Full-relativistic calculation of electronic structure of Zr2AlC and Zr2AlN

We have employed a full-relativistic version of an all-electron full-potential linearized-augmented plane-wave method in the local density approximation to investigate the electronic structure of nanolaminate Zr₂AlX (X=C and N). The Zr 4d electrons are treated as valence electrons. We have investigated the lattice parameters, bulk moduli, band structures, total and partial densities of states and charge densities. It is demonstrated that the strength and electrical transport properties in these materials are principally governed by the metallic planes.     

The dielectric constant of barium mono-chalcogenides and their improved band gap results

We have applied Engel-Vosko exchange energy within density functional theory, to calculate the electronic structure and the optical properties of BaX (X = Te, Se, and S) compounds via full potential linearized augmented plane wave method. We have found that this improves the band gap results comparing to our previous work in which we had made use of Perdew et al. exchange energy functional. We have also calculated the dielectric constant of these compounds, using both Perdew et al. and Engel-Vosko schemes. It is shown that Engel-Vosko exchange energy functional leads to a better result. We have also reported the effect of spin-orbit coupling on the results.     

Optical properties of in situ doped and undoped titania nanocatalysts and doped titania sol-gel nanofilms

In this paper we present spectroscopic properties of doped and undoped titanium dioxide (TiO₂) as nanofilms prepared by the sol-gel process with rhodamine 6G doping and studied by photoacoustic absorption, excitation and emission spectroscopy. The absorption spectra of TiO₂ thin films doped with rhodamine 6G at very low concentration during their preparation show two absorption bands, one at 2.3 eV attributed to molecular dimmer formation, which is responsible for the fluorescence quenching of the sample and the other at 3.0 eV attributed to TiO₂ absorption, which subsequently yields a strong emission band at 600 nm. The electronic band structure and optical properties of the rutile phase of TiO₂ are calculated employing a fully relativistic, full-potential, linearized, augmented plane-wave (FPLAPW) method within the local density approximation (LDA). Comparison of this calculation with experimental data for TiO₂ films prepared for undoped sol-gels and by sputtering is performed.     

Ab-initio simulation of elastic constants for some ceramic materials

Athermal elasticity for some ceramic materials (α-Al₂O₃, SiC (α and β phases), TiO₂ (rutile and anatase), hexagonal AlN and TiB₂, cubic BN and CaF₂, and monoclinic ZrO₂) have been investigated via density functional theory. Energy-volume equation-of-state computations to obtain the zero pressure equilibrium volume and bulk modulus as well as computations of the full elastic constant tensor of these ceramics at the experimental zero pressure volume have been performed. The present results for the single crystal elasticity are in good agreement with experiments both for the aggregate properties (bulk and shear modulus) and the elastic anisotropy. In contrast, a considerable discrepancy for the zero pressure bulk modulus of some ceramics evaluated from the energy-volume fit to the computational zero pressure volume has been observed.     

Tunneling of two interacting electrons in a quantum wire

A model calculation is reported for the tunneling probability of one as well as two interacting electrons from a quantum well within a narrow channel. We discuss the cases when the two electrons are spin polarized or unpolarized by transforming the system to a noninteracting one with the use of quantal density functional theory to obtain an effective single-particle confining potential. A semiclassical approach is used to obtain the tunneling probability from this effective potential. The calculation is motivated by recent measurements of the conductance of an electron gas in a narrow channel but is not meant to explain the anomalous behavior that has been reported since, for example, we deal with a simplified two-level system. Numerical results for the tunneling probability are presented.     

Neutron diffraction study of crystal structure and antiferromagnetic ordering in the heavy fermion compound CePd2Al3

Nuclear and magnetic structures of an annealed polycrystalline sample of the heavy-fermion compound CePd₂Al₃ prepared by arc-melting were investigated by neutron powder diffraction. The chemical structure corresponds well to the ordered hexagonal PrNi₂Al₃-type structure. The antiferromagnetic structure of CePd₂Al₃ with an ordered magnetic moment _Ce=0.47(2) _B at saturation is remarkably similar to that in the heavy fermion superconductor UPd₂Al₃. The additional incommensurate magnetic structures reported previously both for UPd₂Al₃ and CePd₂Al₃ are not observed in the present sample of CePd₂Al₃. At 1.4 K the magnetoresistivity of CePd₂Al₃ measured up to 14 T indicates only one field-induced phase transition at 3.0 T.     

The new hydrides CeNiGeH1.6 and CeCuGeH1.0 crystallizing in the derivative hexagonal ZrBeSi-type structure

The ternary germanides CeNiGe and CeCuGe absorb hydrogen in the temperature range 393-473 K. X-ray powder diffraction and transmission electron microscopy show that the hydride CeNiGeH_1.6(1) adopts the hexagonal ZrBeSi-type whereas CeCuGeH_1.0(1) crystallizes in a superstructure of this type. Magnetization, electrical resistivity and thermopower measurements reveal that CeNiGeH_1.6(1) is an intermediate valence compound having a Kondo temperature ≅220(10) K smaller than that observed ≅600(20) K for CeNiGe. On the contrary, a transition from ferromagnetism to non-magnetic ordering above 1.8 K is evidenced during the hydrogenation of CeCuGe.     

Stabilization of ferromagnetism in Mn doped ZnO with C co-doping

We present a method for stabilizing ferromagnetism in Mn doped ZnO. We find that Mn doped ZnO show anti-ferromagnetic order in the absence of additional carriers. When Mn doped ZnO is co-doped with C atom at O sites ferromagnetic state gets stabilized. The C doping creates holes which leads to stabilization of ferromagnetic state via hole mediated double exchange mechanism.     

Role of the d-f Coulomb interaction in intermediate valence and Kondo systems: a numerical renormalization group study

Using the numerical renormalization group method, the dependences on temperature of the magnetic susceptibility χ(T) and specific heat C(T) are obtained for the single-impurity Anderson model with inclusion of d-f the Coulomb interaction. It is shown that the exciton effects caused by this effect (charge fluctuations) can significantly change the behaviour of C(T) in comparison with the standard Anderson model at moderately low temperatures, whereas the behaviour of χ(T) remains nearly universal. The ground-state and temperature-dependent renormalizations of the effective hybridization parameter and f-level position caused by the d-f interaction are calculated, and satisfactory agreement with the Hartree-Fock approximation is derived.     

Er‐ and Eu‐doped GaP‐oxide porous composites for optoelectronic applications

We demonstrate the controlled preparation of Er‐ and Eu‐doped GaP‐oxide porous composites. The fabrication procedure entails the use of porous semiconductor templates and the impregnation of rare earth ions from a rare earth salt solution in alcohol and thermal treatment. The composites exhibit strong green and red emission that comes from finely dispersed ErPO₄ and EuPO₄ oxide submicron phases in the composite. These materials may prove useful in future generations of optoelectronic and photonic devices. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)