Volume 116, Number 3, November 2016
|Number of page(s)||7|
|Section||Condensed Matter: Electronic Structure, Electrical, Magnetic and Optical Properties|
|Published online||05 December 2016|
Infrared optical properties of α quartz by molecular dynamics simulations
1 DMMT, Università di Brescia - Viale Europa 11, 25123 Brescia, Italy
2 Dipartimento di Matematica, Università degli Studi di Milano - Via Saldini 50, 20133 Milano, Italy
Received: 26 July 2016
Accepted: 16 November 2016
This paper is concerned with theoretical estimates of the refractive-index curves for quartz, obtained by the Kubo formulæ in the classical approximation, through MD simulations for the motions of the ions. Two objectives are considered. The first one is to understand the role of nonlinearities in situations where they are very large, as at the structural phase transition. We show that, on the one hand, they do not play an essential role in connection with the form of the spectra in the infrared. On the other hand, they play an essential role in introducing a chaoticity which involves a definite normal mode. This might explain why that mode is Raman active in the α phase, but not in the β phase. The second objective concerns whether it is possible in a microscopic model to obtain normal mode frequencies, or peak frequencies in the optical spectra, that are in good agreement with the experimental data for quartz. Notwithstanding a lot of effort, we were unable to find results agreeing better than about 6%, as apparently also occurs in the whole available literature. We interpret this fact as indicating that some essential qualitative feature is lacking in all models which consider, as the present one, only short-range repulsive potentials and unretarded long-range electric forces.
PACS: 78.20.Ci – Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity) / 42.70.Ce – Glasses, quartz / 63.20.Ry – Anharmonic lattice modes
© EPLA, 2016
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