Transport anomalies and magnetic properties of Sn x Eu1.2−x Mo6S8 alloys

We have measured the resistance R, magnetoresistance, and magnetization of sintered samples of the compounds Sn _x Eu_1.2–x Mo₆S₈, where x = 0, 0.12, 0.24, and 0.48. A large resistance anomaly beginning at T ~ 100 K and extending to low temperatures is found in all of the alloys except x = 0.48, which had a high superconducting transition temperature T _c ~ 11.3 K. The resistance anomaly, which appears to be correlated with the depression of T _c, is also correlated with a large negative magnetoresistance at temperatures T 12 K. For all samples except x = 0.48, that part of the magnetoresistance attributed to spin-flip scattering is proportional to the square of the magnetization M ² and is 30–40% of R (0) for some samples. Magnetization measurements are consistent with a spin-7/2 paramagnetic behavior of Eu²⁺ ions when allowance is made for a concentration of 20–30% nonmagnetic (J = 0) Eu³⁺ ions as determined from Mössbauer measurements. The experimental results are inconsistent with predictions of dilute magnetic alloy Kondo models. We speculate that the resistance anomaly and suppression of T _c for x 0.24 are associated with a semiconductor-like behavior.Work performed under the auspices of the U.S. DOE.Work supported by NSF Research Grant DMR 78-2428.     

Etching behavior of PbTe and Pb1 − x Sn x Te crystal surfaces in aqueous H2O2-HBr-tartaric acid solutions

The chemical polishing of the surface of single crystals of PbTe and Pb_{1 ? x} Sn _x Te solid solutions by H₂O₂-HBr-tartaric acid bromine-releasing etchants has been studied for the first time under reproducible hydrodynamic conditions. The dissolution rate of the crystals has been determined as a function of etchant composition, solution stirring rate, and temperature. The polished surfaces have been examined by microstructural analysis and electron microscopy. We have located the composition boundaries of solutions for the dynamic chemical polishing of the semiconductor materials studied.     

Non-substitutional Sn Defects in Ge1−x Sn x Alloys for Opto- and Nanoelectronics

Important technological applications are envisaged for Ge_1?x Sn _x alloys. They provide a route to obtain direct-gap group IV materials, tuneable by concentration. Therefore, these alloys are ideal candidates for optoelectronic devices, highly compatible with Si integrated circuits. Contrary to other binary alloys with group IV elements, homogeneous Ge_1?x Sn _x alloys, as required for device applications, have proven difficult to form above a certain temperature-dependent critical Sn concentration. Through a detailed ab-initio local defect study, and the proposal of a statistical model for the formation of these alloys, we predicted that a new type of Sn defect (β-Sn), consisting of a single Sn atom in the centre of a Ge divacancy, might be formed. The environment of this defect relaxes towards a cubic octahedral configuration, facilitating the nucleation of white tin and its segregation, as found in amorphous samples. We confirmed that Sn would enter substitutionally in the Ge lattice, but above a temperature-dependent critical concentration, non-substitutional β-Sn defects should be formed, consistent with experimental observations. In this paper we introduce a two-site substitutional equivalent for the non-substitutional β-Sn defect in Ge, as needed in order to be able to include β-Sn in electronic structure calculations with effective-field electronic models for disorder, like the Virtual Crystal Approximation (VCA). The equivalent substitutional model is derived in order to take into account the different symmetries in the immediate environment of the substitutional α and non-substitutional β defect sites and their effect on the electronic structure.     

Thermal conductivity of tellurium-enriched Pb1 − x Mn x Te single crystals

We have studied the effect of excess tellurium atoms (up to 0.5 at %) on the thermal conductivity of single-crystal Pb_{1 ? x} Mn _x Te (x = 0.04) samples annealed at 570K for 120 h. We have evaluated the lattice and electron thermal conductivities of the samples, their additional thermal resistance due to structural defects, and the coefficient in the expression for the effective phonon scattering cross section. We assume that some of the excess Te atoms fill lead vacancies, thereby reducing the effective phonon scattering cross section.     

Investigation of the gapless state induced by pressure in Hg1−x Cd x Te alloys

We report the results of an experimental study of the gapless state (GS) induced inp-type semimetallic alloys Hg _1–x Cd _x Te (0<x<0.15) by pressure. Galvanomagnetic effects in weak magnetic fields (2T<300 K) and the Shubnikov-de Haas effect have been investigated in the pressure interval 1p<15 kbar. Direct evidence for the existence inp-type semimetallic alloys of an impurity hole band overlapping with the conduction band by 3–4 me V is obtained. At liquid helium temperatures the Fermi level is located in the impurity band, so that two groups of carriers take part in transport effects: light electrons in the conduction band and heavy holes in the impurity band. The electron Fermi energyE _F is proved to be essentially constant during the transition to the GS. A linear dependence of the electron effective mass at the band edgem* (0) upon the gapE _g is obtained. A significant role of scattering of electrons into the impurity band at liquid helium temperatures is revealed.     

Crystallization kinetics in glassy Ge20Se80−x In x alloys

Crystallization kinetics is studied in glassy Ge₂₀Se_80–xIn_x (0 x 20) using isothermal annealing at temperatures between the glass transition and melting. D.c. conductivity is taken as a parameter to estimate the extent of crystallization (). The activation energy of crystallization (E) and the order parameter (n) are calculated by fitting the values of in the Avrami equations of isothermal crystallization. The results indicate that E is highly composition-dependent, which is explained in terms of the stable phases in the Ge-Se-In system.     

First-principles investigations of structural, elastic, electronic and magnetic properties of Ga1−x Mn x P and In1−x Mn x P

We employ the full-potential linearized augmented plane wave plus local orbital (FP L/APW + lo) method based on the density functional theory (DFT) in order to investigate the structural, elastic, electronic, and magnetic properties of ordered dilute ferromagnetic semiconductors Ga_1?x Mn _x P and In_1?x Mn _x P at (x = 0.25) in the zinc blende phase, using generalized gradient approximation, GGA (PBE). To our knowledge the elastic constants of these compounds have not yet been measured or calculated, hence our results serve as a first quantitative theoretical prediction for future study. Results of calculated electronic structures and magnetic properties reveal that both Ga_0.75Mn_0.25P and In_0.75Mn_0.25P have stable ferromagnetic ground state, and they are ideal half-metallic (HM) ferromagnetic at their equilibrium lattice constants. Also we show the nature of the bonding from the charge spin-densities calculations. The calculated total magnetic moments are 4.0 μ_B per unit cell for both Ga_0.75Mn_0.25P and In_0.75Mn_0.25P, which agree with the Slater–Pauling rule quite well, and we observe that p–d hybridization reduces the local magnetic moment of Mn from its free space charge value and produces smaller local magnetic moments on the nonmagnetic Ga, In and P sites. The values of N ₀α and N ₀β exchange constants confirm the magnetic nature of these compounds. From the robust half-metallicity of Ga_0.75Mn_0.25P and In_0.75Mn_0.25P as a function of lattice constant is also investigated.     

Point defect redistribution in Si1−x Ge x alloys

Using high resolution secondary ion mass spectrometry (SIMS) measurements we have studied the effects of germanium content on antimony diffusion in strained Si_1–xGe_x layers. Samples were molecular beam epitaxy (MBE) grown Si_1–xGe_x buried layers incorporating in situ doped antimony delta-layers. These were annealed for a variety of times and temperatures, following which SIMS profiles were taken and from these diffusivities were calculated. The results of a diffusivity versus germanium content study and a diffusivity versus time study are presented; equilibrium antimony diffusion coefficients in alloys of up to 30% germanium are given along with possible transient effects. Changes in antimony diffusivity with composition are attributed to changes in the vacancy population and the vacancy enthalpy of migration. Comparison with the diffusivity of boron versus germanium content in silicon-germanium alloys leads to the proposal that, in silicon-rich alloys, boron diffuses predominantly via the interstitialcy mechanism. The dependence of diffusivity on germanium content for boron is different from that of antimony and it is proposed that the boron diffusion mechanism changes from largely interstitialcy in silicon to vacancy in germanium—the change occurring at around 40% germanium.     

The mechanisms of yield and plastic flow in HgTe and Cd x Hg1−x Te

Single crystals of HgTe and Cd _x Hg_1–x (0.18<x<0.30), oriented for single slip, have been deformed in four-point bending at strain rates 10^–4 sec^–1 and temperatures from –11 to +84° C for HgTe, and 20 to 195° C for Cd _x Hg_1–x Te. At the lowest temperatures, the stress-strain curve exhibits a sharp yield relaxation and subsequent zero work hardening regime, as commonly observed for other semiconductors. Experiments show that the yielding mechanism is that proposed by Johnston and Gilman for LiF. Possible explanations for the post-yield zero work hardening phenomenon are discussed. The influence of composition, temperature and strain rate on the stress-strain behaviour are reported. At 20° C, the upper and lower yield stresses ( _uy and _1y ) increase with increasingx in qualitative agreement with our earlier hardness results. For Cd_0.2Hg_0.8Te, _1y varies with temperature,T, at a strain rate of 10^–4 sec^–1, according to _1y exp (Q/kT) whereQ is 0.16 eV. For HgTe the comparable value is 0.11 eV. Atx=0.25 and constant temperature, _1y depends on strain rate as _1y ^1/n wheren is 4. The stress level for deformation of Cd_0.2Hg_0.8Te at 10^–4 sec^–1 and 20° C is 2–3 kg mm^–2, comparable with that for InSb at 300° C or Si at 1000° C. Strain rate cycling tests on Cd _x Hg_1–x Te give values of activation volumeV* around 10b³ at 20° C, independent of plastic strain (up to 2–3%), suggesting that deformation in these alloys is controlled by the Peierls mechanism, as observed in other II–VI compounds.     

Andreev reflection on the Ag−BaPb1−x Bi x O3 microconstriction: Temperature and magnetic field dependence

The point-contact spectroscopy has been performed on the normal metal-superconductor microconstriction Ag?BaPb_1?xBi_xO₃. The conductance curves of our junctions reveal almost ideal Andreev reflection behavior described by theBlonder-Tinkham-Klapwijk theory, with the in-gap conductance twice higher than the conductance at higher voltages. We studied the effect of the temperature and magnetic field on the Andreev reflection. The temperature dependence of the energy gap (determined from the BTK theory) follows the BCS prediction. The value of the reduced gap yields a weak coupling strength in BaPb_1?xBi_xO₃. ¿From magnetic measurements up to 10 Tesla the temperature dependence of the upper critical field H_c2 has been determined and found to follow the Werthamer-Helfand-Hohenberg prediction.     

Electrical properties of In/Yb/n-Hg1 − x Cd x Te contacts

The specific contact resistance, _c, and the modified sheet R _sk, of In/Hg_{1 – x} Cd _x Te contacts incorporating a Yb diffusion barrier were measured as a function of the layer thickness and composition (x = 0.32–0.65). Significant increases in _c, were evident only for x 0.56 and at Yb thicknesses between 2.5 and 6.0 nm, depending on the x-value. Analytical examination of the interfaces by Rutherford backscattering spectrometry (RBS) also showed a progressive reduction in the extent of inward diffusion of In with increasing thickness of the Yb interlayer.     

Crystal structure and dielectric properties of (Pb1−x Ca x )(Zr1−y Sn y )O3 ceramics

There has been an increasing demand for dielectric resonator materials that operate in the microwave frequency range for applications in microwave communications. (Pb,Ca)ZrO₃ ceramics have a dielectric constant (_r), high quality factor (Q) and a small temperature coefficient of resonant frequency (_f). However, basic properties such as its crystal structure, temperature characteristics and the nature of its phase transformation are not yet fully understood. The temperature coefficient of resonant frequency can be controlled fairly well with the temperature coefficient of the dielectric constant. In this paper, we report the results of investigated crystal structure and the dielectric properties of (Pb_1–x Ca _x )(Zn₁– _y Sn _y )O₃ ceramics with the objective of elucidating the relationship between the crystal structure and the dielectric properties. The crystal structure refinement was performed by the Rietveld method. The dielectric properties were measured from-150–350 °C. The phase transformation was analysed from high and low temperature XRD data.     

First-principles study of ZrC x N1−x alloys with electron concentration modulation

The structural and mechanical properties of the ZrC _x N_1?x alloys with electron concentration modulation were systematically investigated by using the pseudopotential plane wave method within the generalized gradient approximation. The obtained equilibrium structure parameters and mechanical properties are in good agreement with the available experimental and other theoretical results. In addition, the theoretical Vickers hardness of ZrC _x N_1?x was calculated with the empirical formula. With an increase in carbon content in this alloys the hardness increases as carbon content up to 70 % then decreases a little, which has the same hardness change trend comparing with that of TiC _x N_1?x . Also, it was found that ZrC_0.7N_0.3 alloy with a valence-electron concentration of 8.3 per unit cell possesses the greatest hardness among the whole composition range. Furthermore, the analysis of electronic properties revealed the metallic character of ZrC_0.7N_0.3 alloy. The thermodynamic stability of the designed alloy was also estimated by the calculated mixing enthalpy.     

Structural, transport, magnetic, and dielectric properties of La1−x Te x MnO3 (x = 0.10 and 0.15)

In this study, we have investigated the structure, temperature-dependent resistivity, magnetization, and dielectric properties of La_1?x Te _x MnO_3±δ (x = 0.10 and 0.15). X-ray diffraction analysis confirms the rhombohedral crystal symmetry with space group R $ \overline{3} $ c. For both the samples, the temperature dependence of magnetization plots show paramagnetic-to-ferromagnetic phase transition. The Curie temperature (T _c) and magnitude of magnetization increase with the Te concentration. Field-dependent magnetization produces the asymmetric hysteresis loop that has been attributed to the magneto crystalline anisotropy induced by lattice distortion and the rare earth spin coupling at room temperature. Temperature-dependent resistivity plots exhibit metal–insulator transition (MIT) and charge-ordering state. These plots have been fitted using variable range hopping model, and the density of states [N(E_F)] has been estimated. Magnetoresistance is measured as a function of temperature in the field of 1T, 5T, and 8T. The dielectric constant shows an anomaly near MIT. The dielectric constant exhibits a peaking behavior with the applied frequency and the temperature dependence of dielectric constant attains colossal values at high temperatures.     

Electronic structure and low-temperature properties of V x Nb1−x N alloys

The vanadium and niobium nitrdes V _x Nb_1–x N are studied theoretically and experimentally for varying compositionsx. Self-consistent linear muffin-tin orbital band calculations are performed for various ordered supercells corresponding tox=0.0, 0.25, 0.50, 0.75, and 1.0. The band results are used to calculate electron-phonon coupling and spin susceptibility enhancement factors in order to study the variations of specific heat and superconducting transition temperatures with composition. A relative localization of, in particular, the V 3d states is found for the mixed compositions. The superconducting temperatures are found to be more reduced for intermediate compositions than what is expected from a simple interpolation between pure VN and NbN. Effects of spin fluctuations are important for the alloys containing vanadium, but are probably larger than what has been found in our theoretical study, and also containingq0 contributions. A small peak in the density of states near the Fermi energy is associated with a flat band near theW point, and causes a shoulder in density of state-dependent properties forx near 0.9.     

Fabrication and anisotropic electronic property for oriented Li1+x−yNb1−x−3yTix+4yO3 solid solution by slip casting in a high magnetic field

The Li_1+x?yNb_1?x?3yTi_x+4yO₃ (0.06 ≤ x ≤ 0.33, 0 ≤ y ≤ 0.09) (hereafter LNT), forms a unique and periodical structure in the Li₂O-Nb₂O₅-TiO₂ ternary system. In this work, toward application of the unique qualities of an electro-ceramic, we fabricated oriented LNT balk ceramics by slip casting in a strong magnetic field of 12 T using various sizes of particles. The c-axis of the LNT powders was aligned parallel to the magnetic field. As a result, we found anisotropic- and unique- electric properties which were caused by a superstructure with intergrowth layers of corundum-type [Ti₂O₃]²⁺. The Qf value parallel to the c-axis was about five times greater than that of perpendicular to the c-axis. We first clarified the mechanism showing that the anisotropic Qf value was caused by the anisotropic electron conductivity and the anisotropic bonding strength in the superstructure.     

Precipitation of (PdxAg1−x)3 In in Ag-25% Pd-In alloys and the partial phase diagram of ternary Ag-Pd-In alloys

A study was carried out to determine the partial phase diagram for the system Ag-25 wt % Pd–y wt% In with y 8 by X-ray powder diffraction and differential thermal analysis. The liquidus and solidus temperatures decreased with increasing indium content up to 8 wt % In for a given 25 wt % Pd. The solvus curve of phase identified as Pd_xAg_1-x)₃ In was determined by the parametric method and the temperature dependence of In-solubility in Ag-25 wt % Pd was very strong above 900 °C. ©2000 Kluwer Academic Publishers     

Theoretical prediction of structural parameters, band-gap energies, and mixing enthalpies of Sc1−x In x As alloys

Structural parameters, band-gap energies, and mixing enthalpies of Sc_1?x In _x As alloys were calculated using the full-potential linearized–augmented plane wave method. These calculations are based on density functional theory, within local density approximation, and generalized gradient approximation for the exchange and correlation potential. Given that the binary precursor compounds ScAs and InAs crystallize in rock-salt and zinc-blende, respectively, we made calculations for the ternary alloys in these two phases. The effect of composition x on structural parameters, band-gap energies, and mixing enthalpies was analyzed for x = 0, 0.25, 0.5, 0.75, 1. The effect of atomic composition on lattice constant, bulk modulus, and band-gap energy shows nonlinear dependence on concentration x. Deviations of the lattice constant from Vegard’s law and deviations of the bulk modulus and gap-energy from linear concentration dependence were found. We have found a metallic character for rock-salt Sc_1?x In _x As alloys, while the zinc-blende Sc_1?x In _x As alloys are semiconductors. Our results show that the band-gap undergoes a direct ( $X \rightarrow X$ )-to-direct ( $\Upgamma\rightarrow \Upgamma$ ) transition at a given indium composition. The physical origin of the band-gap bowing in zinc-blende Sc_1?x In _x As alloys was investigated. To study the thermodynamic stability of Sc_1?x In _x As alloys, a regular-solution model was used. This resulted in lower mixing enthalpies for zinc-blende Sc_1?x In _x As alloys.     

Crystal structure and magnetic properties of Sr1 − x Sm x Fe12 − x Co x O19 solid solutions

Sr_{1 ? x} Sm _x Fe_{12 ? x} Co _x O₁₉ (0 ≤ x ≤ 0.5) ferrites have been prepared by solid-state reactions in air at 1470 K using mixtures of samarium oxide, ferric oxide, Co₃O₄, and strontium carbonate. X-ray diffraction characterization showed that the samples with x < 0.2 were single-phase, whereas the samples with 0.2 ≤ x ≤ 0.5 contained α-Fe₂O₃ and those with 0.3 ≤ x ≤ 0.5 contained SmFeO₃, CoFe₂O₄, and Sm₂O₃ as well. The highest degree of Sm³⁺ and Co²⁺ substitutions for Sr²⁺ and Fe³⁺ (x) in the SrFe₁₂O₁₉ ferrite at 1470 K was determined to be slightly less than 0.2. This substitution only slightly decreases the a and c parameters of the hexagonal lattice and the Curie temperature (T _C) of the material. At temperatures of 5 and 300 K in magnetic fields of up to 14 T, we obtained magnetic hysteresis loops, which were used to evaluate the spontaneous magnetization (σ₀), specific saturation magnetization (σ_s), and coercive force (_σ H _c) of the ferrites. The experimentally determined 5-K spontaneous magnetization per formula unit (n ₀) of the x = 0.1 ferrite is 20.86μ_B, which coincides with the theoretical value calculated as n ₀ = (8 × 5) ? (3.9 × 5 ? 0.1 × 3) = 20.8μ_B. At 300 K, the n ₀ and _σ H _c of Sr_0.9Sm_0.1Fe_11.9Co_0.1O₁₉ exceed those of SrFe₁₂O₁₉ by 7.7 and 9.9%, respectively.