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Pickard C.J.,University College London | Martinez-Canales M.,University College London | Needs R.J.,Theory of Condensed Matter Group
Physical Review Letters | Year: 2013

Computational searches for stable and metastable structures of water ice and other H:O compositions at TPa pressures have led us to predict that H 2O decomposes into H2O2 and a hydrogen-rich phase at pressures of a little over 5 TPa. The hydrogen-rich phase is stable over a wide range of hydrogen contents, and it might play a role in the erosion of the icy component of the cores of gas giants as H2O comes into contact with hydrogen. Metallization of H2O is predicted at a higher pressure of just over 6 TPa, and therefore H2O does not have a thermodynamically stable low-temperature metallic form. We have also found a new and rich mineralogy of complicated water ice phases that are more stable in the pressure range 0.8-2 TPa than any predicted previously. © 2013 American Physical Society.


Pickard C.J.,University College London | Martinez-Canales M.,University College London | Needs R.J.,Theory of Condensed Matter Group
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We have studied solid hydrogen up to pressures of 300 GPa and temperatures of 350 K using density functional theory methods and have found "mixed structures" that are more stable than those predicted earlier. Mixed structures consist of alternate layers of strongly bonded molecules and weakly bonded graphene-like sheets. Quasiharmonic vibrational calculations show that mixed structures are the most stable at room temperature over the pressure range 250-295 GPa. These structures are stabilized with respect to strongly bonded molecular phases at room temperature by the presence of lower frequency vibrational modes arising from the graphene-like sheets. Our results for the mixed structures are consistent with the experimental Raman data. We find that mixed phases are reasonable structural models for phase IV of hydrogen. © 2012 American Physical Society.


Pickard C.J.,University College London | Needs R.J.,Theory of Condensed Matter Group
Physical Review Letters | Year: 2011

First-principles density-functional-theory calculations show that compression of alkali metals stabilizes open structures with localized interstitial electrons which may exhibit a Stoner-type instability towards ferromagnetism. We find ferromagnetic phases of the lithium-IV-type, simple cubic, and simple hexagonal structures in the heavier alkali metals, which may be described as s-band ferromagnets. We predict that the most stable phases of potassium at low temperatures and pressures around 20 GPa are ferromagnets. © 2011 American Physical Society.


Martinez-Canales M.,University College London | Pickard C.J.,University College London | Needs R.J.,Theory of Condensed Matter Group
Physical Review Letters | Year: 2012

Phases of carbon are studied up to pressures of 1 petapascal (PPa) using first-principles density-functional-theory methods and a structure searching algorithm. Our extensive search over the potential energy surface supports the sequence of transitions diamond→BC8→simple cubic under increasing pressure found in previous theoretical studies. At higher pressures we predict a soft-phonon driven transition to a simple hexagonal structure at 6.4 terapascals (TPa), and further transitions to the face centered cubic electride structure at 21 TPa, a double hexagonal close packed structure at 270 TPa, and the body centered cubic structure at 650 TPa. © 2012 American Physical Society.


Pickard C.J.,University College London | Needs R.J.,Theory of Condensed Matter Group
Nature Materials | Year: 2010

Studying materials at terapascal (TPa) pressures will provide insights into the deep interiors of large planets and chemistry under extreme conditions. The equation of state of aluminium is of interest because it is used as a standard material in shock-wave experiments and because it is a typical sp-bonded metal. Here we use density-functional-theory methods and a random-searching approach to predict stable structures of aluminium at multiterapascal pressures, finding that the low-pressure close-packed structures transform to more open structures above 3.2TPa (nearly ten times the pressure at the centre of the Earth), with an incommensurate host guest structure being stable over a wide range of pressures and temperatures. We show that the high-pressure phases may be described by a two-component model consisting of positive ions and interstitial electron blobs ™, and propose that such structures are common in sp-bonded materials up to multiterapascal pressures. © 2010 Macmillan Publishers Limited. All rights reserved.


Pickard C.J.,University College London | Needs R.J.,Theory of Condensed Matter Group
Journal of Physics Condensed Matter | Year: 2011

It is essential to know the arrangement of the atoms in a material in order to compute and understand its properties. Searching for stable structures of materials using first-principles electronic structure methods, such as density-functional-theory (DFT), is a rapidly growing field. Here we describe our simple, elegant and powerful approach to searching for structures with DFT, which we call ab initio random structure searching (AIRSS). Applications to discovering the structures of solids, point defects, surfaces, and clusters are reviewed. New results for iron clusters on graphene, silicon clusters, polymeric nitrogen, hydrogen-rich lithium hydrides, and boron are presented. © 2011 IOP Publishing Ltd Printed in the UK & the USA.


Cooper N.R.,Theory of Condensed Matter Group
Physical Review Letters | Year: 2011

We show that simple laser configurations can give rise to "optical flux lattices," in which optically dressed atoms experience a periodic effective magnetic flux with high mean density. These potentials lead to narrow energy bands with nonzero Chern numbers. Optical flux lattices will greatly facilitate the achievement of the quantum Hall regime for ultracold atomic gases. © 2011 American Physical Society.


Beri B.,Theory of Condensed Matter Group | Cooper N.R.,Theory of Condensed Matter Group
Physical Review Letters | Year: 2011

The success of tensor network approaches in simulating strongly correlated quantum systems crucially depends on whether the many-body states that are relevant for the problem can be encoded in a local tensor network. Despite numerous efforts, strongly correlated projective states, including fractional quantum Hall states, have not yet found a local tensor network representation. Here we show that one can encode the calculation of averages of local operators in a Grassmann tensor network which is local. Our construction is explicit and allows the use of physically motivated trial wave functions as starting points in tensor network variational calculations. © 2011 American Physical Society.


Conduit G.J.,Theory of Condensed Matter Group
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2012

The gauge field of a uniform line of magnetic monopoles is created using a single Laguerre-Gauss laser mode and a gradient in the physical magnetic field. We study the effect of these monopoles on a Bose condensed atomic gas, whose vortex structure transforms when more than six monopoles are trapped within the cloud. Finally, we study this transition with the collective modes. © 2012 American Physical Society.


Ahnert S.E.,Theory of Condensed Matter Group
Molecular BioSystems | Year: 2013

We introduce a framework for the discovery of dominant relationship patterns in transcription networks, by compressing the network into a power graph with overlapping power nodes. Our application of this approach to the transcription networks of S. cerevisiae and E. coli, paired with GO term enrichment analysis, provides a highly informative overview of the most prominent relationships in the gene regulatory networks of these two organisms. © 2013 The Royal Society of Chemistry.

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