Drag exerted on dislocation kinks in solid solutions

The theory of drag exerted on dislocation kinks by statistical density fluctuations of impurity atoms is generalized to the case of solid solutions. The conditions of the transition to anomalous kink drift, which is characterized by a nonlinear time (t) dependence of the displacement x (x ～ t ^δ, δ < 1), are obtained. The related consequences for the dislocation mobility and the plastic flow of a solid solution are derived. The theory is illustrated through calculating the concentration dependence of the deforming stress in a Ge_1?c Si _c solid solution.     

Susceptibility of mixed magnetic systems and spin glasses in the presence of chemical short range order

The paramagnetic susceptibility of a magnetic system diluted with non-magnetic atoms is considered. While for the ideal case of random mixing the paramagnetic Curie temperature θ (c_B) varies linearly with concentration c_B of magnetic atoms, already small amounts of chemical short range order lead to pronounced deviations from linearity for c_B → 0. Model calculations of θ (c_B) are presented both for the case of mixed systems with an unmixing and with superstructure ordering transition, calculating the respective Cowley short range order parameters by Monte Carlo methods from pairwise interaction models.     

Concentration dependence of the lattice constant of disordered interstitial solid solutions

The lattice constant of the A-(C) interstitial solid solutions is calculated, and its dependence on the solution composition is determined for arbitrary concentration c_C of the C atoms and their arbitrary distribution over interstices of different types. For c _c ≪1, a linear dependence of the lattice constant on c_C is established, which is also observed experimentally. The influence of temperature redistribution of interstitial atoms over interstices of different types, leading to a specific nonlinear temperature dependence of the lattice constant, is investigated. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 3–5, December, 2005.     

The Curie temperature of dilute ferromagnetic alloys near the percolation threshold

Upper limit for the Curie temperature T_c(x) of the dilute Heisenberg ferromagnet near percolation threshold is derived. Qualitative arguments are put forward that this limit gives a correct dependence of T_c on the concentration of magnetic atoms x.     

Bulk magnetization study of a DyNi5 single crystal

Magnetization and susceptibility measurements were performed on a single crystal of DyNi₅ along the three main symmetry axes of the ortho-hexagonal cell. Below its ordering temperature (T_c = 11.6 K), b and c are respectively the easy and hard magnetization axes. The strong anisotropy originates from the crystalline electric field acting on the 4f electrons of the Dy³⁺ ions. A small magnetization is induced on nickel atoms by the applied field and the exchange interactions with the dysprosium atoms. The crystal field parameters, the molecular field coefficients and the susceptibility of nickel atoms are determined from the experimental data.     

Magnetic structure and preferential occupation of Fe in the composite compound Nd2Co6Fe

The crystal and magnetic structures of the composite compound Nd₂Co₆Fe have been investigated by high-resolution neutron powder diffraction and X-ray powder diffraction. The compound crystallizes in the hexagonal Ce₂Ni₇-type structure consisting of Nd(Co,Fe)₂ and Nd(Co,Fe)₅ structural blocks alternately stacked along the c-axis. Multi-pattern Rietveld refinement of neutron diffraction and X-ray diffraction data at room temperature reveal that substitution of Fe for Co occurs exclusively in the Nd(Co,Fe)₅ structural blocks. The preferential occupation of the Fe atoms in the structure is discussed based on the mixing enthalpy between Nd and Fe atoms and on the lattice distortions. In agreement with the reported magnetic phase diagram of the Nd₂Co_7−xFe_x compounds, magnetic structure models with the moments of all atoms in the ab plane at 300 K and along the c-axis at 450 K provide a satisfactory fitting to the experimental neutron diffraction data. The refinement results show that the atomic moments of (Co,Fe) atoms within the Nd(Co,Fe)₅ blocks decrease slightly with temperature, whereas the atomic moments of Nd in the compound and of (Co,Fe) atoms at the interface between the Nd(Co,Fe)₂ and Nd(Co,Fe)₅ blocks are reduced significantly.     

Photoemission studies of oxygen adsorbed on a LiAl(1 1 0) alloy surface: Role of Li segregation

We investigated that the effect of the number of segregated Li atoms on the rate of oxidation on a LiAl alloy surface. Oxygen molecules adsorbed on the LiAl alloy react with the surface atoms to form stable oxides. The segregated Li atoms at reconstructed surfaces (c(2×2) and (2×1)) enhance the oxidation rate and form stable LiAlO_x and Li₂O. The degree of enhancement of oxidation by segregated Li atoms varies as a function of O₂ exposure and annealing temperature, where the latter is directly related to the mode of surface reconstruction by Li segregation.     

Oxygen ordering and superconductivity in YBa2Cu3O7-δ

Samples of YBa₂Cu₃O_7-δ in which concentration of oxygen was varied by annealing at different temperatures between 200 and 900C followed by quenching to 77 K have been investigated by carrying out measurements of oxygen stoichiometry, room temperature resistivity, superconductivity and crystal structure. It is shown that the overall oxygen stoichiometry alone does not adequately characterize the superconducting and normal state behaviour;T _c, ΔT _cand room temperature resistivity also vary with the heat treatment conditions. This implies a dependence of the physical properties on the details of the distribution of the oxygen atoms. The results show a definite correlation betweenT _cand resistivity hitherto not reported.     

Mössbauer and X-ray diffraction studies of the high-temperature superconductor YBa2(Cu0.99Fe0.01)3O7−δ

The high-T _c superconductor YBa₂(Cu_0.99Fe_0.01)₃O_7?δ was studied by Mössbauer spectroscopy and X-ray diffraction. Spectra taken at room temperature were fitted with four components. Annealing experiments showed that changes in the spectra at various temperatures are related to the nearest environment of Fe atoms rather than to the global lattice symmetry. Based on the results obtained, a model of lattice site assignment of Fe atoms has been proposed.     

Effect of chemisorption on superconductivity of thin metal films

The mechanism of effective attraction of electrons in a thin metal film due to interaction with oscillations of adsorbed atoms is considered. At a sufficiently narrow resonance electron level close to the Fermi level this interaction has a resonance character and, therefore, makes a great contribution to the effective attraction constant. The temperature of the superconducting transition T_c with due regard to this mechanism has been calculated and shown to lead to a considerable increase of T_c.     

Superconducting properties and structural transformations of nitrogen implanted molybdenum films

Evaporated Mo-layers have been implanted with nitrogen ions to maximum fluences corresponding to a concentration of 33 at.%. The implantations lead to an enhancement of the superconducting transition temperature T_c up to 9.2 K and to structural transformations. Both effects are attributed to radiation induced disorder stabilized by the implanted impurity atoms; in particular the increase of T_c is thought to be due to the formation of impurity-defect complexes; channelling experiments support the idea of locally stabilized complexes.     

Structural distortion in the primitive cubic phase of the superconducting/magnetic ternary rare-earth rhodium stannides

A structural distortion in the primitive cubic phase of the SnM₃Rh₄Sn₁₂ compounds with M = La, Ce, Pr, Nd, Sm, and Gd has been detected by electron diffraction and studied by X-ray diffraction. On the contrary, no distortion has been detected for the compounds with M = Eu, Yb, Ca, Sr, and Th. The La, Yb, Ca, Sr, and Th compounds become superconductors with T_c ranging between 8.7 K and 1.9 K, whereas those of Eu and Gd have a magnetic transition at about 11 K. Long exposure (200 hrs.) precession photographs revealed the existence of superstructure spots which can be indexed either on a body-centered cubic cell with a_I = 2a_cp or on a tetragonal cell with a_T = ∫2 a_cp and c_T = c_cp. In this latter case the sample must be considered as twinned and the extra spots would come from three different individuals each having the tetragonal axis along one of the three cubic fourfold axes. However, electron microscope photographs have failed so far to reveal the existence of domains. From the systematic absences it has been determined that the cubic distortion belongs to space group I2₁3 while the tetragonal one to space group P4₂22. The main effect of the distortion is the lowering of the site symmetries which favors the disorder between the cationic tin and the M atoms. The size of these latter atoms does not seem to be an important factor for the distortion. The only feature which separates the distorted compounds from the undistorted ones is the valence of the M atoms. However, no explanation can be offered why the valence plays an important role for the distortion.     

Magnetic properties of NdCo2Ge2, ErCo2Ge2 and PrFe2Ge2 compounds

Magnetometric and neutron diffraction studies of polycrystalline NdCo₂GE₂, ErCo₂Ge₂ and PrFe₂Ge₂ compounds were carried out in the temperature range between 4.2 and 300 K. All samples are antiferromagnetic with Néel temperature 26.5, ～ 4.2 and 13 K, respectively. The RECo₂Ge₂ compounds have collinear antiferromagnetic order of +?+? type. For PrFe₂Ge₂ a sinusoidal magnetic structure is observed. Magnetic moment is localized on RE atoms only and is equal to that of RE³⁺ free ion value. In ErCo₂Ge₂ the magnetic moment of Er atoms is perpendicular to the c-axis, whereas for remaining compounds it is parallel to the c-axis.     

A possible role of the surface free enthalpy excess of solid chemical elements in their melting and critical temperature

A 5–20% increase in the average number of neighbors of an atom (n_avrg) in the surface phase between 0 K and melting temperature T_m makes the solid surface “geometrically impossible” to exist at some temperature called the melting temperature. The latter results in the collapse of crystal structure, beginning with the formation of surface layers of liquid a few atoms thick, in agreement with recently published studies. The critical temperature of solid chemical elements is also discussed. The derivation yields expressions for the heat of melting (ΔH_m), entropy of melting (ΔS_m), melting temperature (T_m) and critical temperature (T_c) in case of pure metals.     

Trapped Bose condensate in a gravitational field

The Bose condensation of atoms in finite 1D and 2D parabolic traps placed in a gravitational field is considered. The distortion of the trap potential in this field is modeled by a combination of two rectangular 1D and 2D traps. The change in the critical temperature T _c is found with regard to the cutoff and renormalization of the spectrum of these model potentials. The shift of the critical temperature T _c in the gravitational field is calculated. The shift sign and magnitude depend on the way of introducing the gravitational field. For a certain choice, three critical temperatures can be sequentially observed. These temperatures can be attributed to three Bose condensations that occur in the cyclic motion of the trap along the Earth (I)-space (II)-Earth (III) route.     

Specific features in the behavior of the central peak in Raman spectra of lithium tantalate

Low-frequency Raman spectra of a LiTaO₃ ferroelectric crystal are studied in the temperature range 300–1273 K. The central peak associated with the relaxation susceptibility of the crystal lattice is recorded throughout the temperature range covered, including the temperatures both substantially lower and considerably higher than the Curie point (T _c = 900 K). The critical slowing down of the relaxation response time predicted by the Ginzburg-Landau-Devonshire theory is observed in the temperature range from 0.9T _c to 1.1T _c . In contrast to the critical slowing down, the width of the central peak γ_R increases below 0.9T _c and decreases above 1.1T _c with increasing temperature.     

On the superconducting transition temperature for metallic nanocrystals

A new approach is proposed for calculating the Debye temperature of a nanocrystal in the form of an n-dimensional rectangular parallelepiped with an arbitrary microstructure and the number of atoms N ranging from 2ⁿ to infinity. The geometric shape of the system is determined by the lateral-to-basal edge ratio of the parallelepiped. The size dependences of the Debye and melting temperatures for a number of materials are calculated using the derived relationship. The theoretical curves thus obtained agree well with the experimental data. The calculated dependences of the superconducting transition temperature T _c on the size d of aluminum, indium, and lead nanocrystals are also in reasonable agreement with the experimental estimates of T _c (d). It is demonstrated that, as the nanocrystal size d decreases, the greater the deviation of the nanocrystal shape from an equilibrium shape (in our case, a cube), the higher the temperature of the superconducting transition T _c (d). The superconducting transition temperature is calculated as a function of the thickness (diameter) of a plate (rod) with an arbitrary length. It is found that a decrease in the thickness (diameter) of the plate (rod) leads to an increase in the temperature T _c (z): the looser the microstructure of the metallic nanocrystal, the higher the temperature T _c (z).     

Mössbauer effect of tin-119 doped in highT c Y−Ba−Cu−O superconductor

Mössbauer measurements at temperatures ranging from 12 K to 300 K have been carried out on highT _c superconductor YBa₂Cu₃O_6+δ samples doped with Sn (5,2 and 0.5 wt%). The temperature dependent recoilless fractionf(T) has a narrow and small drop nearby transition temperatureT _c which is dependent on the Sn-concentration and has a drop between 200–220 K. This indicates the occurrence of phonon softening at these temperatures. The Debye temperature θ_D of the samples containing 5, 2 wt% Sn atoms was determined as (290±25) K and (320±13) K respectively. These results show that phonons may play an important role for electron pairing in novel oxide superconductor.     

Heat capacity, root-mean-square displacements of atoms and thermal expansion coefficient of europium hexaboride

The temperature dependences of the specific heat capacity c _p(T), thermal expansion coefficient α(T) of europium hexaboride, and root-mean-square displacements of Eu and B atoms are determined in the temperature range from helium to room temperature (5–300 K).     

The concentration dependence of the ferroelectric transition temperature of the solid solution of TSCC and TSCB

The concentration dependence of the ferroelectric transition temperature T_c of single crystals of the solid solution tris-sarcosine calcium chloride_1?x bromide_x is studied experimentally by measurement of the static dielectric constant ?_b and described by the empirical relation $\text{T}{}_{\mathrm{c}}\text{(X) = T}{}_{\mathrm{c}}\text{(0) [1?(X/X}{}_{\mathrm{c}}\text{)]}\text{1}\text{2}$. The Ising model with transverse field and the model of coupled anharmonic oscillators are used to explain the concentration dependence of the transition temperature T_c.