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.     

Plastic behaviour of Cd x Hg1−x Te (0⩽ x⩽1) crystals

Cd _x Hg_1–x Te (0x1) single crystals were strained by microhardness and by constant strain rate uniaxial compression tests, in the temperature range 300 to 600 K. Hardness curves as function of temperature can be described by empirical relations. Stress-strain curves, relaxation tests and dislocation observations using transmission electron microscopy show that the deformation is controlled by a thermally activated Peierls mechanism. Moreover, dislocations are dissociated with a stacking fault energy which does not depend on thex composition.     

Dynamics of Local Bond Correlations in FeSe x Te1−x by Inelastic Neutron Scattering

The dynamics of the local atomic pair correlations in the FeSe _x Te_1?x ( x = 0.1, 0.5, and 0.9) system were investigated using the dynamic pair density function analysis obtained from inelastic neutron scattering. Distinct differences are observed between the superconducting and nonsuperconducting compositions, particularly involving the local dynamics of the Fe-Fe correlations. The energy scale of the observed differences is between 25 and 30 meV. In the nonsuperconducting FeSe_0.1Te_0.9, the Fe-Fe correlations are enhanced with cooling that may signal stronger exchange interactions, while in the superconducting compositions of FeSe_0.9Te_0.1 and FeSe_0.5Te_0.5, they are suppressed.     

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.     

Structural and optical properties of Cd x Zn(1 − x)Te thin films

Seven Cd _x Zn_{(1 ? x} Te solid solutions with x = 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 were synthesized by fusing stoichiometric amounts of CdTe and ZnTe constituents in silica tubes. Each composition was used in the preparation of a group of thin films of different thicknesses. Structural investigation of the obtained films indicates they have a polycrystalline structure with predominant diffraction lines corresponding to (111) (220) and (311) reflecting planes, which can be attributed to the characteristics of growth with the (111) plane. The optical constants (the refractive index n, the absorption index k, and the absorption coefficient α) of Cd _x Zn_{(1 \s -x)} Te thin films were determined in the spectral range 500–2000 nm. At certain wavelengths it was found that the refractive index, n, increases with increasing molar fraction, x. It was also found that plots of α² (hv) and α^1/2 (hv) yield straight lines, corresponding to direct and indirect allowed transitions respectively obeying the following two equations: $$\begin{gathered} E_g^d = 1.583 + 0.277x + 0.197x^2 \hfill \\ E_g^{ind} = 1.281 + 0.111x + 0.302x^2 \hfill \\ \end{gathered}$$      

Composition and predominant electron scattering mechanisms in Hg1 − x − y Cd x Eu y Se crystals

The surface of Hg_{1 ? x ? y} Cd _x Eu _y Se crystals has been examined by electron microscopy using backscattered and secondary electron imaging, and the composition of the crystals has been determined. Using transport and optical measurements, we have identified the predominant electron scattering mechanisms in the Hg_{1 ? x ? y} Cd _x Eu _y Se crystals.     

Magnetic, optical, and kinetic properties of Hg1 − x − y Mn x Dy y Te crystals

We have studied the magnetic, optical, and kinetic properties of Hg_{1 ? x ? y} Mn _x Dy _y Te crystals. The behavior of their magnetic susceptibility can be accounted for by the presence of clusters of various sizes. The Hg_{1 ? x ? y} Mn _x Dy _y Te crystals are shown to be n-type. Absorption data are used to determine the optical band gap of the crystals.     

Electrical and microstructural properties of Bi2−x Sb x Te and Bi2−x Sb x Te2 foils obtained by the ultrarapid quenching process

This paper is concerned with the feasibility and reproducibility of the ultrarapid quenching process used to fabricate Bi_2–x Sb _x Te and Bi_2–x Sb _x Te₂ alloys for thermoelectric applications. Microstructural properties of the materials, obtained in the shape of foils, were studied concerning the phase analysis, cell parameters, texture, and microstructure observations. The Bi_2–x Sb _x Te alloys were found to have the (2 0 3) texture. The (2 0 4) texture, with an additional (1 1 0) component for x values greater than 0.4, was predominant for Bi_2–x Sb _x Te₂ foils. The electrical properties of these materials were then characterized by measuring the Seebeck coefficient, Hall coefficient, and electrical resistivity. It was found that Bi_2–x Sb _x Te foils changed from n- to p-type for an x-value of about 1.2. A maximum Seebeck coefficient, ||, of 36×10^–6 V K^–1 was measured for Bi₂Te. In the case of Bi_2–x Sb _x Te₂ foils, the change from n- to p-type was observed for an x value of about 1. A maximum Seebeck coefficient, ||, of 32×10^–6 V K^–1, was measured for Bi_1.4Sb_0.6Te₂. Measurements of the temperature-dependent electrical resistivity, Hall and Seebeck coefficients of the foils were carried out and the analysis revealed a semi-metallic behavior.     

MOCVD of p-Cd x Hg1 – x Te/GaAs Heteroepitaxial Structures

Metalorganic chemical vapor deposition from Cd and Te alkyl compounds and Hg vapor is used to grow p-type Cd _x Hg_{1 – x} Te epitaxial layers on semi-insulating GaAs(111)Bsubstrates by the interdiffused multilayer process (alternating CdTe and HgTe layers) at a substrate temperature of 350°C, followed by postgrowth annealing. Layers are obtained with x = 0.2–0.4, 77-K carrier concentrations in the range (1–5) × 10¹⁶ cm^–3, and 77-K carrier mobilities from 200 to 400 cm²/(V s). The rocking curves of the epilayers have a full width at half maximum in the range 2–4 min of arc.     

Magnetocaloric Effect in La1−x Cd x MnO3

Calculations of the magnetocaloric effect for La_1?x Cd _x MnO₃ (LSCM) upon 0.05 T magnetic field variation have been carried out. It is found that magnetic entropy change distribution of the LSCM is much more uniform than that of gadolinium. This feature is desirable for an Ericson-cycle magnetic refrigerator. Furthermore, at different concentrations of Cd, the temperature range between 150 K and room temperature can be covered using the La_1?x Cd _x MnO₃ system. Therefore, the LSCM system is beneficial for manipulating magnetocaloric refrigeration that occurs in various temperatures.     

Investigation of the lithium-rich boundary of the Li1+x Mn2−x O4 cubic spinel phase in air

Several compositions of the cubic spinel Li_1+x Mn_2?x O_4?δ phase in the lithium–manganese–oxygen (Li–Mn–O) system were synthesized at 700, 750, and 800 °C in air ( $ p_{{{\text{O}}_{2} }} $  = 0.2 atm) to investigate the Li-rich boundary of the cubic spinel phase at these temperatures. The lattice parameters of the several compositions were determined by Rietveld analysis of the measured X-ray patterns, and the Li and Mn contents of the samples were measured using inductively coupled plasma with optical emission spectroscopy (ICP-OES). A Vegard-like dependence of the measured lattice parameter of the cubic spinel phase with Li to Mn ratio exists in the homogeneity range of the cubic spinel. This dependence could be used to derive the boundary of the single phase cubic spinel field in the Li–Mn–O system at 700 and 750 °C at $ p_{{{\text{O}}_{2} }} $  = 0.2 atm and to estimate the Li-rich boundary at 800 °C. The results of the present study are compared with two other experimental studies on the homogeneity range of the cubic spinel phase in an attempt to resolve the contradiction between these two studies.     

Investigation of the functional properties of Mg x Ni1−x Fe2O4 ceramics

The paper presents a complex study of the effect of Mg substitution on the functional properties of Ni-ferrite ceramics prepared by self-combustion sol–gel method. The sintered ceramics have pure cubic spinel structures with an increase of the lattice parameter and the grain size with Mg content. The electrical properties of Mg _x Ni_1?x Fe₂O₄ (x = 0; 0.17; 0.34; 0.5; 0.66; 1) ceramics have been investigated. The complex impedance spectra suggest a grain boundary contribution in the conduction process and reveal that the real part of impedance and the imaginary component (reactance) increase with increasing the Mg amount. The temperature dependence of dielectric properties shows that the hopping of charge carriers is thermally activated. The resistivity as a function of frequency for different degree of humidity was also investigated. All the investigated samples show a typical ferrimagnetic character with a strong non-linearity, small coercitive field (~50 Oe) and a saturation field of ~1kOe, typical to the investigated Mg _x Ni_1?x Fe₂O₄ ceramics. The Curie temperature determined from magnetic susceptibility versus temperature dependences presents a decrease with the addition of non-magnetic Mg²⁺ ion concentration from 603 °C (for x = 0) to 384 °C (for x = 1).     

Composition and Electrical Properties of Hg x Cd1 – x S Whiskers

Data are presented on the morphology, composition, and electrical and photoelectric properties of Hg _x Cd_{1 – x} S (x = 0.25–0.6) whiskers.     

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.     

Electronic properties and charge density of Be x Zn1 − x Te alloys

Electronic band structure calculations are performed for the Be _x Zn_1???x Te (0≤x≤1 in steps of 0·2) alloys following the empirical pseudopotential method. The alloying effects are modelled through the modified virtual crystal approximation. Throughout the composition, valence band maximum resides at the Γ point. The conduction band minimum, however, shifts from Γ to X point of symmetry when x?= 0·27. The observed crossover from direct to indirect bandgap is well in accordance with the experimental observations. Effect of alloying on the density of states is also discussed. The charge density distribution along a few major planes is computed and discussed. The electronic band structure related parameters like bandwidths, bandgaps and ionicity are reported and compared with experimental data wherever available. We also give estimates of cohesive energy and bulk modulus for the alloys.     

Band edge motion in quantizing magnetic field and nonequilibrium states in Pb1−x Sn x Te alloys doped with In

Galvanomagnetic and oscillation effects in Pb_1–x Sn _x Te single crystals doped with 0.5 at % In have been studied in magnetic fields up to 60 kOe at temperatures from 4.2 to 30 K under hydrostatic pressure up to 18 kbar. Beyond the ultraquantum magnetic field limit (H _uql) for the metallic state of Pb_1–x Sn _x Te(In) alloys, Fermi level pinning by high-density quasilocal states takes place. In a strong fieldH>H _uql the equationE _F = const is valid instead of the equationn = const which is usual for degenerate semiconductors (E _F is the electron or hole Fermi energy, andn is their concentration). This makes it possible to determine the direction of the band edge motion in the Pb_1–x Sn _x Te energy spectrum in a quantizing magnetic field in the direct and inverse spectral regions. It is found that the charge carrier transitions between quasilocal and band states are of anomalously long duration (10⁵ sec atT=4.2 K). By the application of a quantizing magnetic field we obtained a nonequilibrium metallic state of the system with a frozen or slowly diminishing Fermi surface. The characteristic time of the transition was found as a function of temperature and pressure. The relaxation kinetics of the nonequilibrium states induced by a quantizing magnetic field and infrared irradiation is discussed.     

Influence of the Oxygen Excess in the Synthesis of NdFeAsO1−x F x Superconductors

The influence of the oxygen excess in the nucleation of the NdFeAsO_1?x F _x superconducting phase was studied. The experimental results indicate that a high excess of oxygen inside the quartz ampoule, where the phase is synthesized, promotes the formation of spurious phases, and prevents the formation of the NdFeAsO phase. It is possible to obtain pure NdFeAsO phase with good crystalline properties if the oxygen excess inside the quartz ampoule is compensated via oxygen deficiency in nominal precursors. Nevertheless, the synthesis of pure NdFeAsO phase does not guaranties the fluorine incorporation and the emergency of superconductivity. The efficient fluorine incorporation occurs only after an oxygen deficiency that directly produces oxygen vacancies in the NdFeAsO structure. The detailed study for different samples with various fluorine doping levels x allows fixing the lower doping limit for the emergency of superconductivity at a values of [F]～13 %.     

Field-Induced Magnetostructural Transitions in Antiferromagnetic Fe1+y Te1−x S x

Transport and structural properties of Fe_1+y Te_1?x S _x were studied in pulsed magnetic fields. Application of high magnetic fields induces first order transitions showing positive magnetoresistance effects in the antiferromagnetic phase. Polarizing microscope images taken at high magnetic fields revealed the concomitant melting of the orbital order. These results indicate the importance of crossed coupling between spin and lattice or orbital degrees of freedom in this compound.