Atomistic study of the pressure-induced phase-transition mechanism in GaAs by Möbius inversion potentials

In this work, the phase transition from zincblende (B3) to rocksalt (B1) structure in bulk GaAs is investigated by using Möbius inversion potentials. A Cm transition path is proposed from the molecular static simulation results, even though it is different from previously proposed transition paths Imm2, R3m and P3₂. The present Cm path is quite close to the previously suggested Imm2 path according to the crystal cell geometries and activation enthalpies. By comparison, the activation enthalpies along the R3m and P3₂ path are relative high. Therefore, the Imm2 mechanism, as a simplified model of the Cm one, is suggested to describe the microscopic process of the B3-B1 phase transition of GaAs. In this way, we investigate the changes of the system features during the transition process characterized by Imm2 mechanism and obtain a concise picture for the common B3-B1 transition. All the calculated results are compared to relevant experimental observations and other calculations.     

Consistency tests of Ampcalculator and chiral amplitudes in SU(3) Chiral Perturbation Theory: A tutorial-based approach

The positive-parity yrast bands of ^{79, 81, 83, 85, 87, 89}Y isotopes have been studied using the projected shell model (PSM). Nuclear-structure properties like yrast spectra, transition energies, band diagrams, kinetic moment of inertia, rotational frequencies and reduced transition probabilities (B(M1) and B(E2) are calculated. The results obtained from the PSM calculations are also compared with the available experimental as well as theoretical data and, in general, a reasonable agreement is obtained between them. Calculations in the present work also predict that these isotopes have multi-quasiparticle structure.     

Transition probabilities of lines belonging to the 5p-nd (n = 4,5,6,7) transition arrays of Kr(I)

Relative transition probabilities for lines belonging to the 5p-nd (n = 4,5,6,7) transition arrays of neutral krypton have been determined by means of emission line-intensity measurements. The experimental values of the present work are compared with the experimental and theoretical data given by other authors. The relative transition probabilities for lines arising from some levels belonging to the 4p⁵nd (n = 6,7) configurations have been put on an absolute scale, taking into account experimental and theoretical data.     

Electronic band structure,stability, structural,and elastic properties of IrTi alloys

The structural properties and mechanical stabilities of B2-IrTi have been investigated using first-principle calculations. The elastic constants calculations indicate that the B2-IrTi is unstable to external strain and the softening of C₁₁−C₁₂ triggers the B2-IrTi (cubic) to L1₀-IrTi (tetragonal) phase transformation. Detailed electronic structure analysis revealed a Jahn–Teller-type band split that could be responsible for elastic softening and structure phase transition. The cubic–tetragonal transition is accompanied by a reduction in the density of states (DOS) at the Fermi level and the d-DOS of Ti at Fermi level plays a decisive role in destabilizing the B2-IrTi phase.     

Transition probabilities of some lines of Ne(I)

Relative transition probabilities for twenty-four spectral lines belonging to 3p-3d transition array of neutral neon have been determined by means of emission line-intensity measurements. The lifetime of the 3d(3/2)₂ level has been measured by the delayed coincidence method obtaining a value of 20.5±2.0 ns. With this lifetime value and taking into account the 3887.1 Å forbidden line, the transition probabilities of the lines with origin in the 3d(3/2)₂ level were put on an absolute scale. The experimental values of the present work are compared with theoretical calculations obtained by use of the Coulomb approximation and intermediate coupling scheme.     

Electromagnetic Transition Rates and Deformation Feature in 130Ce

Lifetimes of the excited states in light rare-earth nucleus ¹³⁰Ce have been measured using heavy ion reaction ¹¹⁶Sn(¹⁶O,2n)¹³⁰Ce at beam energy of 73MeV through analyzing the Doppler-broadened line shapes. The reduced transition probabilities B(E2) are extracted from these measurements. The experimental results show that the previously reported anomalously high B(E2) value has not been observed in the present work. The transition quadrupole moment for the yrast band varies with spin and corresponds to a triaxial deformation with γ≈7°.     

Features of the semiconductor-metal transition in GaAs at ultrahigh pressures: New intermediate phases

Using the thermopower method (Seebeck effect), the semiconductor-metal transition that occurs in gallium arsenide single crystals of n and p types at ultrahigh pressures P above ～11–18 GPa has been studied. It has been found that the transition in n-type samples begins at lower pressures. In the region of the semiconductor-metal phase transition, features have been observed on the thermopower dependences S(P). These features indicate that lattices intermediate between the initial semiconductor structure of zinc blende and the Cmcm high-pressure orthorhombic metallic phase are formed. By analogy with ZnTe, one intermediate phase (semiconductor with hole conductivity) is suggested to have the cinnabar structure and the second intermediate phase (semimetallic with electron conductivity) possibly has the SC16 structure. A model of the semiconductor-metal transition is discussed. The behavior of the thermoelectric properties in GaAs under pressure is compared with the behavior of these properties in other A_NB_8?N semiconductors, which also undergo the transition to the metallic state.     

Structural and thermodynamic properties of Os from first-principles calculations

We investigate the structural, thermodynamic and electronic properties of Os by plane-wave pseudopotential density functional theory method. The obtained lattice constants, bulk modulus and cell volumes per formula unit are well consistent with the available experimental data. Especially, from our calculated bulk modulus, we conclude that Os is more compressible than diamond. Moreover, the temperature induced phase transition of Os from HCP structure to FCC structure has been obtained. It is found that the transition temperature of Os at zero pressure is 2702 K. However no transition pressure is found in our calculations. The effect of bulk modulus B as well as other thermodynamic properties of Os (including the thermal expansion α and the Grüneisen constant γ) on temperatures have also been studied. Our calculated thermal expansion α=1.510×10⁻⁵ K⁻¹ and the Grüneisen constant γ=2.227 for HCP structure at room temperature agree very well with the experimental data. The density of states for HCP structure at 0 K and FCC structure at transition temperature 2702 K are also investigated in our work.     

3D dependence of the dielectric dispersion in a BaTiO3 single crystal

The 3D dependences ?′(log f, T) and tanδ(logf, T) of a perfect BaTiO₃ single crystal grown by the Remeika method have been studied in the ranges f = 1–2 × 10⁷ Hz and T = ?80–130°C. These dependences characterize a transition from the paraelectric phase (121.5°C) as a near-antiferroelectric transition followed by the transition to the tetragonal phase at ～79.5°C. According to a number of signs, the range 121.5–79.5°C corresponds to a metastable phase typical of first-order phase transitions. The unexpected result of this work has been discussed with invoking the hypothesis on the BaTiO₃ structure in the paraelectric phase, according to which it consists of three antiferroelectric states oriented along the crystallographic axes. Using the dielectric properties of BaTiO₃ as an example, the method of direct correct determination of the temperatures of the structural transformations from the anomaly of tanδ(logf, T) has also been demonstrated.     

Band structure and high pressure study of Rh3Sc, Rh3Y and Rh3La

The electronic structure of the Rhodium based intermetallic compounds (A₃B) such as Rh₃Sc, Rh₃Y and Rh₃La are studied by the Self Consistent Tight Binding Linear Muffin Tin Orbital (TB-LMTO) method. In the present work, an attempt has been made to understand why the compounds namely Rh₃Y and Rh₃La crystallize in hexagonal structure, rather than the cubic structure, where as some of the similar rhodium based A₃B compounds namely Rh₃Ti, Rh₃Zr, Rh₃Hf, Rh₃V, Rh₃Nb, Rh₃Ta and Rh₃Sc are found to stabilize in cubic structure. In this work a prediction has been made about the structural phase transition in Rh₃Y and Rh₃La, from Hexagonal phase to Cubic phase. A report of the lattice constant, bulk moduli, cohesive energy and electronic specific heat coefficient is made and is compared with the available experimental data. Band structure and density of states histograms are also plotted. An electronic topological transition is predicted in Rh₃La, which may lead to the changes in the Fermi surface topology and hence changes the physical properties of Rh₃La.     

Mössbauer spectroscopy in modern permanent magnet alloys

Intermetallic compounds involving the rare earths and a transition metal, especially iron, aroused great interest in the past twenty five years with particular attention been paid to their magnetic properties, due to the fact that these compounds have been used as a permanent magnet materials. Their study using different techniques has given new information about the mechanisms of the magnetic interactions, which are present in these compounds. Among them Mössbauer Spectroscopy (MS) has been proven to be an indispensable tool, due to the fact that information can be obtained either from the spectra of the iron sublattice or from the spectra of the rare earth sublattice. Thus information on local moments, crystal field effects, single ion anisotropy and exchange interactions can be extracted from such spectra and compared with results from other techniques. Among the best alloys for permanent magnet applications are the ones based on the Nd₂Fe₁₄B type structure. Very interesting magnetic properties are also present in the recently discovered series of RFe_12?x M _x , where M=V, Ti, Mo, Si. We will review their intrinsic and extrinsic magnetic properties, as they have been measured using (MS) and correlate them with the findings from other techniques.     

Neutron diffraction study of antiferromagnetism in YbVO4

The antiferromagnetism of YbVO₄ has been studied by neutron diffraction down to 50 mK. The structure is collinear with the spins along the c-axis. The Néel point is at 93 mK and the saturated moment on the ytterbium is (3.1 ± 0.16)μB. The results are in reasonable agreement with Mössbauer work.     

Spontaneous spin-reorientation transition in (NdSmDy)(FeCo)B alloys

A magnetic transition accompanied by a sharp decrease in magnetization has been detected in an (NdSmDy)(FeCo)B alloy at temperature T = 110 K. It is found that the sample undergoes a spin-reorientation transition accompanied with a change in the type of magnetic anisotropy. The “easy axis”-type anisotropy corresponds to high temperatures T > 110 K. A magnetic structure of the type of “the cone of easy-magnetization axes” forms at low temperatures T < 110 K.     

Magnetic and piezoelectric properties of the Bi1 − x La x FeO3 system near the transition from the polar to antipolar phase

The crystal structure, piezoelectric and magnetic properties of the Bi_{1 ? x} La _x FeO₃ solid-solution system near the structural transition between the rhombohedral and orthorhombic phases (0.15 ≤ x ≤ 0.2) have been investigated. The regions of existence of the polar rhombohedral and orthorhombic phases have been determined, and the sequence of structural transitions as a function of the lanthanum ion concentration and temperature has been studied. The maximum piezoelectric signal is found for the solid solution with the composition x = 0.16, which has a single-phase rhombohedral structure. The relation between the type of crystal structure distortions and the increase in the magnetization upon the concentration-driven structural transition from the polar to antipolar phase has been established.     

Comparative study of the nature of chemical bonding of corrugated graphene on Ru(0001) and Rh(111) by electronic structure calculations

Recently, atomic resolved scanning tunneling microscopy investigations revealed that, depending on the substrate (Ni(111), Ru(0001), Ir(111), Pt(111), Rh(111)), graphene overlayer might present regular corrugation patterns, with periodically repeated units of a few nanometers. Variations of the interactions at the interface and the modulation of the local electronic properties are associated with the exact atomic arrangement of the carbon pairs with respect to the metal atoms of the substrate. Better understanding of the atomic structure and of the chemical bonding between graphene and the underlying transition metal is motivated by the fundamental scientific relevance of such systems, but it is also crucial in the perspective of possible applications. With the present work, we propose model systems for the two interfaces showing the most pronounced corrugation patterns, i.e. graphene/Ru(0001) and graphene/Rh(111). Our goal is to understand the nature of the interactions by means of electronic structure calculations based on Density Functional Theory. Our simulations qualitatively reproduce very well experimental results such as the STM topographies and the electrostatic potential maps, and quantitatively provide the closest agreement that has been published so far. The detailed analysis of the electronic structure at the interface highlights similarities and differences by changing the supporting transition metal. Our results point to a fundamental role of the hybridization between the π orbitals of graphene with the d band of the metal in determining the specific corrugation of the adsorbed monolayer. It is shown that differences in the response of the graphene electronic structure to the interaction with the metal can hinder the hybridization and lead to substantially different structures.     

Delocalization in one-dimensional tight-binding models with fractal disorder II: existence of mobility edge

In the previous work, we investigated the correlation-induced localization-delocalization transition (LDT) of the wavefunction at the band center (E = 0) in the one-dimensional tight-binding model with fractal disorder [H.S. Yamada, Eur. Phys. J. B 88, 264 (2015)]. In the present work, we study the energy (E ≠ 0) dependence of the normalized localization length (NLL) and the delocalization of the wavefunction at different energy in the same system. The mobility edges in the LDT arise when the fractal dimension of the potential landscape is larger than the critical value depending on the disorder strength, which is consistent with the previous result. In addition, we present the distribution of individual NLL and Lyapunov exponents in the system with LDT.     

Temperature dependent structural,electrical and electronic investigation of VO2 (B) thin film

Vanadium dioxide can exist in several polymorphs and among these the layered polymorph VO₂ (B) with monoclinic symmetry has numerous applications. In this work, VO₂ (B) phase thin film was prepared on quartz substrate via sputtering technique and its temperature dependent structural, electrical and electronic properties were investigated. We have witnessed a broader structural phase transition around 220 K; which encounter significant changes in the lattice constants yet the monoclinic symmetry is retained over a temperature range from 100 K to 380 K. Temperature dependent resistance measurement also exhibited a semi-metal to insulator like transition near 220 K displaying over 2 order of magnitude change in resistance across the transition. Small changes in the oxygen K-edge x-ray absorption spectrum were seen with change in temperature. At low temperature, an additional peak (d_|| band) has emerged in the XAS spectra at energy higher than the σ* peak. The appearance of d_|| band density of states is associated to the enhanced electron correlation effects driven by the localization of V–V pair's interactions at low temperature.     

A comparative study of the electronic structure and spectra of tetraoxa[8]circulene and octathio[8]circulene

The electronic structure and spectra of two highly symmetric molecules of circulenes, namely, tetraoxa[8]circulene and octathio[8]circulene, which belong to the symmetry point groups D _4h and D _8h , respectively, have been investigated by the density functional theory (DFT) method using the hybrid functional B3LYP in the 6–31G(d) basis set. The infrared (IR) spectra of these molecules in the ground and excited triplet states have been compared. The comparison of the electronic absorption spectra of both molecules has revealed that the first electronic transition is forbidden and determined by the electronic-vibrational interaction due to the degenerate e _u modes. The ability of the studied circulenes to fluoresce and phosphoresce has been analyzed, because these compounds are of interest as promising materials for organic light-emitting diodes.     

Synthesis and dielectric properties of modified potassium sodium niobate solid solutions

In the present work processing microstructure and dielectric properties of lead free KNN based ceramics have been studied. Compositions with a stoichiometric formula (1 ? x)(K_0.5Na_0.5)Nb_{1 ? y} Sb _y O₃-xBaTiO₃ (x = 0.01, 0.02, 0.04; y = 0.04, 0.07) were produced by solid state sintering method. The addition of manganese oxide MnO₂ after synthesis promoted the sintering and densification of ceramic samples. The influence of BaTiO₃ on the microstructure, density and electrical properties was investigated. X-ray diffraction analysis confirmed that obtained samples had a pure perovskite structure with no traces of secondary phase. Phase transition peak at T _c is broad indicating the diffuse phase transition.