Neutron spin-echo study of the critical dynamics of spin-5/2 antiferromagnets in two and three dimensions

We report a neutron spin-echo study of the critical dynamics in the $S=5/2$ antiferromagnets MnF$_2$ and Rb$_2$MnF$_4$ with three-dimensional (3D) and two-dimensional (2D) spin systems, respectively, in zero external field. Both compounds are Heisenberg antiferromagnets with a small uniaxial anisotropy resulting from dipolar spin-spin interactions, which leads to a crossover in the critical dynamics close to the N\'eel temperature, $T_N$. By taking advantage of the $\mu\text{eV}$ energy resolution of the spin-echo spectrometer, we have determined the dynamical critical exponents $z$ for both longitudinal and transverse fluctuations. In MnF$_2$, both the characteristic temperature for crossover from 3D Heisenberg to 3D Ising behavior and the exponents $z$ in both regimes are consistent with predictions from the dynamical scaling theory. The amplitude ratio of longitudinal and transverse fluctuations also agrees with predictions. In Rb$_2$MnF$_4$, the critical dynamics crosses over from the expected 2D Heisenberg behavior for $T\gg T_N$ to a scaling regime with exponent $z = 1.387(4)$, which has not been predicted by theory and may indicate the influence of long-range dipolar interactions

NbSe3: Effect of Uniaxial Stress on the Threshold Field and Fermiology

We have measured the effect of uniaxial stress on the threshold field ET for the motion of the upper CDW in NbSe3. ET exhibits a critical behavior, ET ~ (1 - e/ec)^g, wher e is the strain, and ec is about 2.6% and g ~ 1.2. This ecpression remains valid over more than two decades of ET, up to the highest fields of about 1.5keV/m. Neither g nor ec is very sensitive to the impurity concentraction. The CDW transition temperature Tp decreases linearly with e at a rate dTp/de = -10K/%, and it does not show any anomaly near ec. Shubnikov de-Haas measurements show that the extremal area of the Fermi surface decreases with increasing strain. The results suggest that there is an intimate relationship between pinning of the upper CDW and the Fermiology of NbSe3.Comment: 4 pages, 5 figure

Lagrangian Floer superpotentials and crepant resolutions for toric orbifolds

We investigate the relationship between the Lagrangian Floer superpotentials for a toric orbifold and its toric crepant resolutions. More specifically, we study an open string version of the crepant resolution conjecture (CRC) which states that the Lagrangian Floer superpotential of a Gorenstein toric orbifold $\mathcal{X}$ and that of its toric crepant resolution $Y$ coincide after analytic continuation of quantum parameters and a change of variables. Relating this conjecture with the closed CRC, we find that the change of variable formula which appears in closed CRC can be explained by relations between open (orbifold) Gromov-Witten invariants. We also discover a geometric explanation (in terms of virtual counting of stable orbi-discs) for the specialization of quantum parameters to roots of unity which appears in Y. Ruan's original CRC ["The cohomology ring of crepant resolutions of orbifolds", Gromov-Witten theory of spin curves and orbifolds, 117-126, Contemp. Math., 403, Amer. Math. Soc., Providence, RI, 2006]. We prove the open CRC for the weighted projective spaces $\mathcal{X}=\mathbb{P}(1,\ldots,1,n)$ using an equality between open and closed orbifold Gromov-Witten invariants. Along the way, we also prove an open mirror theorem for these toric orbifolds.Comment: 48 pages, 1 figure; v2: references added and updated, final version, to appear in CM

Project Energize: intervention development and 10 years of progress in preventing childhood obesity

Prevention of childhood obesity is a global priority. The school setting offers access to large numbers of children and the ability to provide supportive environments for quality physical activity and nutrition. This article describes Project Energize, a through-school physical activity and nutrition programme that celebrated its 10-year anniversary in 2015 so that it might serve as a model for similar practices, initiatives and policies elsewhere. The programme was envisaged and financed by the Waikato District Health Board of New Zealand in 2004 and delivered by Sport Waikato to 124 primary schools as a randomised controlled trial from 2005 to 2006. The programme has since expanded to include all 242 primary schools in the Waikato region and 70 schools in other regions, including 53,000 children. Ongoing evaluation and development of Project Energize has shown it to be sustainable (ongoing for >10 years), both effective (lower obesity, higher physical fitness) and cost effective (one health related cost quality adjusted life year between $18,000 and$30,000) and efficient ($45/child/year) as a childhood 'health' programme. The programme's unique community-based approach is inclusive of all children, serving a population that is 42 % Ma¯ori, the indigenous people of New Zealand. While the original nine healthy eating and seven quality physical activity goals have not changed, the delivery and assessment processes has been refined and the health service adapted over the 10 years of the programme existence, as well as adapted over time to other settings including early childhood education and schools in Cork in Ireland. Evaluation and research associated with the programme delivery and outcomes are ongoing. The dissemination of findings to politicians and collaboration with other service providers are both regarded as priorities

Experimental evidence for the interplay between individual wealth and transaction network

We conduct a market experiment with human agents in order to explore the structure of transaction networks and to study the dynamics of wealth accumulation. The experiment is carried out on our platform for 97 days with 2,095 effective participants and 16,936 times of transactions. From these data, the hybrid distribution (log-normal bulk and power-law tail) in the wealth is observed and we demonstrate that the transaction networks in our market are always scale-free and disassortative even for those with the size of the order of few hundred. We further discover that the individual wealth is correlated with its degree by a power-law function which allows us to relate the exponent of the transaction network degree distribution to the Pareto index in wealth distribution.Comment: 6 pages, 7 figure

Modelling Unsteady Processes with the Direct Simulation Monte Carlo Technique

Over the past 40 years, the Direct Simulation Monte Carlo (DSMC) technique has been developed into a flexible and effective solver for flow problems in the rarefied to near continuum regime. However, even with modern parallelised code, the efficient computation of unsteady near-continuum flows, which are important in processes such as Pulsed Pressure Chemical Vapour Deposition (PP-CVD), remains a challenge. We have developed an unsteady parallel DSMC code (PDSC) utilising advanced features such as transient adaptive sub-cells to ensure nearest neighbour collisions and a temporal-variable time step to reduce computation time. This technique is combined with a unique post-processor called the DMSC Rapid Ensemble Averaging Method (DREAM) which reduces the statistical scatter in the data sets produced by PDSC. The combined method results in a significant memory and computational reduction over ensemble averaging DSMC, while maintaining low statistical scatter in the results. The unsteady code has been validated by simulation of shock-tube flow and unsteady Couette flow, and a number of test cases have been demonstrated including shock impingement on wedges. The technique is currently being used to model the development of an underexpanded jet in a PP-CVD reactor