On the magnetic susceptibility of Pd1−x Ag x in the range 0.5≦x≦1

The weak variation of the magnetic bulk susceptibility of Pd_1–x Ag _x with temperature T and silver mole fractionx within 0.5x1 has been investigated in the range 5KT400K. Experimental evidence can be given for an intersection point of the susceptibility isotherms (T=const,x) atx=0.55. The observed dependence of on T andx is interpreted by means of a semiphenomenological alloy susceptibility function (T,x).     

Structural deformations of the cubic lattice of the Zn1 ? x Fe x Se (x = 0.001) crystal

The structural state of a Zn_{1 ? x} Fe _x Se (x = 0.001) crystal has been studied using thermal neutron diffraction. The diffraction patterns of the cubic crystal have been found to contain diffuse scattering regions concentrated in the vicinity of the strong Bragg reflections. It has been shown that the diffuse scattering effects are due to local transverse displacements of the crystal lattice atoms, and these displacements are induced by iron ions that demonstrate the static Jahn-Teller effect of the tetragonal type in the ZnSe compound.     

On the microstructure of the charge density wave observed in La1− x Ca x MnO3

We have used low temperature (90?K) transmission electron microscopy to investigate the ‘charge ordering’ modulation in the mixed valent manganite, La_{1? x} Ca _x MnO₃. It has been stated that Mn³⁺ and Mn⁴⁺ ions order at low temperature to produce a structural modulation composed of supercells whose size is an integer multiple of the unmodulated unit cell. Here, we use convergent beam electron diffraction to show that the periodicity of the modulation need not be an integer multiple of the undistorted cell, even on the smallest scales. We therefore suggest that this modulation is a charge density wave with a uniform periodicity. We show that the modulation wavevector lies close to the a* axis of the crystal but need not be exactly collinear. A typical grain of size 0.5?µm in La_0.48Ca_0.52MnO₃ had a wavevector which varied on a scale of tens of nanometres with an average of ?q??=?0.450a * and a standard deviation Δq?=?0.004?a* in its magnitude and Δθ?=?0.56° in its direction at 90?K. The magnitude of the wavevector in this composition fell by 20% as the temperature was increased from 90?K to room temperature. This change occurred by nucleation and growth. Although weak, the modulation was still present at room temperature, some 30?K above the ‘charge ordering temperature’.     

On the magnetic susceptibility of the liquid Pd1−x Si x alloy system at 1825 K

The isothermal magnetic susceptibility (x) of the completely miscible liquid Pd_1–x Si _x alloy system shows a rapid monotonous decrease withx from strong paramagnetism to weaklyx-dependent diamagnetism. The measured susceptibility isotherm at 1825 K is analysed within 0x1 by using a semiphenomenological method of decomposing the magnetic susceptibility into its constituent parts. Because of the empirical similarity between liquid and glassy metals this interpretation also provides assertions about the magnetism of glassy Pd_1–x Si _x aroundx=0.2.     

Studying the effects of temperature and radiation on the Al x O1 − x /TiNC system

The penetration of hydrogen through protective layers of aluminum oxide fixed on the surface of nanocrystalline (NC) titanium is studied. A film 400 nm thick is prepared by the magnetron sputtering. Radiation- and thermally-induced gas release are employed. It is found that the Al _x O_1?x film prevents the release of hydrogen from a sample under both radiation and thermal effects. The temperature of hydrogen extraction from metal hydride accumulators can be reduced by 200–250°C, provided that heating is performed under conditions of surface irradiation by electrons with energies of ～30 keV and current densities of 2 to 3 μA cm^?2.     

Enhancement of the colossal magnetoresistance in Eu1 − x Ca x B6

The transport and magnetic properties of single crystal samples of substitutional solid solutions Eu_{1 ? x} Ca _x B₆ (0 ≤ x ≤ 0.26) have been studied at temperatures 1.8–300 K in magnetic fields up to 80 kOe. It has been shown that an increase in the calcium concentration results in the suppression of the charge transport accompanied by an increase in the amplitude of the colossal magnetoresistance (CMR) up to the value (ρ(0) ? ρ(H))/ρ(H) ≈ 7 × 10⁵ detected for x = 0.26 at liquid-helium temperature in a field of 80 kOe. The transition from the hole-like conductivity to the electron-like conductivity has been observed in the Eu_0.74Ca_0.26B₆ solid solution in the CMR regime at T < 40 K. The Hall mobility values μH = 200?350 cm²/(V s) estimated for charge carriers in the strongly disordered matrix of the Eu_0.74Ca_0.26B₆ solid solution are comparable with the charge carrier mobility μH = 400?600 cm²/(V s) for the undoped EuB₆ compound. The anomalous behavior of the transport and magnetic parameters of the Eu_{1 ? x} Ca _x B₆ solid solutions is discussed in terms of a metal-insulator transition predicted within the double exchange model for this system with low carrier density.     

Structure and magnetic properties of the Co x Pt100−x nanowire arrays

The development of photocathodes materials has become an important task for X-ray free electron laser and new generation of particle accelerator. The choice of the optimum cathode type and its further improvement is a fundamental issue for the progress in radio-frequency photoinjectors. Metallic photocathodes offer several advantages over the semiconductor ones, e.g. long lifetime and prompt response time on the photoemission. This paper reviews the requirements and the current status of metallic photocathodes prepared by pulsed laser ablation deposition technique. Magnesium, yttrium and lead are proposed as good alternative to copper photocathode which is generally used in radio-frequency photoinjectors. Parametric studies of the irradiation conditions are demanded to optimize the metallic thin film deposition. The main achievements on the morphology and structure characterization as well as photoemission testing of metallic photocathodes are presented and discussed.     

Pressure Dependence of the Bandgap Bowing in Zinc-Blende ZnTe 1− x Se x

We report on the pressure dependence of the bandgap bowing in the ZnTe 1 m x Se x alloy, in the whole composition range. The bandgap bowing parameter is shown to increase almost linearly with pressure from 1.23 at ambient pressure to 1.6 at 7 GPa. Saturation effects observed in the pressure dependence for x =0.1 and x =0.2 are shown to be related to the direct-to-indirect crossover. Results are discussed and interpreted in the framework of structural relaxation models for gap bowing. A prediction of these models (the negative bowing of the o 15 m ;X 1 transition) is shown to be compatible with the fact that the direct-to-indirect crossover pressure increases with the Se content.     

Anomalies of the static dielectric constant of Pb1−x Ge x Te

The static dielectric constant of Pb_1–x Ge _x Te (0x0.05) has been determined from differential capacitance measurements on Schottky-barriers in the temperature range of 4.2–300 K. A comparison with data deduced from the phonon frequencies via the Lyddane-Sachs-Teller relation shows substantial discrepancies which are attributed to lattice defects.Work supported by Jubiläumsfonds der Österreichischen Nationalbank     

Bandgap evolution of GaN1−x As x in the whole composition range

The bandgap evolution of GaN_1?x As _x in the whole composition range is investigated and a model describing its bandgap energy is developed. It is found that the bandgap evolution is due to two factors. One is the interaction between the impurity band and the Γ conduction band or the Γ valence band of the host materials. The other one is the intraband coupling within the conduction band and separately within the valence band. The former is dominant in the As-rich GaNAs and the N-rich GaNAs. The latter plays an important part in the N-rich range and the moderate composition range.     

Thermal conductivity of single crystals of the Ca1 − x Y x F2 + x solid solution

The thermal conductivity of single crystals of the solid solution of yttrium fluoride in calcium fluoride Ca_{1 ? x} Y _x F_{2 + x} with the fluorite structure (x ≤ 0.20) and the Ca_0.27Y_0.73F_2.73 phase with the tisonite structure has been studied by the absolute steady-state longitudinal heat flow method in the temperature range 50–300 K. It has been established that the thermal conductivity drops sharply with increasing yttrium trifluoride concentration, especially in the low-temperature region.     

Anomalies of the refractive index and the optical energy gap of ferroelectric Pb1−x Ge x Te

The refractive index and the optical energy gap of Pb_1–x Ge _x Te (0x0.11) have been determined from transmission- and reflectivity measurements in the temperature range from 4.2 K to 300 K. At the ferroelectric phase transition a change of the temperature coefficient of both quantities is observed. A two bandk·p model calculation demonstrates a correlation of the optical energy gap with the high frequency dielectric constant. For higher values ofx (x=0.09) a splitting of absorption edge and birefringence have been observed.     

Cluster glass-like behavior of the diluted antiferromagnet Fe x Mg1−x TiO3 (x=0.2,x=0.3)

A diluted antiferromagnet Fe _x Mg_1–x TiO₃ has been shown to behave as a spin glass (x=0.2) and a reentrant spin glass (x=0.3) near the Fe percolation concentrationx 0.25. In order to obtain microscopic information on these samples, we performed Mössbauer measurements. At considerably higher temperatures than the transition temperatures, magnetically broadened spectra appear superimposed upon the paramagnetic doublets. A remarkable feature is that the intensity of the magnetic spectra increases accompanying the decrease of their linewidth. This behavior can be ascribed to the gradual slow-down of fluctuations of the antiferromagnetic clusters formed at high temperatures. To investigate the temperature variations of the relaxation time of the clusters, we analyzed the Mössbauer spectra using the method formulated by Blume. It has been shown that becomes long with decreasing temperature and the rate of the slow-down of is hastened aroundT _SG andT _N.     

Factors determining the morphology of Cd x Hg1−x Te films in the course of molecular beam epitaxy

Using atomic force microscopy, the dependence of the micromorphology of CdHgTe(301) films grown by molecular beam epitaxy on growth conditions and micromorphology of the buffer CdTe layer is studied. Transmission and high-resolution electron microscopy were used to reveal the interrelation between the micromorphology and microstructure of CdHgTe(301) films. It is found that the roughness of the surface of buffer CdTe layers is inherited during growth of CdHgTe films and initiates the capture of excess tellurium by macrosteps under non-optimal growth conditions, thereby leading to nucleation of growing-in macrodefects. In CdHgTe(301) films grown at the elevated temperature, a periodic wave-like profile and associated lateral modulation of the composition in the [1?03] direction normal to the direction of profile lines [010] are observed. According to the model suggested in the study, formation of a wave-like profile and lateral modulations of the composition are caused by the character of distribution of stresses in the CdHgTe(301) film at the stage of pseudomorphic growth on the buffer CdTe layer.     

Calculation of the electronic structure and hyperfine fields for Fe1 − x Co x B and (Fe1 − x Co x )2B compounds by the Korringa-Kohn-Rostoker method

The magnetic properties of Fe_{1 ? x} Co _x B and (Fe_{1 ? x} Co _x )₂B disordered compounds were investigated using first-principles calculations of the electronic structure in the framework of the density functional theory with the Korringa-Kohn-Rostoker method. The concentration dependences of the magnetic moments and the electron density were calculated for the Fe_{1 ? x} Co _x B solid solutions. The results obtained were used to analyze in detail and to interpret the transition from a magnetic phase to a nonmagnetic phase, which was previously revealed from the experiments in the compounds under investigation. The performed analysis of the calculated hyperfine fields induced by the electronic shells at the iron and cobalt atoms in the (Fe_{1 ? x} Co _x )₂B borides made it possible to explain the experimentally observed magnetic anisotropy.     

Magnetic correlations in Eu x Sr1−x S and the ferromagnet-spin glass transition

Neutron (Bragg and small angle) scattering and susceptibility measurements are used to study magnetic ordering in Eu _x Sr_1–x S with ferromagnetic nearest neighbor exchangeJ ₁ and antiferromagnetic next-nearest neighbor exchangeJ ₂. We present data for 0.50x0.70 which cannot be analyzed within the merely geometrical treatments of percolation theory. Breakdown of ferromagnetism occurs atx _c =0.51, far above the percolation thresholdx _p =0.136, and a spin-glass phase is observed in the intermediate concentration regime. Close tox _c , the ferromagnetic state is also displaced by the spinglass phase at lower temperatures. Both properties are a general characteristic of diluted systems with competing interactions. An effective decoupling of finite magnetic clusters from the ferromagnetic net arises from frustration, which enhances the ground-state entropy. Anomalous properties below the Curie temperatureT _c as well as atT _c support this microscopic picture.     

Pressure Effects in the Raman Spectra of La 2−x Sr x CuO 4

Micro-Raman measurements under hydrostatic pressures up to 6 GPa have been carried out on high-quality La 2 m x Sr x CuO 4 polycrystalline compounds with Sr concentration up to x =0.45. The zz scattering polarization has been investigated, where two strong modes due to La/Sr and the apex oxygen, and (in the low Sr concentrations) the soft mode at ～100 cm m 1 are observed. The frequency of the strong modes increases almost linearly with pressure for the Sr concentrations studied. Modifications in the increment rate d y /d p and the phonon width have been detected depending on the amount of doping. For x =0.45 a considerable increase in the width of both strong phonons with pressure was found, which must reflect a separation into phases, since this concentration is close to the solubility limit. The relative intensity of the strong phonons was investigated in connection with its correlation to the transition temperature.     

Luminescence of the compounds Y0.5 − x Li1.5PO4:Eu x

In this paper for the first time, the compounds Y_{0.5 ? x} Li_1.5PO₄:Eu _x (YLP:Eu) were synthesized by the simple method of solid state reaction. The technology takes much time dry low temperature and little time calcined to synthesize rare earth phosphates. Their structures were analyzed by X-ray powder diffraction experiment. The excitation and emission spectra were measured at room temperature. Y_{0.5 ? x} Li_1.5PO₄:Eu _x shows the characteristice emissions of Eu³⁺ (⁵ D ₀-⁷ F _{1, 2, 3, 4} transitions dominated by ⁵ D ₀-⁷ F ₄). Incorporation of Li⁺ ions in the phosphors promote the crystallinity, reduce the amount of Y₂O₃, and lower the cost of production, moreover, transition dominated by ⁵ D ₀-⁷ F ₄ is important for a number of practical applications.     

The electronic structure of the mixed 5f system UC x N1−x

We present results of electronic structure calculations for UC _x N_1–x obtained with the relativistic Korringa-Kohn-Rostocker Greens function (RKKR-GF) method. While on the anion sites the disorder is treated within the Coherent Potential Approximation (CPA) the small change in the cation potential upon alloying is accounted for in the averaget-matrix approximation (ATA). The presence of strong local spin fluctuations forx0.6 restricts our treatment to the carbon rich phase, and even there we find that the observed linear specific heat coefficient is much enhanced over the bandstructure value. The computed X-ray photoemission spectra vary smoothly with composition the main change consisting in the melting away of the side peak at –3 eV binding energy seen in pure UC with the adjunction of nitrogen.