Negative Linear Compressibility

While all materials reduce their intrinsic volume under hydrostatic (uniform) compression, a select few actually \emph{expand} along one or more directions during this process of densification. As rare as it is counterintuitive, such "negative compressibility" behaviour has application in the design of pressure sensors, artificial muscles and actuators. The recent discovery of surprisingly strong and persistent negative compressibility effects in a variety of new families of materials has ignited the field. Here we review the phenomenology of negative compressibility in this context of materials diversity, placing particular emphasis on the common structural motifs that recur amongst known examples. Our goal is to present a mechanistic understanding of negative compressibility that will help inform a clear strategy for future materials design.Comment: Submitted to PCC

Adaptive Optics Simulations for Siding Spring

Using an observational derived model optical turbulence profile (model-OTP) we have investigated the performance of Adaptive Optics (AO) at Siding Spring Observatory (SSO), Australia. The simulations cover the performance for AO techniques of single conjugate adaptive optics (SCAO), multi-conjugate adaptive optics (MCAO) and ground-layer adaptive optics (GLAO). The simulation results presented in this paper predict the performance of these AO techniques as applied to the Australian National University (ANU) 2.3 m and Anglo-Australian Telescope (AAT) 3.9 m telescopes for astronomical wavelength bands J, H and K. The results indicate that AO performance is best for the longer wavelengths (K-band) and in the best seeing conditions (sub 1-arcsecond). The most promising results are found for GLAO simulations (field of view of 180 arcsecs), with the field RMS for encircled energy 50% diameter (EE50d) being uniform and minimally affected by the free-atmosphere turbulence. The GLAO performance is reasonably good over the wavelength bands of J, H and K. The GLAO field mean of EE50d is between 200 mas to 800 mas, which is a noticeable improvement compared to the nominal astronomical seeing (870 to 1700 mas).Comment: 15 pages; accepted for publication in PAS

Characterisation of the Optical Turbulence at Siding Spring

Measurements of the optical turbulence profile above Siding Spring Observatory were conducted during 2005 and 2006. This effort was largely motivated by the need to predict the statistical performance of adaptive optics at Siding Spring. The data were collected using a purpose-built instrument based on the slope-detection and ranging method (SLODAR) where observations of a bright double star are imaged by Shack-Hartmann taken with the Australian National University 24 inch and 40 inch telescopes. The analysis of the data yielded a model consisting of a handful of statistically prominent thin layers that are statistically separated into the ground layer (37.5, 250m) and the free atmosphere (1, 3, 6, 9, 13.5 km) for good (25%), typical (50%) and bad (25%) observing conditions. We found that ground-layer turbulence dominates the turbulence profile with up to 80% of the integrated turbulence below 500 m. The turbulence tends to be non-Kolmogorov, especially for the ground-layer with a power law index of $\beta \sim 10/3$. The mirror/dome seeing can be a significant fraction of the ground-layer turbulence. The median atmospheric seeing, is around 1.2", in agreement with observational reports.Comment: 23 pages; accepted for publication in PAS

Design of crystal-like aperiodic solids with selective disorder--phonon coupling

Functional materials design normally focuses on structurally-ordered systems because disorder is considered detrimental to many important physical properties. Here we challenge this paradigm by showing that particular types of strongly-correlated disorder can give rise to useful characteristics that are inaccessible to ordered states. A judicious combination of low-symmetry building unit and high-symmetry topological template leads to aperiodic "procrystalline" solids that harbour this type of topological disorder. We identify key classes of procrystalline states together with their characteristic diffraction behaviour, and establish a variety of mappings onto known and target materials. Crucially, the strongly-correlated disorder we consider is associated with specific sets of modulation periodicities distributed throughout the Brillouin zone. Lattice dynamical calculations reveal selective disorder-phonon coupling to lattice vibrations characterised by these same periodicities. The principal effect on the phonon spectrum is to bring about dispersion in energy rather than wave-vector, as in the poorly-understood "waterfall" effect observed in relaxor ferroelectrics. This property of procrystalline solids suggests a mechanism by which strongly-correlated topological disorder might allow new and useful functionalities, including independently-optimised thermal and electronic transport behaviour as required for high-performance thermoelectrics.Comment: 4 figure

Alternative Model Selection Using Forecast Error Variance Decompositions in Wholesale Chicken Markets

Although Vector Autoregressive models are commonly used to forecast prices, specification of these models remains an issue. Questions that arise include choice of variables and lag length. This article examines the use of Forecast Error Variance Decompositions to guide the econometrician’s model specification. Forecasting performance of Variance Autoregressive models, generated from Forecast Error Variance Decompositions, is analyzed within wholesale chicken markets. Results show that the Forecast Error Variance Decomposition approach has the potential to provide superior model selections to traditional Granger Causality tests.broiler markets, DAGs, forecasting, market structure, VAR, Agribusiness, Demand and Price Analysis, Livestock Production/Industries, Risk and Uncertainty, C53, D4, L1, Q00,