An Unpublished Chateaubriand Letter

Barth, Gilbert

doi:10.2307/3042699

Cited by 19 publications

(20 citation statements)

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“…The ion core of silicon has been described by a norm-conserving pseudopotential [27,28] created following a scheme by v. Barth and Car [29], which is described in [30]. The exchange-correlation energy is calulated with the use of the local-density approximation [ 31,32].…”

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Phonons in theβ-tin,Imma, and sh phases of silicon fromab initiocalculations

Gaál-Nagy

Strauch

2006

Phys. Rev. B

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We present a new interpretation of measured Raman frequencies of a high-pressure structure of Silicon which was assigned previously to the β-tin phase. Our results show that the β-tin→Imma→sh phase transitions have been already indicated in this experiment which was performed before the discovery of the Imma phase. We have calculated phonon-dispersion curves for the β-tin, Imma, and sh phases of silicon using the plane-wave pseudopotential approach to the density-functional theory and the density-functional perturbation theory within the local density approximation. With the new assignment, the calculated phonon frequencies display an excellent agreement with the experimental data, and can be also used to determine precisely the transition pressure for the Imma→β-tin phase transition. The sh→Imma transition is accompanied by soft modes. In 1993 a new high-pressure phase was found between the β-tin (SiII, body centered tetragonal structure) and the sh (SiV, simple hexagonal structure) phase of silicon [1]. This phase was called Imma phase after its space group and has a body-centered orthorhombic structure (SiXI). In the diffraction experiment, the phase transitions to and from the Imma phase appear as of first order with a discontinuity in the volume and the lattice parameters [2]. Previous ab initio calculations of the phase transitions β-tin→Imma→sh have indicated both phase transitions to be of second order [3] or both of first order [4]. A theoretical investigation of elastic stabilities has shown a second-order phase transition β-tin→Imma [5]. From group-theoretical arguments [6], the phase transition β-tin→Imma can be of second order, whereas the phase transition Imma→sh has to be of first order. This conclusion was strongly supported by an ab initio calculation in our previous work [7].An intermediate phase similar to the Imma phase, with a structure more general than the β-tin and the sh phase was considered theoretically before [8] with a prediction that the phase transitions might be accompanied by soft phonon modes. Since the Imma structure results from a distortion and a relative sublattice shift of β-tin along the c-direction, which corresponds to a soft Γ phonon displacement [9], it is worthwhile to investigate the pressure dependence of the phonon-dispersion curves. Especially, it was speculated that the superconductivity of these metallic high-pressure phases of silicon might be enhanced by these soft modes [10,11]. Available experimental and theoretical data for superconducting properties [12,13,14,15,16] did not consider the Imma phase, because its existence was not known at that time. In the experiment, the superconducting temperature as a function of pressure shows a kink [11,15,16] within the assumed stability range of the sh phase. In the corresponding theoretical work this kink was traced back to some soft modes from calculations of a few phonon modes under pressure for the sh structure with a frozen-phonon technique [13]. However, in our opinion this kink corresponds to the phase transiti...

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Phonons in theβ-tin,Imma, and sh phases of silicon fromab initiocalculations

Gaál-Nagy

Strauch

2006

Phys. Rev. B

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“…In our calculations, we have adopted the generalized gradient approximation [16] to the local density approximation. A Car-Von Barth-type pseudopotential [17] was used for carbon and a numerical Troullier-Martins [18] for oxygen, both in Kleinman-Bylander factorized form [19]. The integration time step was fixed to 1.2 3 10 216 s. The electronic wave functions, assumed to have the same periodicity of the (deformable) simulation cell, were expanded on a plane wave basis set, up to a kinetic energy cutoff of 80 Ry.…”

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Decomposition and Polymerization of Solid Carbon Monoxide under Pressure

et al. 1998

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By performing constant-pressure deformable-cell ab initio molecular dynamics simulations we have studied the pressure-induced chemical instability of CO above 5 GPa. The simulation shows that, contrary to previous speculations, polymerization proceeds without CO bond dissociation. The resulting polymer consists of a disordered network of small polycarbonyl ͑CO͒ n chains connecting fivefold C 4 O cycles. The computed vibrational spectra and electronic gap agree very well with (and shed light on) very recent experimental data. [S0031-9007(98)07059-8] PACS numbers: 62.50. + p

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“…The around aerodynamic configurations, recent research efalgorithm is based on standard concepts of hp finite elements as forts have been focused into developing high-order discretwell as tensorial spectral elements. This new formulation employs isation procedures on unstructured meshes [3,4]. Also, a hierarchical/modal basis constructed from a new apex co-ordinate new fields such as computational electromagnetics for system which retains a generalised tensor product.…”

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confidence: 99%