Some physical properties of calcia and yttria stabilized ThO2 ceramics

In this paper the density, electrical conductivity, and dielectric permittivity of vacant ceramics of Ca _x Th_1–x O_2–x and Y _x Th_1–x O_2–x /2 type, respectivelly, were investigated as functions of temperature, frequency, and composition of these systems.The density drops with increasing calcium or yttrium amount, respectively. In the ThO₂-CaO system this is in agreement with the mechanism of formation of vacant phase Ca _x Th_1–x O_2x . The decrease of porosity in the ThO₂-Y₂O₃ system with increasing yttrium content can be due various factors which influence the mobility of Y³⁺ ions and, thus, also the formation of pores.The investigation of transport numbers has shown that whilst the Ca _x Th_1–x O_2–x phase under our experimental conditions is essentially p-type conductor, the Y _x Th_1–x O_2–x/2 phase is essentially ionic conductor.The transport of ion carriers is a diffusion-like process which has activation energy 1·28 to 1·16eV in the Ca _x Th_1–x O_2–x phase and 1·28 to 1·06 in the Y _x Th_1–x O_2–x /2 phase, respectively. The association of impurity ions with anion vacancies provides the maxima in conductivity isotherms.The relative dielectric permittivity of the vacant phase Ca _x Th_1–x O_2–x ranges from 19·2 to 36·2, in the phase Y _x Th_1–x O_2–x /2 from 19·2 to 103, depending on temperature, frequency, and composition of these systems.     

On the superconductivity of the tin-hydrogen system

Conductivity and superconductivity studies of amorphous [Sn_1–y Cu _y–]_1–x H _x samples in connection with¹¹⁹Sn Mössbauer effect experiments on¹¹⁹Sn_1–x H _x give strong evidence that the observed increase of the superconducting transition temperatureT _c in the Sn–H-system is caused by the stabilization of an amorphous structure. Thus the Sn–H-system is very similar to the Sn–Cu-system and no H-specific effect is needed to explain the increase ofT _c.     

Vortex pinning properties of –Ba–Cu–O and –Ba–Cu–O superconducting bulks

We have studied the effect of a small amount of Y-site substitution by La or Pr ions on the vortex pinning in the Y–Ba–Cu–O system. (Y_1-xLa_x)–Ba–Cu–O and (Y_1-xPr_x)–Ba–Cu–O bulks were fabricated by the melt-textured growth, in which x was varied from 0 to 0.01. The critical current density J_c at 77 K is improved in magnetic fields parallel to the c-axis above 2–4.5 T and the corresponding irreversibility field, H_irr, shifts to the higher value in both bulks.     

The electronic structure of bcc-based random solid solutions of transition metals

The electronic structure of Nb _x Zr_1–x , V _x Nb_1–x and Mo _x V_1–x random solid solutions with the bcc lattice structure is investigated in the frame of the first-principles tight-binding muffin-tin orbital method and the coherent potential approximation. The total and component densities of states as well as the Bloch spectral densities are determined over a broad concentration range.     

Magnetic and magnetotransport properties of Co-doped manganites with perovskite structure

A systematic study of the doping of the Mn-sites by cobalt in three series of manganites — La_0.76Ba_0.24(Mn_1−xCo_x)O₃ single crystals, La_2/3Ba_1/3(Mn_1−xCo_x)O₃ and La(Mn_1−xCo_x)O₃ ceramics has been performed. It was found that La(Mn_1−xCo_x)O₃ annealed at 800°C in the range 0.4x0.9 is a mixture of ferromagnetic domains with ordered Mn and Co ions and ionically disordered spin-glass domains. In the quenched samples the fraction of spin-glass-type component increases strongly. The La_2/3Ba_1/3(Mn_1−xCo_x)O₃ solid solutions exhibit also an evidence for phase separation in the range 0.5x0.8. All the La(Mn_1−xCo_x)O₃ samples show an insulating behavior, however, magnetoresistance reduces strongly when the cobalt content rises to x=0.5. The La_0.76Ba_0.24(Mn_1−xCo_x)O₃ single crystals show first-order phase transition below their Curie points associated with a change of ground state of the Co²⁺ ions. The magnetic phase diagrams are depicted. The results are discussed in terms of positive Mn³⁺–O–Mn⁴⁺, Mn³⁺–O–Mn³⁺, Mn⁴⁺–O–Co²⁺ and negative Mn⁴⁺–O–Mn⁴⁺, Co²⁺–O–Co²⁺, Co²⁺–O–Mn³⁺ superexchange interactions as well as Co²⁺ and Mn⁴⁺ ionic ordering.     

Magnetic Parameters of Hg1–x Mn x Se1–y S y and Hg1–x Mn x Te1–y S y Crystals

The magnetic susceptibility of Hg_1–x Mn _x Se_1–y S _y and Hg_1–x Mn _x Te_1–y S _y crystals is investigated by the Faraday method at H = 3 kOe in the temperature interval T = 77–300 K. It is established that the specific features of are due to Mn–S–Mn–S, Mn–Se–Mn–Se, and Mn–Te–Mn–Te clusters and mixed Mn–Se–Mn–S and Mn–Te–Mn–S clusters of different sizes in which the indirect exchange antiferromagnetic interaction between Mn atoms is realized through chalcogen atoms. Based on the dependences 1/_Mn = f(T), the magnetic parameters are determined and their dependences on the crystal composition (x and y) are established.     

Influence of thermal processing of Mn x Hg1−x Se crystals in mercury and selenium vapor on their magnetic susceptibility

We report a study of the influence of thermal processing of Mn _x Hg_1–x Se crystals in mercury and selenium vapor on the temperature dependence of their magnetic susceptibility. It is shown that thermal processing of Mn _x Hg_1–x Se in mercury vapor leads to diffusion of Hg atoms into the crystal, with a resulting increase in the number of interstitial mercury atoms, which act as donors and increase the electron concentration in the samples. As a consequence, the diamagnetic contribution of the interstitial mercury increases, along with the dia- or paramagnetic contribution of the increasing electron concentration, and there is a change in the contribution of the Mn atoms, which migrate during the anneal, leading to a change in the magnetic susceptibility of the Mn _x Hg_1–x Se crystals. We also show that thermal processing of Mn _x Hg_1–x Se in Se vapor leads to the formation of new clusters in the crystal, or a change in size for ones which preexist (either by increasing or decreasing the size) due to the thermodynamic equilibrium conditions in the Mn _x Hg_1–x Se system, i.e. the relationship between the cluster size and the concentration.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 54–59, August, 1993.     

Competition of magnetic order and superconductivity investigated by positive muon spin rotation in La2−x M x CuO4 (M = Ba,Sr) aroundx=0.12

Muon spin rotation (SR) is applied to La_2–x Ba _x CuO₄ aroundx=0.12 where the superconductivity (SC) is suppressed remarkably. The magnetic ordering of Cu-moments appears below 35 K in the narrow range ofx where a lattice instability from the low temperature orthorhombic (LTO) to the low-temperature tetragonal (LTT) structure exists. The present study suggests strongly that the magnetic ordering of Cu-moments is an important factor in the suppression of high-T _c superconductivity aroundx=0.12 in La_2–x Ba _x CuO₄. Similar results are obtained for the La_2–x Sr _x CuO₄ and La_2–x–y Sr _x Nd _y CuO₄ systems.     

Parametric Frequency Converters Based on New Nonlinear Crystals

Results of investigations into the physical properties of a number of new nonlinear crystals are presented. The parameters of frequency converters manufactured on their basis are compared, and second-harmonic generation of CO₂ lasers first excited by frequency conversion in LiInSe₂, AgGa_{1– x} In_xSe₂, Ag_xGa_xGe_xS_{2(1 + x)}, and Hg_1–x Cd_xGa₂S₄ crystals is investigated. The superiority of HgGa₂S₄, Hg_1–x Cd_xGa₂S₄, and AgGa_1–x In_xSe₂ over the well-known crystals has been demonstrated for frequency conversion both within the middle-IR range and from the visible range to the middle-IR range. Advantages of LiInSe₂ and AgGa_1–x In_xSe₂ crystals are demonstrated for the direct frequency conversion of femtosecond laser radiation to the middle-IR range compared to the cascade frequency conversion and direct frequency conversion in LiInS₂ crystals.     

Concentration Phase Transitions and Structural Disordering in the System of Solid Solutions LixNa1?xTa0.1Nb0.9O3

Concentration phase transitions and structural disordering in the system of solid solutions Li _x Na_1–x Ta_0.1Nb_0.9O₃ (x = 0–0.16) are investigated by the methods of Raman scattering, x–ray structural analysis, and electrophysical measurements. Points of disruption of the translational order in the arrangement of cations at x = 0.05–0.06, 0.09, 0.11, 0.135, and 0.14 that correspond to the boundaries of morphotropic regions and concentration structural anomalies are detected. At points x = 0.11; 0.135, and 0.14, the Raman spectrum in the region of vibrations of the cations located in octahedral and cubooctahedral holes (0–400 cm^–1) consists of two wide maxima and is practically smeared into the wing of the Rayleigh line, and on the concentration dependences of the electrophysical properties one observes sharp anomalies. The oxygen ochahedrons are distorted less significantly as x changes: the corresponding lines broaden, and new lines demonstrating a symmetry change of the oxygen octahedrons appear only for x 0.14. It is shown that the system of solid solutions Li _x Na_1–x Ta_0.1Nb_0.9O₃ for x < 0.015 is characterized by the antiferroelectric type of dipole ordering, and for x 0.015 the structure of a solid solution becomes noncentrosymmetric.     

Diffusion of Au in amorphous Zr x Ni100−x alloys studied by Rutherford Backscattering Spectrometry

The diffusion coefficients (D) of Au in three binary amorphous Zr _x Ni_100–x (x=61, 65, and 67) alloys were measured in the temperature range 549–623 K using the technique of the Rutherford Backscattering Spectrometry (RBS). The D values were found to lie in the range 1.0×10^–21–9.0×10^–20 m²s^–1 for different alloys. The activation energy (Q) was calculated in each case on the basis of an observed Arrhenius temperature dependence of D. The activation energy was found to scale with the crystallization temperature (T _x) of the alloy. Other published measurements for Au diffusion in amorphous Zr-Ni alloys also appear to follow the scaling relation between Q and T _x.     

Magnetic transitions and magnetoresistance of -type () compounds

Magnetic transitions and magnetotransport properties of polycrystalline Tb_1−xGd_xMn₆Ge₆ (x=0.2–1.0) compounds have been investigated by magnetic property and resistivity measurements in an applied magnetic field up to 50 kOe. The cell parameter a,c and cell volume V of compounds (x=0.2–1.0) increase with an increasing Gd content. The compounds (x=0.2–1.0) show a rich variety of magnetic behavior, such as antiferromagnetic, ferrimagnetic and paramagnetic state with increasing temperature. Their Curie temperatures increase almost linearly with an increasing Gd content from 460 K for x=0.2 to 484 K for x=1.0. The compounds (x=0.2–1.0) display the field induced metamagnetic transitions, and the threshold fields first increase and then decrease with an increasing Gd content. The magnetoresistance curves of the Tb_0.4Gd_0.6Mn₆Ge₆ compounds in an applied magnetic field up to 50 kOe are presented and the magnetoresistance effects are related to the metamagnetic transitions.     

The superconductive properties and crystal structure of Ba2(Y1−x Pr x )Cu3O9−y solid solutions

A series of Ba₂(Y_1–x Pr _x )Cu₃O_9–y solid solutions were prepared with the orthorhombic distorted perovskite structure. The compound Ba₂PrCu₃O_9–y belonging to tetragonal system is a good semiconductor. The superconductivity is observed for samples withx<0.6. With the increase of Y content, the distortion of crystal structure is greater and the superconductive transition temperature of zero resistanceTc(0) raises.     

Impurity distribution and electrical characteristics of boron-doped Si1−x Ge x /Si p +/N heterojunction diodes produced using pulsed UV-laser-induced epitaxy and Gas-immersion laser doping

A two-step pulsed UV-laser process which independently controls the metallurgical and electrical junction depth of a Si_1–x Ge _x /Si heterojunction diode has been implemented. Pulsed Laser-Induced Epitaxy (PLIE) combined with Gas-immersion Laser Doping (GILD) are used to fabricate boron-doped heteroepitaxial p ⁺/N Si_1–x Ge _x /Si layers and diodes. Borontrifluoride is used as the gaseous dopant source in the GILD process step. Boron incorporation and activation are investigated as a function of laser energy fluence and the number of laser pulses using SIMS and Halleffect measurements. The dose of incorporated dopant is on the order of 10¹³ cm^–2 per pulse. The B profiles obtained are flat except for a peak at the interface resulting from segregation effects. The B and Ge distributions are compared with shifts in the turn-on voltage of p ⁺/N Si_1–x /Si heterojunction diodes produced by the process. The GILD/PLIE process is spatially selective with the resulting diodes fabricated being quasiplanar. Hole mobilities in the heavily doped Si_1–x Ge _x films are found to be slightly lower than in comparable Si films.Presently at the Oregon Graduate Institute, Beaverton, OR 97006, USA     

Pressure dependence of the electric field gradient in Sb and Sb1−x M x (M=ln,Zn, Ge,Pb, Cd,Sn) at the site of111Cd

Using the¹¹¹Cd-TDPAC (time differential perturbated angular correlation) method, the pressure dependence of the electric field gradient (EFG) in Sb and Sb_1–x M _x (M=ln, Zn, Ge, Pb, Cd, Sn) was investigated. The application of a phenomenological ansatz for the parametrisation of the pressure and temperature dependence of the EFG made it possible to combine temperature data gained in former studies [1], [2] with the pressure dependent data presented in this paper. The resulting pressure dependence of –2±0.2 MHz/kbar is shown to be independent of concentration and element of admixture. Results for the volume and explicit temperature dependence agree with existing information on the mixed system Sb_1–x M _x (M=ln, Zn, Ge, Pb, Cd, Sn); the investigation of the EFG in Sb_1–x–y M _x Pb _y showed that the resulting EFG may be interpreted as the weighted sum of the individual contributions of the two metals.This paper is dedicated to Prof. Dr. W. Kreische on the occasion of his 60th aniversary on 02.02.1995     

Central Limit Theorems for Nonlinear Hierarchical Sequences of Random Variables

Let a random variable x ₀ and a function f:[a, b] ^k [a, b] be given. A hierarchical sequence {x _n :n=0, 1, 2,...} of random variables is defined inductively by the relation x _n =f(x _{n–1, 1}, x _{n–1, 2}..., x _{n–1, k} ), where {x _{n–1, i} :i=1, 2,..., k} is a family of independent random variables with the same distribution as x _n–1. We prove a central limit theorem for this hierarchical sequence of random variables when a function f satisfies a certain averaging condition. As a corollary under a natural assumption we prove a central limit theorem for a suitably normalized sequence of conductivities of a random resistor network on a hierarchical lattice.     

Sulphidation-induced changes in the metallic phase of Fe-Cr alloys

The influence of sulphidation on the metallic phase in a series of Fe_100–x Cr _x alloys (x<15) was investigated by means of⁵⁷Fe Mössbauer spectroscopy. The results obtained gave evidence that the sulphidation at 1073 K is selective in the atmosphere of H₂/H₂S, with the partial pressure of sulphur ranging from 10^–7 to 10^–9 atm. Atx _cr=5.0(1) at.%, a change of the sulphidation preference occurs. Forx<5.0 at.%, the sulphidation results in an enrichment of the metallic phase in chromium, while forx> 5.0 at.%, in iron.     

Near-infrared photorefractivity in Cr-doped potassium sodium strontium barium niobate single crystal

We report on the photorefractive properties of a Cr-doped (K_1–x Na _x )_2A–2 (Sr _y Ba_1–y )_2–A Nb₁₀O₃₀ (x=0.586, y=0.659, A=1.12) single crystal in the near-infrared spectrum. The sample exhibits photorefractivity for wavelengths up to at least 840 nm where the steady-state two-beam coupling gain is found to be larger than 2 cm^–1. Photorefractive gain and decay rate are measured as a function of wavelength, grating spacing and intensity. The wavelength dependence of gain fluctuations in two-beam coupling are also measured.     

Electronic theory for the metal-non-metal transition in simple metal alloys

The metal-non-metal transition in liquid alloys such as Cs_1–x Au _x and Li_1–x Pb _x is explained as resulting from concentration dependent electron charge transfer causing short range atomic order and a change from metallic to ionic bonds. Numerical results for the electronic density of states, the electron charge transfer, the free energy of mixing and the volume change are given.     

The Hall effect in amorphous (Zr0.64Ni0.36)1−x M x alloys (M=Al,Ga)

Values of the room temperature Hall coefficients and electrical resistivity of amorphous melt spun (Zr_0.64Ni_0.36)_1–x Al _x and (Zr_0.64Ni_0.36)_1–x Ga _x alloys forx=0–0.25 are reported. Addition of Al or Ga to Zr_0.64Ni_0.36 dramatically increases the already positive Hall coefficient of this alloy and also increases the electrical resistivity and crystallization temperature.