arXiv subjects

Richard J. Needs

Publications and source records attributed to Richard J. Needs.

46 records · Page 3Linked to original sources

arXiv2014

Anomalous non-additive dispersion interactions in systems of three one-dimensional wires

The non-additive dispersion contribution to the binding energy of three one-dimensional (1D) wires is investigated using wires modelled by (i) chains of hydrogen atoms and (ii) homogeneous electron gases. We demonstrate that the non-additive dispersion contribution to the binding energy is significantly enhanced compared with that expected from Axilrod-Teller-Muto-type triple-dipole summations and follows a different power-law decay with separation. The triwire non-additive dispersion for 1D electron gases scales according to the power law $d^{-β}$, where $d$ is the wire separation, with exponents $β(r_s)$ smaller than 3 and slightly increasing with $r_s$ from 2.4 at $r_s = 1$ to 2.9 at $r_s=10$, where $r_s$ is the density parameter of the 1D electron gas. This is in good agreement with the exponent $β=3$ suggested by the leading-order charge-flow contribution to the triwire non-additivity, and is a significantly slower decay than the $\sim d^{-7}$ behaviour that would be expected from triple-dipole summations.

cond-mat.mes-hall↗

arXiv2013

Stable all nitrogen metallic salt at terapascal pressures

The phase diagram and equation of state of dense nitrogen are of interest in understanding the fundamental physics and chemistry under extreme conditions, including planetary processes, and in discovering new materials. We predict several stable phases of nitrogen at multi-TPa pressures, including a P4/nbm structure consisting of partially charged N2 pairs and N5 tetrahedra, which is stable in the range 2.5-6.8 TPa. This is followed by a modulated layered structure between 6.8 and 12.6 TPa, which also exhibits a significant charge transfer. The P4/nbm metallic nitrogen salt and the modulated structure are stable at high pressures and temperatures, and they exhibit strongly ionic features and charge density distortions, which is unexpected in an element under such extreme conditions and could represent a new class of nitrogen materials. The P-T phase diagram of nitrogen at TPa pressures is investigated using quasiharmonic phonon calculations and ab initio molecular dynamics simulations.

cond-mat.mtrl-sci↗

arXiv2013

Compression algorithm for multi-determinant wave functions

A compression algorithm is introduced for multi-determinant wave functions which can greatly reduce the number of determinants that need to be evaluated in quantum Monte Carlo calculations. We have devised an algorithm with three levels of compression, the least costly of which yields excellent results in polynomial time. We demonstrate the usefulness of the compression algorithm for evaluating multi-determinant wave functions in quantum Monte Carlo calculations, whose computational cost is reduced by factors of between 1.885(3) and 25.23(4) for the examples studied. We have found evidence of sub-linear scaling of quantum Monte Carlo calculations with the number of determinants when the compression algorithm is used.

physics.comp-ph↗

arXiv2013

Excitons and biexcitons in symmetric electron-hole bilayers

Symmetric electron-hole bilayer systems have been studied at zero temperature using the diffusion quantum Monte Carlo method. A flexible trial wave function is used that can describe fluid, excitonic and biexcitonic phases. We calculate condensate fractions and pair correlation functions for a large number of densities rs and layer separations d. At small d we find a one-component fluid phase, an excitonic fluid phase, and a biexcitonic fluid phase, and the transitions among them appear to be continuous. At d = 0, excitons appear to survive down to about rs = 0.5 a.u., and biexcitons form at rs > 2.5 a.u.

cond-mat.str-el↗

arXiv2013

Decomposition and terapascal phases of water ice

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$_2$O decomposes into H$_2$O$_2$ 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 H$_2$O comes into contact with hydrogen. Metallization of H$_2$O is predicted at a higher pressure of just over 6 TPa, and therefore H$_2$O does not have a thermodynamically stable low-temperature metallic form. We have also found a new and rich mineralogy of complicated water ice phases which are more stable in the pressure range 0.8--2 TPa than any predicted previously.

cond-mat.mtrl-sci↗

arXiv2013

Classical and quantum ordering of protons in cold solid hydrogen under megabar pressures

A combination of state-of-the-art theoretical methods has been used to obtain an atomic-level picture of classical and quantum ordering of protons in cold high-pressure solid hydro-gen. We focus mostly on phases II and III of hydrogen, exploring the effects of quantum nuclear motion on certain features of these phases (through a number of ab initio path-integral molecular dynamics (PIMD) simulations at particular points on the phase diagram). We also examine the importance of van der Waals forces in this system by performing calculations using the optB88-vdW density functional, which accounts for non-local correlations. Our calculations reveal that the transition between phases I and II is strongly quantum in nature, resulting from a competition between anisotropic inter-molecular interactions that restrict molecular rotation and thermal plus quantum fluctuations of the nuclear positions that facilitate it. The transition from phase II to III is more classical because quantum nuclear motion plays only a secondary role and the transition is determined primarily by the underlying potential energy surface. A structure of P21c symmetry with 24 atoms in the primitive unit cell is found to be stable when anharmonic quantum nuclear vibrational motion is included at finite temperatures using the PIMD method. This structure gives a good account of the infra-red (IR) and Raman vibron frequencies of phase II. We find ad-ditional support for C2/c structure as a strong candidate for phase III, since it remains transparent up to 300 GPa, even when quantum nuclear effects are included. Finally, we find that accounting for van der Waals forces improves the agreement between experiment and theory for the parts of the phase diagram considered, when compared to previous work which employed the widely-used Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional.

cond-mat.mtrl-sci↗

arXiv2012

Quantum simulation of low-temperature metallic liquid hydrogen

Experiments and computer simulations have shown that the melt-ing temperature of solid hydrogen drops with pressure above about 65 GPa, suggesting that a liquid state might exist at low temperatures. It has also been suggested that this low temperature liquid state might be non-molecular and metallic, although evidence for such behaviour is lacking. Here, we report results for hydrogen at high pressures using ab initio path-integral molecular dynamics methods, which include a description of the quantum motion of the protons at finite temperatures. We have determined the melting temperature as a function of pressure by direct simulation of the coexistence of the solid and liquid phases and have found an atomic solid phase from 500 to 800 GPa which melts at <200 K. Beyond this and up to pressures of 1200 GPa a metallic atomic liquid is stable at temperatures as low as 50 K. The quantum motion of the protons is critical to the low melting temperature in this system as ab initio simulations with classical nuclei lead to a considerably higher melting temperature of \sim300 K across the entire pressure range considered.

cond-mat.mtrl-sci↗

arXiv2012

Density functional theory study of phase IV of solid hydrogen

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. Quasi-harmonic 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 [M. I. Eremets and I. A. Troyan, Nature Mater. 10 927 (2011) and R. T. Howie et al. Phys. Rev. Lett. 108 125501 (2012)]. We find that mixed phases are reasonable structural models for phase IV of hydrogen.

cond-mat.mtrl-sci↗

arXiv2011

High pressure ionic and molecular crystals of ammonia monohydrate within density functional theory

A combination of first-principles density functional theory calculations and a search over structures predicts the stability of a proton-transfer modification of ammonia monohydrate with space group P4/nmm. The phase diagram is calculated with the PBE density functional, and the effects of a semi-empirical dispersion correction, zero point motion, and finite temperature are investigated. Comparison with MP2 and coupled cluster calculations shows that the PBE functional over-stabilizes proton transfer phases because too much electronic charge moves with the proton. This over-binding is partially corrected by using the PBE0 hybrid exchange-correlation functional, which increases the enthalpy of P4/nmm by about 0.6 eV per formula unit relative to phase I of ammonia monohydrate (AMH-I) and shifts the transition to the proton transfer phase from the PBE pressure of 2.8 GPa to about 10 GPa. This is consistent with experiment as proton transfer phases have not been observed at pressures up to ~9 GPa, while higher pressures have not yet been explored experimentally.

cond-mat.mtrl-sci↗

arXiv2011

Persistence and eventual demise of oxygen molecules at terapascal pressures

Computational searches for structures of solid oxygen under pressures in the multi TPa range have been carried out using density-functional-theory methods. We find that molecular oxygen persists to about 1.9 TPa at which it transforms into a semiconducting square spiral-like polymeric structure (I41/acd) with a band gap of about 3.0 eV. Solid oxygen forms a metallic zig-zag chain-like structure (Cmcm) at about 3.0 TPa, but the chains in each layer gradually merge as the pressure is increased and a structure of Fmmm symmetry forms at about 9.5 TPa in which each atom has four nearest neighbors. The superconducting properties of molecular oxygen do not vary much with compression, although the structure becomes more symmetric. The electronic properties of oxygen have a complex evolution with pressure, swapping between insulating, semiconducting and metallic.

cond-mat.mtrl-sci↗