Temperature dependence of the dislocation starting stress in Ni+17 at.% Cr

The K state can [1–7] have a marked effect on the mechanical parameters of homogeneous solid solutions containing transition elements. The K state is seen as a rise in resistance in low-temperature annealing after quenching from high temperatures and has been ascribed to the formation of short-range order [4, 5, 8–10] or of imperfect long-range order [6, 7]. The mechanical properties are also substantially affected. We have examined the effects on the dislocation starting stress as a function of temperature, as measured by mechanical hysteresis [18–20], in the K state range for Ni+17% Cr.     

Elastic scattering of 6He + 27Al and 7Be + 51V at RIBRAS

A double superconducting solenoid system is installed at the Pelletron Laboratory of the University of S?o Paulo, Brazil. This system allows the production of secondary beams of ⁸Li, ⁶He, ⁷Be and others light exotic nuclei. The first results using this facility are presented.     

Critical surface scattering of x-rays at grazing angles

We study the cross-section for x-ray scattering under conditions of total reflection on surfaces of semi-infinite systems undergoing a continuous phase transition in the bulk. Because of their finite penetration, totally reflected x-rays probe the local surface order parameter and its critical fluctuations. The angular and temperature dependence of the scattered intensity gives experimental access to several critical surface exponents.We consider in particular order-disorder transitions in binary alloys and discuss some requirements on angular and temperature resolution which have to be met by experiments.This work was supported by the Bundesministerium für Forschung und Technologie     

Rise Kinetics of Up-Conversion Luminescence of the LiY0.8Yb0.2F4:Tm3+ (0.2 at %) Crystal with Pulsed Excitation

Physics of the Solid State - The regression analysis of the rise kinetics of up-conversion luminescence of the LiY0.8Yb0.2F4:Tm3+ (0.2 at %) crystal is performed. The kinetics curve is obtained...     

Mechanism for the separation of the Ni3Ti phase in the alloy Ni + 16.3% Cr + 5.8% Ti

A metallographic study has been made of the discontinuous separation of the -phase, and many cases of a branching of its laminae have been found. The effect of the quenching temperature on the discontinuous -phase separation has been studied. Measurements have been made of the microhardness of regions with continuous and discontinuous separation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, Vol. 12, No. 5, pp. 90–93, May, 1969.     

Neutral (V + A) currents and weak lepton-lepton scattering

Symmetrical neutral (V + A) currents are introduced to describe weak-scattering processes not accompanied by change in the nature of the primary particles; a study is made of the spin correlations in neutrino-electron (muon) and electron-electron (positron, muon) scattering by means of a current-current neutral (V + A) scheme.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 64–68, August, 1970.     

On the theory of multiple scattering II. Critical scattering

The systematic theory of multiple scattering which we gave in a previous paper is further elaborated for critical scattering. It is shown that in each order the multiple-scattering intensity near the critical point is in essence a contraction of consecutive uncorrelated single-scattering intensities. The anomaly of the critical depolarization factor is calculated and is found to be in quantitative agreement with recent experimental results. Double scattering corrections to the Ornstein-Zernike plot are discussed.     

Critical regime of proton elastic scattering at the LHC

It is shown that the darkness of the interaction region of protons is governed by the ratio of the slope of the diffraction cone to the total cross section. At LHC energies, it becomes completely absorptive at small impact parameters. The lower limit of the ratio is determined. That imposes some restrictions on its energy behavior. It is argued that the black disk terminology should be replaced by the black torus.     

Critical magnetic surface scattering of neutrons at grazing angles

We derive and discuss the cross-section for elastic and inelastic magnetic scattering of neutrons under conditions of total reflection on the surface of antiferromagnets near the bulk critical temperature.This work was supported by the Bundesministerium für Forschung und Technologie     

Critical hyper-Raman scattering in ferroelectrics

It is shown, that the critical hyper Raman scattering (CHRS) due to the polarization fluctuations in ferroelectrics must increase sharply in the vicinity of the phase transition point T_c. The spectral intensity of the CHRS is expressed explicitely in terms of the imaginary part of low-frequency dielectric permeability ε(ω, T) which permits to use the CHRS for investigations of the critical dynamics near T_c, in particular, in centrosymmetric phase, where the ordinary Raman scattering (of the first order) is forbidden. The calculated CHRS intensity in the BaTiO₃ type crystals is found to be larger by 4–5 orders of magnitude than that in the media investigated earlier.     

Critical scattering in the helimagnets

The critical scattering in the cubic noncentrosymmetrical helimagnets with Dzyaloshinskii–Moriya interaction Fe_1-xCo_xSi (x=0.15, 0.2, and 0.5) has been studied. The samples with small Co concentration (x≤0.2) are found to give the critical scattering with the q-dependence of the typical Lorentzian shape. The critical index of the inverse correlation length ν=0.48±0.05 is found for the compound with x=0.15. The critical scattering above T_C has not been observed for x=0.5. The critical index of the order parameter for the compounds with x=0.15 and 0.20 are equal to β=0.220±0.005 and 0.230±0.007, respectively. The value of β could not been established for x=0.50 because of the first order character of the phase transition. In our opinion, the change of the character of the phase transition from the weakly first order to the real first order type with increase of the Co concentration is related to the decrease of the anisotropy and increase of the itinerancy of the compounds under study.     

Structural phase transitions III. Critical dynamics and quasi-elastic scattering

The dynamical behaviour of systems exhibiting structural phase transitions is reviewed, with particular emphasis on the problems posed by the simultaneous existence of quasi-elastic and phonon features in the dynamic response of the soft coordinates. The results of scattering experiments revealing this phenomenon are surveyed in detail, and are used in an assessment of current theories, subdivided into three categories: those which seek to describe the phenomenon within an extended phonon-based theory; those which attempt to trace the effect to the formation of long-lived clusters of precursor order; and those which associate the quasi-elastic scattering with crystal imperfections.     

Critical scattering in heavy ion reactions

Heavy ion reactions at intermediate energies are mostly described by the VUU equations. Using methods of quantum statistics we derive a useful relation between the width of s.p. states and the density — density correlation function. Near the point of instability from this relation there follows the increase of the collision integral and enhanced equilibration.The author thanks Yu. Kalinovsky, R. Nazmitdinov and M. Di Toro for valuable discussions.     

The kinetics of vacancy annihilation and short range order formation in Ni 11.4 at % Cr

In Ni-Cr alloys annealing after quenching from high temperatures leads to an increase of electrical resistivity which is attributed to a short range order state often denoted as K-state. Here the kinetics of the formation of the K-state in a Ni 10% Cr alloy is investigated by resistivity measurements for different quenching and annealing temperatures. The measurements are analyzed by an extended Schulze-Lücke method [22] which assumes that the rate of resistivity change is proportional to the vacancy concentration and that this rate as well as the rate of vacancy annihilation is described by a chemical rate equation.This analysis allowed a quantitative determination of the parameters determining the kinetics of short range order formation under different vacancy concentrations as well as the kinetics of annealing out of the quenched-in surplus vacancies. Among other quantities this treatment yielded the activation energies for vacancy formation H_F = 1.16 eV, for vacancy migration H_M = 1.56 eV and, as an independent cross check, for self diffusion H_D = 2.73 eV in good agreement with H_F + H_M = 2.72 eV.     

Optical spectroscopy of Cr3+ and Cr3+-Tm3+ in alkaline silicate glasses

Conclusions Alkaline silicate glasses seem to be good candidates as host materials for codoping with Cr³⁺ and Tm³⁺. Cr³⁺ ions occupy mainly low-field sites in them, and their broadband emission overlaps Tm³⁺ excitation. As a consequence very high efficiency for the energy transfer Cr³⁺→Tm³⁺ is achieved with moderate concentrations of these ions. Moreover, the average lifetime of the Cr³⁺ ions in codoped glasses is long enough to allow significant energy storage by flashlamp pumping. Published in Zhurnal Prikladnoi Spektroskopii, Vol. 62, No. 5, pp. 115–120, September–October, 1995.