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The impact of europium doping on the electronic and structural properties of the topological insulator Bi2Te3 is studied in this paper. The crystallographic structure studied by electron diffraction and transmission microscopy confirms that grown by Molecular Beam Epitaxy (MBE) system film with the Eu content of about 3% has a trigonal structure with relatively large monocrystalline grains. The X-ray photoemission spectroscopy indicates that europium in Bi2Te3 matrix remains divalent and substitutes bismuth in a Bi2Te3 matrix. An exceptional ratio of the photoemission 4d multiplet components in Eu doped film was observed. However, some spatial inhomogeneity at the nanometer scale is revealed. Firstly, local conductivity measurements indicate that the surface conductivity is inhomogeneous and is correlated with a topographic image revealing possible coexistence of conducting surface states with insulating regions. Secondly, Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) depth-profiling also shows partial chemical segregation. Such in-depth inhomogeneity has an impact on the lattice dynamics (phonon lifetime) evaluated by femtosecond spectroscopy. This unprecedented set of experimental investigations provides important insights for optimizing the process of growth of high-quality Eu-doped thin films of a Bi2Te3 topological insulator. Understanding such complex behaviors at the nanoscale level is a necessary step before considering topological insulator thin films as a component of innovative devices.

Phase-change alloys have seen widespread use, from rewritable optical disks to current interest in their use in emerging neuromorphic computing architectures. In spite of this enormous commercial interest, the physics of the carriers in these materials is still not fully understood. Here, we describe the time and space dependence of the coupling between photoexcited carriers and the lattice in both the amorphous and crystalline states of one phase-change material, GeTe. We study this material using a time-resolved optical technique called the picosecond acoustic method to investigate the in situ thermally assisted amorphous-tocrystalline phase transformation in GeTe. Our work reveals a clear evolution of electron-phonon coupling during the phase transformation, as the spectra of photoexcited acoustic phonons in the amorphous (a-GeTe) and crystalline (α-GeTe) phases are different. In particular, and surprisingly, our analysis of the photoinduced acoustic pulse duration in crystalline GeTe suggests that part of the energy deposited during the photoexcitation process takes place over a distance that clearly exceeds that defined by the skin depth of the pump light. Alternatively, the photoexcitation process remains localized within that skin depth in the amorphous state. We then demonstrate that this is due to supersonic diffusion of photoexcited electronhole plasma in the crystalline state. Consequently, these findings prove the existence of the nonthermal transport of energy, which is much faster than lattice heat diffusion.