{\AA}ngstr\"om-scale chemically powered motors

Like their larger micron-scale counterparts, {\AA}ngstr\"om-scale chemically self-propelled motors use asymmetric catalytic activity to produce self-generated concentration gradients that lead to directed motion. Unlike their micron-scale counterparts, the sizes of {\AA}ngstr\"om-scale motors are comparable to the solvent molecules in which they move, they are dominated by fluctuations, and they operate on very different time scales. These new features are studied using molecular dynamics simulations of small sphere dimer motors. We show that the ballistic regime is dominated by the thermal speed but the diffusion coefficients of these motors are orders of magnitude larger than inactive dimers. Such small motors may find applications in nano-confined systems or perhaps eventually in the cell.Comment: 6 pages, 8 figure

The Ω\Omega Dependence in the Equations of Motion

We show that the equations of motion governing the evolution of a collisionless gravitating system of particles in an expanding universe can be cast in a form which is almost independent of the cosmological density parameter, $\Omega$, and the cosmological constant, $\Lambda$. The new equations are expressed in terms of a time variable $\tau\equiv \ln D$, where $D$ is the linear rate of growth of density fluctuations. The weak dependence on the density parameter is proportional to $\epsilon=\Omega^{-0.2}-1$ times the difference between the peculiar velocity (with respect to $\tau$) of particles and the gravity field. In the general case, the effect of this weak $\Omega$ dependence is to enhance the rate of evolution of density perturbations in dense regions. In a flat universe with $\Lambda\ne 0$, this enhancement is less pronounced than in an open universe with $\Lambda=0$ and the same $\Omega$. Using the spherical collapse model, we find that the increase of the $rms$ density fluctuations in a low $\Omega$ universe relative to that in a flat universe with the same linear normalization is $\sim 0.01 \epsilon(\Omega)$, where $\delta$ is the density field in the flat universe. The equations predict that the smooth average velocity field scales like $\Omega^{0.6}$ while the local velocity dispersion (rms value) scales, approximately, like $\Omega^{0.5}$. High resolution N-body simulations confirm these results and show that density fields, when smoothed on scales slightly larger than clusters, are insensitive to the cosmological model. Halos in an open model simulation are more concentrated than halos of the same $M/\Omega$ in a flat model simulation.Comment: 8 pages, MN style, 9 figures, submitted to MNRA

Clustering of Galaxies in a Hierarchical Universe: III. Mock Redshift Surveys

This is the third paper in a series which combines N-body simulations and semi-analytic modelling to provide a fully spatially resolved simulation of the galaxy formation and clustering processes. Here we extract mock redshift surveys from our simulations: a Cold Dark Matter model with either Omega_0=1 (tauCDM) or Omega_0=0.3 and Lambda=0.7 (LambdaCDM). We compare the mock catalogues with the northern region (CfA2N) of the Center for Astrophysics (CfA) Redshift Surveys. We study the properties of galaxy groups and clusters identified using standard observational techniques and we study the relation of these groups to real virialised systems. Most features of CfA2N groups are reproduced quite well by both models with no obvious dependence on Omega_0. Redshift space correlations and pairwise velocities are also similar in the two cosmologies. The luminosity functions predicted by our galaxy formation models depend sensitively on the treatment of star formation and feedback. For the particular choices of Paper I they agree poorly with the CfA survey. To isolate the effect of this discrepancy on our mock redshift surveys, we modify galaxy luminosities in our simulations to reproduce the CfA luminosity function exactly. This adjustment improves agreement with the observed abundance of groups, which depends primarily on the galaxy luminosity density, but other statistics, connected more closely with the underlying mass distribution, remain unaffected. Regardless of the luminosity function adopted, modest differences with observation remain. These can be attributed to the presence of the ``Great Wall'' in the CfA2N. It is unclear whether the greater coherence of the real structure is a result of cosmic variance, given the relatively small region studied, or reflects a physical deficiency of the models.Comment: 47 pages, LaTex, 17 figures, MNRAS, in press; one figure adde

Ultrafast control of strong-field electron dynamics in solids

We review theoretical foundations and some recent progress related to the quest of controlling the motion of charge carriers with intense laser pulses and optical waveforms. The tools and techniques of attosecond science enable detailed investigations of a relatively unexplored regime of nondestructive strong-field effects. Such extremely nonlinear effects may be utilized to steer electron motion with precisely controlled optical fields and switch electric currents at a rate that is far beyond the capabilities of conventional electronics.Comment: This manuscript was prepared for a book "Ultrafast Ionization Dynamics", which is to be published by Springe

Inter-cluster filaments in a Λ\LambdaCDM Universe

The large--scale structure (LSS) in the Universe comprises a complicated filamentary network of matter. We study this network using a high--resolution simulation of structure formation of a $\Lambda$ Cold Dark Matter cosmology. We investigate the distribution of matter between neighbouring large haloes whose masses are comparable to massive clusters of galaxies. We identify a total of 228 filaments between neighbouring clusters. Roughly half of the filaments are either warped or lie off the cluster--cluster axis. We find that straight filaments on the average are shorter than warped ones. More massive clusters are connected to more filaments than less massive ones on average. This finding indicates that the most massive clusters form at the intersections of the filamentary backbone of LSS. For straight filaments, we compute mass profiles. Radial profiles show a fairly well--defined radius, $r_s$, beyond which the profiles follow an $r^{-2}$ power law fairly closely. For the majority of filaments, $r_s$ lies between 1.5 $h^{-1}$ Mpc and 2.0 $h^{-1}$ Mpc. The enclosed overdensity inside $r_s$ varies between a few times up to 25 times mean density, independent of the length of the filaments. Along the filaments' axes, material is not distributed uniformly. Towards the clusters, the density rises, indicating the presence of the cluster infall regions. In addition, we also find some sheet--like connections between clusters. In roughly a fifth of all cluster--cluster connections where we could not identify a filament or sheet, projection effects lead to filamentary structures in the projected mass distribution. (abridged)Comment: 10 pages, 18 figures; submitted to MNRAS; updated: final version, accepted for publicatio

Comparative dimensions of social housing in Arhus and Newcastle, 1890s - 1979 : the problem of the political culture of two social housing systems

Denmark....being a much smaller country than Britain, has, in absolute terms, a smaller housing problem. Nevertheless, there are surely lessons to be learned from the highly successful system which the Danish people and Government have worked out for themselves. A housing society, or some equivalent organization, provided for each separate region or sub-region in Great Britain might offer a solution to the difficult [sic.] that design for our working-class housing is under the controls of councils of very varying degrees of technical knowledge, which then have to be prodded and supervised to some extent by various Government departments. The housing society seems an admirable compromise, provided that it can be kept on the completely non-profit making basis that is successfully secured in Denmark. Ian Bowen, Housing Policy in Denmark, The Architects' Journal, August 4, 1949, p.133 A generation of competent technicians and fearless, idealistic politicians [in Britain] have been able to make a contribution which will persist as a good example of the capabilities of the present and as an incomparable field of study for others who are working in planning. Aage Jedich, Report from Holme-Tranbjerg Council Committee's visit to England, 12.07.19631 A comparison of the housing provided by two cities within separate nation states may encourage a mutually admiring gaze from each position. Comparisons have provided a tool in learning about new housing practices, understanding one's own position from a different vantage point and throwing light on areas that may have remained unquestioned until a visit abroad revealed different approaches to a similar problem. As the quotes above suggest, professional groups involved in the provision of housing and urban planning in post-war Denmark and Britain held each other's national strategies in high regard as they contemplated their local problems of creating spaces for effective urban communities. It will become clear for the cities studied in this thesis that local councillors, public officials and social housing providers at times sought to explore the wider areas of learning that practices abroad could offer. Yet the main approach adopted in this thesis is the comparative historical approach: the thesis studies the origins and history of social housing systems in Arhus, Denmark, and Newcastle, Britain. The comparison creates contrasts and similarities between the two cities through an urban social history approach. The key theme explored in the work is the notions of local democratic culture arising within the social housing systems of the two cities covering most of the twentieth century, but with an emphasis on the period 1945¬1979. The introduction will discuss themes running through the work and will consider how the structure of the thesis allows for the comparison to illuminate aspects of the local political culture of the two cities that was directly affected by and affected in turn the local provision of social housing. Like most Western European cities in the twentieth century Arhus and Newcastle faced the problems of providing adequate housing for large groups of working people as the cities grew or older housing types became outdated. The study examines the options and strategies that were explored and adopted by the housing authorities in the two cities to recover from slumps in housing provision. It is clear that each city approached housing provision through different groups of facilitators: in Arhus, as in Denmark in general, the housing association was the primary generator of social housing, while Newcastle followed the British pattern — providing social housing through the municipality. Thus the agency of provision was different in the two cases from the outset. How the mediating influence of housing associations between the Arhusian Council and residents in social housing contrasted with the direct provision of council housing in Newcastle is a key issue for the the..

Clustering of Galaxies in a Hierarchical Universe: I. Methods and Results at z=0

We introduce a new technique for following the formation and evolution of galaxies in cosmological N-body simulations. Dissipationless simulations are used to track the formation and merging of dark matter halos as a function of redshift. Simple prescriptions, taken directly from semi-analytic models of galaxy formation, are adopted for cooling, star formation, supernova feedback and the merging of galaxies within the halos. This scheme enables us to study the clustering properties of galaxies and to investigate how selection by type, colour or luminosity influences the results. In this paper, we study properties of the galaxy distribution at z=0. These include luminosity functions, colours, correlation functions, pairwise peculiar velocities, cluster M/L ratios and star formation rates. We focus on two variants of a CDM cosmology: a high- density model with Gamma=0.21 (TCDM) and a low-density model with Omega=0.3 and Lambda=0.7 (LCDM). Both are normalized to reproduce the I-band Tully-Fisher relation near a circular velocity of 220 km/s. Our results depend strongly both on this normalization and on the prescriptions for star formation and feedback. Very different assumptions are required to obtain an acceptable model in the two cases. For TCDM, efficient feedback is required to suppress the growth of galaxies low-mass field halos. Without it, there are too many galaxies and the correlation function turns over below 1 Mpc. For LCDM, feedback must be weak, otherwise too few L* galaxies are produced and the correlation function is too steep. Given the uncertainties in modelling some of the key physical processes, we conclude that it is not yet possible to draw conclusions about the values of cosmological parameters from studies of this kind. Further work on global star formation and feedback effects is required to narrow the range of possibilitiesComment: 43 pages, Latex, 16 figures included, 2 additional GIF format figures, submitted to MNRA