A new extended x-ray-absorption fine structure spectroscopy study of local bonding identifies for the first time significant concentrations of Ge-Ge bonds in amorphous Ge2Sb2Te5. The study provides a new understanding of the local molecular structure of this phase-change material. Application of bond constraint theory indicates that the amorphous phase is an ideal network structure in which the average number of constraints per atom equals the network dimensionality. Analysis within this framework imparts new and significant insights concerning the nature of the reversible optically driven amorphous-crystalline phase transition of Ge2Sb2Te5.

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In this paper we present a theoretical investigation of the focusing of coaxial Gaussian electromagnetic beams and of a Gaussian ripple on an electromagnetic beam of uniform irradiance in a collisional plasma (in thermal equilibrium in the absence of the beams). A self consistent solution of the electromagnetic wave equation, the energy balance equation, and Fourier’s equation of heat conduction has been obtained in the paraxial approximation. The nonuniform distribution of the electron density and thereby the dielectric function on account of the nonuniform electron temperature/density distribution causes the focusing/defocusing of the beams. The effect of thermal conduction on the temperature distribution of the electrons, on the critical curves, and the nature of focusing has been specifically studied. In common with earlier studies, three regions in the initial beam width—initial axial irradiance plane, corresponding to steady divergence, self focusing and oscillatory divergence have been characterized. Numerical computations have been made for an ionic collisions dominated plasma.