Novel photo-conducting state and its perturbation by electrostatic fields in oxide-based two-dimensional electron gas

The two-carrier transport model as proposed for the two-dimensional electron gas formed at the interfaces of oxide heterostructures is investigated by means of a combined perturbation of near ultra-violet radiation and electrostatic field, applied both separately and simultaneously. Comparison of the photo-response of the prototype systems such as the band insulator LaAlO3 and Mott insulator LaTiO3 films on TiO2 terminated SrTiO3 show remarkably similarities. Two types of non-equilibrium carriers are generated in each system, having a signature of a particular type of perturbation characterized by distinctly different relaxation process. While, the photo-conducting state diminishes in a stretched exponential manner, with a temperature dependent activation energy varying from few tens of meV to ~ 1 to 2 meV on lowering the temperature, and a relaxation time of several hours, the recovery from electrostatic gating occurs in milli-seconds time scale. An attempt is also made to explain the experimental observations using the ab-initio density functional calculations. The calculations show that the electronic transitions associated with near ultra-violet radiation emerge from bands located at ~ 2 eV above and below the Fermi energy, which are the Ti 3d states of the SrTiO3 substrate, and that from the AlO2 (TiO2) layers of the LaAlO3 (LaTiO3) films, respectively. The slow decay of the photo-current to the unperturbed state is explained in terms of the closely spaced Ti 3dxy states in the lower conduction band, which are manifested as flat bands (or localized states) in the band structure. Such localization leads to increased carrier life-times, through the energy-time relationship of the uncertainty principle.

Photo-absorption spectra of small hydrogenated silicon clusters using the time-dependent density functional theory

We present a systematic study of the photo-absorption spectra of various Si$_{n}$H$_{m}$ clusters (n=1-10, m=1-14) using the time-dependent density functional theory (TDDFT). The method uses a real-time, real-space implementation of TDDFT involving full propagation of the time dependent Kohn-Sham equations. Our results for SiH$_{4}$ and Si$_{2}$H$_{6}$ show good agreement with the earlier calculations and experimental data. We find that for small clusters (n<7) the photo-absorption spectrum is atomic-like while for the larger clusters it shows bulk-like behaviour. We study the photo-absorption spectra of silicon clusters as a function of hydrogenation. For single hydrogenation, we find that in general, the absorption optical gap decreases and as the number of silicon atoms increase the effect of a single hydrogen atom on the optical gap diminishes. For further hydrogenation the optical gap increases and for the fully hydrogenated clusters the optical gap is larger compared to corresponding pure silicon clusters.

Optical second harmonic generation in Yttrium Aluminum Borate single crystals (theoretical simulation and experiment)

Experimental measurements of the second order susceptibilities for the second harmonic generation are reported for YAl3(BO3)4 (YAB) single crystals for the two principal tensor components xyz and yyy. First principles calculation of the linear and nonlinear optical susceptibilities for Yttrium Aluminum Borate YAl3(BO3)4 (YAB) crystal have been carried out within a framework of the full-potential linear augmented plane wave (FP-LAPW) method. Our calculations show a large anisotropy of the linear and nonlinear optical susceptibilities. The observed dependences of the second order susceptibilities for the static frequency limit and for the frequency may be a consequence of different contribution of electron-phonon interactions. The imaginary parts of the second order SHG susceptibility chi_{123}^{(2)}(omega), chi_{112}^{(2)}(omega), chi_{222}^{(2)}(omega), and chi_{213}^{(2)}(omega) are evaluated. We find that the 2(omega) inter-band and intra-band contributions to the real and imaginary parts of chi_{ijk}^{(2)}l(omega) show opposite signs. The calculated second order susceptibilities are in reasonably good agreement with the experimental measurements.