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Yi X.,Fudan University | Wang Z.,Fudan University | Dong F.,Fudan University | Cheng S.,Fudan University | And 12 more authors.
Journal of Physics D: Applied Physics | Year: 2016

The temperature-dependent structural and optical properties of Ge60Te40 during transformation from amorphous to crystalline phases are studied by spectroscopic ellipsometry, grazing incidence x-ray diffraction (GIXRD), x-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM) as well as ab initio molecular dynamics (MD) simulations. Our findings, derived from GIXRD results and dielectric functions, indicate that the crystallization process of Ge60Te40 occurs at ∼260°C and the crystalline phase adopts the rhombohedral GeTe phase along with the cubic Ge phase. Our calculations further indicate that the Ge atoms also prefer to clump together in amorphous Ge60Te40. Such a segregation of Ge leads to a high-temperature phase-change in Ge60Te40 samples. According to the XPS results, the local atomic environment between amorphous and crystalline states gets a little bit of change. And the result of the atomic cluster alignment suggests that both the amorphous and crystalline phases have the rhombohedral local structure. © 2016 IOP Publishing Ltd.

Yuan Y.,Fudan University | Yuan Y.,Key Laboratory of Micro and Nano Photonic Structures | Wang N.,Fudan University | Wang N.,Key Laboratory of Micro and Nano Photonic Structures | And 2 more authors.
EPL | Year: 2012

By a comprehensive mathematical derivation, we show that the underlying physics of realizing a two-dimensional omnidirectional radiation from multiple sources via radially anisotropic zero-index metamaterials (RAZIM), as numerically and experimentally demonstrated in Cheng Q. et al., Phys. Rev. Lett., 108 (2012) 213903, lies in that the RAZIM shell confines all anisotropic cylindrical modes within the shell, while keeping transparent to isotropic cylindrical modes. From the illustrated underlying physics, it follows straightforwardly that it is impossible to use the RAZIM for generating perfectly coherent isotropic radiation in a three-dimensional free space, contrary to the claim made in the above-mentioned reference. © Copyright EPLA, 2012.

Yu X.,Key Laboratory of Micro and Nano Photonic Structures | Yu X.,Fudan University | Zhang F.,Key Laboratory of Micro and Nano Photonic Structures | Zhang F.,Fudan University | And 12 more authors.
Thin Solid Films | Year: 2013

The optical properties of TiO2 thin films prepared by electron beam evaporation were studied by spectroscopic ellipsometry and analyzed quantitatively using effective medium approximation theory and an effective series capacitance model. The refractive indices of TiO2 are essentially constant and approach to those of bulk TiO2 for films thicker than 40 nm, but drop sharply with a decrease in thickness from 40 to 5.5 nm. This phenomenon can be interpreted quantitatively by the thickness dependence of the void fraction and interfacial oxide region. The optical band gaps calculated from Tauc law increase with an increase of film thickness, and can be attributed to the contribution of disorder effect. © 2013 Elsevier B.V. All rights reserved.

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