Generalized existence conditions for surface states of a semi-infinite Kronig–Penney crystal

A derivation of most general existence conditions for surface states of a semi-infinite Kronig–Penney crystal is reported. They reduce to Steslicka's conditions under a subsidiary restriction and also indicate that the surface-state energies may lie in the allowed bulk bands. Numerical computations supporting the qualitative conclusions are provided.     

Comments on the presence of a peak in the static structure factor of an electron liquid

Emphasis is laid on the fact that the peak in the static structure factor S(k) observed in a recent experiment at k≈2k_F for conduction electrons in beryllium agrees well with the one predicted by us theoretically some time back. The error in the calculation of the pair correlation function g(r) using the experimental data on S(k) is pointed out. The position of the peak obtained in our g(r) clearly indicates that the effect of electron correlation is to condense into a Wigner lattice at a distance equal to the average interparticle separation rather than making a Mott type transition to an atomic-like state.     

Phase coherent photorefractivity in ZnSe single quantum wells

We observe an efficient phase coherent photorefractive effect in ZnSe single quantum wells for ultrashort light pulses resonant to the excitonic transition. The effect is attributed to the formation of an electron grating in the quantum well induced by the interference of coherent excitons that preserve phase and polarization of the incident light fields. All characteristic features of the diffracted signal are explained and reproduced by numerical calculations that are based on the optical Bloch equations for a three-level system.     

Oscillation of Forced Nonlinear Neutral Delay Difference Equations of First Order

Necessary and sufficient conditions are obtained for every solution of

The structure and properties of flavins: Molecular orbital study based on totally optimized geometries. I. Molecular geometry investigations

Molecular orbital calculations (MINDO /3) using energy minimized molecular geometries were performed on oxidized and reduced lumiflavin and related methylated isoalloxazines, including cationic and anionic species. Close agreement with experimental geometry, photoelectron spectra, and NMR data supports the importance of optimized geometries for these molecular systems and provides the basis for interpretation of chemical and biological properties. Oxidized forms are shown to be most stable in the planar configuration but also highly flexible about the N(5)—N(10) axis; only 1 kcal/mol is required for a 10° bend. N(10) is generally out of the plane slightly; also, C(9)-methyl substitution introduces nonplanarity. The unsubstituted isoalloxazine is computed to be 0.76 kcal/mol (ΔH) less stable than its isomer, alloxazine. Calculations were also performed on enol as well as quinone-methide tautomeric forms. Reduced flavin geometry depends on methyl substitution pattern: N(10) substituted forms are bent with typical fold angles around 155°, whereas the unsubstituted reduced form is planar. Both oxidized and reduced forms are also flexible. Proton affinities were calculated for protonation and deprotonation of oxidized and reduced forms. Protonation of oxidized forms is favored at N(1) by 10–12 kcal/mol and produces somewhat nonplanar isoalloxazinium ions. In addition, ΔH for the two-electron reduction of lumiflavin is estimated to be ?19.7 kcal/mol. In this paper investigations of geometric aspects are presented along with introductory and background material. Orbital structure and electron distribution studies are presented in paper II.     

Optical nonlinearity of organic dyes as studied by Z-scan and transient grating techniques

The excited state absorption cross-section of 5,5′-dichloro-11-diphenyl-amino-3,3′-diethyl-10, 12-ethylinethiatricarbocyanine perchlorate (IR140) have been measured by using a single beam transmission technique. Z-scan experiments have been used to find out a few nonlinear parameters. The excited state relaxation times have also been measured by using laser induced transient grating (LITG) technique.     

Anomalous optical transitions in AlInGaN/GaN heterostructures grown by metalorganic chemical vapor deposition

Using temperature-dependent photoluminescence (PL) measurements, we report a comprehensive study on optical transitions in Al_yIn_xGa_1−x−yN epilayer with target composition, x=0.01 and y=0.07 and varying epilayer thickness of 40, 65 and 100 nm. In these quaternary alloys, we have observed an anomalous PL temperature dependence such as an S-shape band-edge PL peak shift and a W-shape spectral broadening with an increase in temperature. With an increase in excitation power density, the emission peak from the AlInGaN epilayers shows a blue shift at 100 K and a substantial red shift at room temperature. This is attributed to the localization of excitons at the band-tail states at low temperature. Compared to 40 and 65 nm thick epilayers, the initial blue shift observed with low excitation power from 100 nm thick AlInGaN epilayer at room temperature is caused by the existence of deeper localized states due to confinement effects arising from higher In and Al incorporation. The subsequent red shift of the PL peak can be attributed by free motion of delocalized carriers that leads to bandgap renormalization by screening. Due to competing effects of exciton and free carrier recombination processes, such behavior of optical transitions leads to two different values of exponent ‘k’ in the fitting of PL emission intensity as a function of excitation power.     

Proton radioactivity with a Yukawa effective interaction

The half-lives of proton radioactivity of proton emitters are investigated theoretically. Proton-nucleus interaction potentials are obtained by folding the densities of the daughter nuclei with a finite-range effective nucleon-nucleon interaction having Yukawa form. The Wood-Saxon density distributions for the nuclei used in calculating the nuclear as well as the Coulomb interaction potentials are predictions of the interaction. The quantum mechanical tunneling probability is calculated within the WKB framework. These calculations provide reasonable estimates for the observed proton radioactivity lifetimes. The effects of neutron-proton effective mass splitting in neutron-rich asymmetric matter as well as the nuclear matter incompressibility on the decay probability are investigated.     

Convergence of H 1-Galerkin mixed finite element method for parabolic problems with reduced regularity on initial data

We study the convergence of H ¹-Galerkin mixed finite element method for parabolic problems in one space dimension. Both semi-discrete and fully discrete schemes are analyzed assuming less regularity on initial data. More precisely, for the spatially discrete scheme, error estimates of order \(\mathcal{O}\) (h ² t ^?1/2) for positive time are established assuming the initial function p ₀ ∈ H ²(Ω) ∩ H ₀ ¹ (Ω). Further, we use energy technique together with parabolic duality argument to derive error estimates of order \(\mathcal{O}\) (h ² t ^?1) when p ₀ is only in H ₀ ¹ (Ω). A discrete-in-time backward Euler method is analyzed and almost optimal order error bounds are established.     

Formation of nanodots on oblique ion sputtered InP surfaces

Using a field emission gun based scanning electron microscopy, we report the formation of nanodots on the InP surfaces after bombardment by 100 keV Ar⁺ ions under off-normal ion incidence (30° and 60° with respect to the surface normal) condition in the fluence range of 1 × 10¹⁶ to 1 × 10¹⁸ ions cm⁻². Nanodots start forming after a threshold fluence of about 1 × 10¹⁷ ions cm⁻². It is also seen that although the average dot diameter increases with fluence the average number of dots decreases with increasing fluence. Formation of such nanostructured features is attributed due to ion-beam sputtering. X-ray photoelectron spectroscopy analysis of the ion sputtered surface clearly shows In enrichment of the sputtered InP surface. The observation of growth of nanodots on the Ar⁺-ion sputtered InP surface under the present experimental condition matches well with the recent simulation results based on an atomistic model of sputter erosion.