Long-Period Structures in Noble-Metal Alloys: New Aspects

Proceeding from precise calculation of the electronic structure and the generalized electronic susceptibility, a systematic study of the relationship between the nesting properties of the Fermi surface (FS) and the character of long-period ordered superstructures (LPSs) in noble-metal alloys, Cu–Au, Cu–Pd, and Ag–Mg, is performed. An answer is provided as to why in Cu–Au, Cu–Pd, and Cu–Pt alloys so-called incommensurate LPSs, characterized by an irrational period and smeared antiphase boundaries, are formed. The reasons for occurrence of such LPSs within a short temperature range only are clarified. The dependence of the long period M on the long-range order parameter in Cu–Au alloys is accounted for. The causes for formation of two-dimensional LPSs in Au₃Cu and Cu₃Pd alloys are found. It is shown that their stability may be explained for by energy gaps opening on coinciding patches of FS in two mutually perpendicular directions. Arguments are drawn in favor of the fact that among quasicrystalline substances, phases with incommensurate LPSs occupy an intermediate position between incommensurate systems and quasicrystals. An attempt is made to identify the reasons for qualitatively different behavior of LPSs with large and small superperiods. It is shown that the dependence of LPS's behavior on the value of 2M is controlled by the quality of nesting on FS and the nesting vector sensitivity (2k _F) to variations in the long-range order parameter .     

Electrical properties of p-type ZnGeP2 irradiated by H+ ions

The properties of p-type ZnGeP₂ [p₀=(5–10)·10¹⁰ cm^–3, ₀=(2–5)·10^–7 (·cm)^–1], irradiated with H⁺ ions [E=5 MeV, T_irr=300 K, D=(1·10¹²–1.7·10¹⁶) cm^–2] are studied. An increase in the resistivity (to gr_max - 5·10¹¹ ·cm) and subsequent reduction in for large currents of H⁺ ions ( - 9·10⁸ ·cm for D - 1.7·10¹⁶ cm^–2), is observed in irradiated crystals. The resistivity of irradiated p-type ZnGeP₂ is found to be very sensitive to hydrostatic pressure [(4–5)·10^–5 bar^–1]. The annealing of radiation defects in the temperature interval (20–600) °C is examined.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 91–93, October, 1991.     

IR Reflection of Thin Bi4Ge3O12 Films

The spectra of IR reflection of the systems thin Bi₄Ge₃O₁₂ film–substrate made of molten -SiO₂ quartz in the region 10–1600 cm^–1 at 295 K are investigatedterpretation of fundamental vibrations in the region 10–800 cm^–1 and two phonon processes in the region 800–1600 cm^–1 are considered.     

Electronic transport properties of amorphous NiTi alloys

Measurements of electrical resistivity, magnetoresistance and Hall effect on amorphous Ni_100–x Ti _x alloys withx=39, 43, 60, 65, 70, and 76 are presented. The resistivity at 4.2 K is in the range 210 cm to 325 cm. The Hall coefficient turns out to be negative for low Ti contents and positive for high Ti contents with values around –6×10^–11 m³/As and +7×10^–11 m³/As, respectively. At low temperatures, the temperature and magnetic field dependences of the quantities measured are analysed and discussed in terms of weak localisation and electron-electron interaction effects.     

Rapid thermal annealing and titanium silicide formation

Titanium silcides have been formed on monocrystalline (111) silicon substrates by rapid thermal annealing (RTA) of Ti layers deposited on Si at 700–800 °C for 1 to 240 s. The phase composition is dependent on the annealing temperature and time: at 700° and 750 °C for short annealing, TiSi and TiSi₂ are observed. At 800 °C and by increasing the exposure time at 700 ° and 750 °C, only TiSi₂ is detected. The growth of the total silicide thickness is found to be faster for RTA than for conventional furnace annealing and governed by two different mechanisms depending on the phases formed: in the range 700–750 °C, and 750–800 °C, activation-energy values of 2.6 ± 0.2 and 1.5 ±0.2 eV are found, respectively.For a thin deposited Ti layer (< 100 nm), the whole Ti is finally transformed into TiSi₂ with 20@ cm resistivity. For thicker Ti thicknesses, titanium oxide stops the reaction.     

Some optimum parameters of golay detector of infra-red radiation

The relation for the sensitivity of the pneumatic detector based on the analysis of the thermal, pneumatic and optical parts of the instrument is derived. From this relation the optimum diameter of the flexible mirror and the optimum gas pressure in the detector cell are calculated.List of symbols used B brightness [sb] - C thermal capacity of absorbing membrane [cal cm^–2 grad^–1] - c specific heat of gas in cell [cal g^–1 grad^–1] - D diameter of grid [cm] - d diameter of flexible mirror [cm] - E illumination [ph] - F ₂ area of light spot [cm²] - F ₃ area of flexible mirror [cm²] - f,f ₁ focal distance [cm] - f ₀ chopping frequency of infra-red radiation [sec^–1] - h depth of cell [cm] - k radiation cooling constant [cal cm^–2 sec^–1 grad^–1] - k ₀ reflectivity of flexible mirror - k ₁,k ₂,k ₃,k ₄ constants - L total length of edge of half of grid [cm] - l distance between lens and flexible mirror [cm] - M, N constants - m refractive power of flexible mirror [cm^–1] - n number of grammolecules of gas in cell - P radiative power [cal sec^–1] - p,p ₀,p pressure, atmospheric pressure, change in pressure [dyne cm^–2] - R gas constant [erg grad^–1] - r coordinate - r ₀ radius of flexible mirror [cm] - r ₁,r ₂ radii of beam of rays [cm] - t tension of flexible mirror [dyne cm^–1] - , T, T _f absolute temperature and its change [grad] - V, V volume and increase in volume [cm³] - v coordinate - v ₀ maximum height of deflection of flexible mirror [cm] - x, x distance of object and image from focal planes [cm] - , angles - specific weight of gas at pressure of 1 atm. [g cm^–3] - absorption coefficient of infra-red radiation - thermal conductivity of gas [cal cm^–1 sec^–1 grad^–1] - v number of lines of grid per 1 cm [cm^–1] - radius of light spot on grid [cm] - T time constant [sec] - ₀, ₁, ₂, ₃ light flux and its change [lm]     

Selectively dopedn-AlxGa1−xAs/GaAs heterostructures with high-mobility two-dimensional electron gas for field effect transistors

The transport properties of warm and hot electrons in selectively dopedn-Al _x Ga_1–x As/GaAs heterostructures created by electric fields up to 500 V/cm were studied by Hall effect, conductivity, and Shubnikov-de Haas measurements at lattice temperatures from 4.2 to 300 K. Hall measurements revealed a substantial decrease of electron mobility and also of sheet electron concentration at 77 K with enhanced electric field. The accelerated 2D electrons are partly scattered into the low-mobility first excited (E ₁) subband, and they are partly trapped in immobile states located in the Al_xGa_1–xAs near the interface. Consequently, two differentv(E) characteristics were obtained at 77 K. The 2D electrons populating only the lowest (E ₀) subband exhibit a velocity of v-2×10⁷ cm/s at 500 V/cm, while the averaged velocity due to all electrons reaches a value of v-1.5×10⁷cm/s at 500 V/cm. The analysis of the Shubnikov-de Haas oscillations and Fast Fourier transformation of the data manifested that the 2D electrons are very rapidly accelerated at 4.2 K and achieve electron temperatures much higher than the lattice temperature at electric fields as low as 1 V/cm. The major cooling process for these electrons is scattering into the low-mobilityE ₁ subband.     

Primary Matter Creation in a Weyl–Dirac Cosmological Model

In the framework of an integrable Weyl–Dirac (W–D) theory a cosmological model is proposed. It describes a universe that began its expansion from a primary pre-Planckian geometric entity containing no matter. During the pre-Planckian period, from R ₀ =5.58×10 ^–36 cm to R_I=5.58×10 ^–34 cm, this embryonic universe has undergone a very rapid expansion and cosmic matter was created by geometry. At R_I the universe was already filled with matter having the Planckian density _P and being in the state of prematter (P=–), while the Weylian geometric elements were insignificant. This state is the Planckian egg that has served as the initial state of the singularity-free cosmological model ⁽¹⁾ considered in the framework of Einstein's general theory of relativity. The W–D character of the geometry and the cosmological constant are significant in the pre-Planckian period during the matter creation. In the dust-dominated period a relic of the W–D geometry causes a global dark matter effect. In between the pre-Planckian and dust period one has Einstein's framework and is negligible.     

Nonlinear refraction and nonlinear absorption of As2S3 aqueous solution

We report for the first time to our knowledge on the preparation of colloidal solution of chalcogenide semiconductor As₂S₃ by laser ablation and the measurements of its nonlinear-optical characteristics using Z-scan method at the wavelength of Nd:YAG laser radiation ( = 1064 nm, = 25 ns). The nonlinear refractive index was measured to be –7.5 × 10^–18 m² W^–1. Nonlinear absorption coefficient of chalcogenide solution was measured to be 1 cm GW^–1.     

The Ising Model on a Quenched Ensemble of c=−5 Gravity Graphs

We study with Monte Carlo methods an ensemble of c=–5 gravity graphs, generated by coupling a conformal field theory with central charge c=–5 to two-dimensional quantum gravity. We measure the fractal properties of the ensemble, such as the string susceptibility exponent _s and the intrinsic fractal dimension d _H. We find _s=–1.5(1) and d _H=3.36(4), in reasonable agreement with theoretical predictions. In addition, we study the critical behavior of an Ising model on a quenched ensemble of the c=–5 graphs and show that it agrees, within numerical accuracy, with theoretical predictions for the critical behavior of an Ising model coupled dynamically to two-dimensional quantum gravity, with a total central charge of the matter sector c=–5.     

Static and dynamic properties of XY systems with extended defects in cubic anisotropic crystallines

Static and dynamic critical behavior ofXY systems in cubic anisotropic crystallines, with extended defects (or quenched nonmagnetic impurities) strongly correlated along _d -dimensional space and randomly distributed ind – _d dimensions, were studied. These extended defects make the systems coordinate anisotropic, resulting in unique critical behavior due to competition between the cubic anisotropy and the coordinate anisotropy. The systems were analyzed by an ^1/2 (4 – d) type of expansion with double expansion parameters based on a renormalization-group (RG) approach. Critical exponents were calculated near the second-order phase transition point and the behavior of the first-order transition was evaluated near the tricritical point.     

The oscillator representation and the stability of three-body Coulomb systems

Within nonrelativistic quantum mechanics the Wick-ordering method, called the oscillator representation, is suggested for calculating the energy spectrum for a wide class of potentials allowing the existence of a bound state. As test cases, anharmonic (V(r)=r ²) and screened Coulomb potentials are considered. In particular, the method is applied to three-body Coulomb systems to obtain the dependence of the bound-state energy on the masses and charges of the particles. The calculations of the bound-state energies for the moleculesH ^–=(pee),H ₂ ⁺ =(ppe), (e ^–e^–e⁺) and (pp), (dd), (dt) prove the accuracy of the zeroth approximation to be better than one per cent. For the three-body Coulomb system with charges +, –, – and arbitrary masses the region of stability is determined. For the systems (pe ^–C⁺), (A ⁺e^–e⁺), and (pB ^–e^–) the critical masses are calculated to beM _c=1.945m_e,M _A=4.350m_e andM _B=1.575m_e. It turns out that the system (pe ^–e⁺) is unstable.     

Critical electric fields of AlGaN in AlGaN-based vertical conducting diodes on -SiC substrates

We have succeeded in obtaining high critical electric fields from AlGaN layers using the p-InGaN/i-Al_xGa_1−xN/n-Al_xGa_1−xN (x=0–0.22) vertical conducting diodes grown on n-SiC substrates by low-pressure metalorganic vapor phase epitaxy (MOVPE). The breakdown voltage (V_B) increases with increasing Al composition of the AlGaN layer. The corresponding critical electric fields are calculated to be 2.4 MV/cm for GaN and 3.5 MV/cm for Al_0.22Ga_0.78N. The critical electric field is proportional to the bandgap energy to a power of 2.5. This bandgap energy dependence is much stronger than that in the empirical expression proposed by Sze and Gibbons. The figure of merit, , increases with increasing Al composition, indicating the AlGaN-based p–i–n diodes are promising for high-power and high-temperature electronic device applications.     

Special features of the operation of an electron-ionization CO2-laser at high pressures

A study was made of the excitation of the working medium of a CO₂-laser at a pressure of up to 8 atm by an electron beam of duration 15 · 10^–9 sec. Results are given of the energy dissipation in a CO₂:N₂-13 gas medium in the whole range of pressures with E₀/p = (18–38) V /cm · mm Hg. The dependences of the time of appearance of the stimulated radiation with respect to the current pulse are given.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 40–44, April, 1976.     

Optical and electron spin resonance spectra ofZ 2(Eu) centers in alkali halide crystals

The ESR spectra ofZ ₂(Eu)-centers in KBr and KCl are presented and correlated to the optical absorption and the electron spin resonance of the Europium vacancy complex. The fine structure of theZ ₂ spectra can be described by a Spin-Hamiltonian with an axialb ₂ ⁰ -parameter only. At room temperature it amounts to 1,761 10^–4 cm^–1 for KBr, 1,854 10^–4 cm^–1 for KCl and 1,453 10^–4 cm^–1 for RbCl. Accordingly theZ ₂(Eu)-center must posses a strict axial 100-symmetry in these crystals.     

The magnetic anisotropy of manganate ferrites

The anisotropy constants K₁ for systems Mn_xFe_3–xO₄, with 1x 1·8, are calculated on the basis of the one-ion model and it is shown that the anomalous temperature dependence of the constant K₁ can be explained by the presence of Mn³⁺ ions in octahedral positions. The influence of the spin order on the magnetic anisotropy and the uniaxial anisotropy in systems Mn_xFe_3–xO₄ are discussed.     

Reflection Spectra of Single Crystals of La2CuO4−x ‐Type Metal‐Oxide Compounds

We have investigated the reflection spectra in polarized light at 80 and 295 K in an energy region of 1.0–4.8 eV for the single crystals of ternary and quaternary copperoxide systems with the participation of rareearth elements (La₂CuO_4–x , LaCuO₄, Pr₂CuO_4–x , GdBa₂Cu₃O_7–x , and LaSrCaCuO).     

Energy-band structure of semiconductors having the chalcopyrite lattice II. MgSiP2, ZnGeP2, ZnSiAs2, and CdSiP2

The pseudopotential method is used to calculate the band structure of MgSiP₂, ZnGeP₂, ZnSiAs₂, and CdSiP₂. The valence band of these compounds is shown to consist of several allowed subbands separated by energy gaps. There are two levels at the top of the valence band, slightly separated in ZnGeP₂ and ZnSiAs₂ and well-separated in MgSiP₂ and CdSiP₂ — in which there is significant tetragonal compression of the lattice. The effective electron masses at the bottom of the conduction band turn out to be anisotropic, with m/m 7–10.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 6, pp. 95–100, June, 1970.     

The ESR spectrum of oxygen vacancies in sodium tellurite glasses

A theoretical analysis is presented for the shape of the ESR spectrum of the radiation-induced centers of oxygen vacancy type in sodium tellurite glasses. It is found that a qualitative agreement between the theoretical and observed curves can be obtained only if one assumes a completely anisotropic g factor and an inhomogeneous structure for the paramagnetic centers, whose parameters are found as g_x = 1.930; g_y = 1.971; g_z = 2.046; x ² >=4–10^–4-2.5·10^–3; y ² > = 4.9·10^–8-1·10^–6; z ² > = 2.2·10^–4-9·10^–4; d ² > = 36–100 G². It is concluded that the glass composition affects the center nonuniformity.Translated from Izvestiya VUZ, Fizika, No. 3, pp. 63–68, March, 1973.