X-ray spectra and electronic band structure of nitrogen in Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N solid solid solutions

The electronic energy structure of the valence band and the x-ray absorption near edge structure (XANES) region of nitrogen in Al _x Ga_1?x N solid solutions and binary crystals of gallium nitride GaN and aluminum nitride AlN are calculated using the local coherent potential method and the cluster version of the muffin-tin approximation within the framework of the multiple scattering theory. It is demonstrated that the calculated electron densities of states correlate with the nitrogen K x-ray emission and nitrogen K x-ray absorption spectra. The electronic energy structure of the top of the valence band and the XANES region in Al _x Ga_1?x N solid solutions are compared with those in the binary crystals of the GaN and AlN nitrides, and an interpretation of their specific features is proposed. An analogy is drawn between the evolution of the electronic energy structure of the valence band and the XANES region in the alloys under investigation and the evolution of the electronic band structure in the Al _x B_1?x N and B _x Ga_1?x N alloys. General trends in the transformation of the structure and variations in properties of these alloys are discussed.     

Charge density of Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Al<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sb

Charge density calculations and electronic band structures for Ga _x Al_{1 − x} Sb with x = 1.0, 0.5 and 0.0 are presented in this work. The calculations are performed using the empirical pseudopotential method. The charge density is computed for a number of planes, i.e. z = 0.0, 0.125 and 0.25 A ₀ by generating the potential through a number of potential parameters available in the literature. The virtual crystal approximation was applied for the semiconducting alloy. The characteristics of the band structure and charge density are observed to be affected by the potential parameters. Calculated band gaps and the nature of gaps are in good agreement with the experimental data reported. The ionicity is also reasonably in good agreement with other scales proposed in the literature; however the formulation needs to be improved. The present work also demands indirect experimental band gap for the alloy.      

The band structures of BSb and B<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sb alloys

The band structures of BSb and B _x Ga_1−x Sb alloys are studied using first-principles calculations in the generalized gradient approximation. By SQS-8 supercells to model a random alloy, the direct transition energy-gap (Γ _15v−Γ _1c) bowing of 3.0 eV is obtained for B _x Ga_1−x Sb alloys in x = 0–50%, in x = 0–11% the energy-gap is the band-gap and increases by 7 meV/%B with boron composition increasing; by SQS-16 supercells the bowing parameter is about 1.9 eV in x = 0–12.5%. The formation enthalpies of mixing, ΔH, are calculated for B _x Ga_1−x As and B _x Ga_1−x Sb alloys. A comparison of enthalpies indicates that B _x Ga_1−x Sb films with boron composition of 7% may be possible. Supported by the National Natural Science Foundation of China (Grant No. 10774031) and the Guangdong Provincial Natural Science Foundation (Grant No. 07001790)     

Metallic-like conductivity of asymmetric <Emphasis Type="Italic">p</Emphasis>-In<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>As quantum wells

The temperature dependence of the conductivity of the two-dimensional hole gas in an asymmetric GaAs/In _x Ga_{1 ? x} As/GaAs quantum well has been investigated. It is shown that fast spin relaxation leads to metallic-like behavior of the temperature dependence of the conductivity.     

Strain accumulation in InAs/In<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>As quantum dots

The effect of strain accumulation in the InAs/In _x Ga_1−x As quantum dots (QDs) system was studied in this work. We found strain in the In _x Ga_1−x As layer with accumulation in the QD layer. This effect resulted in a dramatic reduction of growth-mode transition thickness of the QD layer. For InAs/In_0.25Ga_0.75As QDs, critical thickness is measured to be as low as 1.08 ML. The experimental results in this work highlight the importance of strain accumulation in the design and fabrication of QD-based devices with metamorphic buffer layer involved.     

Phenomenological description of strain relief in step-graded metamorphic buffer layers based on In<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Al<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>As ternary solutions

Spatial distributions of the residual elastic strains in layers of step-graded metamorphic buffers of two different designs, grown via molecular beam epitaxy on the basis of In _x Al_{1 ? x} As ternary solutions, are obtained by means of reciprocal space mapping. It is shown that with allowance for work hardening, which affects strain relief in buffer layers and increases the strain in dislocation-free layers, the mechanism of strain relief in the final buffer steps, and the residual elastic strain in a buffer dislocation-free layer, are governed by the same phenomenological law as in a single-layer heterostructure.     

Strain accumulation in InAs/In<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>As quantum dots

The effect of strain accumulation in the InAs/In _x Ga_1−x As quantum dots (QDs) system was studied in this work. It was found that strain in the In _x Ga_1−x As layer accumulation in the QD layer. This effect resulted in a dramatic reduction of growth mode transition thickness of the QD layer. For InAs/In_0.25Ga_0.75As QDs, critical thickness is measured to be as low as 1.08 ML. The experimental results in this work highlight the importance of strain accumulation in the design and fabrication of QD-based devices with metamorphic buffer layer involved.     

Electronic structure and ground state parameters of Ru<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript><Emphasis Type="Italic">Me</Emphasis><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Al refractory alloys

The electronic structure and ground state parameters of B2 RuAl-based refractory alloys have been investigated in the framework of the density functional theory using the exact muffin-tin orbital method in combination with the coherent potential approximation. It has been demonstrated that the number of states at the Fermi level for the Ru_{1 − x} Me _x Al alloys as a function of the alloying metal content has a minimum, which indicates a change in the Fermi surface topology and the presence of specific features in the behavior of elastic constants. It has been concluded that the electronic structure of the alloys can be described in terms of the rigid band model. The nonlinear variations of the lattice parameters of the alloys has been explained.     

High-temperature study of SrFe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mo<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3−<Emphasis Type="Italic">z</Emphasis></Subscript> perovskites

The structure of high-temperature SrFe_{1 − x} Mo _x O_{3 − z} (0 ≤ x ≤ 0.5) phases was studied. Such studies are necessary to understand the mechanism of oxygen transport in membrane materials used for high-temperature oxygen separation.     

The theory and experiment of very-long-wavelength 256×1 GaAs/Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>As quantum well infrared detector linear arrays

The 256×1 linear array of multiple quantum wells infrared photodetector (QWIP) is designed and fabricated for the peak response wavelength at λ _P = 14.6 μm. The response spectral width is bigger than 2.2 μm. The two-dimensional (2D) diffractive coupling grating has been formed on the top QWIP photosensitive pixel for coupling the infrared radiation to the infrared detective layers. The performance of the device at V _B = 3 V and T = 45 K has the responsibility 4.28×10⁻² (A/W), the blackbody detectivity D _b* = 5.14×10⁹ (cm·Hz^1/2/W), and the peak detectivity D _λ * = 4.24× 10¹⁰ (cm·Hz^1/2/W). The sensor pixels are connected with CMOS read out circuit (ROC) hybridization by indium bumps. When integral time is 100 μs, the linear array has the effective pixel of QWIP FPA N _ef of 99.2%, the average responsibility (V/W) of 3.48×10⁶ (V/W), the average peak detectivity D _λ * of 8.29×10⁹ (cm·Hz^1/2/W), and the non-uniformity U_R of 5.83%. This device is ready for the thermal image application. Supported by the National Natural Science Foundation of China (Grant No. 10374095)     

Optical properties and electronic structure of YNi<Subscript>5 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> intermetallic compounds

The optical properties of hexagonal intermetallic compounds YNi_{5 − x} Cu _x (x = 0, 1, 2) have been investigated by ellipsometry in the spectral range of 0.22–15 μm. It is shown that the replacement of nickel atoms by copper atoms leads to local changes in the optical-conductivity spectra. A new absorption band is found at 3.5–4.5 eV; its intensity depends on the copper content. The plasma and relaxation frequencies of conduction electrons are determined. The electronic structure and interband optical conductivity of these compounds are calculated within the electron density functional theory using the pseudopotential method. The main parameters of the band structure and the total and partial densities of electronic states are determined. Qualitative agreement is obtained between the experimental and theoretical frequency dependences of the optical conductivity.     

Structure and Curie temperature of Y<Subscript>2</Subscript>Fe<Subscript>17−<Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

The structures of Y₂Fe_17−x Cr _x are simulated by theab initio potentials. The site preference of Cr atom in Y₂Fe₁₇ is evaluated and the order is determined as 4f, 12j, which is close to the experimental result. Based on the site preference behavior, the calculated parameters and the atom sites of Y-Fe-Cr system are studied. The result corresponds well to observed data. Further, the DOS of the relaxed structures are calculated and the variation in Curie temperature is explained qualitatively by the spin-fluctuation theory.     

Magnetostriction of Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>S (<Emphasis Type="Italic">x</Emphasis> = 0.27) crystals

The magnetostriction of Fe _x Mn_{1 − x} S (x = 0.27) single crystals in strong magnetic fields up to 120 kOe has been investigated. It has been found that the magnetostriction reaches colossal values (±3 × 10⁻⁴) atypical of compounds of 3d elements. It has been found that the magnetostriction changes sign when varying temperature and magnetic field; this behavior indicates an important role of the spin-phonon interactions in the formation of the magnetic order in solid solutions of iron-manganese sulfides.     

Hall effect in the Ce(Al<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript> heavy-fermion system

The influence of substitution on the binding energy of many-body states and the formation of the magnetically ordered state in a heavy-fermion compound (CeAl₂) have been studied by measuring the transport characteristics (Hall effect, resistivity) in intermetallic compounds of the Ce(Al_1?x M _x )₂ system (M = Ni, Co; x ≤ 0.08). It is established that the Hall coefficient R _H in Ce(Al_1?x Co _x )₂ intermetallides with x = 0.05 and 0.08 grows by more than an order of magnitude as the temperature decreases from 1.8 to 300 K. The experimental data are used to estimate the effective mass of charge carriers, the relaxation time, and the localization radius of many-body states.     

Fractal dimension study of polaron effects in cylindrical GaAs/Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>As core–shell nanowires

Polaron effects in cylindrical GaAs/Al _x Ga_1-xAs core–shell nanowires are studied by applying the fractal dimension method. In this paper, the polaron properties of GaAs/Al _x Ga_1-xAs core–shell nanowires with different core radii and aluminum concentrations are discussed. The polaron binding energy, polaron mass shift, and fractal dimension parameter are numerically determined as functions of shell width. The calculation results reveal that the binding energy and mass shift of the polaron first increase and then decrease as the shell width increases. A maximum value appears at a certain shell width for different aluminum concentrations and a given core radius. By using the fractal dimension method, polaron problems in cylindrical GaAs/Al _x Ga_1-xAs core–shell nanowires are solved in a simple manner that avoids complex and lengthy calculations.     

Competing effects of Hund’s splitting and symmetry-breaking perturbations on electronic order in Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te

We study the effect of a uniform external magnetization on p-wave superconductivity on the (001) surface of the crystalline topological insulator (TCI) Pb_1?xSn_xTe. It was shown by us in an earlier work that a chiral p-wave finite-momentum pairing (FFLO) state can be stabilized in this system in the presence of weak repulsive interparticle interactions. In particular, the superconducting instability is very sensitive to the Hund’s interaction in the multiorbital TCI, and no instabilities are found to be possible for the “wrong” sign of the Hund’s splitting. Here we show that for a finite Hund’s splitting of interactions, a significant value of the external magnetization is needed to degrade the surface superconductivity, while in the absence of the Hund’s interaction, an arbitrarily small external magnetization can destroy the superconductivity. This implies that multiorbital effects in this system play an important role in stabilizing electronic order on the surface.     

Magnetic ordering in La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>x</Subscript>MnO<Subscript>3−<Emphasis Type="Italic">x</Emphasis>/2</Subscript> anion-deficient manganites

The structural, magnetic, and electrotransport properties of La_1?xSr_xMnO_{3? x/2}(0≤x≤0.30) manganites with perovskite structure are investigated experimentally as a function of oxygen deficiency. In the solid solutions La_1?xSr_xMnO₃, a change in the type of symmetry of the unit cell is observed at x=0.125. Samples with x≤0.125 are characterized by an O′-orthorhombic unit cell, whereas samples with x>0.125 are characterized by a rhombohedral unit cell. The structural properties of the anion-deficient solid solutions La_1?xSr_xMnO_3?x/2 are analogous to those of the stoichiometric system. It is assumed that, as the oxygen content decreases, La_{1? x}Sr_xMnO_3?x/2 anion-deficient solid solutions experience a series of successive magnetic phase transformations in the ground state: from an A-type (x=0) antiferromagnet to a cluster spin-glass-type inhomogeneous magnetic state (0.175>x≤0.30) through a two-phase (antiferromagnetic and ferromagnetic) state (0>x≤0.175). The anion-deficient solid solution with x=0.175 has the maximal value of the ferromagnetic component. As the oxygen deficiency increases, the resistivity of La_{1? x}Sr_xMnO_3?x/2 samples first decreases (up to a value of x=0.175), acquiring an activation character, and then increases (up to a value of x=0.30). In this case, none of the anion-deficient solid solutions exhibits a metal-semiconductor transition in the whole range of concentrations considered. A peak of magnetoresistance at a temperature below the point of magnetic ordering is observed only in the sample with x=0.175. The results of experiments carried out with a series of La_1?xSr_xMnO_3?x/2 anion-deficient solid solutions are summarized in the concentration diagrams of the spontaneous magnetic moment and the critical temperature of magnetic phase transitions. Hypothetical magnetic phase states are pointed out. The experimental results obtained can be interpreted in terms of the phase-separation model and the competition between ferromagnetic and antiferromagnetic indirect superex-change interactions. It is assumed that Mn³⁺-O-Mn³⁺ indirect superexchange interactions in the orbitally disordered phase are positive in the case of octahedral coordination of manganese ions and are negative when the coordination of at least one Mn³⁺ ion is pentahedral.     

Spin-wave resonance in Ge<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films exhibiting percolation ferromagnetism

Magnetic properties of Ge_{1 ? x} Mn _x (x = 0.02, 0.04, 0.08) thin films obtained by ion-implanting Mn⁺ ions into single-crystal Ge plates are investigated. The contributions of the subsystem of dispersed Mn²⁺ ions, Ge³Mn⁵ ferromagnetic clusters, and Mn-enriched ferromagnetic domains of Ge to the magnetic moment of Ge_{1 ? x} Mn _x films are distinguished. In the subsystem of dispersed Mn²⁺ ions in Ge_{1 ? x} Mn _x films at temperatures below 10 K, a spin-wave resonance is observed in the magnetically ordered state due to percolation ferromagnetism. It is established that, in the films with percolation ferromagnetism, the exchange integrals determined from static measurements correspond to those determined by dynamic measurements.     

Transport properties and ferromagnetism of Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>S sulfides

We have studied the resistivity and thermoelectromotive force (thermo emf) in a temperature range of T = 80–1000 K, the magnetic susceptibility and magnetization in a temperature range of T = 4.2–300 K at an external magnetic field of up to 70 kOe, and the structural characteristics of Co _x Mn_1?x S sulfides (0 ≤ x ≤ 0.4). Anomalies in the transport properties of these compounds have been found in the temperature intervals ΔT ₁ = 200–270 K and ΔT ₂ = 530–670 K and at T ₃ ～ T _N. The temperature dependences of the magnetic susceptibility, magnetization, and resistivity, as well as the current-voltage characteristics, exhibit hysteresis. In the domain of magnetic ordering at temperatures below the Néel temperature (T _N), the antiferromagnetic Co _x Mn_1?x S sulfides possess a spontaneous magnetic moment that is explained using a model of the orbital ordering of electrons in the t _2g bands. The influence of the cobalt-ion-induced charge ordering on the transport and magnetic properties of sulfides has been studied. The calculated values of the temperatures corresponding to the maxima of charge susceptibility, which are related to a competition between the on-site Coulomb interaction of holes in various subbands and their weak hybridization, agree well with the experimental data.     

Short-range order in Fe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Si<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>(<Emphasis Type="Italic">x</Emphasis>=0.05−0.08) alloys with induced magnetic anisotropy

Previously, iron—silicon alloys were investigated using X-ray diffraction and Mössbauer spectroscopy. It was demonstrated that, at low silicon concentrations, the alloys undergo a local separation into regions of the α iron phase depleted in silicon and silicon-rich clusters with a B2 ordering. The structure of locally ordered regions of the B2 phase is characterized by a pair ordering of silicon atoms: the Si—Si pairs are formed by next-nearest neighbors, and the axes of pairs are oriented along the 〈100〉 directions, which are the easy-magnetization axes. The thermomagnetic treatment in a constant magnetic field applied along the 〈100〉 axis induces an axial magnetic anisotropy, results in the formation of an anisotropic distribution of the B2 phase, and leads to a slight decrease in the volume fraction of the coordination 6: 2 with two silicon atoms in the first coordination shell of the iron atom. Therefore, the formation of an anisotropic local order of pairs of silicon atoms occurs as a result of their reorientation.