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Humpert A.,University of Warwick | Allen M.P.,University of Warwick | Allen M.P.,Hh Wills Physics Laboratory
Physical Review Letters | Year: 2015

We show, by molecular simulation, that for a range of standard, coarse-grained, nematic liquid crystal models, the director bend fluctuation is a propagating mode. This is in contrast to the generally accepted picture of nematic hydrodynamics, in which all the director modes (splay, twist, bend, and combinations thereof) are overdamped. By considering the various physical parameters that enter the equations of nematodynamics, we propose an explanation of this effect and conclude that propagating bend fluctuations may be observable in some experimental systems. © 2015 American Physical Society.

Jack R.L.,University of Bath | Dunleavy A.J.,HH Wills Physics Laboratory | Dunleavy A.J.,University of Bristol | Dunleavy A.J.,Bristol Center for Complexity science | And 3 more authors.
Physical Review Letters | Year: 2014

We analyze connections between structure and dynamics in two model glass formers, using the mutual information between an initial configuration and the ensuing dynamics to compare the predictive value of different structural observables. We consider the predictive power of normal modes, locally favored structures, and coarse-grained measurements of local energy and density. The mutual information allows the influence of the liquid structure on the dynamics to be analyzed quantitatively as a function of time, showing that normal modes give the most useful predictions on short time scales while local energy and density are most strongly predictive at long times. © 2014 American Physical Society.

Royall C.P.,HH Wills Physics Laboratory | Royall C.P.,University of Bristol | Royall C.P.,Center for Nanoscience and Quantum Information | Williams S.R.,Australian National University
Physics Reports | Year: 2015

Amorphous solids, or glasses, are distinguished from crystalline solids by their lack of long-range structural order. At the level of two-body structural correlations, glassformers show no qualitative change upon vitrifying from a supercooled liquid. Nonetheless the dynamical properties of a glass are so much slower that it appears to take on the properties of a solid. While many theories of the glass transition focus on dynamical quantities, a solid's resistance to flow is often viewed as a consequence of its structure. Here we address the viewpoint that this remains the case for a glass. Recent developments using higher-order measures show a clear emergence of structure upon dynamical arrest in a variety of glass formers and offer the tantalising hope of a structural mechanism for arrest. However a rigorous fundamental identification of such a causal link between structure and arrest remains elusive. We undertake a critical survey of this work in experiments, computer simulation and theory and discuss what might strengthen the link between structure and dynamical arrest. We move on to highlight the relationship between crystallisation and glass-forming ability made possible by this deeper understanding of the structure of the liquid state, and emphasise the potential to design materials with optimal glassforming and crystallisation ability, for applications such as phase-change memory. We then consider aspects of the phenomenology of glassy systems where structural measures have yet to make a large impact, such as polyamorphism (the existence of multiple liquid states), ageing (the time-evolution of non-equilibrium materials below their glass transition) and the response of glassy materials to external fields such as shear. © 2014 Elsevier B.V.

Foster D.,HH Wills Physics Laboratory | Krusch S.,University of Kent
Nuclear Physics B | Year: 2015

In this paper, we present a detailed study of Skyrmion-Skyrmion scattering for two B=. 1 Skyrmions in the attractive channel where we observe two different scattering regimes. For large separation, the scattering can be approximated as interacting dipoles. We give a qualitative estimate when this approximation breaks down. For small separations we observe an additional short-range repulsion which is qualitatively similar to monopole scattering. We also observe the interesting effect of "rotation without rotating" whereby two Skyrmions, whose orientations remain constant while well-separated, change their orientation after scattering. We can explain this effect by following preimages through the scattering process, thereby measuring which part of an in-coming Skyrmion forms part of an out-going Skyrmion. This leads to a new way of visualising Skyrmions. Furthermore, we consider spinning Skyrmions and find interesting trajectories. © 2015 The Authors.

Giles P.A.,HH Wills Physics Laboratory | Maughan B.J.,HH Wills Physics Laboratory | Birkinshaw M.,HH Wills Physics Laboratory | Worrall D.M.,HH Wills Physics Laboratory | Lancaster K.,HH Wills Physics Laboratory
Monthly Notices of the Royal Astronomical Society | Year: 2012

We present the results of aChandraobservation of the galaxy cluster Abell 689 (z= 0.279). Abell 689 is one of the most luminous clusters detected in theROSATAll Sky Survey (RASS), but was flagged as possibly including significant point source contamination. The small point spread function of theChandratelescope allows us to confirm this and separate the point source from the extended cluster X-ray emission. For the cluster, we determine a bolometric luminosity ofLbol= (3.3 ± 0.3) × 1044ergs-1 and a temperature ofkT= 5.1+2.2 - 1.3keV when including a physically motivated background model. We compare our measured luminosity for A689 to that quoted in the RASS, and findL0.1-2.4keV= 2.8 × 1044ergs-1, a value ~10 times lower than theROSATmeasurement. Our analysis of the point source shows evidence for significant pile-up, with a pile-up fraction of ≃60 per cent. Sloan Digital Sky Survey spectra andHubble Space Telescope(HST) images lead us to the conclusion that the point source within Abell 689 is a BL Lac object. Using radio and optical observations from the Very Large Array andHSTarchives, we determine αro= 0.50, αox= 0.77 and αrx= 0.58 for the BL Lac, which would classify it as being of 'high-energy peak BL Lac' type. Spectra extracted of A689 show a hard X-ray excess at energies above 6keV that we interpret as inverse-Compton emission from aged electrons that may have been transported into the cluster from the BL Lac. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Berry M.V.,Hh Wills Physics Laboratory | Shukla P.,Indian Institute of Technology Kharagpur
European Journal of Physics | Year: 2011

The spin of a top can be regarded as a fast variable, coupled to the motion of the axis which is slow. In pure precession, the rotation of the axis round a cone (without nutation), can be considered as the result of a reaction from the fast spin. The resulting restriction of the total state space of the top is an illustrative example, at graduate-student level, of the general dynamical concept of the slow manifold. For this case, the slow manifold can be calculated exactly, and expanded as a series of reaction forces (ofmagnetic type) in powers of slowness, corresponding to a modified precession frequency. The forces correspond to a series for the Hannay angle for the fast motion, describing the location of a point on the top. © 2011 IOP Publishing Ltd.

Bremer M.N.,Hh Wills Physics Laboratory | Silk J.,Denys Wilkinson Building | Davies L.J.M.,Hh Wills Physics Laboratory | Lehnert M.D.,Laboratoire dEtudes des Galaxies
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2010

We have carried out a redshift survey using the VIMOS spectrograph on the VLT towards the cosmic microwave background cold spot. A possible cause of the cold spot is the Integrated Sachs-Wolfe effect imprinted by an extremely large void (hundreds of Mpc in linear dimension) at intermediate or low redshifts. The redshift distribution of over seven hundred z < 1 emission-line galaxies drawn from an I-band flux limited sample of galaxies in the direction of the cold spot shows no evidence of a gap on scales of Δz ≳ 0.05 as would be expected if such a void existed at 0.35 < z < 1. There are troughs in the redshift distribution on smaller scales (Δz ≈ 0.01) indicating that smaller scale voids may connect regions separated by several degrees towards the cold spot. A comparison of this distribution with that generated from similarly sized subsamples drawn from widely spaced pointings of the VVDS survey does not indicate that the redshift distribution towards the cold spot is anomalous or that these small gaps can be uniquely attributed to real voids. © 2010 The Authors. Journal compilation © 2010 RAS.

Bonamente M.,University of Alabama in Huntsville | Bonamente M.,NASA | Landry D.,University of Alabama in Huntsville | Maughan B.,HH Wills Physics Laboratory | And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We report the first Chandra detection of emission out to the virial radius in the cluster Abell 1835 at z = 0.253. Our analysis of the soft X-ray surface brightness shows that emission is present out to a radial distance of 10 arcmin or 2.4 Mpc, and the temperature profile has a factor of 10 drop from the peak temperature of 10 keV to the value at the virial radius. We model the Chandra data from the core to the virial radius and show that the steep temperature profile is not compatible with hydrostatic equilibrium of the hot gas, and that the gas is convectively unstable at the outskirts. A possible interpretation of the Chandra data is the presence of a second phase of warm-hot gas near the cluster's virial radius that is not in hydrostatic equilibrium with the cluster's potential. The observations are also consistent with an alternative scenario in which the gas is significantly clumped at large radii. © 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Le Feber B.,FOM Institute for Atomic and Molecular Physics | Rotenberg N.,FOM Institute for Atomic and Molecular Physics | Beggs D.M.,FOM Institute for Atomic and Molecular Physics | Beggs D.M.,HH Wills Physics Laboratory | Kuipers L.,FOM Institute for Atomic and Molecular Physics
Nature Photonics | Year: 2014

Control of light-matter interactions at the nanoscale has advanced fields such as quantum optics, photovoltaics and telecommunications. These advances are driven by an improved understanding of the nanoscale behaviour of light, enabled by direct observations of the local electric fields near photonic nanostructures. With the advent of metamaterials that respond to the magnetic component of light, schemes have been developed to measure the nanoscale magnetic field. However, these structures interact not only with the magnetic field, but also with the electric field of light. Here, we demonstrate the essential simultaneous detection of both electric and magnetic fields with subwavelength resolution. By explaining our measurements through reciprocal considerations, we create a route towards designing probes sensitive to specific desired combinations of electric and magnetic field components. Simultaneous access to nanoscale electric and magnetic fields will pave the way for new designs of optical nanostructures and metamaterials. © 2013 Macmillan Publishers Limited. All rights reserved.

Thomas M.R.,University of Bristol | Klein S.,Down The Avenue | Greasty R.J.,Hh Wills Physics Laboratory | Mann S.,University of Bristol | And 2 more authors.
Advanced Materials | Year: 2012

Self-assembled disc-shaped clusters of hexagonally packed, thiol-functionalized gold nanorods are prepared and dispersed in thermotropic nematic liquid crystals. The resultant hybrid complex fluids exhibit colloidal anisotropy with very high orientational order and are characterized by SAXS as shown in the figure. Precise, reconfigurable control of the cluster orientation at very low electric field strengths (0.18 V μm-1) is achieved. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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