Dynamics of Si-H vibrations in an amorphous environment

We present results of the first vibrational photon-echo, transient-grating, and temperature dependent transient-bleaching experiments on a-Si:H. Using these techniques, and the infrared light of a free electron laser, the vibrational population decay and phase relaxation of the Si-H stretching mode were investigated. Careful analysis of the data indicates that the vibrational energy relaxes directly into Si-H bending modes and Si phonons, with a distribution of rates determined by the amorphous host. Conversely, the pure dephasing appears to be single exponential, and can be modeled by dephasing via two-phonon interactions.     

Magnetism at spinel thin film interfaces probed through soft X-ray spectroscopy techniques

Magnetic order and coupling at the interfaces of highly spin polarized Fe₃O₄ heterostructures have been determined by surface sensitive and element specific soft X-ray spectroscopy and spectro-microscopy techniques. At ambient temperature, the interface between paramagnetic CoCr₂O₄ or MnCr₂O₄ and ferrimagnetic Fe₃O₄ isostructural bilayers exhibits long range magnetic order of Co, Mn and Cr cations which cannot be explained in terms of the formation of interfacial MnFe₂O₄ or CoFe₂O₄. Instead, the ferrimagnetism is induced by the adjacent Fe₃O₄ layer and is the result of the stabilization of a spinel phase not achievable in bulk form. Magnetism at the interface region is observable up to 500 K, far beyond the chromite bulk Curie temperature of 50-95 K.     

Imaging the phase separation in atomically thin buried SrTiO(3) layers by electron channeling

A phase-separation instability, resulting in the dewetting of thin SrTiO(3) films grown on Si(100) is shown by scanning transmission electron microscopy. Plan-view imaging of 1-nm thick, buried SrTiO(3) films was achieved by exploiting electron channeling through the substrate to focus the incident 0.2 nm beam down to a 0.04 nm diameter, revealing a nonuniform coverage by epitaxial SrTiO(3) islands and 2 x 1 Sr-covered regions. Density-functional calculations predict the ground state is a coexistence of 2 x 1 Sr-reconstructed Si and Sr-deficient SrTiO(3), in correspondence with the observed islanding.     

Ion implantation,luminescence, and cluster growth in silica layers

To activate silica optically our investigations are extended to ion implantation, mainly to overstoichiometric injection or isoelectronic substitution of the both constituents silicon or oxygen, i.e. by ions of the group IV (C, Si, Ge, Sn, Pb) or the group VI (O, S, Se). Such implantation produce new luminescence bands in silica layers, partially with optical electronic–vibronic transitions and respective multimodal spectra. In this context, special interest should be directed to low-dimension nanocluster formation in silica layers. Cathodoluminescence, high resolution transmission (HR-TEM) and scanning transmission electron microscopy (STEM), and energy dispersive X-ray analysis (EDX) have been used to investigate Si and Ge cluster formation in amorphous silicon dioxide layers and their respective luminescence behavior.     

Valence changes and structural distortions in "charge ordered" manganites quantified by atomic-scale scanning transmission electron microscopy

We investigate the microscopic nature of the "charge ordering" modulation in mixed-valent manganites in real space using scanning transmission electron microscopy. The modulation in Bi0.5Sr0.4Ca0.1MnO3 has a uniform periodicity appearing as stripes in high angle annular dark field images. Geometric phase analysis shows the modulation to be a displacement wave with transverse amplitude (0.008+/-0.001)a and longitudinal amplitude (0.003+/-0.001)a. Series of energy loss spectra taken across the stripes show no periodic changes and place an upper bound of +/-0.04 on any valence changes of the Mn ions.