Optical Study of Plasmon–LO Phonon Modes in Ga1?xMnxAs

Ga_1–xMn _x As layers with Mn fractions 0 x 2.8 % grown on GaAs(001) substrates by low-temperature molecular beam epitaxy were investigated using micro-Raman spectroscopy and far-infrared (FIR) reflectance spectroscopy. The Raman and FIR spectra are strongly affected by the formation of coupled modes of longitudinal optical phonons and hole plasmons. A full line-shape analysis of the spectra was performed within a Lindhard–Mermin model for the dielectric function, including intraband and interband hole transitions. Annealing at temperatures between 250 and 500°C results in a decrease of the hole density with increasing annealing temperature and total annealing time. Simultaneously, a reduction of the number of Mn atoms on Ga lattice sites is deduced from high-resolution X-ray diffraction. After annealing at 450°C the Raman lines of elemental As are observed, which are due to the precipitation of As on the sample surface.     

Local Structure and Valence State of Mn in Ga1?xMnxN Epilayers

Mn has been incorporated in epilayers of the large-gap semiconductor GaN grown by molecular beam epitaxy using a nitrogen plasma cell. Detailed extended X-ray absorption fine structure (EXAFS) studies of a Ga_0.98Mn_0.02N epilayer confirm that the Mn atoms substitute the Ga atoms, with an increase by 2.7% of the distance to the nearest nitrogen atoms. Near-edge spectroscopy results tend to indicate that the valence state of Mn is slightly higher than 3+, while EXAFS analysis suggests an electron transfer to the N neighbors.     

Magnetic Phase Diagram of Ca1?xMnxO

Alternating current susceptibility and direct current magnetization have been studied for polycrystalline Ca_1–x Mn _x O. On increasing the Mn content, magnetic ordering changes from spin glass behavior for 0.25 x 0.4 to antiferromagnetic order. The paramagnetic/antiferromagnetic transition is of second order for 0.5 x 0.65 and of first order for x 0.7. For low Mn concentrations, the high-temperature alternating current susceptibility can be described by a diluted Heisenberg magnet model developed for diluted magnetic semiconductors.     

Thermodynamic evaluation and potentiometric study of Cd1?xMnxTe and Cd1?xZnxTe dissolution in acid and alkaline solutions

We have performed thermodynamic calculations and have constructed Pourbaix diagrams representing redox processes in the MnTe-H₂O, ZnTe-H₂O, Cd_1−x Mn _x Te-H₂O, and Cd_1−x Zn _x Te-H₂O systems. Analysis of the Pourbaix diagrams and potentiometric data attests to different etching behaviors of Cd_1−x Mn _x Te and Cd_1−x Zn _x Te solid solutions in alkaline and acid solutions. Original Russian Text ? S.G. Dremlyuzhenko, A.G. Voloshchuk, Z.I. Zakharuk, I.N. Yurijchuk, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 1, pp. 26–34.     

Electrical and magnetic properties of the diluted magnetic semiconductors Cd1 ? xMnxGeP2 and Cd1 ? xMnxGeAs2 at high pressures

The structural and magnetic phase transitions and magnetoresistance of the diluted magnetic semiconductors Cd_{1 − x} Mn _x GeAs₂ and Cd_{1 − x} Mn _x GeP₂ have been studied at high hydrostatic pressures, up to 7 GPa. The normal and anomalous Hall coefficients of the samples have been determined graphically from experimental data.     

Giant Zeeman Effects and Spin Dynamics of Excitons in Dense Self-Organized Quantum Dots of CdSe and Cd1?xMnxSe

Giant Zeeman effects and spin dynamics of excitons are studied in dense self-organized quantum dots (QDs) of CdSe and Cd_1–xMn_xSe. Microphotoluminescence (PL) measurements for each individual dot reveal the typical dot diameter of 3.5 ± 0.2 nm and the density of 5000 m^–2 in the CdSe QDs. The exciton lifetime is shorter in smaller dots with higher energies, indicating energy transfer and tunneling processes among the dots. Circular polarization of excitonic PL is observed at 0 T with an opposite sign to that of the excited light and with the rise time of 50 ps. The CdSe QDs coupled with a Zn_1–xMn_xSe layer show the giant Zeeman shift of exciton, arising from overlapping of exciton wavefunctions in the dots with Mn ions. Spin polarization dynamics in the coupled QDs is also studied.     

Relation of Structure and Superconductivity in Self-compensated Y1?xCaxBa2Cu3?xAlxOz System

The co-doped compounds of Y_1−x Ca _x Ba₂Cu_3−x Al _x O _z , with x from 0.1 to 0.4, were synthesized through a solid-state reaction method. Structural and superconducting properties have been investigated by X-ray diffraction, Rietveld refinement, and DC magnetization measurement. The lattice constant a decreases while b increases with the addition of x. The difference between a and b diminishes gradually. Careful study of the crystalline structure shows that the critical temperature (T _c ) changes monotonically with some local structural parameters, such as the difference between Ba and Cu(2) atoms’ Z coordinates, the bond length of Cu(2)–O(4), and the bond angle of Cu(2)–O(2)–Cu(2), which are all closely related to the interaction between the perovskite block and the rock salt block in the unit cell. The results indicated that the influence of the crystalline structure on superconductivity is important and independent of the carrier concentration.     

Thermal conductivity of Ca1 ? xHoxF2 + x optical ceramics

The thermal conductivity of Ca_{1 − x} Ho _x F_{2 + x} (x ≤ 0.03) optical ceramics has been studied experimentally in the temperature range 50–300 K. With increasing holmium content, the thermal conductivity of the ceramics decreases, especially at low temperatures: from 10.2 to 2.3 W/(m K) at 300 K and from 250 to 4.5 W/(m K) at 50 K.     

Carbon-coated nano-sized LiFe1?xMnxPO4 solid solutions (0?≤?x?≤?1) obtained from phosphate–formate precursors

LiFe_1−x Mn _x PO₄ solid solutions in the whole concentration range (0 ≤ x ≤ 1) are obtained at 500 °C by a phosphate–formate precursor method. The method is based on the formation of homogeneous lithium–iron–manganese phosphate–formate precursors by freeze-drying of aqueous solutions containing Li(I), Fe(II), Mn(II), phosphate, and formate ions. Thermal treatment of the phosphate–formate precursors at temperatures at 500 °C yields nano-sized LiFe_1−x Mn _x PO₄ coated with carbon. The structure and the morphology of the LiFe_1−x Mn _x PO₄ compositions are studied by XRD, IR spectroscopy, and SEM analysis. The in situ formed carbon is analyzed by Raman spectroscopy. The electrochemical performance of LiFe_1−x Mn _x PO₄ is tested in model lithium cells using a galvanostatic mode. All LiFe_1−x Mn _x PO₄ compositions are characterized with an ordered olivine-type structure with a homogeneous Fe²⁺ and Mn²⁺ distribution in the 4c olivine sites. The morphology of LiFe_1−x Mn _x PO₄ consists of plate-like aggregates which are covered by in situ formed carbon. Inside the aggregates nano-sized isometric particles with narrow particles size distribution (between 60 and 100 nm) are visible. The structure of the deposited carbon presents a considerable disordered graphitic phase and does not depend on the Fe-to-Mn ratio. The solid solutions LiFe_1−x Mn _x PO₄ deliver a good reversible capacity due to the Fe²⁺/Fe³⁺ and Mn²⁺/Mn³⁺ redox-couples at 3.5 and 4.1 V, respectively.     

Molecular beam epitaxy of Pb1 ? xEuxTe and Pb1 ? xSnxTe layers and related periodic structures

Layers and periodic Bragg structures based on Pb_{1 − x} Eu _x Te (0 < x < 1) and Pb_{1 − x} Sn _x Te (0 < x < 0.1) ternary solid solutions have been grown on (111) BaF₂ and (111) Si substrates by molecular beam epitaxy. X-ray diffraction measurements show that the layers in three-period EuTe/Pb_0.94Eu_0.06Te structures maintain the [111] crystallographic orientation normal to the substrate plane. In the interface plane, the [110] directions of the layers and substrate are parallel to each other. The full width at half maximum of the rocking curve of each layer of both the binary and ternary compounds is about 20′. The large optical contrast (40%) in such structures allowed us to reach 99.9% mid-IR reflectivity.     

X-ray diffraction study of multicomponent oxides in the Zn2 ? x(TiaZrb)1 ? xFe2xO4 system

The multicomponent refractory oxide system Zn_{2 − x} (Ti _a Zr _b )_{1 − x} Fe_2x O₄ (a + b = 1; a: b = 1: 5, 1: 4, 1: 3, 1: 2, 1: 1, 2: 1, 3: 1, 4: 1; x = 0–1.0; Δx = 0.05) has been studied by X-ray diffraction, using samples prepared by melting appropriate metal oxide mixtures in a low-temperature hydrogen-oxygen plasma. Two phases, both with wide homogeneity ranges, have been identified: α-phase, with a cubic inverse spinel structure, and β-phase, with a tetragonal inverse spinel structure. The phase boundaries in the system have been determined. Structural data are presented for about 100 solid solutions.     

Magnetic and transport properties of Mn3+xGa1?xN compounds

Mn_3+x Ga_1−x N compounds with x = 0.0 and 0.1 were prepared by re-sintering Mn₂N_0.86, Ga bulk and Mn powders. These compounds are deduced to be the N-deficiency ones. In Mn₃GaN, a step-like magnetic transition, from frustrated antiferromagnetism to paramagnetism with increasing temperature, occurs at 370 K, while the same magnetic transition of Mn_3.1Ga_0.9N is far above 380 K. The enhanced magnetization of Mn₃GaN at low temperatures is ascribed to the fast lowering of antiferromagnetism. The electrical resistivity of Mn₃GaN exhibits a typically metallic conducting behavior with a positive magnetoresistance of 4–7%.     

Photoluminescence of i-GaxIn1 ? xAs/n-GaAs heterostructures containing a random InAs quantum dot array

i-Ga _x In_{1 − x} As/n-GaAs heterostructures containing InAs quantum dots have been grown by ion beam deposition and their photoluminescence has been studied. The photoluminescence spectrum of the heterostructures contains three characteristic peaks. The peak at 1.1 eV has a large width and is due to the array of InAs quantum dots of different sizes, randomly arranged on the surface of the i-Ga _x In_{1 − x} As layer. From the position of another peak (hν = 0.94 eV), we have evaluated the composition of the Ga _x In_{1 − x} As solid solution: x = 0.64. The third photoluminescence peak corresponds to the intrinsic absorption edge of the n-GaAs substrate. The key features of luminescence photoexcitation in the heterostructures are discussed. We show that the Ga_0.64In_0.36As quantum well may accumulate not only photogenerated electrons but also electrons that come from a thin interfacial n-GaAs layer through ballistic transport.     

Effect of Y Doping on Superconductivity and Spin-Density-Wave States in Sm1?xYxFeAsO0.8F0.2 and Sm1?xYxFeAsO

Y doping effects on crystal structure, resistivity, superconductivity, and spin-density-wave states in the parent and F-doped SmFeAsO compounds have been studied. Y doping leads to shrinkage of the lattice parameters in both systems. The superconductivity of Sm_1−x Y _x FeAsO_0.8F_0.2 is suppressed by Y doping with onset T _c decreasing monotonically from 52.6 K at x=0 to 25 K at x=0.5. A weak resistivity anomalies around 120 K ascribed to the spin-density-wave instability is observed in the sample of x=0.5. In parent SmFeAsO compound, the resistivity anomaly temperature was detected to be around 150 K and shifted towards lower temperatures with the Y doping level increased. It is concluded that a negative pressure effect takes responsibilities for the decrease of T _c in Sm_1−x Y _x FeAsO_0.8F_0.2.     

Microscopic Scenario for x = 1/8 Anomaly in La2 ? xSrxCuO4

We adopt a t ₁-t ₂-t ₃-J-G model for explanation of x = 1/8 anomaly in La_{2 – x} Sr _x CuO₄ family compound. The calculated charge susceptibility shows a maximum near Q = (, ) at intermediate temperatures and near (, /2) as temperature approaches zero, in agreement with neutron scattering experiments. Coulomb repulsion G between the first neighbors turns out to be the source of Charge Density Waves (CDW) in narrow band t ^eff ₁, t ^eff ₂, t ^eff ₃ < G. For physically realistic hopping values we obtain the CDW amplitude e _Q = x. The in-phase domain structure as a candidate for stripe picture is proposed.     

Static Ferromagnetic Modulation Revealed by Neutron Scattering in La1?xCaxMnO3

Mn perovskites La_1–x Ca _x MnO₃ (0x0.1) have been studied by small angle neutron scattering (SANS) and elastic neutron scattering (ESANS). At low temperature, both experiments reveal a broad modulation centered at the same q _m whatever the q direction. This scattering pattern is typical of a liquid-like distribution of similar magnetic droplets, or large magnetic polarons.     

Superconducting Properties of Carbon and Zn Double-doped Mg1?xZnx(B1?xCx)2

A series of polycrystalline samples of Mg_1−x Zn _x (B_1−x C _x )₂ (x=0.00, 0.02, 0.04, 0.06, 0.08, and 0.10) were synthesized by a conventional solid-state reaction method under a background pressure about 10⁻³ Pa. Phase identification, crystal structure and superconducting transition temperature (T _c) were studied by means of X-ray diffraction (XRD) and resistivity measurements. The results indicated that the lattice parameters a and c show no clear trend in their changes with increasing doping level, and it turned out that the dopants had a marked effect on the crystal-lattice parameters and changed the crystal structure of the samples. The T _c for Mg_1−x Zn _x (B_1−x C _x )₂ decreased with C and Zn doping, but the rate of decrease was slower than single C-doped. We propose that the suppression of T _c by doping originates largely from the structural change.     

GaxIn1 ? xBiyAszSb1 ? y ? z/InSb and InBiyAszSb1 ? y ? z/InSb heterostructures grown in a temperature gradient

Heterophase equilibria in the Ga _x In_{1 − x} Bi _y As _z Sb_{1 − y − z} -InSb and InBi _y As _z Sb_{1 − y − z} -InSb systems have been analyzed within the simple solution approximation. Ga _x In_{1 − x} Bi _y As _z Sb_{1 − y − z} /InSb and InBi _y As _z Sb_{1 − y − z} /InSb heterostructures have been grown in a temperature gradient, and their composition and structural perfection have been assessed.     

Electrical Characterization of p-Type Pb1?xEuxTe

The transport properties of p-type Pb_1–x Eu _x Te epitaxial layers were studied as a function of Eu content, temperature, and magnetic field. The low-temperature hole mobility is drastically reduced when the Eu concentration is increased from 0 to 6%, while the hole concentration remains almost constant. A metal-insulator transition was observed for x 0.04, which is probably induced by the disorder caused by the introduction of Eu. For temperatures down to 10 K, only positive magnetoresistance has been observed at low magnetic fields. An anomalous behavior of the resistivity as a function of temperature has been detected for a Eu content about 5%, which is attributed to the resonance between the localized Eu 4f level and the valence band maximum.     

Crystal structure and magnetic properties of high-coercivity Sr1 ? xPrxFe12 ? xZnxO19 solid solutions

Sr_1−x Pr _x Fe_{12 − x} Zn _x O₁₉ ferrites with x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5 have been prepared by solid-state reactions between praseodymium, iron, and zinc oxides and strontium carbonate in air at 1470 K. According to X-ray diffraction results, the samples with x ≤ 0.2 were single-phase and those with 0.3 ≤ x ≤ 0.5 contained, in addition to the magnetoplumbite phase, small amounts of α-Fe₂O₃, ZnFe₂O₄, and PrFeO₃. The mixed-phase samples further fired twice at 1470 K for 4 and 2 h contained no impurity phases at x = 0.3 and contained only α-Fe₂O₃ at x = 0.4 and 0.5. In the composition range 0 ≤ x ≤ 0.3, the a and c cell parameters, unit-cell volume V, and X-ray density ρ_x of the magnetoplumbite phase vary linearly according to the relations a(?) = 5.8869 − 0.0162x, c(?) = 23.027 + 0.449 x, V(?³)= 691.10 + 9.65x, and ρ_x(g/cm³) = 5.102 + 0.230 x. The highest degree of combined heterovalent substitution of Pr³⁺ for Sr²⁺ and Zn²⁺ for Fe³⁺ in the SrFe₁₂O₁₉ ferrite (formation of Sr_1−x Pr _x Fe_{12 − x} Zn _x O₁₉ solid solutions) at 1470 K is x = 0.32−0.36. The saturation magnetization per formula unit (n _s) of the x = 0.1 ferrite exceeds that of SrFe₁₂O₁₉ by 1.7% at 6 K and by 15.2% at 308 K. The 308-K n _s and coercive force (_σ H _c) of the x = 0.2 ferrite exceed those of SrFe₁₂O₁₉ by 7.6 and 8.5%, respectively.