Structural Change in Urban Systems

This paper outlines a conceptual background for understanding the dynamics of metropolitan regions and urban systems in general. An essential part of the paper reviews existing theoretical explanations of urban change processes with special attention being paid to long term cycles and waves as well as discontinuities and qualitative changes in the evolution of metropolitan regions. The paper introduces a distinction between constrained and structural dynamics. With reference to this distinction, the authors describe and classify various forms of urban oscillations and changes in urban structures. Relations between technological development and infrastructural change are also discussed

Observations of stratospheric source gas profiles during the Arctic winter

An international campaign was performed at ESRANGE rocket base, near Kiruna, Sweden (68 N) from January 4 to February 15 in order to investigate the Chemistry of Ozone in the Polar Stratosphere (CHEOPS). Within the framework of this campaign two sets of large stratospheric air samples were collected by means of a balloon borne cryogenic air sampler. The two balloons were launched on February 1, and February 10, 1988. At present the samples are analyzed in our laboratory for their contents of several long lived trace gases such as CH4, N2O, H2, CO2, CO and the major halocarbons CH3Cl, CFCl3, CF2Cl2, CCl4, CH3CCl3, and C2F3Cl3. The vertical profiles derived from these samples will be presented and compared with previous observations made in February 1987. The data will be discussed in view of the dynamical evolution of the Arctic polar vortex during this winter

Real-time observation of interfering crystal electrons in high-harmonic generation

Accelerating and colliding particles has been a key strategy to explore the texture of matter. Strong lightwaves can control and recollide electronic wavepackets, generating high-harmonic (HH) radiation which encodes the structure and dynamics of atoms and molecules and lays the foundations of attosecond science. The recent discovery of HH generation in bulk solids combines the idea of ultrafast acceleration with complex condensed matter systems and sparks hope for compact solid-state attosecond sources and electronics at optical frequencies. Yet the underlying quantum motion has not been observable in real time. Here, we study HH generation in a bulk solid directly in the time-domain, revealing a new quality of strong-field excitations in the crystal. Unlike established atomic sources, our solid emits HH radiation as a sequence of subcycle bursts which coincide temporally with the field crests of one polarity of the driving terahertz waveform. We show that these features hallmark a novel non-perturbative quantum interference involving electrons from multiple valence bands. The results identify key mechanisms for future solid-state attosecond sources and next-generation lightwave electronics. The new quantum interference justifies the hope for all-optical bandstructure reconstruction and lays the foundation for possible quantum logic operations at optical clock rates

Higgs boson pair production in gluon fusion at NLO with full top-quark mass dependence

We present the calculation of the cross section and invariant mass distribution for Higgs boson pair production in gluon fusion at next-to-leading order (NLO) in QCD. Top-quark masses are fully taken into account throughout the calculation. The virtual two-loop amplitude has been generated using an extension of the program GoSam supplemented with an interface to Reduze for the integral reduction. The occurring integrals have been calculated numerically using the program SecDec. Our results, including the full top-quark mass dependence for the first time, allow us to assess the validity of various approximations proposed in the literature, which we also recalculate. We find substantial deviations between the NLO result and the different approximations, which emphasizes the importance of including the full top-quark mass dependence at NLO.Comment: Version published in PRL, v2: results at 13 TeV (v1 was at 14 TeV), minor correction to virtual part included, conclusions unchange

Environmental Quality, Abatement, and Urban Development

The purpose of this paper is to derive a simultaneous equation model of an urban economy in which the impacts of environmental policy on the suburbanization-reurbanization process can be assessed. The variable to show these effects is a measure of the density of land-use activity. The model consists of essentially four components -- an urban land market, a market for an aggregate good, a description of a process of emission of residuals as a necessary by-product of land-use activities (production, consumption, commuting), and their accumulation and diffusion in a receiving medium (e.g., air and noise pollution). Environmental policy is then introduced into this simultaneous equation model (emission standards and fines, ordinances to install abatement equipment, emission fees, etc.). A discussion follows on the hypothesis that a successful antipollution policy could introduce a trend toward reurbanization with the claim that clear air could induce land users to relocate closer to the center of a city. The policies considered include fines for exceeding emission standards, inducing land users to purchase abatement devices in the market. The study attempts to derive the impacts of these measures on residuals' concentration, the roles of abatement devices, and urban density based on some a prior; assumptions