Band Structure

We present the study of half metallacity of Co 2 MnAl as half-Heusler alloy in L2 1 structure which consists of four inter-penetrating fcc sub-lattices. Density functional theory based electronic structure calculations will be performed by using the full-potential linear augmented plane wave (FP-LAPW). We will use the general gradient approximation method (GGA) and local density approximation method called LDA+U. Density of states and band structure results will be presented in this paper.

Samples of SrAl 2 O 4 and SrAl 2 O 4 :Cr 3+ were prepared by mixing the powder materials SrCO 3 , Al 2 O 3 , and Cr 2 O 3. The crystal structures of the undoped and doped samples were analyzed by X-ray diffraction (XRD) measurements. The diffraction patterns reveal a dominant phase, characteristic of the monoclinic SrAl 2 O 4 compound and another unknown secondary phase, in small amount, for doped samples. The data were fitted using the Rietveld method for structural refinements and lattice parameter constants (a, b, c, and b) were determined. Luminescence of Cr 3+ ions in this host is investigated for the first time by excitation and emission spectroscopy at room temperature. Emission spectra present a larger band and a smaller structure associated to the 4 T 2 (4 F) ! 4 A 2 (4 F) and 2 E (2 G) ! 4 A 2 (4 F) electronic transitions, respectively. The obtained results are analyzed by crystal-field theory and the crystal-field parameter, Dq, and Racah parameters, B and C, are determined from the excitation measurements.

The Boltzmann equation for transport in semiconductors is projected onto spherical harmonics in such a way that the resultant balance equations for the coefficients of the distribution function times the generalized density of states can be discretized over energy and real space by box integration. This ensures exact current continuity for the discrete equations. Spurious oscillations of the distribution function are successfully suppressed by stabilization based on a maximum entropy dissipation principle avoiding the H-transformation. The derived formulation can be used on arbitrary grids as long as box integration is possible. The new approach works not only with analytical bands but also with full-band structures in the case of holes. Results are presented for holes in bulk silicon based on a full band structure and electrons in a Si NPN BJT. For the first time the convergence of the spherical harmonics expansion is shown for a device and it is found that the quasiballistic transport in nanoscale devices requires an expansion of considerably higher order than the usual first one. The stability of the discretization is demonstrated for a range of grid spacings in the real space and bias points which produce huge gradients in the electron density and electric field. It is shown that the resultant large linear system of equations can be solved memory efficiently by the numerically robust package ILUPACK.

In this Letter we present an experimental analysis of the acoustic transmission of a two-dimensional periodic array of rigid cylinders in air with two different geometrical configurations: square and triangular. In both configurations, and above a certain filling fraction, we observe an overlap, in the range of the audible frequencies, between the attenuation peaks measured along the two high-symmetry directions of the Brillouin zone. This effect is considered as the fingerprint of the existence of a full acoustic gap. Nevertheless, the comparison with our calculation of band structures shows that the triangular lattice has band states in that frequency range. We call them deaf bands. This contradictory result is explained by looking at the symmetry of the deaf bands; they cannot be excited by experiments of sound transmission.

The intermetallic compound MnGaGe, which crystallizes in the Fe2As structure shows a large and positive pressure dependence of the Curie temperature. To study this behaviour we perform band structure calculations whose results enter a spin fluctuation model. It is shown that the peculiar properties of MnGaGe can be traced back to the quasi two-dimensional magnetism of the Mn layers.

The de Haas-van Alphen effect was observed in the underdoped cuprate YBa2Cu3O6.5 via a torque technique in pulsed magnetic fields up to 59 T. Above a field of approximately 30 T the magnetization exhibits clear quantum oscillations with a single frequency of 540 T and a cyclotron mass of 1.76 times the free electron mass, in excellent agreement with previously observed Shubnikov-de Haas oscillations. The oscillations obey the standard Lifshitz-Kosevich formula of Fermi-liquid theory. This thermodynamic observation of quantum oscillations confirms the existence of a well-defined, closed, and coherent, Fermi surface in the pseudogap phase of cuprates.

The theoretical electronic densities of states for both the valence and conduction bands are presented for the tetrahedral semiconductors CdTe, HgTe and their alloy Hg 0:5 Cd0:5Te based on band structures computed using the empirical pseudopotential method. For the ternary alloy HgCdTe, we have coupled the virtual crystal approximation with the pseudopotential method. Various quantities such as the energy levels, the refractive index and the transverse e ective charge are calculated. The spin-orbit splitting is included in our calculations.

A set of heavily doped Al0.48In0.52As samples grown by molecular beam epitaxy on InP (Fe) substrates was investigated using the photoreflectance (PR) technique. The spectra at 300 K are characterized by a transition in the vicinity of the InP energy gap, followed by strongly damped Franz-Keldysh oscillations (FKOs) which do not appear when the spectra are obtained at 77 K. The builtin electric field estimated from FKOs shows a small doping dependence but is substantially affected by the inclusion of a thin layer of AlxGayIn1-x-yAs (x≡0.22) at the interface between InP (Fe) and AlInAs:Si. In order to explain these results, a model based on the discontinuity of the energy bands in the InP/AlInAs and InP/AlGaInAs/AlInAs systems and also on the matching of the Fermi levels between the different materials is suggested.

A systematic theoretical investigation is undertaken in order to identify a two-dimensional periodic dielectric structure that has a complete in-plane photonic band gap for both polarizations. Of the various structures studied, only a triangular lattice of air columns is found to have the desired band-gap properties. Microwave transmission experiments are performed to test the theoretical predictions.

A double-walled carbon nanotube is used to study the radial charge distribution on the positive inner electrode of a cylindrical molecular capacitor. The outer electrode is a shell of bromine anions. Resonant Raman scattering from phonons on each carbon shell reveals the radial charge distribution. A self-consistent tight-binding model confirms the observed molecular Faraday cage effect, i.e., most of the charge resides on the outer wall, even when this wall was originally semiconducting and the inner wall was metallic.

Motivated by exploring spin-orbit coupled magnetism in 5d-based transition metal oxides (TMOs) beyond the iridates, we present a powder inelastic neutron scattering study of magnetic excitations in Ba2FeReO6-a member of the double perovskite family of materials which exhibit half-metallic behavior and high Curie temperatures Tc. We find clear evidence of two well-defined dispersing magnetic modes in its low temperature ferrimagnetic state. We develop a local moment model, which incorporates the interaction of Fe spins with spin-orbital locked magnetic moments on Re, and show that it captures our experimental observations. This allows us to extract moment sizes and exchange couplings, explain the magnitude of Tc, and to infer that magneto-structural locking terms are weak. Our study further opens up Re-based compounds as model systems to explore the interplay of strong correlations and spin-orbit coupling in 5d TMOs.

We show here in a graphic and simple way the relation between the periodic and chaotic regions in the Mandelbrot set. Since the relation between the periodic and chaotic regions in a onedimensional (1D) quadratic set is already well known, we shall base on it to extend the results to the Mandelbrot set. We shall see that in the same way as the hyperbolic components of the perioddoubling cascade determines the chaotic bands structure in 1D quadratic sets, the periodic region determines the chaotic region in the Mandelbrot set.

We study the quantum dynamics of a particle bound on a torus in the presence of an external magnetic field. We derive and discuss the effective Schrödinger equation for the surface eigenmodes and the corresponding quantum effective potential which is periodic and exhibits quantum anti-centrifugal force for nonvanishing angular momentum quantum numbers. An energy band structure of pure geometrical origin emerges naturally which can be tested experimentally in semiconducting as well as carbon nanotori.

We report calculations of the optical and magneto-optical properties of GdFe(2) and GdCo(2) using the full-potential linear-augmented-plane-wave method. Calculations with the Coulomb corrected local spin density approximation (LSDA+U) give a better representation of the band structure, density of states and magnetic moments compared to LSDA alone. However, both LSDA and LSDA+U approximations give fairly good agreement with experiment for the diagonal optical conductivity. The calculated results for GdCo(2) are in better agreement with the 'oxide corrected' data. Our results suggest that the data for GdFe(2) are most likely influenced by surface oxidation, due to the high reactivity of these compounds. For the much smaller off-diagonal components and Kerr rotation, LSDA results are better than the LSDA+U results, particularly in the energy range 0-3 eV. We show that the unphysical negative ellipticity values are taken care of by the use of stronger relaxation, which also improves the qualitative agreement with experimental data. Overall we have obtained a fair agreement with the experimental data for both optical and magneto-optical properties. We feel that measurements over a larger energy range are required for facilitating an exhaustive and decisive comparison and also to strengthen the bond between theory and experiments.

Spin-polarized photoemission was employed to investigate the temperature dependence of the exchange splitting of Gd(0001) for both the surface state near the Fermi level Fr and the 5d bulk bands. The bulk bands at 1-2 eV below EF show Stoner-like behavior where two peaks with opposite spin polarization shift toward each other as the temperature increases. In contrast, the temperature dependence of the surface state indicates spin-mixing behavior due to fluctuating local moments. These differences are attributed to the degree of itinerancy of the 5d electrons in the two cases.

Theoretical results are presented regarding the incorporation of Scandium into wurtzite GaN and InN binaries. The electric, optical and piezoelectric properties of the resulting ScGaN and ScInN systems are reported by using first-principles Local-density approximation (LDA) within density functional theory (DFT), Berry phase approach within modern theory of polarization and phonon calculations within the density functional perturbation theory. Our results predict the existence of breaking-symmetry structural phase transition in ordered Sc0.5Ga0.5N and Sc0.5In0.5N alloys when subjected to a compressive or tensile strain. Moreover, our results demonstrate the existence of symmetry preserving pressure-induced isostructural phase transitions in ordered ScGaN and ScInN systems for different Sc concentrations. It has been shown that the existence of isostructural phase transitions leads to dramatic changes in optical, acoustic, and piezoelectric properties of ordered ScGaN and ScInN systems under high pressure. In particular, this study demonstrates that the existence of first-order isostructural phase transitions in Sc1Ga1N2 at a critical hydrostatic pressure of 12.3 GPa leads to a huge enhancement of piezoelectricity (i.e., the e33 piezoelectric coefficient adopts a huge value as large as 13 C/m 2). In addition, It has been shown that ordered Sc0.5Ga0.5N and Sc0.5In0.5N alloys exhibit tremendous piezoelectric response, associated with a breakingsymmetry phase transition from nonpolar P 63/mcc(D 6h) space group to a polar P 63mc(C6v) structure, at fixed Ga, In and Sc compositions, as a function of the in-plane compressive and tensile strains. We also reveal the reason behind, and consequences of, these unusual properties associated with the strain-induced and pressure-induced structural phase transitions in the novel ScGaN and ScInN ordered structures.

New strontium titanyl phosphate Sr 2 TiO(PO 4) 2 (1) was synthesized and characterized by X ray powder diffraction, electron diffraction, high resolution electron microscopy, and band structure calculations. Titanyl phosphate 1 is isostructural with vanadyl phosphate Sr 2 VO(PO 4) 2 and has a layered structure. The titanium atoms are shifted from the centers of the TiO 6 octahedra and form short (1.74 Å) titanyl bonds. The structure of 1 is an unusual example of the disordered orientation of the chains formed by TiO 6 octahedra in complex titanium phosphates.

The Mossbauer spectra of ' Au in CsAu at 4.2 K and 1 bar, 27, 30, and 40 kbar suggest a phase transition from the CsCl-type structure to a structure of lower symmetry, at a pressure somewhat below 27 kbar. The transition is indicated by an increase of Debye temperature, a decrease of the Au isomer shift, and the occurrence of a nuclear quadrupole splitting of ' Au in the highpressure phase. The results are interpreted in terms of recent band-structure calculations.

Copper sulfide (Cu2-xS) is a class of low-cost, environment friendly p-type semiconductor, where electronic structure and the thus induced optoelectronic properties can be significantly varied through the creation of copper deficiency. To this end, varying composition of Cu2-xS (i.e., Cu2S, Cu1.96S, Cu1.8S, Cu1.8S+ Cu1.6S and CuS) films were grown here by using a low temperature molecular solution based deposition method, following which a wide range of characterization tools were used to understand their microstructure, electronic structure and optoelectronic properties. The hole concentration of these films are found to vary from 3.32 × 1019 cm−3 to 2.54 × 1022 cm−3 as Cu2-xS composition changes from Cu2S to CuS. This is because of the induced Cu deficiency in Cu2-xS films with decreasing Cu/S-molar ratio, which reduced the Cu d-band width in the valence band, thus pushing the Fermi level deep into the valence band. This leads the optical and transport gap to increase from 1.36 eV to 2.23 eV and 1.31 eV to 2.02 eV respectively with increasing copper deficiencies from Cu2S to CuS. Moreover, in this work, both the valence and conduction band edge positions are found to shift negatively with increasing Cu deficiency in these films.

The opticaf absorption edge of CuInTer. CuGaS, and CuGaSe, single crystafs was measured as a function of hydrostatic pressure up to 2OCPa. In all cases the energy gap increases with pressure. From the data the pressure dependence of the energy gap (dE ,/dP) was determined as 22 meV GPa-l for CuInTel, 40meVGPa-'

We present results of the band structure and density of states for the chalcopyrite compounds CuAlX 2 (X = S, Se, Te) using the state-of-the-art full potential linear augmented plane wave (FP-LAPW) method. Our calculations show that these compounds are direct band gap semiconductors. The energy gap decreases when S is replaced by Se and Se replaced by Te in agreement with the experimental data. The values of our calculated energy gaps are closer to the experimental data than the previous calculations. The electronic structure of the upper valence band is dominated by the Cu-d and X-p interactions. The existence of Cu-d states in the upper valence band has significant effect on the optical band gap.

In this work the complete valence-band structure of the molybdenum dichalcogenides MoS2, MoSe2, and α-MoTe2 is presented and discussed in comparison. The valence bands have been studied using both angle-resolved photoelectron spectroscopy (ARPES) with synchrotron radiation, as well as, ab-initio band-structure calculations. The ARPES measurements have been carried out in the constant-final-state (CFS) mode. The results of the calculations show in general very good agreement with the experimentally determined valence-band structures allowing for a clear identification of the observed features. The dispersion of the valence bands as a function of the perpendicular component k ⊥ of the wave vector reveals a decreasing three-dimensional character from MoS2 to α-MoTe2 which is attributed to an increasing interlayer distance in the three compounds. The effect of this k ⊥ dispersion on the determination of the exact dispersion of the individual states as a function of k is discussed. By performing ARPES in the CFS mode the k -component for off-normal emission spectra can be determined. The corresponding k ⊥ -value is obtained from the symmetry of the spectra along the ΓA, KH, and ML line, respectively.

The carbon-doped titania with high surface area was prepared by temperature-programmed carbonization of K-contained anatase titania under a flow of cyclohexane. This carbon-doped titania has much better photocatalytic activity for gas-phase photo-oxidation of benzene under irradiation of artificial solar light than pure titania. The visible light photocatalytic activity is ascribed to the presence of oxygen vacancy states because of the formation of Ti 3+ species between the valence and the conduction bands in the TiO 2 band structure. The co-existence of K and carbonaceous species together stabilize Ti 3+ species and the oxygen vacancy state in the as-synthesized carbon-doped titania.

We report a detailed study of the electronic and structural properties of the 39K superconductor \mgbtwo and of several related systems of the same family, namely \mgalbtwo, \bebtwo, \casitwo and \cabesi. Our calculations, which include zone-center phonon frequencies and transport properties, are performed within the local density approximation to the density functional theory, using the full-potential linearized augmented plane wave

The group theoretical treatment of bound and scattering state problems is extended to include band structure. We show that one can realize Hamiltonians with periodic potentials as dynamical symmetries, where representation theory provides analytic solutions, or which can be treated with more general spectrum generating algebraic methods. We find dynamical symmetries for which we derive the transfer matrices and dispersion relations. Both compact and non-compact groups are found to play a role.

Meso-scale characteristics of disturbances that bring about atmospheric disasters in pre-and mature monsoon seasons in Bangladesh are analyzed. Several types of meteorological instruments capable of observations with high temporal and spatial resolutions were introduced for the first time in this area to capture the meso-scale structure of rainfall systems. We installed an automatic weather station (AWS) and several automatic raingauges (ARGs) and utilized the weather radar of Bangladesh Meteorological Department (BMD). From the radar image in the summer of 2001 (16-18 July), a striking feature of the systematic diurnal variation in this area was elucidated. In these 3 days, the diurnal evolutions of convective activity were remarkably similar to each other, implying that this pattern can be understood as a typical response of local cloud systems to the diurnal variation of insolation under some summer monsoon situations. The ARG data show the difference in characteristics of rainfall between pre-and mature monsoon seasons. The short intense downpour tends to occur more frequently in the pre-monsoon season than in the mature monsoon season. The pre-monsoon rainfall also has clear diurnal variation with a peak that is more strongly concentrated in time. In the northern part the rainfall peak is found in between midnight and early morning, while it is observed in the daytime in central to western parts of the country. Two disaster cases caused

Nanocrystal quantum dot photovoltaics and photodetectors with performance optimized by engineering the nanocrystals size and the optoelectronic properties of the nanocrystal's chemical coating are reported. Due to the large surface-tovolume ratio inherent to nanocrystals, the surface effects of ligands used to chemically coat and passivate nanocrystals play a significant role in device performance. However, the optoelectronic properties of ligands are difficult to ascertain, as the band structure of the ligand-capped nanoparticle system is complex and difficult to model. Using density-of-states measurements, we demonstrate that modeling of electropositive and electronegative substituents and use of the Hammett equation, are useful tools in optimizing nanocrystal detector performance. A new particle, the Janus-II nanoparticles, developed using 'charge-donating' and 'charge-withdrawing' ligands distributed over opposite surfaces of the nanocrystal, is described. The polarizing ligands of the Janus-II nanoparticle form a degeneracy-splitting dipole, which reduces the overlap integral between excitonic states, and thus reduces the probability of carrier recombination, allowing carrier extraction to take place more efficiently. This is shown to allow increased photodetection efficiencies and to allow the capture of multiple exciton events in working photodetectors.

A numerical study of second harmonic generation (SHG) in one-dimensional nonlinear photonic crystals based on full nonlinear system of equations, implemented by a combination of the method of finite elements and fixed-point iterations, is reported. This model is derived from a nonlinear system of Maxwell's equations, which partly overcomes the known shortcoming of some existing models relied on the undepleted-pump approximation. We derive a general solution of SHG in one-dimensional nonlinear photonic crystals structures. The convergence of our method is fast. Numerical simulations also show the conversion efficiency of SHG can be significantly enhanced when the frequencies of the fundamental wave are located at the photonic band edges or are assigned to the designed defect states.

The new antimonides MFe_1-xSb can be synthesized by arc-melting of M, Fe, and MSb_2 (M=Zr, Hf). All title compounds crystallize in the TiNiSi structure type (space group Pnma, Z=4). The lattice parameters of the new phases MFe_1-xSb, as obtained from the bulk samples of the nominal compositions MFeSb, are (a=681.4(1) pm, b=417.87(7) pm, c=740.3(1) pm for ZrFe_1-xSb and a=674.0(1) pm, b=412.0(2) pm, c=729.7(2) pm for HfFe_1-xSb. Under the reaction conditions used, the occupancy factors of the iron position content of ZrFe_1-xSb does not exceed 68(1)% (i.e. x=0.32(1)). Extended Hückel calculations, performed on the hypothetical model structures ``ZrFeSb'' and ``ZrFe_0.75Sb'', point to the phase ZrFe_1-xSb being metallic, independent of the x value. The band structure of ``ZrFeSb'', obtained with ab initio LMTO calculations, reveals a three-dimensional metallic conductivity and a nonmagnetic ground state.

The energy spectrum and density of states of graphene bilayer superlattices (SLs) are evaluated. We take into account doping and/or gating of the layers as well as tunnel coupling between them. In addition, we evaluate the transmission through such SLs and through single or double barriers. The transmission exhibits a strong dependence on the direction of the incident wave vector.

The complex dielectric function ͑DF͒ of wurtzite InN and GaN as well as zinc blende GaN was measured by spectroscopic ellipsometry between 14 and 32 eV with synchrotron radiation. In this spectral region, the DF of InN and GaN originates from Ga 3d and In 4d core level transitions to unoccupied conduction-band states. The Ga 3d and In 4d electronic states are highly localized and show almost no dispersion. We use these core states as a reference in order to probe the conduction bands. For this purpose, the imaginary part is compared to the density of empty p-orbital-like electronic states located around the cation atomic site, as calculated by densityfunctional theory in the local density approximation. The constant splitting of absorption features in the DF is attributed to the spin-orbit splitting of the d states. ⌬d 5/2−3/2 is found to be 0.82 eV for the In 4d and 0.41 eV for the Ga 3d level, respectively. On wurtzite samples with the c axis in the surface plane, ellipsometry measurements give access to both independent dielectric tensor components ʈ and Ќ , respectively. The observed anisotropy is induced by a directional dependence of empty p states yielding a p ʈ -DOS ͑density states͒ different to the p Ќ -DOS.

Using first-principles linear muffin-tin orbital density functional band structure calculations, the ordering of the states in the wurtzite ZnO valence-band maximum, split by crystal-field and spin-orbit coupling effects, is found to be ⌫ 7(5) Ͼ⌫ 9(5) Ͼ⌫ 7(1) , in which the number in parentheses indicates the parent state without spin-orbit coupling. This results from the negative spin-orbit splitting, which in turn is due to the participation of the Zn 3d band. The result is found to be robust even when effects beyond the local density approximation on the Zn 3d band position are included. Using a Kohn-Luttinger model parametrized by our first-principles calculations, it is furthermore shown that the binding energies of the excitons primarily derived from each valence band differ by less than the valence-band splittings even when interband coupling effects are included. The binding energies of nϭ2 and nϭ1 excitons, however, are not in a simple 1/4 ratio. Our results are shown to be in good agreement with the recent magneto-optical experimental data by Reynolds et al. ͓Phys. Rev. B 60, 2340, in spite of the fact that on the basis of these data these authors claimed that the valence-band maximum would have ⌫ 9 symmetry. The differences between our and Reynolds' analysis of the data are discussed and arise from the sign of the Landé g factor for holes, which is here found to be negative for the upper ⌫ 7 band.

The results of self-consistent, spin-polarized LMTO band structure calculations are shown for the compounds Ni3Pt and NiPt3, of L 12 (Cu3Au) structure. Lattice constants are reported together with bulk moduli, and the electronic structure is studied in relation to magnetism in both cubic compounds. Covalent magnetism is shown to act against the magnetization in Ni3Pt.

Orthorhombic CaGeO 3 is studied using density-functional theory (DFT) considering both the local density and generalized gradient approximations, LDA and GGA, respectively. The electronic band structure, density of states, dielectric function and optical absorption are calculated. Two very close indirect (S ! G) and direct (G ! G) band gap energies of 1.68 eV (2.31 eV) and 1.75 eV (2.41 eV) were obtained within the GGA (LDA) approximation, as well as the effective masses for electrons and holes. Comparing with orthorhombic CaCO 3 (aragonite), the substitution of carbon by germanium changes the localization of the valence band maximum of the indirect transition, and decreases by almost 2.0 eV the Kohn-Sham band gap energies. r