Real-space renormalization group investigation of pure and disordered mixed spin Ising models ond-dimensional lattices

The mixed spin Ising model (spins =1/2 andS=1) ond-dimensional hypercubic lattices with nearest-neighbour exchange interactions is studied via a renormalization group transformation in position space. The phase diagrams in (L, K) space, i.e. in dependence of the bilinear (K) and the biquadratic (L) interaction coefficients, are qualitatively different ford=2 andd>2. For any dimensiond however it is found that all transitions are of second order. At zero-temperature (K=,L=), the ferromagnetic order disappears at (L/K)₀=2, which does not depend ond. Using an extension of this real-space renormalization group analysis we study the two-dimensional random disordered version of the above model.L is kept homogeneous and the bilinear interactionsK _ij are assumed to be independent random variables with distributionP(K _ij )=p(K _ij –K)+(1–p)(K _ij –K); whereK>0. The phase diagrams for different values ofp are obtained. At zero temperature, it is found that in the bond diluted model (=0) the value (L/K)₀ depends continuously onp, whereas in the random ±K interactions (=–1) (L/K)₀ is unique and does not depend onp.Supported by the agreement of cooperation between the DFGW. Germany and the CNR-Maroc     

Dielectric properties of (K,Li)TaO3 crystals at superhigh frequencies

The dielectric-resonator method was used to study the dielectric constant and loss of K_1–xLi_xTaO₃ crystals for lithium contents x=0.02, 0.05, and 0.1 in the frequency of 0.5 GHz (in the temperature interval from 4.2 to 300 K) the dielectric constant, the static nonlinearity, and the dynamic nonlinearity were studied, the dielectric properties of K_1–xLi_xTaO₃ are determined simultaneously by the relaxation of impurity ions and by the soft mode, but at helium temperatures the contribution of the soft mode to predominates. The addition of lithium leads to a reduction of at 4.2 K and to a lessening of the steepness of the (t) curve. KTaO₃ crystals alloyed with lithium exhibited exceptionally low losses at helium temperatures (at 4.2 K in the range 12–14 GHz, tan =2 · 10^–5 for x=0.1). With a rise in temperature the loss increased just as in the case of unalloyed KTaO₃, but less steeply. Measurements of crystals with x=0.1 revealed considerable dispersion of the nonlinearity: the coefficient of static nonlinearity was an order of magnitude of magnitude higher than the coefficient of dynamic nonlinearity ₁ ^dyn, which was determined according to the efficiency of generation of the second harmonic.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 3–7, January, 1988.     

Temperature dependence of anisotropy in monocrystalline lithium ferrite films

A study of crystallographic and uniaxial anisotropy in monocrystalline Li-ferrite films in the temperature range 4.2–550°K is presented. The experimental results K₁(T) agree well with calculations based on the one ion model with crystalline field coefficients of a_A=–2.77 ·10^–2 cm^–1, a_B=3.34 · 10^–2 cm^–1. An experimental function K_u(T) is obtained which does not contradict the assumption that anisotropic stresses are responsible for the development of uniaxial anisotropy in Li-ferrite films.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 113–116, August, 1973.     

Spatial distribution of elements of charged Frenkel defect pairs in alkali-halide crystals

Ion vibrations in alkali-halide crystals are investigated by the molecular dynamics method during decay of the molecular ion X ₂ ^– . The high efficiency of the athermal separation of the -, I-pair by displacement of the dynamic crowdion that is formed during decay of the molecular halide ion from the pre defect state or during charge-transfer of the F-, H-pair during their separation is proved. It is established that -, I-pairs are stable to annihilation if at the time of formation they turn out to be separated by a distance exceeding two interstial spacings.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 21–25, March, 1989.     

Preparation and basic physical properties of BiTeI single crystals

A modified Bridgman method is described, which makes it possible to prepare homogeneous BiTeI crystals using excess iodine. At room temperature the values of the electrical conductivity of the crystals range around 2000 ^–1 cm^–1, the Hall constant value about 0·09 cm^–3 coul^–1, the Seebeck coefficient about 50 V K^–1. In connection with the assumption of super-stoichiometric iodine content we expect there exist point defects in the crystals, where Te atoms are replaced with I atoms, which gives rise to electric conductivity. On the basis of the temperature dependence of the electron mobility one can suppose a mixed mechanism of the scattering of the free carriers by the acoustic branch of lattice vibrations and by ionized impurities.     

Mechanism of the influence of ultrasonic and electric fields on the vibrational motion of dislocations in the ultrasonic vibration of alkali-halide crystals

The influence of electric and ultrasonic fields on the vibrational motion of disolocations in alkali-halide crystals was investigated by the method of current-voltage characteristics I(V) and the measurement of the amplitude dependence of the damping Q^–1(₀) and the defect of the modulus M/M(₀) in a composite piezoelectric oscillator. An abrupt decrease of I(t) is observed at amplitudes of the relative deformation ₀=10^–6–10^–5. The internal friction at the same ₀ reveals an amplitude dependence, while establishment of steady values is attained by passage through the maximum of Q^–1(t). The electric field shifts the amplitude of the onset of the abrupt decrease of I(t) into the region of smaller ₀. The displacement of the vibrating dislocation at an amplitude ₀ corresponding to the onset of the singularity of I(t) is near the value of the radius of the Debye-Hückel cloud surrounding it. The characteristic times of the process correspond to the relaxation time of a free charge in an insulator.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 92–98, May, 1987.     

Properties of MOS-structures based on anode-oxidized CdSnAs2

The properties of metal (Al, Au)-anodic oxide-CdSnAs₂ monocrystal structures are studied. It is established that MOS-structures using undoped CdSnAs₂ crystals show a high positive fixed charge in the anodic oxide (N_S 5·10¹² cm^–2) and high surface state density on the oxide-CdSnAs₂ boundary surface (N_SS 2·10¹³ cm^–2·eV^–1). In MOS-structures using diffusion-doped (copper) crystals the sign of the fixed charge is negative (N_S 10¹¹ cm^–2, N_SS 2·10¹² cm^–2·eV^–1). The latter structures show a definite photosensitivity and photomemory. The possibility of effective control of the fixed charge value within the oxide by illumination is shown. The surface state distribution over energy, time constant, and capture section is determined.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 90–93, September, 1982.     

Calculation of the electron-transfer parameters of thin polycrystalline films of metals

Electron transfer parametrs (scattering coefficient R, crystal-boundary transmission coefficient r, film-surface reflection coefficient) are calculated at temperatues t_msm=120, 293, and 573 K using Sc and Re films as an example. The conditions L d and L < d are satisfied, respectively, for these films (L is the mean diameter of the crystals and d is the thickness of the specimens). The films were obtained in a vacuum of 10^–6-5·10^–7 Pa. In the thickness ranges d200–500 Å (Re) and 300–800 Å (Sc), the films had a mean crystallite size of 250 and 600 Å, respectively. Experimental data on the dimensional effect of the temperature coefficient of resistance was analyzed within the framework of the Mayadas-Shatzkes (MS) theory, the model of isotropic carrier scattering, and the three-dimensional Tellier-Tosser-Pichard (TTP) model. It was concluded that the electrophysical properties of Sc films are satisfactorily described by the TTP model, while the MS theory yields exaggerated values of the coefficients r and p. In the case of films of Re, use was made of the isotropic scattering model and an approximation of the three-dimensional model for polycrystalline films. It was found that the coefficients r and R are independent of temperature.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 42–47, June, 1988.We thank V. B. Lobode and L. G. Kulemzina for their assistance in conducting the investigation.     

About X0 (958) spin determination

The likelihood analysis of all the production and decay information about the reaction K^–pX⁰ is suggested, instead of the one-dimensional Adair analysis. It is shown that even the analysis of the present data could yield the likelihood ratio of the 0^– and 2^– hypotheses less than 10^–5.The author is much grateful to V. I.Ogievetsky, V. I.Rud, W.Tybor and A. N.Zaslavsky for valuable discussions.     

Critical behavior of the two-dimensional first passage time

We study the two-dimensional first passage problem in which bonds have zero and unit passage times with probabilityp and 1–p, respectively. We prove that as the zero-time bonds approach the percolation thresholdp _c, the first passage time exhibits the same critical behavior as the correlation function of the underlying percolation problem. In particular, if the correlation length obeys(p) ¦p–p _c¦^–v, then the first passage time constant satisfies(p)¦p–p _c¦^v. At p_c, where it has been asserted that the first passage time from 0 tox scales as ¦x¦ to a power with 0<<1, we show that the passage times grow like log ¦x¦, i.e., the fluid spreads exponentially rapidly.     

Internal friction of alkali-halide crystals with small strains

A study is made of the strain-amplitude dependence of internal friction (IF) and the Young's modulus defect in alkali-halide crystals at amplitudes ₀ 10^–7-10^–6. It is found that If is amplitude-independent at room temperature but becomes dependent on strain amplitude and time when measured in an electric field and at elevated temperatures. In these cases, an asymmetric hysteretic peak is seen on the curve (₀). The peak is due to the thermally activated release of dislocations from weak pinning points as the dislocations vibrate inside ionic atmospheres. The energies of the ruptured bonds (0.03–0.11 eV) are typical of cationic vacancies.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 44–50, January, 1995.     

Two-dimensional mixed spin Ising models with bond dilution and random ±J interactions

Using the finite cluster approximation we study a mixed spin model (spins =1/2 andS=1) on a square lattice with nearest-neighbour and crystal field interactions. The nearest-neighbour couplingsK _ij are assumed to be independent random variables with distribution,P(K _ij )=p(K _ij –K)+(1–p)(K _ij –K), whereK>0. We investigate the cases =0 and =–1 corresponding to bond dilution and to random ±J interactions, respectively. In certain ranges ofp the phase diagrams exhibit tricritical behaviour and reentrance.Supported by the agreement of cooperation between the DFG and the CNR-MarocOn leave from Faculté des Sciences I, Université Hassan II, Casablanca, Morocco; and Laboratoire de Magnétisme Université de Rabat, Morocco     

Characterization of multilayer systems by high-resolution x-ray diffraction

The x-ray diffraction pattern of MBE-grown Ga_1–x Al _x As/GaAs superlattices has been measured withd-spacing resolutiond/d approaching 17000, corresponding to about 8500 Å^–1. The resolution was achieved by employing a highly dispersive monochromator consisting of a pair of fivefold reflecting grooved silicon crystals which delivers about 10⁴ photons s^–1 to the sample. Detailed information like the presence of buffer layers, the molar fractionx, the numberp of layers, the superlattice periodt _p , and the component layer thicknessest ₁ ,t ₂ are extracted from the measured pattern by comparing it with the pattern calculated from the dynamical theory for layer structures. In addition, the influence of disorder on the determination of the above superlattice parameters is investigated by including two different models of disorder.     

Partition temperatures ofK 0 andΛ 0 fromπ + p collisions

The rapidity distributions of inclusive ⁺ p ^–,K ⁰, ⁰ andK ^* (892) atP _1ab=16 GeV/c of CERN experiments, are analyzed using the covariant partition temperatureT _p model. In the fireball system, apart fromK ^* (892),T _p ^* m, no indication of thermal equilibrium, the average momentum ofK ⁰ and ⁰ being approximately equal and in the opposite direction, the entropy density is practically the same forK ⁰ and ⁰ but less than that of ^–. It is found that the principle of equipartition holds rather for the momenta of ,K, and in the fireball system.     

Optical and Photoelectric Inhomogeneity of CdTe:V Crystals

The optical and photoelectric properties of CdTe:V crystals with the doping impurity concentration N _V = 5·10¹⁸–5·10¹⁹ cm^–3 are investigated and the possibility of their use as a photorefractive material is considered. As is seen from the spectra of optical transmission, the crystals of both types possess high transparency (50–65%), which for CdTe:V specimens with N _V = 5·10¹⁹ cm^–3 decreases sharply and in the range 12–14 m does not exceed 5%, whereas for CdTe:V crystals with vanadium concentration of 5·10¹⁸ cm^–3 such a value of transmission remains unchanged up to 25 m, implying a good optical quality of the latter crystals and their possible application in the spectral range 1.06–1.25 m in modern fiber-optic communication lines.     

Gap exponents for percolation processes with triangle condition

A study is made of the gap exponents for percolation processes with the triangle condition in the subcritical region. It is show that the gaps are given by _t =2 fort=2, 3,. Scaling theory predicts thatP _p (¦C ₀¦S(p))–(p _c –p) andE _p (1/¦C ₀¦; ¦C ₀¦S(p))–(p _c –p)³, whereS(p) is the typical cluster size. It is found that (p _c –p)P _p (|C ₀S(p) ^1–)(p _c –p)^1–2 and (p _c –p)³E _p (1/|C ₀|;|C ₀|S(p) ^1–))(p _c –p)^3–4.     

Time component of the mirror beta decays of12B and12N

Beta-ray angular distributions from aligned¹²B and¹²N have been observed in order to study the meson-exchange effect in a nucleus. Alignment-correlation coefficients for¹²B and¹²N were determined to be _–=–(0.0174±0.0059)%/MeV and ₊=–(0.2774±0.0086)%/MeV, respectively. These values are fairly consistent with the previous values, while the reliability was greatly improved. From these coefficients, the nuclear parametery, i.e. the ratio of the time component of the axial vector currents of the Gamow-Teller matrix element, was determined to bey=4.13±0.15. The value is enhanced by 33% from that calculated by use of the impulse approximation, which clearly shows a considerable effect due to meson-exchange currents.     

Spin-orbit splitting in σ-hypernuclei

The shell model analysis of experimental 0⁺, 2⁺ spectra of and as obtained from (K ^–, ⁺) in-flight and stoppedK ^– reactions, is performed. It suggests a large (twice that for nucleons) spin-orbit splitting of hyperon levelsp _3/2 andp _1/2. Certain constraints on effective central N interaction arise, as well. Some experiments are put forward to further test this value and to extend it to hypernuclei.Presented at the symposium Mesons and Light Nuclei, Bechyn, Czechoslovakia, May 27–June 1, 1985.     

Nonlinear Optical Characteristics of BSO and BGO Photorefractive Crystals in Visible and Infrared Ranges

Nonlinear refraction, nonlinear absorption and optical limiting in photorefractive crystals Bi₁₂SiO₂₀(BSO) and Bi₁₂GeO₂₀(BGO) at the wavelengths of 1064 and 532 nm were investigated. It was shown that both BSO and BGO crystals possess by positive nonlinear refraction in two investigated spectral ranges (n ₂ ^BSO=(2.5 ± 0.5)× 10^–12 esu, n ₂ ^BGO=(6.3 ± 1.3)× 10^–12 esu at equals 1064 nm; n ₂ ^BSO=(4.4 plusmn; 0.9)× 10^–12 esu, n ₂ ^BGO=(7.4 ± 1.5)× 10^–12 esu at = 532 nm). The nonlinear absorption was due to three-photon absorption at the wavelength of 1064 nm ( ⁽³⁾ _BSO=(2.5 ± 0.8)× 10^–20cm³W^–2, ⁽³⁾ _BSO=(4.4 ± 1.3)× 10^–20cm³W^–2) and two-photon absorption at the wavelength of 532 nm ( ⁽²⁾ _BSO=(2 ± 0.4)× 10^–9cm W^–1, ⁽²⁾ _BGO=(3.7 ± 0.7)× 10^–9cm W^–1).     

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.