Dynamics of atomic spin-orbit-state wave packets produced by short-pulse laser photodetachment

We analyse the experiment by Hultgren et al. [Phys. Rev. A {\bf 87}, 031404 (2013)] on orbital alignment and quantum beats in coherently excited atomic fine-structure manifolds produced by short-pulse laser photodetachment of C$^-$, Si$^-$ and Ge$^-$ negative ions, and derive a formula that describes the beats. Analysis of the experimental data enables us to extract the non-coherent background contribution for each species, and indicates the need for a full density matrix treatment of the problem

Comment on "Direct photodetachment of F−^- by mid-infrared few-cycle femtosecond laser pulses"

Multiphoton detachment of F$^-$ by strong few-cycle laser pulses was studied by Shearer and Monteith using a Keldysh-type approach [Phys. Rev. A 88, 033415 (2013)]. We believe that this work contained errors in the calculation of the detachment amplitude and photoelectron spectra. We describe the necessary corrections to the theory and show that the results, in particular, the interference features of the photoelectron spectra, appear noticeably different.Comment: 9 pages, 4 figure

Wave attenuation and dispersion due to floating ice covers

Experiments investigating the attenuation and dispersion of surface waves in a variety of ice covers are performed using a refrigerated wave flume. The ice conditions tested in the experiments cover naturally occurring combinations of continuous, fragmented, pancake and grease ice. Attenuation rates are shown to be a function of ice thickness, wave frequency, and the general rigidity of the ice cover. Dispersion changes were minor except for large wavelength increases when continuous covers were tested. Results are verified and compared with existing literature to show the extended range of investigation in terms of incident wave frequency and ice conditions

Topological Excitations in Spinor Bose-Einstein Condensates

We investigate the properties of skyrmion in the ferromagnetic state of spin-1 Bose-Einstein condensates by means of the mean-field theory and show that the size of skyrmion is fixed to the order of the healing length. It is shown that the interaction between two skyrmions with oppositely rotating spin textures is attractive when their separation is large, following a unique power-law behavior with a power of -7/2.Comment: 4 pages, 5 figure

An empirical study on applying community detection methods in defining spatial housing submarkets in London

Housing submarkets can be defined as a set of dwellings that are reasonably close substitutes with one another, but poor substitutes between other submarkets. This research argues similarities within submarkets are not only captured by its building and location characteristics but also in how each dwelling is inter-connected within its local area and embedded to the rest of the system. This research conjectures that spatial network local-areas as defined by community detection methods can be used to identify spatial housing submarkets. In order to test this conjecture, the hedonic approach will be used as an empirical strategy on the case study of London. The study found spatial network local areas correspond with planned known local area boundaries and that greater house price similarity is found within spatial network local-areas than between. The study also found that spatial network local area as defined by community detection technique can be used to identify spatial housing submarkets to explain house price. The contribution of this research is it represents a proof of concept in the use of community detection techniques in the definition of spatial housing submarket. Importantly it illustrates the significance in how spatial configuration influences housing market not just in terms of accessibility (Law et al. 2013) but also in terms of housing submarket. Further research will be carried out to study the spatial configuration of the spatial network local areas in understanding severances and connectivity between them. By understanding cities through multiple spatial representations will allow more informed policies at the local-area level

Encounter and its configurational logic: Understanding spatiotemporal co-presence with road network and social media check-in data

Public space facilitates the social interaction between people. It is widely accepted that the connection between spaces creates the possibility of the mutual visibility between people. The relationship between spatial configuration and the spatiotemporal encounters, however, has rarely been investigated explicitly in empirical cases. The focus of this study is two folded: firstly, it examines the way to measure spatiotemporal encounters between different groups of people based on their mobility records; secondly, it investigates how the design of the built environment contributes to physical co-presence on spatial and temporal dimensions. Using ubiquitous individual social media check-in data in Central Shanghai, China, this study proposes a framework for quantifying physical face-to-face co-presence patterns between the defined local random walkers and the remote visitors across time in every street. In the introduced People-Space-Time (PST) model, social capital is conceptualised as an integration among social difference, spatial distance (metric and geometrical distance) and time distance. The reliability of the applied data and the effectiveness of the introduced methods are validated by the investigations of the scaling nature of the extracted mobility patterns and the correlation between the outputs and surveyed data. The produced spatiotemporal patterns of face-toface co-presence reveal that city centres and the large-scale urban complexes (e.g., transport hubs, shopping malls, stadiums, etc.) are ideal places for people to encounter. The results of the regression analyses demonstrate that spatial and functional centrality measures are significant variables for predicting spatiotemporal co-presence in streets, but in which the functional centrality structures maintain a higher standard of explanatory power than the spatial network. The temporal complexity of the co-presence is revealed by the temporally shifting performance of the integrated regression models across time. The findings in this study yield that it is the spatio-functional interaction influencing spatiotemporal variation of the physical encounter between people, and reclaim the necessity of adding fine-scale land-use patterns in the traditional configurational analysis for deeply understanding the social processes with urban big data in the contemporary digitalised cities

Stability of Filters for the Navier-Stokes Equation

Data assimilation methodologies are designed to incorporate noisy observations of a physical system into an underlying model in order to infer the properties of the state of the system. Filters refer to a class of data assimilation algorithms designed to update the estimation of the state in a on-line fashion, as data is acquired sequentially. For linear problems subject to Gaussian noise filtering can be performed exactly using the Kalman filter. For nonlinear systems it can be approximated in a systematic way by particle filters. However in high dimensions these particle filtering methods can break down. Hence, for the large nonlinear systems arising in applications such as weather forecasting, various ad hoc filters are used, mostly based on making Gaussian approximations. The purpose of this work is to study the properties of these ad hoc filters, working in the context of the 2D incompressible Navier-Stokes equation. By working in this infinite dimensional setting we provide an analysis which is useful for understanding high dimensional filtering, and is robust to mesh-refinement. We describe theoretical results showing that, in the small observational noise limit, the filters can be tuned to accurately track the signal itself (filter stability), provided the system is observed in a sufficiently large low dimensional space; roughly speaking this space should be large enough to contain the unstable modes of the linearized dynamics. Numerical results are given which illustrate the theory. In a simplified scenario we also derive, and study numerically, a stochastic PDE which determines filter stability in the limit of frequent observations, subject to large observational noise. The positive results herein concerning filter stability complement recent numerical studies which demonstrate that the ad hoc filters perform poorly in reproducing statistical variation about the true signal