Research of the behaviour of O chemisorption on the (110) surface of Rhx--Pt1－x alloy

An atomic group model of the disordered binary alloy Rhx-Pt1-x has been constructed to investigate surface segregation. According to the model, we have calculated the electronic structure of the Rhx-Pt1-x alloy surface by using the recursion method when O atoms are adsorbed on the Rhx-Pt1-x （110） surface under the condition of coverage 0.5. The calculation results indicate that the chemical adsorption of O changes greatly the density of states near the Fermi level, and the surface segregation exhibits a reversal behaviour. In addition, when x 〈 0.3, the surface on which O is adsorbed displays the property of Pt; whereas when x 〉 0.3 it displays the property of Rh.     

Surface segregation in CuxNi1−x alloy

The tight-bonding Hartree Hamiltonian and a mixed Bethe-lattice approximation is used to study the surface segregation phenomenon of Cu_xNi_1−x alloy. It is found that Ni atoms do segregate to the surface layer in the Cu-rich range (0.75 < x < 1). This is in contrast with previous theories which predict the enrichment of Cu atoms at the surface for all bulk Cu concentrations. However, the present theory agrees reasonably well with the recent experimental observation of a crossover at x = 0.84 by Hashizume et al. [1].     

Influence of Covalence on the Metal-Insulator Temperature in the CuIr 2 (S 1− x Se x ) 4 Spinels

Influence of the covalence on the metal-insulator (M-I) temperature in the CuIr 2 (S 1 m x Se x ) 4 spinel series is considered. For this purpose a vacancy model and the effective number of valence electrons per molecule are used. Calculations of the vacancy model parameters, i.e., the ion packing coefficients, the filling coefficient of the unit cell, the difference of the Pauling electronegativities and the vacancy parameter suggest that the covalence increases with increasing selenium concentration x , leading to a decrease of the metal-insulator temperature. It was observed that the metal-insulator temperature is related to the effective number of valence electrons per molecule, suggesting that with increasing covalence, less and less valence electrons take part in the metal-insulator transition. These effects are explained within the framework of the mixed valence of the iridium ions including structural defects.     

Influence of C on Ge incorporation in the growth of Ge-rich Ge1−x−ySixCy alloys on Si (100)

Amorphous carbon nitride powders are successfully produced by ammonothermal synthetic routes. The existence of the chemical bonding of C≡N in the powders is confirmed by Raman and infrared spectra. Raman spectra of various samples show that the G-band frequency shifts to higher energies, and the intensity ratio I_D/I_G decreases as the N content increases. The Knoop hardness of the sintering sample is up to 1200 kgf/mm². The hardness of the sample increases with increasing of the N content. Received: 24 May 2000 / Accepted: 26 May 2000 / Published online: 2 August 2000     

High-pressure calculations of the elastic properties of ZnSx Se 1−x alloy in the virtual-crystal approximation

We report first-principles calculation results on the structural and elastic properties of ZnS _x Se_1?x alloy. The calculations done using density functional theory within the local density approximation and employing the virtual-crystal approximation. It is found that the lattice parameter, the phase transition pressure, and the elastic constants (and their derivative with respect to the pressure) follow a quadratic law in x.     

Amplification and generation of surface plasmon polaritons in a dielectric−HTS film−dielectric structure

Propagation of surface polaritons in the dielectric?HTS?dielectric structure is investigated in the temperature region of T <T _c/2, where the structure can be considered nonabsorbing. It is shown that in the region where the polariton wave is appreciably slowed down it can effectively interact with a flux of charged particles and be amplified due to the energy transfer from the drift current wave. It is also shown that a periodic substrate can cause generation of the surface polariton wave in the structure.     

Antihysteresis of the electrical resistivity of graphene on a ferroelectric Pb(Zr x Ti1 − x )O3 substrate

A quantitative model is developed to explain the antihysteretic behavior of the electrical resistivity of graphene on a ferroelectric Pb(Zr _x Ti_{1 ? x} )O₃ substrate as a function of the gate voltage. The model takes into account the trapping of the electrons from the graphene layer by the states related to the graphene-ferroelectric interface. The finite energy gap of impurity states is also taken into account, which makes it possible to describe the well-known experimental dependences, including an increase and the subsequent saturation of a “memory window” with the switching gate voltage. The obtained estimates can be important for creating next-generation nonvolatile memory elements, which use the two stable values of electrical resistivity (one of them is attributed to logical “0” and the other, to “1”) that result from the antihysteresis effect.     

Dependence of the irreversible magnetization in Cu1−xCdxFe2O4 on the magnetizing field

The mean potential difference of Barkhausen jumps (V_B) and the frequency of Barkhausen jumps (F_B) were studied for polycrystalline samples of Cu_1−xCd_xFe₂O₄, where X = 0, 0.1, 0.2, 0.3 and 0.5. Both V_B and F_B have been measured against the magnetizing field. CuFe₂O₄ shows the highest values of V_B and F_B. These values decrease with an increase of cadmium content. Also the effect of the magnetizing frequency on V_B has been studied. V_B vanishes at certain values of the magnetizing frequency. The magnetizing frequency at which V_B vanishes decreases with the increase of cadmium content. The behaviour of V_B and F_B is attributed to the change of the magnetic ordering of the sample and to the effect of eddy current on the domain wall motion.     

The growth of Ag films on a TiO2(110)-(1×1) surface

We report here the growth of Ag film and its thermal stability on the TiO₂(1 1 0)-(1×1) surface using combination techniques of low-energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). At a surface temperature as low as 125 K, a 2D growth of Ag films seems to occur for submonolayer coverages up to ∼0.8 ML. Annealing of low temperature grown Ag films to 500 K for coverage of 1–2.4 ML would result in the formation of metastable Ag layers with rest of Ag forming 3D needle-like islands on top of this Ag film.     

Discontinuity of the magnetization in one-dimensional 1/¦x−y¦2 Ising and Potts models

Results from percolation theory are used to study phase transitions in one-dimensional Ising andq-state Potts models with couplings of the asymptotic formJ _x,y const/¦x–y¦². For translation-invariant systems with well-defined lim _x x ² J _x =J ⁺ (possibly 0 or ) we establish: (1) There is no long-range order at inverse temperatures withJ ⁺1. (2) IfJ ⁺>q, then by sufficiently increasingJ ₁ the spontaneous magnetizationM is made positive. (3) In models with 0<J ⁺< the magnetization is discontinuous at the transition point (as originally predicted by Thouless), and obeysM( _c )1/( _c J ⁺)^1/2. (4) For Ising (q=2) models withJ ⁺<, it is noted that the correlation function decays as xy()c()/|x–y|² whenever< _c . Points 1–3 are deduced from previous percolation results by utilizing the Fortuin-Kasteleyn representation, which also yields other results of independent interest relating Potts models with different values ofq.     

The influence of the local surrounding on the hyperfine interactions in Zr(Fe1−x Ni x )2 compounds

Mössbauer spectroscopy and the TDPAC method have been used to study Zr(Fe_1–x Ni _x )₂ compounds forx0.30. The hyperfine magnetic field at the Fe sites and the quadrupole splitting as functions of nickel concentration were analysed by use of⁵⁷Fe Mössbauer spectroscopy. Values of the internal magnetic field on¹⁸¹Ta nuclei have been found by means of the TDPAC method.     

Band-gap bowing calculation of SixSn1−x alloy

Using ab initio pseudopotential method within the local density approximation we have investigated the electronic properties of Si_xSn_1−x semiconducting alloy. The bowing parameter of the band-gap energy variation with alloy concentration is found to be large. We also analyzed its origin in terms of chemical and structural effects.     

Observation of negative photoconductivity in (ZnO)x(CdO)1−x films

In this study, (ZnO)_x(CdO)_1?x films were prepared by ultrasonic spray pyrolysis (USP) technique at a substrate temperature of 400 °C. X-ray diffraction patterns of the films indicate that the (ZnO)_x(CdO)_1?x films have hexagonal wurtzite and cubic structure for the constituent materials. A decrease in the average transmission with increasing quantity of the cadmium acetate dehydrate in the sprayed solution was observed. The photoconductivity transients were performed using UV light, which has 360 nm wavelength. After light cut off, conductivity changed slowly, and the decay time was thousands of seconds. The films with x=0.2 and 0 exhibited negative photoconductivity. Temperature-dependent photoconductivity and dark conductivity measurements were performed and negative photoconductivity was also observed for the same films (x=0.2 and 0). Photoluminescence measurements were performed and band-to-band excitation energies of (ZnO)_x(CdO)_1?x films were determined. Band gap of the pure CdO film was found as 3.11 eV, interestingly.     

Separation of the contributions to the magnetization of Tm1 − x Yb x B12 solid solutions in steady and pulsed magnetic fields

The magnetization of substitutional Tm_{1 ? x} Yb _x B₁₂ solid solutions is studied in the composition range 0 < x ≤ 0.81. The measurements are performed at low temperatures (1.9–300 K) in steady (up to 11 T) and pulsed (up to 50 T, pulse duration of 20–100 ms) magnetic fields. An analysis of the experimental data allowed the contributions to the magnetization of the paramagnetic phase of the Tm_{1 ? x} Yb _x B₁₂ compounds to be separated. These contributions include a Pauli component, which corresponds to the response of the heavy-fermion manybody states that appears in the energy gap in the vicinity of the Fermi level (density of states (3?4) × 10²¹ cm^?3 meV^?1), and a contribution with saturation in high magnetic fields attributed to the localized magnetic moments ((0.8–3.7)μ_B per unit cell) of the nanoclusters formed by rare-earth ions with an antiferromagnetic interaction.     

Studies on the complex behavior of optical phonon modes in wurtzite (ZnO)1−x (Cr2O3) x

This paper reports on the use of phonon spectra obtained with laser Raman spectroscopy for the uncertainty concerned to the optical phonon modes in pure and composite ZnO_1?x (Cr₂O₃) _x . Particularly, in previous literature, the two modes at 514 and 640 cm^?1 have been assigned to ZnO are not found for pure ZnO in our present study. The systems investigated for the typical behavior of phonon modes with 442 nm as excitation wavelength are the representative semiconductor (ZnO)_1?x (Cr₂O₃) _x (x = 0, 5, 10 and 15 %). Room temperature Raman spectroscopy has been demonstrated polycrystalline wurtzite structure of ZnO with no structural transition from wurtzite to cubic with Cr₂O₃. The incorporation of Cr³⁺ at most likely on the Zn sub-lattice sites is confirmed. The uncertainty of complex phonon bands is explained by disorder-activated Raman scattering due to the relaxation of Raman selection rules produced by the breakdown of translational symmetry of the crystal lattice and dopant material. The energy of the E ₂ (high) peak located at energy 53.90 meV (435 cm^?1) due to phonon–phonon anharmonic interaction increases to 54.55 meV (441 cm^?1). A clear picture of the dopant-induced phonon modes along with the B ₁ silent mode of ZnO is presented and has been explained explicitly. Moreover, anharmonic line width and effect of dislocation density on these phonon modes have also been illustrated for the system. The study will have a significant impact on the application where thermal conductivity and electrical properties of the materials are more pronounced.     

Experimental and theoretical investigations of the Knight shift of Pd and Ag in the alloy system Ag x Pd1−x

The Knight shift of Pd in Ag _x Pd_1–x has been determined for concentrationsx0.2. In full accordance with the expectations based on the behaviour of the magnetic susceptibility, it was found that the Knight shift of Pd is rapidly reduced in magnitude by adding Ag to Pd. To allow for a detailed interpretation of this finding, we have performed Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) band structure calculations for Ag _x Pd_1–x . These calculations clearly demonstrate that the decrease in spin susceptibility with increasingx is accompanied with a decrease in core polarization. In contrast to Pd, the negative Knight shift of Ag on the Pd-rich side of the system is caused by the valence band contribution, as it is demonstrated by our calculations. This is caused by an intersite effect in analogy to the transferred hyperfine field found for non-magnetic elements dissolved in a magnetic host.     

On the nature of a lattice vibrational mode at a frequency of 135 cm−1 in Hg1 − x Cd x Te alloys

The vibrational spectrum of a cadmium impurity atom in the HgTe crystal has been calculated using the microscopic theory of lattice dynamics in the approximation of a low impurity concentration. Within this theory, the behavior of the local and quasi-local modes induced upon substitution of the lighter Cd atom for the Hg atom in the region of the zero or very low one-phonon density of states in the HgTe crystal has been considered. It has been found that, apart from the local mode at a frequency of 155 cm^?1, the calculated vibrational spectra exhibit a weak (but clearly pronounced) feature at a frequency of 134 cm^?1, which coincides with the experimentally observed vibrational mode (the “minicluster” mode) at a frequency of 135 cm^?1 in the Hg_{1 ? x} Cd _x Te (x = 0.2–0.3) alloys at 80 K.     

Mössbauer and magnetization studies on the ferromagnet Fe3−x Ru x Si

⁹⁹Ru and⁵⁷Fe Mössbauer spectroscopic studies were carried out on ternary intermetallic compounds containing ruthenium, Fe_3–x Ru _x Si, within the concentration range 0.1x1.5. Magnetization of the samples was also measured in the temperature range between 4 K and room temperature.⁹⁹Ru Mössbauer spectra ofx=0.5 and 1.0 were fitted satisfactorily with a broad component ofH _hf, the peak positions of which were 340 and 270 kOe, respectively.