Phonons of Phase-Change Materials

peer reviewedPhase-change materials (PCMs) undergo a reversible transition from a semiconducting to a metallic state, nicely termed “incipient metal.” This transition is driven by a Peierls distortion. A model of covalent bonding is developed in a tightbinding scheme that accounts for the drastic evolution of some properties of PCMs, among which the Grüneisen parameter. The original vibrational properties of PCM and related Peierls distorted materials are analyzed. A three-body covalent interaction describes the PCM properties. It explains the simultaneous softening of the optical phonons and the hardening of the acoustic phonons upon crystallization of the PCMs

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peer reviewedPhase-change material (PCMs) store data using the contrast (electrical or optical) between two phases: a conductive crystalline phase and a weakly conductive amorphous phase. Most PCMs have a distorted octahedral structure. The contrast comes mainly from the electronic structure. In PCMs, a spontaneous symmetry breaking mechanism, the Peierls distortion, transforms the metallic crystalline structure into a lower-density semiconducting structure. In a simple tight-binding model of the covalent bond, the parameters that control this dis- tortion, characterized by a parameter η, are analyzed. The effective interatomic potential E(η) is developed in a Landau-type series in η : E(η) 1⁄4 E0 þ E2η2 þ E4η4. The PCMs with the largest contrast are those for which the effective potential E(η) of the crystalline phase has a disappearing harmonic contribution (E2 1⁄4 0) and a vanishing electronic gap. This is called as an “incipient Peierls distortion.” It coincides with the so-called “incipient metal”. The hardness of the repulsive potential and the number of electrons per atom play an important role. The vibrational properties and the anomalous Grüneisen parameter, specific to PCMs, are also studied

Structure of liquid semiconductors

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Confluence in UnTyped Higher-Order Theories by means of Critical Pairs

User-defined higher-order rewrite rules are becoming a standard in proof assistants based on intuitionistic type theory. This raises the question of proving that they preserve the properties of beta-reductions for the corresponding type systems. We develop here techniques that reduce confluence proofs to the checking of various forms of critical pairs for higher-order rewrite rules extending beta-reduction on pure lambda-terms. The present paper concentrates on the case where rewrite rules are left-linear and critical pairs can be joined without using beta-rewrite steps. The other two cases will be addressed in forthcoming papers

HD 172555: Detection of 63 μ m [OI] emission in a debris disc

Astronomy and Astrophysics 546 (2012): L8 Reproduced with permission from Astronomy & AstrophysicsContext. HD 172555 is a young A7 star belonging to the β Pictoris moving group that harbours a debris disc. The Spitzer/IRS spectrum of the source showed mid-IR features such as silicates and glassy silica species, indicating the presence of a warm dust component with small grains, which places HD 172555 among the small group of debris discs with such properties. The IRS spectrum also shows a possible emission of SiO gas. Aims. We aim to study the dust distribution in the circumstellar disc of HD 172555 and to asses the presence of gas in the debris disc. Methods. As part of the GASPS open time key programme, we obtained Herschel/PACS photometric and spectroscopic observations of the source.We analysed PACS observations of HD 172555 and modelled the spectral energy distribution with a modified blackbody and the gas emission with a two-level population model with no collisional de-excitation. Results. We report for the first time the detection of [OI] atomic gas emission at 63.18 μm in the HD 172555 circumstellar disc. We detect excesses due to circumstellar dust toward HD 172555 in the three photometric bands of PACS (70, 100, and 160 μm).We derive a large dust particle mass of (4.8 ± 0.6) × 10−4 M⊕ and an atomic oxygen mass of 2.5 × 10−2R2 M⊕, where R in AU is the separation between the star and the inner disc. Thus, most of the detected mass of the disc is in the gaseous phaseThis research has been funded by Spanish grants AYA 2010-21161-C02-02, CDS2006-00070 and PRICIT-S2009/ESP-1496. J.-C. Augereau and J. Lebreton thank the ANR (contract ANR-2010 BLAN-0505-01, EXOZODI) and the CNES-PNP for financial support. C. Pinte, F. Menard and W.-F. Thi acknowledges funding from the EU FP7-2011 under Grant Agreement nr. 284405. G. Meeus is supported by RYC-2011-07920. G. Meeus, C. Eiroa, I. Mendigutía and B. Montesinos are partly supported by AYA-2011-26202. F.M. acknowledges support from the Millennium Science Initiative (Chilean Ministry of Economy), through grant ÒNucleus P10-022-F