Classical to quantum mapping for an unconventional phase transition in a three-dimensional classical dimer model

We study the transition between a Coulomb phase and a dimer crystal observed in numerical simulations of the three-dimensional classical dimer model, by mapping it to a quantum model of bosons in two dimensions. The quantum phase transition that results, from a superfluid to a Mott insulator at fractional filling, belongs to a class that cannot be described within the Landau-Ginzburg-Wilson paradigm. Using a second mapping, to a dual model of vortices, we show that the long-wavelength physics near the transition is described by a U(1) gauge theory with SU(2) matter fields.Comment: 15 pages, 5 figures; v2: added appendi

Excited state spectra at the superfluid-insulator transition out of paired condensates

We describe gapped single-particle and collective excitations across a superfluid to insulator quantum phase transition of particles (bosons or fermions) in a periodic potential, with an even number of particles per unit cell. We demonstrate that the dynamics is controlled by a quantum impurity problem of a localized particle interacting with the bulk critical modes. Critical exponents are determined by a renormalization group analysis. We discuss applications to spin oscillations of ultracold atoms in optical lattices, and to the electronic phases in the cuprate and related compounds.Comment: 4 pages, 1 figure; fixed referenc

Virtual communities and professional learning across a distributed remote membership

Headteachers, or Principals, of schools work in isolation from each other yet share common practice and domain of leadership and management. They exhibit the characteristics of a community of practice yet are remote from other members of their community. Similar communities of practice can be identified for other types of school leaders, subject co-ordinators for example, and for professionals in other disciplines – consultant registrars in health, optometrists working in dispensing opticians, museum curators, and so on. This paper explores ways of using virtual communities to develop professional learning in these communities of practice. We discuss our work in the context of education and formal and informal learning communities of school leaders and explore how the lessons learnt have general application. We present a model for professional learning through online collaboration and communication, and look, in particular, at the concept of time and its effects in the virtual community

The Cotton and Sugar Subsidies Decisions: WTO's Dispute Settlement System Rebalances the Agreement on Agriculture

Agricultural and Food Policy, International Relations/Trade,

An alternative method to access diverse N,N′-diquaternised-3,3′-biquinoxalinium “biquinoxen” dications

An alternative synthetic route for the design of N,N′-diquaternised-3,3′-biquinoxalinium “biquinoxen” dications is reported, involving oxidative radical coupling of dithionite reduced quinoxaline quaternary salts. Although the reaction is not regioselective, leading to relatively modest yields (up to 32%), the advantages of this new synthetic protocol lie in a simple potentially gram scale synthesis using inexpensive easily accessible reagents with no metal catalysts and no purification steps. Thus whereas the method reported previously to access the N,N′-dimethyl-3,3′-biquinoxalinium, “methylbiquinoxen” precursor gave higher yield than the new method reported here, this new method avoids the limitation of using scarce oxonium reagents. Overall, the new protocol is a robust synthetic strategy which offers new design possibilities

Mean-field theory for confinement transitions and magnetization plateaux in spin ice

We study phase transitions in classical spin ice at nonzero magnetization, by introducing a mean-field theory designed to capture the interplay between confinement and topological constraints. The method is applied to a model of spin ice in an applied magnetic field along the [1 0 0] crystallographic direction and yields a phase diagram containing the Coulomb phase as well as a set of magnetization plateaux. We argue that the lobe structure of the phase diagram, strongly reminiscent of the Bose–Hubbard model, is generic to Coulomb spin liquids