Femtosecond dynamics of coherent optical phonons in GeTe/ Sb 2 Te 3 superlattices ͑SLs͒, a new class of semiconductor SLs with three different states, have been investigated by using a reflection-type pump-probe technique at various lattice temperatures. The time-resolved transient reflectivity obtained in as-grown SLs exhibits the coherent A 1 optical modes at 5.10 and 3.78 THz while only the single A 1 mode at 3.68 THz is observed in annealed SLs. The decay rate of the A 1 mode in annealed SLs is strongly temperature dependent while that in as-grown SLs is not temperature dependent. This result indicates that the damping of the coherent A 1 phonons in amorphous SLs is governed by the phonon-defect ͑vacancy͒ scattering rather than the anharmonic phonon-phonon coupling. DOI: 10.1103/PhysRevB.79.174112 PACS number͑s͒: 78.47.JϪ, 63.20.kp, 63.50.Ϫx, 68.35.Rh One of the most common materials for optical recording media is Ge 2 Sb 2 Te 5 ͑GST͒ in which phase transition between crystalline and amorphous phases serve rewritable recording. 1,2 Recently, extensive theoretical investigation on the mechanism of the phase change in GST have been made using molecular-dynamics simulations. [3][4][5] In addition, experimental studies using extended x-ray absorption fine structure ͑XAFS͒ and Raman-scattering measurements have examined dynamics of phase transition in GST, [6][7][8][9] suggesting that the structure of amorphous GST can be described as a cross section of a distorted rocksalt structure with vacancies and the amorphization of GST is due to an umbrella flip of Ge atoms from an octahedral position into a tetrahedral one. Moreover, Sun et al. 4 theoretically proposed that the vacancies in the crystalline ͑cubic͒ GST are highly ordered and layered followed by the recent prediction of the formation of large voids in the amorphous GST films. 10 The experimental information on the existence of vacancies from the lattice dynamical point of view, however, has not been explored.One of the advantages of GST as the optical recording media is its high-speed switching of read-write characteristics whose time scale has been believed to be less than a nanosecond. In order to understand and control the rapid phase change in GST, a time-resolved study of phonon dynamics in GST is strongly demanded, however, the timeresolved studies are still very few.11 Moreover, a new class of semiconductor superlattices ͑GeTe/ Sb 2 Te 3 ͒ with three different states has recently been proposed which will enable us to realize reversible transition among the three states by the means of the irradiation of laser pulses. 12The coherent phonon spectroscopy ͑CPS͒ is a powerful tool to study ultrafast dynamics of structural phase transitions, occurring within picosecond and femtosecond time scales and in fact it has been applied to semimetals, 13 ferroelectric materials, 14,15 and Mott insulators. 16,17 In the CPS, the pump pulse impulsively generates coherent lattice vibration through real or virtual electronic transitions. The pulse length ͑⌬͒ used s...

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