Deposition and properties of highly C-oriented GaN films on diamond substrates

High-quality GaN films are deposited on freestanding thick diamond films by electron cyclotron resonance plasma enhanced metal organic chemical vapor deposition (ECR-PEMOCVD). The TMGa flux dependent structural, morphological, and electrical characteristics of GaN films were investigated by x-ray diffraction analysis (XRD), reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM) and Hall effect measurement. The results indicate that it is feasible to deposit GaN films on freestanding thick diamond films under the proper deposition procedures. The high-quality GaN films with small surface roughness of 4.9 nm and high c-orientation are successfully achieved at the optimized TMGa flux of 0.5 sccm. The GaN/diamond structure has great potential for the development of SAW devices with high frequencies and low insertion.     

Electronic structure and optical properties of III–VI crystals

The polarized spectra of the full set of optical functions of GaS, GaSe, InSe, GaTe, InS, and TlSe crystals are determined in a wide range of fundamental-absorption energies. The ?₂ and -Im?^?1 spectra are decomposed into elementary components. The main parameters of the components were determined and the main features of the spectra and transition components are established. The results obtained are explained on the basis of the theoretical band calculations.     

Magnetic and structural properties of Pd–Mn–Sn intermetallics compounds

Magnetic and structural properties of Heusler Pd_0.5Mn_{0.5- x} Sn _x with x = 0.05, 0.10, 0.17, 0.20 and 0.25, have been studied by magnetisation and X-ray diffraction measurements at room and low temperatures. The crystal structure at room temperature is L2₁ cubic phase for x = 0.17, 0.20, 0.25 and B2 cubic phase for x = 0.10. Martensite structure 10M, was observed at room temperature for x = 0.05. X-ray measurements at low temperatures revealed a structural transformation from B2 to 14M for the x = 0.10 case. The lattice parameter of the L2₁ phase decreases linearly with the concentration, x. A ferromagnetic behaviour has been detected for L2₁ compounds, but the ferromagnetic exchange characteristic of each composition is of different strength. This gives rise to different Curie temperatures.     

Coupled structural and magnetic properties of ferric fluoride nanostructures: Part II,a Monte Carlo–Heisenberg study

We present a numerical study of the magnetic structure of nanostructured iron fluoride, using the Monte Carlo Metropolis simulated annealing technique and a classical Heisenberg Hamiltonian with superexchange angle dependent interactions. The parameters are adjusted on experimental results, and the atomic structure and topology taken from a previous atomistic model of grain boundaries in the same system. We find perfect antiferromagnetic crystalline grains and a disordered magnetic configuration (speromagnetic) at the grain boundary, in agreement with experimental features. Both the lowest magnetic energy and the rate of magnetic frustration are found to be dependent on the relative disorientation of crystalline grains, i.e. on the cationic topology. We conclude on possible extensions of the model.     

Contribution from structural Jahn–Teller ions to the elastic and ferroelectric properties of lithium niobate and lithium tantalate

Elastic and ferroelectric characteristics of single crystals of lithium niobate and tantalate are investigated in a wide range of temperatures by complex acoustooptic means. The contribution from Jahn–Teller NbO₆ and TaO₆ systems to the characteristics of elastic moduli, ultrasonic attenuation, and nonlinear optical coefficients is analyzed using a new phenomenological model. It is hypothesized that the displacement of Nb⁵⁺ and Ta⁵⁺ ions, which exhibit the second-order Jahn–Teller effect along trigonal axis C, and the subsequent ordering of octahedral, result in unusual elastic and ferroelectric properties.     

Effect of doping Zn atom on the structural stability,mechanical and thermodynamic properties of AlLi phase in Mg–Li alloys from first-principles calculations

ABSTRACT

This work uses first-principles total energy calculations on the basis of density functional theory to predict the structural stability, mechanical and thermodynamic properties of Zn atom doped AlLi phase in Mg–Li–Al–Zn alloy. The values of the equilibrium lattice parameters and the formation of enthalpy are highly consistent with the experimental and previous calculations results available. Negative enthalpies of formation ΔH are predicted for all AlLi phase doped concentrations which have positive consequences for its structural stability. The elastic modulus is deduced by Voigt–Reuss–Hill arithmetic approximation. The bulk modulus of the Al–Li–Zn compounds increases as the doping concentrations increase, which are larger than the value of the AlLi phase. In particular, the stability and mechanical anisotropy of the Al–Li–Zn compounds are discussed. The charge density cloud map is drawn to reveal the bonding characteristics of four compounds. The changes in thermodynamic properties are derived by the phonon frequencies within the quasi-harmonic approximation.     

Comparison of spectroscopic properties of Yb:YAP and YbiYAG crystals

The Yb:YAG and Yb:YAP crystals have been grown by Czochralski method. The absorption spectra and the fluorescence spectra of Yb:YAG and Yb:YAP crystals have been investigated. It is shown that the Yb:YAG crystal has better laser properties and smaller threshold power than Yb:YAP crystal. In addition, the absorption cross-section of the Yb:YAP crystal is 2.16 times of that of the Yb:YAG crystal, so laser diode pumped Yb:YAG lasing can be easily realized. Because YAP single crystal is anisotropic, it is provided with polarization characteristics.     

Effect of calcination temperature on phase transformation,structural and optical properties of sol–gel derived ZrO2 nanostructures

Zirconia (ZrO₂) nanostructures of various sizes have been synthesized using sol–gel method followed by calcination of the samples from 500 to 700 °C. The calcined ZrO₂ powder samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infra-red spectroscopy (FT-IR), UV–visible spectroscopy (UV–vis.), Raman spectroscopy (RS) and thermogravimetric analysis (TGA). The phase transformation from tetragonal (t) to monoclinic (m) was observed. The average diameter of the ZrO₂ nanostructures calcined at 500, 600 and 700 °C was calculated to be 8, 17 and 10 nm, respectively. The ZrO₂ sample calcined at 500 °C with tetragonal phase shows a direct optical band gap of 5.1 eV. The value of optical band gap is decreased to 4.3 eV for the ZrO₂ calcined at 600 °C, which contains both tetragonal (73%) and monoclinic (27%) phases. On further calcination at 700 °C, where the ZrO₂ nanostructures have 36% tetragonal and 64% monoclinic phases, the optical band gap is calculated to be 4.8 eV. The enhancement in optical band gap for ZrO₂ calcined at 700 °C may be due to the rod like shape of ZrO₂ nanostructures. The tetragonal to monoclinic phase transformation was also confirmed by analyzing Raman spectroscopic data. The TG analysis revealed that the ZrO₂ nanostructure with dominance of monoclinic phase is found to be more stable over the tetragonal phase. In order to confirm the phase stability of the two phases of ZrO₂, single point energy is calculated corresponding to its monoclinic and tetragonal structures using density functional theory (DFT) calculations. The results obtained by theoretical calculations are in good agreement with the experimental findings.     

Investigation of structural,half-metallic and elastic properties of a new full-Heusler compound–Ir2MnSi

The electronic structure, magnetic and elastic properties of Ir₂MnSi full-Heusler compound is studied within the framework of Density Functional Theory (DFT). The ferromagnetic (FM) and non-magnetic (NM) states are compared in Cu₂MnAl and Hg₂CuTi prototype structures. The ferromagnetic state in Cu₂MnAl structure has been found energetically more stable than non-magnetic state in these two types of structures. Due to this stability, all calculations are carried out for FM-state. The spin-polarized calculations show that the spin-up electrons of Ir₂MnSi compound have metallic nature, but the spin-down electrons have semiconducting behavior with 0.55 eV energy gap around the Fermi level. The calculated Cauchy pressure and Poisson's ratio indicated that Ir₂MnSi compound is a ductile material. Ir₂MnSi compound is a half-metallic ferromagnet (HMF) and it has 5µ_B magnetic moment. This study will theoretically lead to experimental works in the spintronic field and its applications.     

Phase transformations and structural characteristics of AL–Cu–Co(–Si) decagonal phase

The phase transformations and structural characteristics of the Al-Cu-Co-Si alloy have been studied by neutron diffraction and high-resolution electron microscopy. The Al₆₅Cu_17.5Co_17.5 decagonal phase is stable in the temperature range between 973 K and 1350 K. At the low-temperature end, it relaxes to a microcrystalline approximant structure. At the high-temperature end, it melts directly into liquid. Two distinct orthorhombic phases are identified in the Al₆₃Cu_17.5Co_17.5Si₂ microcrystalline structure. They are composed of several structure units that can also construct the Penrose tiling. Because of lack of units, a single orthorhombic phase cannot undergo the transformation towards the high-temperature decagonal phase. An analysis of the orientation relationships between the CsCl and orthorhombic phases leads to the definition of Penrose tiling-like subnetworks inside the orthorhombic unit cells so that these orthorhombic phases can be regarded as the periodic patchworks of quasiperiodic subnetworks.     

Dielectric properties and phase transitions in crystals of TlInS x Se2 − x solid solutions

This paper reports on a study of the dielectric permittivity and electrical conductivity of single-crystal TlInS _x Se_{2 − x} solid solutions as functions of temperature and composition. It has been found that the dielectric permittivity ɛ and electrical conductivity σ decrease with increasing x and increase with growing temperature. The temperature dependences ɛ = f(T) and σ = f(T) for TlInS _x Se_{2 − x} crystals have been demonstrated to reveal anomalies in the form of maxima, which suggests that these crystals undergo phase transitions. The temperatures of the phase transitions increase with increasing x.     

Axial crystals – macroscopic symmetry and tensor properties

ABSTRACT

Axial crystals have axial symmetry which keeps invariant straight line with a fixed point. Axial symmetry groups include 27 non-cubic crystallographic point groups and 5 limit groups describing symmetry of textures and liquid crystals. We show that, except for four cases, each axial symmetry belongs to one of five axial types: polar, chiral, pseudopolar (three basic axial types), directional (possessing none of characteristic properties of basic types) or rotational (exhibiting characteristic properties of all basic types). Each basic type can appear in two structurally different variants with the same symmetry. These variants can coexist and form a mesoscopic structure (antiparallel ferroelectric or chiral domain structure, mixture of enantiomers). We examine macroscopic properties of axial types and variants, and experimental accessivity of their characteristic features.     

Some electronic properties and morphological features of hybrid material DNA–SWCNT thin films

Thin films of hybrid material (ssDNA–SWCNT) prepared by ultrasonic mixing of both materials in water have been examined from the viewpoint of electrical, magnetic and optical properties, and morphological features to probe the possibly induced novel functions due to mutual interactions. Enclosing of DNA molecules inside a SWCNT is confirmed by the image and its motion. The temperature dependence of electrical resistivities is best-fitted to the two-dimensional variable-range-hopping regime. The radial breathing mode of the Raman spectra of the SWCNT exhibits significant modification due to interaction with DNA. The expected change in magnetic susceptibilities is not found in this case.     

Changes of thermo-morphologic and thermotropic properties of liquid crystals at direct and reverse nematic mesophase ↔ isotropic liquid phase transitions: surface-induced effect

In this work, the effect of thin films on the thermo-morphologic and thermotropic properties of the phase transitions between the nematic mesophase and isotropic liquid has been investigated. Investigations have been carried out for both the heating and cooling processes. The temperature and linear widths of the biphasic regions of the direct and reverse phase transitions in nematic liquid crystals versus thickness of the thin films have been calculated with a high accuracy. The shift of the nematic mesophase–isotropic liquid and the isotropic liquid–nematic mesophase phase transition temperatures to higher temperatures and the enlargement of the temperature and linear widths of the biphasic regions as the effect of surfaces have been found.     

Structural properties,phase stability,elastic properties and electronic structures of Cu–Ti intermetallics

The structural properties, phase stabilities, anisotropic elastic properties and electronic structures of Cu–Ti intermetallics have been systematically investigated using first principles based on the density functional theory. The calculated equilibrium structural parameters agree well with available experimental data. The ground-state convex hull of formation enthalpies as a function of Cu content is slightly symmetrical at CuTi with a minimal formation enthalpy (–13.861 kJ/mol of atoms), which indicates that CuTi is the most stable phase. The mechanical properties, including elastic constants, polycrystalline moduli and anisotropic indexes, were evaluated. G/B is more pertinent to hardness than to the shear modulus G due to the high power indexes of 1.137 for G/B. The mechanical anisotropy was also characterized by describing the three-dimensional (3D) surface constructions. The order of elastic anisotropy is Cu₄Ti₃ > Cu₃Ti₂ > α-Cu₄Ti > Cu₂Ti > CuTi > β-Cu₄Ti > CuTi₂. Finally, the electronic structures were discussed and Cu₂Ti is a semiconductor.     

Correlation between structural and giant magnetoresistance properties of Fe–Ag nanogranular films

Fe _x Ag_1?x granular thin-films, with the atomic Fe concentration, x, ranging from 0 up to 0.5, were deposited by dc magnetron co-sputtering. The giant magnetoresistance (GMR) intensity is maximum at x _I  = 0.32, while the maximum of GMR efficiency, γ, i.e., the change of GMR intensity for a unit change of reduced squared magnetization, is observed at x _γ = 0.26. Owing to the spin-dependent scattering features, the GMR intensity and γ depend on both the concentration and the arrangement of the magnetic material. Therefore, to explain the difference between x _I and x _γ and to understand how the structural properties affect the magnetoresistive behaviour, we performed magnetization, Mössbauer and X-ray diffraction measurements as a function of x. X-ray data indicate that the granular films exhibit three different regimes: for x < 0.2, they can be described as a Fe–Ag solid solution; for 0.2 < x < 0.32 the Fe–Ag solid solution is still observed and very small Fe precipitates are found; finally, for x > 0.32, a Fe–Ag saturated solid solution is detected, containing bcc Fe clusters whose size is about 10 nm. Differently, for all the concentrations, magnetization data show the presence of Fe precipitates, whose size increases with x, and the Mössbauer investigation confirms this picture. We find that the samples grown at x = x _γ display the finest Fe dispersion within the Ag matrix, as the Fe–Ag solid solution is nearly at saturation and the Fe cluster size is of the order of a few nanometers; this arrangement possibly maximizes the magnetic/non-magnetic interface extension thus enhancing the GMR efficiency. If x is slightly increased, the increase in total Fe content compensates the GMR efficiency reduction, so the GMR intensity maximum is observed.     

Relativistic effects on the structural and transport properties of III–V compounds: A first-principles study

We have performed ab initio self-consistent calculations based on the full potential linear augmented plane-wave method (FP-LAPW) with the local density approximation (LDA) and the Generalised Gradient Approximation (GGA) to investigate the relativistic effects, on the structural, and transport properties of III–V compounds. We found that the stabilisation (destabilisation) of s, p^∗(p,d) orbital energies (i) reduces the lattice parameters of III–V compounds, considerably reduces the band gaps of the III–V compounds, (ii) reduces the effective masse, and (iii) induces strong spin orbit splitting of heavier III–V compounds. Furthermore we circumvent the negative gap problem by combining non relativistic and Engel–Vosko approximations. These approaches open the gap of the most III–V compounds, and leads to a realistic band structure.