M. Piasecki

Effect of U on the Electronic Properties of Neodymium Gallate (NdGaO3): Theoretical and Experimental Studies

We have performed a density functional calculation for the centrosymmetric neodymium gallate using a full-potential linear augmented plane wave method with the LDA and LDA+U exchange correlation. In particular, we explored the influence of U on the band dispersion and optical transitions. Our calculations show that U = 0.55 Ry gives the best agreement with our ellipsometry data taken in the VUV spectral range with a synchrotron source. Our LDA+U (U = 0.55) calculation shows that the valence band maximum (VBM) is located at T and the conduction band minimum (CBM) is located at the center of the Brillouin zone, resulting in a wide indirect energy band gap of about 3.8 eV in excellent agreement with our experiment. The partial density of states show that the upper valence band originates predominantly from Nd-f and O-p states, with a small admixture of Nd-s/p and Ga-p B-p states, while the lower conduction band prevailingly originates from the Nd-f and Nd-d terms with a small contribution of O-p-Ga-s/p states. The Nd-f states in the upper valence band and lower conduction band have a significant influence on the energy band gap dispersion which is illustrated by our calculations. The calculated frequency dependent optical properties show a small positive uniaxial anisotropy.

Optical and photoconductivity spectra of novel Ag2In2SiS6 and Ag2In2GeS6 chalcogenide crystals

Complex spectral studies of near-band gap and photoconductive spectra for novel Ag(2)In(2)SiS(6) and Ag(2)In(2)GeS(6) single crystals are presented. The spectral dependences of photoconductivity clearly show an existence of spectral maxima within the 450 nm-540 nm and 780 nm-920 nm. The fundamental absorption edge is analyzed by Urbach rule. The origin of the spectral photoconductivity spectral maxima is discussed. Temperature dependences of the spectra were done. The obtained spectral features allow to propose the titled crystals as photosensors. An analysis of the absorption and photoconductivity spectra is given within a framework of oversimplified spectroscopic model of complex chalcogenide crystals.

Absorption and photoconductivity spectra of Ag2GeS3 crystal: Experiment and theory

Spectral features of polycrystalline Ag(2)GeS(3) samples synthesized from high-purity elements (at least 99.99 wt.% purity) in quartz ampoules evacuated to 0.1 Pa were explored. The band energy gap of Ag(2)GeS(3) crystals estimated from the fundamental absorption edge and photoconductivity spectra were found to be equal to 1.98 eV and 2.16 eV, respectively. Simultaneously we have performed calculations of the band structure, total and partial density of states and the electron charge density using the ab initio FP-LAPW method. All the calculations were performed with four different exchange-correlation (xc) potentials. It was found that the effect of using different xc is very marginal on the valence band maximum (VBM) while it is dramatically shifted the conduction band minimum (CBM) towards higher energies with respect to the Fermi energy position. Our theoretical results have given a band energy gap equal to 0.40 eV (for LDA), 0.42 eV (GGA), 1.03 eV (EVGGA) and 1.30 eV (mBJ) xc potentials. Thus the underestimation of the energy gap in LDA and GGA was partially corrected in EVGGA and mBJ model. As a remarkable fact mBJ did not bring the energy band gap very close to the experimental once. We have discovered that the Ag-s states have only a small effect on the conduction bands shifts whereas Ge-s states have a strong effect in extending of the gap, while remaining the valence bands unchanged.

IR laser induced spectra in novel crystals CdTe–CuInTe2

The novel crystalline alloys CdTe-CuInTe2 were synthesized. The photoinduced spectral changes of the anharmonic phonon modes were explored by cw CO2 laser at power about 2 kW in the vicinity of the 1650 cm(-1) mode. The changes of the intensities for principal phonon modes were found. These modes were assigned both to harmonic as well as anharmonic modes. All the measurements are studied after the Ir illumination. The performed quantum chemical calculations with application of the norm-conserving pseudopotential method and Green functions allow to identify the origin of the content dependent anharmonic phonon modes. Some correlation between the intensities of the corresponding phonon modes at about 1600-1700 cm(-1) and the corresponding IR induced changes were found.