Phonon spectroscopy of submicron ceramic materials based on Ce1–xGdxO2–y solid solutions

The specific features of the kinetic characteristics of subterahertz phonons in ceramic samples based on Ce_1–x Gd _x O_2–y electrolyte solid solutions have been investigated in the range of liquid-helium temperatures. It has been demonstrated that the observed anomalies of the transport characteristics of thermal phonons in this system are caused by the formation of structural defects associated with the position of vacancies in the anion sublattice with respect to impurity cations. The activation energy of the two-level system Δ = 8.53 K has been determined from an analysis of the experimental results.     

Effect of biaxial strain on the band gap of wurtzite AlxGa1?xN

First-principles calculations are applied to investigate the effect of biaxial strain on the band gap of wurtzite Al _x Ga_1−x N. The band gap and band gap bowing parameter increase with compressive strain and decrease with tensile strain. The strain-induced changes in the band gap of Al _x Ga_1−x N are linear in the strain range of about −1% to 1% while the linearity is invalid out of the range. The linear coefficient B(x), characterizing the relationship between the band gap and the biaxial stress, with a quadratic form is obtained. The value of the band gap bowing parameter decreases from 1.0 eV for −2% strain to 0.91 eV for unstrained and to 0.67 eV for 2% strain.     

Anomalous effect of magnetic field on the low-temperature thermal expansion of the Bi2Sr2?xLaxCuO6+δ and Nd2?xCexCuO4?δ HTSC systems in the non-superconducting phase

The thermal expansion of single crystals of the Bi₂Sr_2−x La _x CuO_6+δ high-temperature superconducting (HTSC) system in the insulating phase with compositions having no superconducting transition to a temperature of 1.8 K (x ≥ 0.8) is measured in an arbitrary direction in the (ab) plane in the temperature range of 7–50 K. Temperature regions of material compression upon heating are found. The study of anomalies in magnetic fields of 3 and 6 T, parallel and perpendicular to the c-axis revealed an anisotropic and nonmonotonic effect of the field on thermal expansion. Such anomalies for the n-type Nd_2−x Ce _x CuO_4−δ HTSC sample also having no superconducting transition are detected for the first time. The results show that the anomaly nature is caused by anisotropic electronic ordering, probably, by the charge density wave in the CuO₂ plane and superconductivity fluctuations in the insulating phase.     

Charge state of manganese ions and the nonstoichiometry of Ca1 ? xLayMnO3 ? δ single crystals

The charge states and effective spins of manganese ions in nonstoichiometric single crystals of Ca_{1 − x} La _y MnO_{3 − δ} (x = 0, 0.05, 1) manganites have been determined from the exchange splitting of Mn 3s X-ray photoelectron spectra. The chemical composition of the manganite samples under investigation, including the concentration of oxygen, has been determined using the data of the X-ray microanalysis of cations. It has been found that the manganite single crystals under investigation have large defectiveness in both the cation and oxygen sublattices. The efficiency of the complex use of 3s photoelectron spectroscopy and X-ray microanalysis in the determination of the charge state of manganese ions and violations of stoichiometry in doped and defected manganites has been demonstrated.     

The effect of the Mn delta layer on the photosensitivity spectra of structures with InxGa1 ? xAs/GaAs quantum wells

The effect of embedding a Mn delta layer in heteronanostructures with an In _x Ga_{1 − x} As/GaAs single quantum well on the photosensitivity spectra of the quantum well in Schottky barrier diode structures has been investigated. It is shown that the embedding of a Mn delta layer results in the broadening of exciton peak and decrease in the photosensitivity of the quantum well until complete disappearance of photosensitivity at the spacer thickness of 1.5 nm. The suppression of the quantum well photosensitivity is mainly due to the increase in the concentration of defects such as recombination centers in the quantum well during embedding of a Mn delta layer.     

Effect of the spin and valence states of cobalt ions on the kinetic properties of Ho1 ? xSrxCoO3 ? δ and Er1 ? xSrxCoO3 ? δ compounds

The electrical conductivity and Seebeck effect in ceramics based on cobaltites Ho_{1 − x} Sr _x CoO_{3 − δ} (x = 0.65, 0.75, 0.85, 0.95) and Er_{1 − x} Sr _x CoO_{3 − δ} (x = 0.75, 0.85, 0.95) with a perovskite-like structure have been investigated in the temperature range T > 77 K. All the compounds under study are characterized by the variable-range-hopping conductivity with the temperature dependence of the electrical resistivity corresponding to the Mott law. It has been found that, in the Ho_0.35Sr_0.65CoO_{3 − δ} compound, thermally excited Co³⁺ ions contribute to the electrical conductivity with an increase in temperature to 250 K. The Seebeck coefficient of the systems studied decreases as the strontium concentration and temperature increase. It has been shown that, for an adequate explanation of this behavior, proper allowance must be made for the splitting of the 3d levels, as well as for the charge disproportionation of the cobalt ions.     

Tailoring the optical properties of MgxZn1?xO thin films by nitrogen doping

Thin films of Mg _x Zn_1−x O and Mg _x Zn_1−x O doped with nitrogen were deposited by Radio Frequency plasma beam assisted Pulsed Laser Deposition (RF-PLD) in oxygen or oxygen-nitrogen discharge with different nitrogen/oxygen ratios. A Nd:YAG laser working at a wavelength of 266 nm, having a 10 Hz repetition rate was used for the depositions. The energy density of the incident beam was 3 J/cm² and the RF power was set to 100 W for all the samples. X-ray Diffraction (XRD) and Spectroscopic Ellipsometry (SE) were employed to investigate the samples. The degree of crystallinity is fount to decrease with increasing the Mg concentration, while the solubility of Mg in ZnO increases by 30% in the N-doped Mg _x Zn_1−x O thin films grown by RF-PLD. Segregation of MgO phase at a Mg concentration of 30% for Mg _x Zn_1−x O thin film is detected both by XRD and SE. The band gap of the samples increases from 3.37 up to 3.57 eV with increasing the Mg concentration and the nitrogen/oxygen ratio for each Mg concentration. A dependence of the dielectric function (refractive index) on both stoichiometry and degree of crystalinity is also found, the refractive index having values between 1.7 and 2 in visible spectral range.     

Thermal conductivity of single crystals of Ca1 ? xErxF2 + x and Ca1 ? xTmxF2 + x solid solutions

The thermal conductivity of single crystals of Ca_{1 − x} Er _x F_{2 + x} (x = 0.01, 0.05, 0.07, and 0.10) and Ca_{1 − x} Tm _x F_{2 + x} (x = 0.02, 0.04, and 0.06) solid solutions is studied in the temperature ranges 50–300 and 298–673 K. With increasing content of rare-earth elements, the behavior of thermal conductivity in these solid solutions changes from the characteristic of defect single crystals to glasslike. The concentration dependences of thermal conductivity for the two systems differ insignificantly.     

Influence of the illumination on the subband structure and?occupation in AlxGa1?xN/GaN heterostructures

The subband structure and occupation in the triangular quantum well at Al _x Ga_1−x N/GaN heterointerfaces have been investigated by means of temperature dependent Shubnikov–de Haas (SdH) measurements at low temperatures and high magnetic fields under illumination. After the illumination of the heterostructures, the total two-dimensional electron gas concentration increases, and the SdH oscillation amplitudes are enhanced when there is no additional subband occupation. It is also found that the energy separation between the subbands decreases after the illumination. We suggest that the illumination decreases the electric field and thus weakens the quantum confinement of the triangular quantum well at Al _x Ga_1−x N/GaN heterointerfaces. The GaN layer is thought to be the primary contributor of the excited electrons by the illumination.     

Anomalous behavior of the coercive force of dilute nickel ferrites NiGaxAlxFe2 ? 2 xO4 with a frustrated magnetic structure

The behavior of the spontaneous magnetization σ _s (T) and coercive force H _c (T) of dilute nickel ferrites NiGa _x Al _x Fe_{2 − 2x} O₄ (x = 0, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, and 0.8) has been studied. Above the transition temperature T _t , the coercive force H _c is found to reveal an anomalous behavior for compositions with x ≥ 0.4, namely, the temperature dependence of the coercive force H _c (T) exhibits a maximum in the range from T _t to T _C. For the reduced temperature θ₂ = 0.8 T _C, at which ferrites with the substitution x ≥ 0.4 reside in the spin glass state, the coercive force H _c is observed to increase sharply with x. The assumption is made that the clusters prevailing in the spin glass state are no larger than 3 nm in size. Original Russian Text ? L.G. Antoshina, A.B. Korshak, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 5, pp. 900–903.     

Synthesis and characterization of ZnxHg1?xSeyS1?y quantum dots

This article describes the synthesis of highly water-soluble Zn _x Hg_1−x Se _y S_1−y quantum dots (QDs) in aqueous solution through a simple photo-assisted reaction between ZnSe QDs and mercury(I) nitrate dihydrate [Hg₂(NO₃)₂·2H₂O]. In order to deduce the optimal synthesis conditions, we varied several parameters, including the concentrations of mercaptosuccinic acid (MSA) and Hg₂(NO₃)₂·2H₂O, the illumination time, and the reaction temperature. When irradiated at temperatures below 80 °C, the ZnSe QDs reacted with the S²⁻ ions formed rapidly from MSA and the Hg²⁺ ions formed from Hg₂ ²⁺ ions to form Zn _x Hg_1−x Se _y S_1−y QDs through a process of photo-etching and surface combination. Under different conditions, we prepared a series of Zn _x Hg_1−x Se _y S_1−y QDs that emit fluorescence at the maximum wavelengths ranging from 405 to 760 nm. Inductively coupled plasma-mass spectrometry and transmission electron microscopy/energy dispersive spectrometry revealed that the content of Hg in the Zn _x Hg_1−x Se _y S_1−y QDs was greater when the synthesis was conducted at higher temperature. The Zn_0.88Hg_0.12Se_0.44S_0.56 QDs exhibit improved photostability than crude ZnSe QDs and possess long lifetimes (τ₁ ~ 38 ns and τ₂ ~ 158 ns).     

Transformation of the acoustic mode near the structural and magnetic phase transitions in La1–xSrxMnO3 (x = 0.175) crystal

The peculiarities of the phase transitions in La_0.825Sr_0.175MnO₃ single crystal have been investigated by acoustic methods at the frequency f = 770 MHz. Generation of a magnetoelastic wave near the phase transition at 305 K is revealed, which is assigned to quasi-transverse modes. The most likely cause of the phase transition near T = 305 K and the formation of the quasi-transverse mode is the suppression of local Jahn-Teller distortions.     

Determination of the probability of existence of pair interactions in the formation of M2tX2t?1Superstructures in MXy nonstoichiometric compounds

An analytical method has been proposed for calculating the probabilities P _i ^(2)(s) of existence of X-X, X−□, and □−□ pair interactions in the nonmetal sublattice of M _2tX_2t−1 superstructures formed in strongly nonstoichiometric compounds MX _y (MX _y □_1−y) and M ₂X _y (MX_y/2□_1−y/2) with a high content of structural vacancies □. The main characteristics necessary for the quantitative determination of the probabilities P_i(2)(s) as functions of the composition, degree of long-range order, symmetry, and structure type have been determined for all the known superstructures M _2tX_2t−1.     

Spin-wave resonance in a multilayer Co1 ? xPx/Co1 ? yPy structure as a method of detection of the Bragg gaps in a spin wave spectrum

The method of spin-wave resonance has been used to detect in multilayer (Co₉₈P₂/Co₉₅P₅) _N structures a modification of the exchange spin wave spectrum due to the formation of the first, second, and third Brillouin zones in a one-dimensional magnon crystal formed by a periodic modulation of the exchange. The band gaps have been measured for wave vectors k _b = π/(d ₁ + d ₂) and 2k _b .     

Brittle-plastic relaxation of misfit stresses in the Si(001)/Si1 ? xGex system

It is found that stressed luminescent samples of Si(001)/Si_{1 − x} Ge _x /Si with a germanium concentration of up to 16% contain microcracks. In contrast to ordinary cracks, microcracks are characterized by partial cracks of the substrate that are not perpendicular to the plane of the plate and are detected by specially designed X-ray analysis techniques. In a 60-nm-thick layer of SiGe with a germanium concentration of 5%, individual microcracks arise in samples near the edges of the same type, and the traces of these microcracks coincide with the ripple profiles of the corrugated growth surface. As the germanium concentration increases, first, the number of microcracks near the edges of a sample increases, while the corrugated growth relief nearly completely disappears, and then microcracks appear even in the central region of the sample, thus leading to the vanishing of the curvature of the structure in this region without visible signs of plastic relaxation. At the same time, even a thin (20 nm) layer of SiGe exhibits a layered structure, and diffuse scattering near the peak of the SiGe layer increases, which points to the presence of fragments of the layer that are misoriented by ±0.015°; the intensity of the diffuse scattering may amount to 0.5% of the layer intensity. A spatial analysis of the luminescence of samples with microcracks shows that the emergence of microcracks hardly affects the peak position and the half-width of the emission line of the SiGe layer. At the same time, the intensity of exciton emission from both the SiGe layer and the bulk of silicon is significantly (several times) changed when passing to the regions with microcracks. All the phenomena observed can be accounted for under the assumption that, between the stages of the loss of the plane crystallization front and the development of plastic relaxation of misfit stresses, there exists an earlier unknown stage of growth in which the concentration of nonequilibrium vacancies increases by four to five orders of magnitude and then the vacancies condensate into micropores.     

Limits on scalar-leptoquark masses from lepton-flavor-violating processes of the li → ljγ type

The contributions of scalar leptoquarks to lepton-flavor-violating processes of the l _i → l _j γ type are investigated on the basis of the minimal model relying on four-color symmetry and involving the Higgs mechanism of quark- and lepton-mass generation. It is shown that experimental data on the decay processes μ → eγ, τ → μγ, and τ → eγ are compatible with the existence of light scalar leptoquarks belonging to the type being considered and having masses around 1 TeV or below.     

Study of high-temperature oxygen permeability in Sr1 ? xLaxCo0.8 ? yNbyFe0.2O3 ? z perovskites

The oxygen permeability of ceramic SrCo_{0.8 − y} Fe_0.2Nb _y O_{3 − z} (0 ≤ y ≤ 0.2) and La_0.3Sr_0.7Co_0.6Fe_0.2Nb_0.2O_{3 − z} disc membranes as a function of temperature and oxygen partial pressure was studied. Kinetic analysis was performed based on the experimental data on oxygen permeability as a function of oxygen partial pressure.     

Effect of boundary scattering on the thermal conductivity of a nanostructured semiconductor material based on the BixSb2 ? xTe3 solid solution

This paper reports on a theoretical and experimental investigation of the behavior of the electrical and thermal conductivities of a nanostructured material based on Bi _x Sb_{2 − x} Te₃ solid solutions. The effect of boundary scattering has been taken into account by introducing the scattering mechanism with a constant mean free path equal to the nanoparticle size. A comparison with the results of the measurements has demonstrated that one can describe satisfactorily the experimental dependences of the electrical and thermal conductivities on the nanoparticle size by using only the parameters of the initial solid solution and its pure constituents. The estimates have revealed that the lattice thermal conductivity of nanostructured materials can be reduced by 20–30% as compared to the initial solid solution with nanoparticle sizes of the order of 20 nm, which should produce a favorable effect on the magnitude of the thermoelectric figure-of-merit.     

Influence of the Jahn—Teller distortion on magnetic ordering in TbMn1-xFexO3

The Jahn-Teller distortion plays an important role in determining the exchange interaction in rare-earth manganites.In this work we study the influence of the Jahn-Teller distortion on the magnetic structures of TbMn1-xFexO3（x = 0,0.02,0.05,0.10,and 0.20） single crystals in the basal MnO2 plane.The decrease in the quadruple splitting with the increasing Fe doping indicates the reduction of the Jahn-Teller distortion,which makes the nearest neighboring（NN） FM interaction dominant over the next nearest neighbor（NNN） AFM interaction.This alteration is favorable for the development of A-type AFM ordering instead of the spiral magnetic ordering,which collapses when x ≥ 0.05.The analysis of dielectric data indicates that the ferroelectricity is arising from the peculiar spiral magnetic ordering.     

Analysis of the ΛQ baryons in the nuclear matter with the QCD sum rules

In this article, we study the Λ c and Λ b baryons in the nuclear matter using the QCD sum rules, and obtain the in-medium masses M\varLambda c*=2.335 GeVM_{\varLambda _{c}}^{*}=2.335~\mathrm{GeV}, M\varLambda b*=5.678 GeVM_{\varLambda _{b}}^{*}=5.678~\mathrm{GeV}, the in-medium vector self-energies \varSigma \varLambda cv=34 MeV\varSigma ^{\varLambda _{c}}_{v}=34~\mathrm{MeV}, \varSigma \varLambda bv=32 MeV\varSigma ^{\varLambda _{b}}_{v}=32~\mathrm {MeV}, and the in-medium pole residues l\varLambda c*=0.021 GeV3\lambda_{\varLambda _{c}}^{*}=0.021~\mathrm{GeV}^{3}, l\varLambda b*=0.026 GeV3\lambda_{\varLambda _{b}}^{*}=0.026~\mathrm{GeV}^{3}. The mass-shifts are M\varLambda c*-M\varLambda c=51 MeVM_{\varLambda _{c}}^{*}-M_{\varLambda _{c}}=51~\mathrm{MeV} and M\varLambda b*-M\varLambda b=60 MeVM_{\varLambda _{b}}^{*}-M_{\varLambda _{b}}=60~\mathrm{MeV}, respectively.