Dispersive magnetic excitations in the S=1 antiferromagnet Ba3_3Mn2_2O8_8

We present powder inelastic neutron scattering measurements of the S=1 dimerized antiferromagnet Ba$_3$Mn$_2$O$_8$. The $T=1.4$ K magnetic spectrum exhibits a spin-gap of $\Delta \approx 1.0$ meV and a dispersive spectrum with a bandwidth of approximately 1.5 meV. Comparison to coupled dimer models describe the dispersion and scattering intensity accurately and determine the exchange constants in Ba$_3$Mn$_2$O$_8$. The wave vector dependent scattering intensity confirms the proposed S=1 dimer bond. Temperature dependent measurements of the magnetic excitations indicate the presence of both singlet-triplet and thermally activated triplet-quintet excitations.Comment: 8 pages, 8 figures, Submitted to Physical Review B, Resubmited versio

Tony Blair and John Howard: comparative predominance and 'Institution Stretch' in the UK and Australia

It has recently been argued that the UK premier enjoys a level of executive power unavailable to US presidents, but how does he or she compare to another prime minister operating within a broadly similar system? Commonalities of intra-executive influence and capacity exist under the premierships in the UK and Australia. Discrete institutional constraints and deviations are evident, but trends and similarities in resource capacity can be clearly identified. These include: the growth of the leaders' office; broadening and centralising of policy advice and media operations; and strengthening of the role and function of ministerial advisers. I contend that this amounts to 'institution stretch', with new structures, processes and practices becoming embedded in the political system by the incumbents. © 2007 The Author. Journal compilation © 2007 Political Studies Association

The Velocity Distribution of the Nearest Interstellar Gas

The bulk flow velocity for the cluster of interstellar cloudlets within about 30 pc of the Sun is determined from optical and ultraviolet absorption line data, after omitting from the sample stars with circumstellar disks or variable emission lines and the active variable HR 1099. Ninety-six velocity components towards the remaining 60 stars yield a streaming velocity through the local standard of rest of -17.0+/-4.6 km/s, with an upstream direction of l=2.3 deg, b=-5.2 deg (using Hipparcos values for the solar apex motion). The velocity dispersion of the interstellar matter (ISM) within 30 pc is consistent with that of nearby diffuse clouds, but present statistics are inadequate to distinguish between a Gaussian or exponential distribution about the bulk flow velocity. The upstream direction of the bulk flow vector suggests an origin associated with the Loop I supernova remnant. Groupings of component velocities by region are seen, indicating regional departures from the bulk flow velocity or possibly separate clouds. The absorption components from the cloudlet feeding ISM into the solar system form one of the regional features. The nominal gradient between the velocities of upstream and downstream gas may be an artifact of the Sun's location near the edge of the local cloud complex. The Sun may emerge from the surrounding gas-patch within several thousand years.Comment: Typographical errors corrected; Five tables, seven figures; Astrophysical Journal, in pres

Regulating Clothing Outwork: A Sceptic's View

By applying the strategies of international anti-sweatshop campaigns to the Australian context, recent regulations governing home-based clothing production hold retailers responsible for policing the wages and employment conditions of clothing outworkers who manufacture clothing on their behalf. This paper argues that the new approach oversimplifies the regulatory challenge by assuming (1) that Australian clothing production is organised in a hierarchical ‘buyer-led’ linear structure in which core retail firms have the capacity to control their suppliers’ behaviour; (2) that firms act as unitary moral agents; and (3) that interventions imported from other times and places are applicable to the contemporary Australian context. After considering some alternative regulatory approaches, the paper concludes that the new regulatory strategy effectively privatises responsibility for labour market conditions – a development that cries out for further debate

Distinguishing among Scalar Field Models of Dark Energy

We show that various scalar field models of dark energy predict degenerate luminosity distance history of the Universe and thus cannot be distinguished by supernovae measurements alone. In particular, models with a vanishing cosmological constant (the value of the potential at its minimum) are degenerate with models with a positive or negative cosmological constant whose magnitude can be as large as the critical density. Adding information from CMB anisotropy measurements does reduce the degeneracy somewhat but not significantly. Our results indicate that a theoretical prior on the preferred form of the potential and the field's initial conditions may allow to quantitatively estimate model parameters from data. Without such a theoretical prior only limited qualitative information on the form and parameters of the potential can be extracted even from very accurate data.Comment: 15 pages, 5 figure

Constraining dark energy with Sunyaev-Zel'dovich cluster surveys

We discuss the prospects of constraining the properties of a dark energy component, with particular reference to a time varying equation of state, using future cluster surveys selected by their Sunyaev-Zel'dovich effect. We compute the number of clusters expected for a given set of cosmological parameters and propogate the errors expected from a variety of surveys. In the short term they will constrain dark energy in conjunction with future observations of type Ia supernovae, but may in time do so in their own right.Comment: 5 pages, 3 figures, 1 table, version accepted for publication in PR

Probing Dark Energy with Supernovae : Bias from the time evolution of the equation of state

Observation of thousands of type Ia supernovae should offer the most direct approach to probe the dark energy content of the universe. This will be undertaken by future large ground-based surveys followed by a space mission (SNAP/JDEM). We address the problem of extracting the cosmological parameters from the future data in a model independent approach, with minimal assumptions on the prior knowledge of some parameters. We concentrate on the comparison between a fiducial model and the fitting function and adress in particular the effect of neglecting (or not) the time evolution of the equation of state. We present a quantitative analysis of the bias which can be introduced by the fitting procedure. Such bias cannot be ignored as soon as the statistical errors from present data are drastically improved.Comment: 22 pages, 10 figures, submitted to Phys. Rev.