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Vanishing‐Harmonicity and Phase‐Change Materials
Author(s) -
Gaspard Jean-Pierre
Publication year - 2021
Publication title -
physica status solidi (rrl) – rapid research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.786
H-Index - 68
eISSN - 1862-6270
pISSN - 1862-6254
DOI - 10.1002/pssr.202000536
Subject(s) - materials science , condensed matter physics , distortion (music) , phase (matter) , amorphous solid , octahedron , electronic structure , atom (system on chip) , charge density wave , band gap , tight binding , electron , crystallography , crystal structure , physics , chemistry , quantum mechanics , amplifier , superconductivity , optoelectronics , cmos , computer science , embedded system
Phase‐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 distortion, characterized by a parameter η , are analyzed. The effective interatomic potential E ( η ) is developed in a Landau‐type series in η : E ( η ) = E 0 + E 2 η 2 + E 4 η 4 . The PCMs with the largest contrast are those for which the effective potential E ( η ) of the crystalline phase has a disappearing harmonic contribution ( E 2 = 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.