J.A.P. da Costa

Structural, electronic, and optical absorption properties of orthorhombic CaSnO3 through ab initio calculations

Density functional theory ab initio calculations of the structural parameters, electronic structure, carriers effective masses, and optical absorption of orthorhombic CaSnO3 were performed within the local density and generalized gradient approximations, LDA and GGA, respectively. A good agreement between the calculated lattice parameters and experimental results was obtained, and a direct energy gap of 1.95 eV (2.92 eV) is estimated in the GGA (LDA) for orthorhombic CaSnO3. The computed carriers effective masses are small and practically isotropic for electrons, allowing us to suggest that direct gap orthorhombic CaSnO3 is a semiconductor with potential for optoelectronic applications. The optical absorption increases for photon energies larger than 7.0 eV due to the appearance of transitions involving O 2p valence states and Ca 3d conduction states.

Lateral shift of far infrared radiation on normal incidence reflection off an antiferromagnet

A lateral shift, similar to a Goos-Hanchen shift, of a normally incident electromagnetic beam reflected off an antiferromagnet in the presence of an external magnetic field is predicted. This shift is interpreted in terms of nonreciprocity of the reflected phase, and is confirmed using numerical simulation. There is also a lateral displacement of the field within the antiferromagnet, but not of the beam transmitted through an antiferromagnetic slab.

Triclinic CdSiO3 structural, electronic, and optical properties from first principles calculations

Quantum ab initio simulations were carried out to study the CdSiO3 triclinic crystal. Unit cell parameters and atomic positions were optimized to find a minimum total energy within the density functional theory (DFT) formalism in both the local density and generalized gradient approximations, LDA and GGA, respectively. Analysis of the Kohn?Sham electronic band structure shows that there are two very close indirect band gaps Eg(Z ? ?) = 2.57?eV (2.79?eV) and Eg(Q ? ?) = 2.59?eV (2.81?eV) for the GGA-PBE (LDA-CAPZ) computations, and a direct band gap Eg(? ? ?) = 2.57, 2.63?eV (2.85?eV). Effective masses for holes and electrons were estimated by parabolic fitting along different directions at the valence band maximum and conduction band minimum, and they are very anisotropic. A comparison with previously reported data for triclinic CaSiO3 (wollastonite) using the LDA-CAPZ exchange-correlation functional reveals that the substitution of calcium by cadmium changes the localization of the valence band maximum in reciprocal space and decreases the band gap energies. Optical properties (dielectric function, optical absorption) for incident light polarized along different crystalline planes were computed, the optical absorption for incident light with polarization along the 0?1?0 crystalline plane being the smallest for energies near the main band gap due to the spatial disposition of the SiO4 tetrahedra and CdO6 octahedra chains that build up the structure of triclinic CdSiO3.

Ionic nitriding in cathodic cage of AISI 420 martensitic stainless steel

Abstract Cylindrical shape samples of AISI 420 martensitic stainless steel were nitrided by cathodic cage plasma nitriding technique at temperatures of 623, 673 and 773 K for 5 h. In this technique the samples are placed on an insulating plate inside a cage that shields the cathodic potential. A systematic study was made to test the efficiency of this technique, when compared with conventional ionic nitriding, related to the elimination of defects such as edge effect. The process present a bigger nitriding rate and the samples nitrided using this new technique displayed crystalline phases and hardness comparable with those obtained using conventional ionic nitriding. Moreover, it was possible to eliminate completely the erosion rings often present in conventionally ionic nitrided samples due to edger effects.

First-principles calculations of structural, electronic and optical properties of orthorhombic CaPbO3

Calculations within the density functional theory approach were performed to obtain structural parameters, electronic band structure, carrier effective masses and optical absorption spectra in orthorhombic CaPbO3. Both local density and generalized gradient approximations, LDA and GGA, respectively, were considered. A comparison reveals good agreement of the calculated lattice parameters with experimental results. A direct Γ → Γ one-electron energy band gap of 0.84 eV (0.94 eV) was obtained within the GGA (LDA) level of calculation, in contrast to a previous interpretation of experimental data pointing to a gap of only 0.43 eV. Comparting our results with band gap energies previously obtained for CaXO3 crystals (X = C in calcite, X = Si in wollastonite and X = Ge,Sn,Pb in the orthorhombic phase), we note that the energy gap oscillates, but with an overall trend to decrease, as the atomic number of the X atomic species increases.

Uniformity of temperature in cathodic cage technique in nitriding of austenitic stainless steel AISI 316

Abstract A series of austenitic stainless steel AISI 316 cylindrical samples with different heights and samples with the same height placed at different positions were simultaneously nitrided using the cathodic cage technique. In this technique the samples are placed on an insulate plate inside a metallic cage which shield the cathodic potential, therefore it stays under a floating potential. A systematic study of the nitriding temperature variation effects was carried out to evaluate the temperature uniformity inside the cage. The samples were characterised through optical microscopy, X-ray diffraction and microhardness measurement. The results were compared with those obtained through conventional ionic nitriding, and it was verified that the samples nitrided by conventional technique present similar surface hardness, less uniformity and lower nitrided layer thickness than the samples treated under similar conditions through this new technique.

Cathodic Cage Plasma Nitriding: An Innovative Technique

Cylindrical samples of AISI 1020, AISI 316, and AISI 420 steels, with different heights, were simultaneously treated by a new technique of ionic nitriding, entitled cathodic cage plasma nitriding (CCPN), in order to evaluate the efficiency of this technique to produce nitrided layers with better properties compared with those obtained using conventional ionic nitriding technique. This method is able to eliminate the edge effect in the samples, promoting a better uniformity of temperature, and consequently, a smaller variation of the thickness/height relation can be obtained. The compound layers were characterized by X-ray diffraction, optical microscopy, and microhardness test profile. The results were compared with the properties of samples obtained with the conventional nitriding, for the three steel types. It was verified that samples treated by CCPN process presented, at the same temperature, a better uniformity in the thickness and absence of the edge effect.

Nitriding using cathodic cage technique of martensitic stainless steel AISI 420 with addition of CH4

AISI 420 martensitic stainless steel samples were nitrided by cathodic cage technique with addition of methane in the atmosphere aiming to reduce chromium nitride precipitation, to increase hardness and wear resistance without the presence of characteristic defects inherent to the ionic nitriding process. Microhardness measurements and X-ray analysis confirm the formation of a high hardness double-layer constituted by two regions: one internal region composed of carbon and another composed of nitrogen.

High lattice temperature effects on the ultrafast electron transport in 4H‐SiC

High lattice temperature effects on the electron transport transient in the 4H‐SiC c-parallel direction are studied within a single equivalent isotropic valley picture in the momentum and energy relaxation time approximation. The ultrafast transport regime occurs in a subpicosecond scale (<0.2ps), during which an overshoot in the electron drift velocity starts to be evident for high electric fields (≳60kV∕cm), depending on the lattice temperature. An increase of the electric field strength shifts the overshoot peak of the electron drift velocity to an earlier time. For a strong enough electric field, a higher lattice temperature cannot eliminate the electron drift overshoot effect, but can reduce it considerably due to a stronger electron-phonon scattering.

The normal incidence Goos-Hänchen shift

We predict a lateral shift of the reflected beam when an electromagnetic beam is normally incident on an antiferromagnet in the presence of an external field. This shift is confirmed using simulations for Gaussian beams.