Spin dynamics across the superfluid-insulator transition of spinful bosons

Bosons with non-zero spin exhibit a rich variety of superfluid and insulating phases. Most phases support coherent spin oscillations, which have been the focus of numerous recent experiments. These spin oscillations are Rabi oscillations between discrete levels deep in the insulator, while deep in the superfluid they can be oscillations in the orientation of a spinful condensate. We describe the evolution of spin oscillations across the superfluid-insulator quantum phase transition. For transitions with an order parameter carrying spin, the damping of such oscillations is determined by the scaling dimension of the composite spin operator. For transitions with a spinless order parameter and gapped spin excitations, we demonstrate that the damping is determined by an associated quantum impurity problem of a localized spin excitation interacting with the bulk critical modes. We present a renormalization group analysis of the quantum impurity problem, and discuss the relationship of our results to experiments on ultracold atoms in optical lattices.Comment: 43 pages (single-column format), 8 figures; v2: corrected discussion of fixed points in Section V

Organisational Identity as a Barrier to Widening Access in Scottish Universities

Widening access policy has historically focused on tackling the socioeconomic barriers to university access faced by prospective students from under-represented groups, but increasingly policy makers are seeking to also address the barriers to wider access posed by undergraduate admissions policies. In this vein, the Scottish Government has recently called upon universities to set separate academic entry requirements for socioeconomically disadvantaged applicants which recognise that “the school attainment of disadvantaged learners often does not reflect their full potential” and which better reflect the minimum needed to succeed in higher education. In this paper, we draw on in-depth interviews with admissions personnel at eighteen Scottish universities to explore the scope for more progressive admissions policies of this kind in light of universities’ identities as organisations and in light of corresponding organisational strategies for position-taking in global and national higher education fields. We present a theoretical model and an empirical illustration of three hierarchically-ordered ideal types of organisational identity—globally competitive, nationally selective, and locally transformative—and show that the more dominant of these tend to constrain the development of more progressive admissions policies. This is because globally competitive and, to a lesser extent, nationally selective organisational identities are understood to require admission of the ‘brightest and best’, conceptualised as those with the highest levels of prior academic attainment who can be expected to succeed at university and beyond as a matter of course. We conclude that universities must recognise and redress the implicitly exclusionary nature of their organisational identities if genuine progress on widening access is to be made

A phenomenological model of the superconducting state of the Bechgaard salts

We present a group theoretical analysis of the superconducting state of the Bechgaard salts, e.g., (TMTSF)_2PF_6 or (TMTSF)_2ClO_6. We show that there are eight symmetry distinct superconducting states. Of these only the (fully gapped, even frequency, p-wave, triplet) 'polar state' is consistent with the full range of the experiments on the Bechgaard salts. The gap of the polar state is d(k) (psi_uk,0,0), where psi_uk may be any odd parity function that is translationally invariant.Comment: 4 pages, no figure

An earth pole-sitter using hybrid propulsion

In this paper we investigate optimal pole-sitter orbits using hybrid solar sail and solar electric propulsion (SEP). A pole-sitter is a spacecraft that is constantly above one of the Earth's poles, by means of a continuous thrust. Optimal orbits, that minimize propellant mass consumption, are found both through a shape-based approach, and solving an optimal control problem, using a direct method based on pseudo-spectral techniques. Both the pure SEP case and the hybrid case are investigated and compared. It is found that the hybrid spacecraft allows consistent savings on propellant mass fraction. Finally, is it shown that for sufficiently long missions (more than 8 years), a hybrid spacecraft, based on mid-term technology, enables a consistent reduction in the launch mass for a given payload, with respect to a pure SEP spacecraft

The use of access thresholds to widen participation at Scottish universities

The Scottish Government has set ambitious targets for widening access to full-time undergraduate degree programmes. Meeting these targets will be a real challenge, not least because young people from socioeconomically disadvantaged contexts continue to lag substantially behind their more advantaged peers when it comes to achievement at Higher level. Following the recommendations of the Commission on Widening Access, the Scottish Government has mandated Scottish universities to set separate entry requirements for contextually disadvantaged applicants, known as ‘access thresholds’. In this article, we draw on the findings of a research project commissioned by the Scottish Funding Council to develop an empirical evidence base for the use of access thresholds to widen participation in higher education. We show that access thresholds are mathematically necessary if wider access is to be achieved, and we present evidence demonstrating that applicants admitted with Higher grades lower than the market rate have a high probability of success at degree level. We welcome the widespread use of access thresholds but highlight the scope to be much bolder than is currently the case. We also show that the use of the Scottish Index of Multiple Deprivation (SIMD) to identify contextually disadvantaged learners runs a high risk of failure to reach the intended beneficiaries. We argue strongly in favour of the use of administratively verified individual level measures of contextual disadvantage instead, specifically receipt of free school meals and low household income

Non-linear Microwave Surface Impedance of Epitaxial HTS Thin Films in Low DC Magnetic Fields

We have carried out non-linear microwave (8 GHz) surface impedance measurements of three YBaCuO thin films in dc magnetic fields $H_{dc}$ (parallel to c axis) up to 12 mT using a coplanar resonator technique. In zero dc field the three films, deposited by the same method, show a spread of low-power residual surface resistance, $R_{res}$ and penetration depth, $\lambda$ (T=15 K) within a factor of 1.9. However, they exhibit dramatically different microwave field, $H_{rf}$ dependences of the surface resistance, $R_s$, but universal $X_s(H_{rf})$ dependence. Application of a dc field was found to affect not only absolute values of $R_s$ and $X_s$, but the functional dependences $R_s(H_{rf})$ and $X_s(H_{rf})$ as well. For some of the samples the dc field was found to decrease $R_s$ below its zero-field low-power value.Comment: 4 pages, 4 figures. To be published in IEEE Trans. Appl. Supercond., June 199

Radiationless Travelling Waves In Saturable Nonlinear Schr\"odinger Lattices

The longstanding problem of moving discrete solitary waves in nonlinear Schr{\"o}dinger lattices is revisited. The context is photorefractive crystal lattices with saturable nonlinearity whose grand-canonical energy barrier vanishes for isolated coupling strength values. {\em Genuinely localised travelling waves} are computed as a function of the system parameters {\it for the first time}. The relevant solutions exist only for finite velocities.Comment: 5 pages, 4 figure

Improving the efficiency of the detection of gravitational wave signals from inspiraling compact binaries: Chebyshev interpolation

Inspiraling compact binaries are promising sources of gravitational waves for ground and space-based laser interferometric detectors. The time-dependent signature of these sources in the detectors is a well-characterized function of a relatively small number of parameters; thus, the favored analysis technique makes use of matched filtering and maximum likelihood methods. Current analysis methodology samples the matched filter output at parameter values chosen so that the correlation between successive samples is 97% for which the filtered output is closely correlated. Here we describe a straightforward and practical way of using interpolation to take advantage of the correlation between the matched filter output associated with nearby points in the parameter space to significantly reduce the number of matched filter evaluations without sacrificing the efficiency with which real signals are recognized. Because the computational cost of the analysis is driven almost exclusively by the matched filter evaluations, this translates directly into an increase in computational efficiency, which in turn, translates into an increase in the size of the parameter space that can be analyzed and, thus, the science that can be accomplished with the data. As a demonstration we compare the present "dense sampling" analysis methodology with our proposed "interpolation" methodology, restricted to one dimension of the multi-dimensional analysis problem. We find that the interpolated search reduces by 25% the number of filter evaluations required by the dense search with 97% correlation to achieve the same efficiency of detection for an expected false alarm probability. Generalized to higher dimensional space of a generic binary including spins suggests an order of magnitude increase in computational efficiency.Comment: 23 pages, 5 figures, submitted to Phys. Rev.

Glutathione diminishes tributyltin- and dibutyltin-induced loss of lytic function in human natural killer cells

This study investigated whether reduced glutathione (GSH) was able to alter the negative effects of tributyltin (TBT) or dibutyltin (DBT) on the lytic function of human natural killer (NK) cells. NK cells are an initial immune defense against the development of tumors or viral infections. TBT and DBT are widespread environmental contaminants, due to their various industrial applications. Both TBT and DBT have been shown to decrease the ability of NK cells to lyse tumor cells (lytic function). The results indicated that the presence of GSH during the exposure of NK cells to TBT or DBT diminished the negative effect of the butyltin on the lytic function of NK cells. This suggests that the interaction of TBT and DBT with functionally relevant sulfhydryl groups in NK cells may be part of the mechanism by which they decrease NK lytic function