Reversal of coherently controlled ultrafast photocurrents by band mixing in undoped GaAs quantum wells.

It is demonstrated that valence-band mixing in GaAs quantum wells tremendously modifies electronic transport. A coherent control scheme in which ultrafast currents are optically injected into undoped GaAs quantum wells upon excitation with femtosecond laser pulses is employed. An oscillatory dependence of the injection current amplitude and direction on the excitation photon energy is observed. A microscopic theoretical analysis shows that this current reversal is caused by the coupling of the light- and heavy-hole bands and that the hole currents dominate the overall current response. These surprising consequences of band mixing illuminate fundamental physics as they are unique for experiments which are able to monitor electronic transport resulting from carriers with relatively large momenta.

High-resolution Raman spectroscopy study of phonon modes in LiBH4 and LiBD4.

We have performed micro Raman measurements on LiBH4 and LiBD4 powders for temperatures between 5 and 300 K. At the lowest temperature, the peak energies agree very well with the results of a calculation within the density functional theory for the orthorhombic structure. The spectra are dominated by three separated bands: the external modes, the internal bending, and the internal stretching vibrations. Internal refers to vibrations within the BH 4 tetrahedrons, whereas external modes imply motions of Li and BH 4. The temperature dependence of the observed phonons corroborates the strong anharmonicity of the system. Due to the anharmonicity, Fermi resonances occur between the first order stretching modes and the second order bending modes of LiBH4. Moreover, the linewidths of the observed modes in LiBH4 have an Arrhenius-like component, with activation energies ranging from 250 to 500 K.

Hydrogen-deuterium exchange in bulk LiBH4.

Because of its apparent simplicity, diffusion of hydrogen in solids can be regarded as a general model system for diffusion. However, only rudimentary knowledge exists for the dynamics of hydrogen in complex hydrides. Insight into the specific diffusion process is given by hydrogen-deuterium exchange experiments. Thermogravimetry and Raman spectroscopy are used to measure the hydrogen-deuterium exchange during the decomposition of LiBH4. At a temperature of 523 K the self-diffusion constant of deuterium in LiBH4 is estimated to be D approximately 7 x 10(-14) m(2) s(-1). A careful analysis of the Raman spectra shows that hydrogen is statistically exchanged by deuterium in LiBH4; i.e., the diffusing species is assumed to be the single hydrogen atom.

Strong correlations in YH(3-delta) evidenced by Raman spectroscopy.

Temperature dependent Raman measurements on insulating YH(3-delta) thin films are reported. With increasing temperature we observe a huge broadening of a line corresponding to an yttrium mode. This particular mode is assigned to a breathing vibration of the yttrium atoms around an octahedral hydrogen position. The line broadening is discussed in terms of a coupling between this breathing mode and the electron excited from an octahedral H vacancy into the 4d conduction band of Y, corroborating the strong correlation models for the electronic structure of YH3.