changing game. To deal with that problem, we provide some suggestions for future policy as well as autonomous vehicle controller designs, offering solutions to reduce the impact of disturbances and crashes caused by inappropriate lane changes, and also, inspire further research about more complex cases

Temporal Variations in Activity Network Using Smart Card Data

This study explores temporal variations in activity networks for four million passengers, differentiated as workers and non-workers, using public transport based on a large-scale smart card dataset generated over 105 days in Beijing. We aim to capture their day-to-day transition and cumulative temporal expansion in activity network using transit over days, weeks, and months. Particularly, workers and non-workers are automatically identified based on their different daily routines, whose activity networks are characterized by six features concerning space coverage, distance coverage, and frequency coverage in two ways, namely, on a per-day transition and with an accumulation of days. The transition features of the networks are statistically analyzed and compared by time, while how the expansion features evolve with time are modeled. Results show that, on weekdays, workers are more likely to travel longer (have larger distance coverage), but cover less area (have smaller space coverage) than non- workers. While opposite patterns occur on weekends. Traveling in the ‘North-South’ direction is weakly correlated with traveling in the ‘East-West’ direction. Workers on weekdays, as well as non-workers on weekends, make longer ‘North-South’ trips. Manhattan distance, trip count, and perimeter present a ∩ shape in their probability density functions, while the remaining features decline dramatically, with probability density functions fit by the exponential distribution. The distance coverage expands faster than that of space coverage. Most passengers increase coverage of space and distance when time expands (obviously no one decreases coverage over time, but some don’t change). The research enables findings on temporal load-balancing, long-term cumulative expansion in travel demands of workers and non-workers, re-balancing the distribution of existing workplace and residential location opportunities, and constructing transit-oriented developments with mixed functions over time.Chinese Scholarship Council TransportLa

A Schematic for Focusing on Youth in Investigations of Community Design and Physical Activity

This paper provides a first step in addressing special considerations for youth in a relatively new area of physical activity research. After reviewing the urgent need for novel approaches to increasing physical activity, the growing interest in the effects of community design are discussed. Although most discussion on this topic has focused on adults, there are important differences between youth and adults that warrant a special focus on youth and need to be accounted for. This article presents a schematic that accounts for how and where youth spend their time, decomposing the day into time spent in travel and time spent at destinations, and identifying portions of those times that are spent engaged in physical activity. By focusing on both spatial and behavioral dimensions of youth time, the schematic may help organize and advance scientific inquiry into the relationships between community design and physical activity specifically for youth

Governing for Access

The only reason to locate anywhere is to be near some people, places, and things (opportunities) and be far from others. Access quantifies the ability to reach, or be reached by, people, places, and things. It explains much of the variation in real estate prices and development density. It does so in large part because real estate capitalises into land value the ease of travel to desired opportunities. Physical infrastructure networks like roads and rails exist to connect within and between places faster than travel without them. Transport agencies often plan networks as if the land use is given, and regulators plan and zone development as if the network were unchangeable. Since the efficiency of a transport network depends on the land use pattern and the efficiency of the land use pattern depends on the network configuration, systems which coordinate these may be more efficient than those where transport and land use are planned independently. This is especially pertinent for long-term capital investments which are largely irreversible. While compared to some peer cities, Sydney has done a good job coordinating transport services and land development, it can do much better. This would lead to shorter commutes lengths, greater public and active transport mode shares, higher employment and incomes, and greater productivity. Overall Sydney would be a more desirable and convenient city. This is also important as better coordinating transport with land development while better balancing jobs and housing will thereby reduce motor vehicle travel. Reducing private vehicle travel will remain critical to addressing environmental problems such as CO2 emissions and air and water pollution, and increasing traveler safety. Modern urban planning confronts the challenge of coordinating policies in transport investment, land use and development regulation, budgets and taxation, and capital spending so that they reinforce instead of undermine each other.1 Several inter-related problems with transport - land use planning processes in New South Wales are apparent: 1. Mobility-centric transport planning and density-centric land use planning. 2. The uncoordinated and disjointed nature of decision making, wherein transport and land use decisions made by different organisations assume the other is unchanged. 3. Lack of systematic feedback in the infrastructure/land development cycle, so the gains in land value from new transport facilities don’t generate revenue could have helped fund the infrastructure in the first place. 4. The political cycle reversing long-term strategic planning decisions. 5. Lack of institutional knowledge caused by lack of long-term stability in senior staff and organisational structure. 6. Lack of domain expertise within operating agencies leading to: • Very high costs (and unexpectedly high costs) for infrastructure, reducing the capacity for investment. • Under-utilisation (over-forecast of demand) of many major new infrastructure projects. 7. Lack of transparency and authentic public participation in decision processes. This report contains several major parts. The next part, The Value of Access to Opportunity (chapter 2) discusses the framework of the Fundamental Model of Access. Then chapter 3 of the report, Operationalising Access describes formally how access is measured. This is followed be an examination of how Infill Stations Expand Access (chapter 4), including an example that illustrates how access might change between two public transport service scenarios when an infill station is added, and a sample of some potential sites for Infill Stations on the Sydney network are provided. Access-Oriented Planning Globally: Case Studies (chapter 5), discusses how integrated transport and land use planning is conducted in several key metropolitan regions globally. It looks at the governance structures in the Randstad (Amsterdam, Rotterdam, Utrecht, and The Hague) in the Netherlands, Singapore, Hong Kong, and Tokyo. The subsequent section, Governing for Access in New South Wales: A Future Sydney Commission (chapter 6), describes how long-term strategic transport - land use planning might be conducted in Greater Sydney, with an aim to address several of those inter-related problems. The final section (chapter 7) concludes the report, showing how the proposed strategy addresses the key problems identified above

Circuity in Urban Transit Networks

This paper investigates the circuity of transit networks and examines auto mode share as a function of circuity and accessibility to better understand the performance of urban transit systems. We first survey transit circuity in the Minneapolis–St. Paul, Minnesota, region in detail, comparing auto and transit trips. This paper finds that circuity can help to explain mode choices of commuters. We then investigate thirty-five additional metropolitan areas in the United States. The results from these areas show that transit circuity exponentially declines as travel time increases. Moreover, we find that the circuity of transit networks is higher than that of road networks, illustrating how transit systems choose to expand their spatial coverage at the expense of directness and efficiency in public transportation networks. This paper performs a regression analysis that suggests the circuity of transportation networks can estimate transit accessibility, which helps to explain mode share

A Model of the Rise and Fall of Roads

Transportation network planning decisions made at one point of time can have profound impacts in the future. However, transportation networks are usually assumed to be static in models of land use. A better understanding of the natural growth pattern of roads will provide valuable guidance to planners who try to shape the future network. This paper analyzes the relationships between network supply and travel demand, and describes a road development and degeneration mechanism microscopically at the link level. A simulation model of transportation network dynamics is developed, involving iterative evolution of travel demand patterns, network revenue policies, cost estimation,and investment rules. The model is applied to a real-world congesting network – the Twin Cities transportation network which comprises nearly 8,000 nodes and more than 20,000 links, using network data collected since year 1978. Four experiments are carried out with different initial conditions and constraints, the results from which allow us to explore model properties such as computational feasibility, qualitative implications, potential calibration procedures, and predictive value. The hypothesis that road hierarchies are emergent properties of transportation networks is confirmed, and the underlying reasons discovered. Spatial distribution of capacity, traffic flow, and congestion in the transportation network is tracked over time. Potential improvements to the model in particular and future research directions in transportation network dynamics in general are also discussed

A model of two-destination choice in trip chains with GPS data

Studying trip chaining behavior has been a challenging endeavor which requires the support of microscopic travel data. New insights can be gained given real-time GPS travel data. This research introduces a framework that considers two-destination choice in the context of home-based trip chains. We propose and empirically compare three alternatives of building choice sets where we consider various relationships of the two destinations (such as major-minor destinations, selecting one first, and select- ing two concurrently). Our choice set formation alternatives use survival models to determine the selection probability of a destination. Our results reveal that trip chaining behavior is shaped by the features of retail clusters, spatial patterns of clusters, transportation networks, and the axis of travel. This research supports our hypothesis that not only the spatial relationship but also the land use relationship of the destinations in a trip chain affect the decision making process