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Cambridge, United Kingdom

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.


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.


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.

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