Bruno Rousseau

Interstitial Electronic Localization

We investigate the ground-state properties of a collection of N noninteracting electrons in a macroscopic volume Omega also containing a crystalline array of N spheres of radius rc, each taken as largely impenetrable to electrons and with proximity of neighboring excluding regions playing a key physical role. The sole parameter of this quantum system is the ratio rc/rs, where rs is the Wigner-Seitz radius. Two lattices (fcc and bcc) are selected to illustrate the behavior of the system as a function of rc/rs. As this ratio increases valence electrons localize in the interstitial regions and the relative bandwidth epsilonF/epsilonF0 is found to decrease monotonically for both. The system is motivated by the behavior of the alkali metals at significant compression. It accounts for band narrowing, leads to electronic densities with interstitially centered maxima, and can be taken as a model which may be improved upon by perturbation and other methods.

Anharmonic stabilization of the high-pressure simple cubic phase of calcium.

The phonon spectrum of the high-pressure simple cubic phase of calcium, in the harmonic approximation, shows imaginary branches that make it mechanically unstable. In this Letter, the phonon spectrum is recalculated by using density-functional theory ab initio methods fully including anharmonic effects up to fourth order at 50 GPa. Considering that the perturbation theory cannot be employed with imaginary harmonic frequencies, a variational procedure based on the Gibbs-Bogoliubov inequality is used to estimate the renormalized phonon frequencies. The results show that strong quantum anharmonic effects make the imaginary phonons become positive even at zero temperature so that the simple cubic phase becomes mechanically stable, as experiments suggest. Moreover, our calculations find a superconducting T(c) in agreement with experiments and predict an anomalous behavior of the specific heat.

Efficient dielectric matrix calculations using the Lanczos algorithm for fast many-body

We present a

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implementations

implementation that assesses the two major bottlenecks of traditional plane-waves implementations, the summations over conduction states and the inversion of the dielectric matrix, without introducing new approximations in the formalism. The first bottleneck is circumvented by converting the summations into Sternheimer equations. Then, the novel avenue of expressing the dielectric matrix in a Lanczos basis is developed, which reduces the matrix size by orders of magnitude while being computationally efficient. We also develop a model dielectric operator that allows us to further reduce the size of the dielectric matrix without accuracy loss. Furthermore, we develop a scheme that reduces the numerical cost of the contour deformation technique to the level of the lightest plasmon pole model. Finally, the use of the simplified quasi-minimal residual scheme in replacement of the conjugate gradients algorithm allows a direct evaluation of the

Ab initio analysis of plasmon dispersion in sodium under pressure

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Enhanced Anharmonicity Under Pressure

corrections at the desired real frequencies, without need for analytical continuation. The performance of the resulting

Isotope effect in the superconducting high-pressure simple cubic phase of calcium from first principles

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Anharmonicity in aluminum hydride at high pressures

implementation is demonstrated by comparison with a traditional plane-waves implementation, which reveals a 500-fold speedup for the silane molecule. Finally, the accuracy of our

Ab initio superconducting temperature of BaSi2 at ambient pressure

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