On electron-electron scattering in molybdenum

Results of the doppleron damping measurements of the temperature dependence of the scattering rate $\text{v}$(T) for carriers on the non-central sections of the different sheets of the Fermi surface (FS) of molybdenum are presented. The αT² law has been found over the temperature range 1.5–5 K, with α practically the same for the electron jack and hole octahedron. The values of α are in a good agreement with those obtained from the radio-frequency size effect data for the central sections of the FS. From the analysis of these data and results obtained by other experimental methods it was concluded that the quadratic terms in the temperature dependence of the scattering rate in molybdenum is due to the electron-electron scattering. A discussion of the observation of electron-electron scattering in the non-transition metals is given.     

Ising ferromagnet with biquadratic exchange

A spin one Ising system with biquadratic exchange, is investigated, using Green's function technique in random phase approximation (RPA). Transition temperature T_c and <(S^z)²> at T_c, are found to increase with biquadratic exchange parameter α for sc, bcc and fcc lattices. The variation of <(S^z)²> at T_c with α is found to be the same for the above lattices.     

Temperature dependence of quantal and thermal dampings of the hot giant dipole resonance

A systematic study of the damping of the giant dipole resonance (GDR) in ⁹⁰Zr, ¹²⁰Sn and ²⁰⁸Pb as a function of temperature T is performed. The double-time Green function technique is employed to determine the single-particle and GDR dampings. The single-particle energies, obtained in the Woods-Saxon potential for these nuclei, are used in the calculations. The results show that the coupling of collective vibration to the pp and hh excitations, which causes the thermal damping width, is responsible for the enlargement of the total width with increasing temperature up to T ≈ 3MeV and its saturation at higher temperatures. The quantal width, which arises from the coupling of the collective mode to the ph excitations decreases slowly with increasing temperature. The effect of single-particle damping on the GDR width is small. The results are found in an overall agreement with the experimental data for the GDR width, obtained in the inelastic α scattering and heavy-ion fusion reactions at excitation energies E^* ⩽ 450 MeV. At high excitation energies (E^* > 400 MeV) a behavior similar to the transition from zero to ordinary sounds is observed.     

The temperature dependence of the angular variation of,and the critical point exponent for the EPR linewidth in the two-dimensional canted antiferromagnetic (C2H5NH3)2MnBr4: Evidence for a structural phase transition

The angular variation of the EPR linewidths in single crystals of (C₂H₅NH₃)₂MnBr₄ has been measured as a function of temperature. The angular dependence is well characterized by δH(θ) = a + b(3 cos²θ ? 1) + c(3 cos²θ ? 1)². The temperature dependence of the expansion coefficients is reported, and the effect of critical point fluctuations near the Néel temperature as well as a linear temperature dependence at high temperature are observed. A sharp decrease in linewidths at 160°K is attributed to a structural phase transition. The Néel temperature is determined to be 46°K (± 1°) from linewidth measurements of a powder sample. The linewidths diverge exponentially near the Néel temperature with a critical point exponent of 1.5.     

Centroid-shift measurement of the mean lifetimes of the 316 and 612 keV states of 192Pt

The mean lifetimes of the 316 and 612 keV levels in ¹⁹²Pt were measured by means of a new variation of the centroid-shift method. The technique uses a triple coincidence among a β-particle and two γ-rays. Values obtained for the lifetimes are τ(316) = 61.7 ± 2.1 and τ(612) = 38.2 ± 2.2psec. The effect of these results on the analysis of the g-factors for these two states is discussed.     

Fission accompanied by alpha particle emission in the12C(85MeV)+232Th reaction

Inclusive and coincident spectra of alpha particles and fission fragments were measured for the²³²Th+¹²C (85 MeV) reaction to study the influence of the excitation energy and the angular momentum on the fission of the compound nucleus and to separate different alpha particle emission mechanisms. At backward angles α emission can be accounted for by the evaporation processes. At forward angles the most important contribution is given by the break-up fusion process. Mass distributions for compound nuclei²⁴⁴Cm (E^*=58 MeV,ff coincidences), and²⁴⁰Pu (E^*=37 MeV,ff α coincidences) were obtained. In the case of²⁴⁰Pu mass distribution has a shape different from those obtained in light ion reactions at the same excitation energy, indicating the strong influence of the entrance channel. The dependence of the mass distribution shape on the α particle energy is also examined.     

Thermal expansion measurements for estimating vacancy concentration in X-ray irradiated KCl single crystals

A sensitive capacitance technique is used for measuring changes in length (Δl) of KCl single crystals with temperature in the region 30–300°C. These measurements have been taken on KCl in (i) as-cleaved (ii) X-ray irradiated (iii) quenched and X-ray irradiated conditions (X-ray irradiation was always done at room temperature (≈ 30°C). The linear coefficient of thermal expansion (α) of the as-cleaved sample is 40.8 × 10^-6°C^-1. Variation of Δl with temperature in X-ray irradiated crystal shows two regions: (a) 30–180°C where α is 48.1 × 10^-6°C^-1, (b) 180–300°C where α is 40.4 × 10^-6°C^-1. Similar behaviour is exhibited by quenched and later X-ray irradiated KCl the first region is up to 140°C, beyond which the second region takes over. From these data, concentration of vacancies in X-ray irradiated KCl at room temperature is calculated to be 3.4 × 10¹⁷ cm^-3 which is in fairly good agreement with the value obtained from F-band absorption measurements on the sample. An attempt has been made to understand these results.     

Proprietes electrogalvaniques et electroniques du sulfure d'argent β: Domaine d'existence

The study of electrogalvanic and electronic properties of the form β of silver sulfide has been performed on polycrystalline samples whose non stoichiometry is controled by coulometric titration with the cell Ag/RbAg₄I₅/Ag₂₊?S/Pt.The data obtained lead to the adoption of an electronic model according to which the Frenkel defects of the silver sublattice are completly ionized. The variation with temperature of the electronic properties near the stoichiometric composition between 23 and 176°C, the temperature of transition between the two forms β and α of Ag₂S, enables the forbidden gap E_i = (1,35?1,5 × 10^?3T) eV to be computed. The existence range has been drawn between 69°C and the α ? β transition temperature.The study of the ionic conductivity has made it possible to detect two ranges of preferential conduction and to determine the mechanism of migrations of the intersticial silver ions.     

Free-carrier absorption in p-type Pb0.79Sn0.21Te

Results are presented of the room-temperature absorption coefficient (α) of Pb_0.79Sn_0.21Te (p_77K = 2 × 10¹⁶-6 × 10¹⁷cm^-3) in wavelength (λ) range 6–15 μm. At long wavelengths α exhibits a λ² dependence, in agreement with the classical free-carrier absorption expression, and in this region α is proportional to carrier density.     

Anomalously slow relaxation of a nonwetting liquid in the disordered confinement of a nanoporous medium

The time evolution of the water–disordered nanoporous medium Libersorb 23 (L23) system has been studied after complete filling at elevated pressure followed by full release of overpressure. It is established that relaxation of the L23 rapidly flows out during the overpressure relief time, following the variation in pressure. At a temperature below that of the dispersion transition (T < T _d = 284 K), e.g., at T = 277 K, the degree of filling θ decreases from 1 to 0.8 within 10 s. The degree of filling varies with time according to the power law θ ~ t ^–α with the exponent α < 0.1 over a period of t ~ 10⁵ s. This process corresponds to slow relaxation of a metastable state of a nonwetting liquid in a porous medium. At times t > 10⁵ s, the metastable state exhibits decay, manifested as the transition to a power dependence of θ(t) with a larger exponent. The relaxation of the metastable state of nonwetting liquid in a disordered porous medium is described in the mean field approximation as a continuous sequence of metastable states with a barrier decreasing upon a decrease in the degree of filling. Using this approach, it is possible to qualitatively explain the observed relaxation process and crossover transition to the stage described by θ(t) with a larger exponent.     

Heat capacity of the La<Subscript>0.9</Subscript>Ag<Subscript>0.1</Subscript>MnO<Subscript>3</Subscript> manganite near the curie temperature

The heat capacity of the La_0.9Ag_0.1MnO₃ manganite is measured in the temperature range 77–350 K and studied in detail in the vicinity of the Curie temperature for the first time. The regularities of the variation in the universal critical parameters in the vicinity of the phase transition point are established. The critical exponent and the amplitude of the heat capacity are calculated to be α = ?0.127 and A ⁺/A ^? = 1.146 with due regard for the scaling corrections. These parameters correspond to the critical behavior within the three-dimensional Heisenberg model. The size of ferromagnetic droplets in the paramagnetic range at T > T _C is estimated as ξ ≈ 19 Å. The results obtained are analyzed thoroughly and compared with theoretical data for a number of model systems.     

On the temperature dependence of the magnetic susceptibility of rhodium

A semiphenomenological function for the temperature variation of the magnetic susceptibility X(T) of Rh up to the melting point is presented. It interpolates between Pauli-Stoner (itinerant) and Curie (localized) behaviour. The low temperature expansion of X(T) up to T² can be correlated with the band structure dependent spin fluctuation model of Béal-Monod.     

Effect of surface temperature on the sorption of hydrogen by Pd(111)

Temperature programmed desorption (TPD) subsequent to various hydrogen exposure conditions indicates the formation of chemisorption, solid solution, and hydride phases of hydrogen in the near surface region of Pd(111). Variation of the sample exposure temperature (T_e) between 80 and 300 K has a strong effect on the subsequent TPD spectra. At T_e = 80 K a single desorption peak, β, appears at 310 K. Coverage variation of the β peak is consistent with second-order recombinative desorption of chemisorbed hydrogen. For T_e between 90 and 140 K a slight enhancement of the β peak occurs and a new peak, α, appears initially near 170 K. It does not saturate, exhibits near-zeroth-order desorption kinetics, and is assigned to the decomposition of a near surface palladium hydride phase. Population of the α peak is thermally activated with a maximum at T_e ≈ 115 K. For T_e, greater than 140 K, α disappears while the total amount of absorbed hydrogen increases significantly. At these temperatures, the concentration of absorbed hydrogen decreases significantly if the sample is held in vacuo at T_e after completion of the hydrogen exposure. At all exposure temperatures there is also a broad desorption feature near 800 K which is enhanced by higher T_e and is associated with hydrogen in solid solution.     

Author index

The mean adsorption lifetimes of F, Cl and Br on (100) and (111) Mo surfaces have been obtained from the first order desorption kinetics observed in low converage conditions (θ < 10^?2 of a monolayer) using a pulsed ionic beam method. The mean adsorption lifetimes τ fit a general expression $\text{τ = τ}{}_0\text{exp}\text{(}\text{E}\text{kT}\text{)}$ in a large temperature range (1700–2400 K) allowing the determination of the binding energies E. The main results of this study are (1) the binding energies decrease from F through Br; (2) the binding energies on both (100) and (111) orientations are similar, E(F)～4.65 eV, E(Cl)～4.15 eV, and E(Br)～3.65 eV. These results are discussed and compared with those previously reported on (100) and (111) Nb and W surfaces. The close binding energies of F, Cl and Br on (100) and (111)^?Nb and Mo surfaces suggest that halogens have a different chemisorption behaviour with respect to O and N.     

Phase transformations in equiatomic alloy TiZr at pressure up to 70 kbar

The effect of pressure on the α ? β and ω ? β transformations in the equiatomic alloy TiZr is studied by the differential thermal analysis (DTA) and calorimetric technique. The α-β equilibrium at atmospheric pressure occurs at a temperature of 579°C, and the heat of transition ΔH is 40.9±2.0 J/g. As the pressure increases up to 28 kbar, the temperature of the α-β equilibrium linearly decreases, dT/dP=?2.2±0.3 K/kbar. In the pressure range 28–48 kbar, the β-phase undergoes a transition to the two-phase (α + ω) state upon cooling to room temperature. At pressures above the triple point with the coordinates P=49±3 kbar and T=460±30°C, the cooling of the β-phase gives rise to only the hexagonal ω-phase with the unit cell parameters a=4.843 Å, c=2.988 Å, and c/a=0.617 under normal conditions. The slope of the ω-β equilibrium boundary is positive at pressures up to 70 kbar, dT/dP≈0.46 K/kbar. The ω → α transformation at atmospheric pressure proceeds in the temperature range T=425–470°C with the enthalpy of transition ΔH=2.8 J/g.     

Density of states and specific heat of elastic vibrations in layer structures

On the basis of a determination of normal modes in materials consisting of periodic arrangements of macroscopic layers (of period d), the low frequency density of states and the corresponding low temperature specific heat were calculated numerically. The average temperature dependence of the latter changes in the vicinity of a characteristic temperature T₀ (proportional to 1/d), from a low temperature (α₀T³)- to a higher temperature (αT³+βT²)- law. Depending on the material parameters, β may be positive (especially if Stonely waves are present) or negative. The coefficients α and α₀ can differ by a factor larger than 2. Characteristics of thick and thin layers and the implications of the results on the interpretation of experimental data are discussed.     

Nonradiative decay from 5d states of rare earths in crystals

Nonradiative decay from 4f^n?1 5d states was investigated for trivalent rare earths in Y₃Al₅O₁₂. The rates of both 5d→4f and 5d→5d transitions were determined from measurements of the lifetimes and intensities of 5d fluorescence from Ce³⁺ and Pr³⁺. Because of the stronger ion-lattice coupling, nonradiative decay rates for transitions involving 5d states are much faster than those between 4f states. Decay rates are dependent upon the temperature and the energy gap to the next-lower level. The temperature dependences of the 5d fluorescence lifetimes from 77 to 700°K are reported.     

Mechanical properties and the evolution of matrix molecules in PTFE upon irradiation with MeV alpha particles

The morphology, chemical composition, and mechanical properties in the surface region of α-irradiated polytetrafluoroethylene (PTFE) have been examined and compared to unirradiated specimens. Samples were irradiated with 5.5 MeV ⁴He²⁺ ions from a tandem accelerator to doses between 1 × 10⁶ and 5 × 10¹⁰ Rad. Static time-of-flight secondary ion mass spectrometry (ToF-SIMS), using a 20 keV C₆₀⁺ source, was employed to probe chemical changes as a function of α dose. Chemical images and high resolution spectra were collected and analyzed to reveal the effects of α particle radiation on the chemical structure. Residual gas analysis (RGA) was utilized to monitor the evolution of volatile species during vacuum irradiation of the samples. Scanning electron microscopy (SEM) was used to observe the morphological variation of samples with increasing α particle dose, and nanoindentation was engaged to determine the hardness and elastic modulus as a function of α dose.The data show that PTFE nominally retains its innate chemical structure and morphology at α doses <10⁹ Rad. At α doses ≥10⁹ Rad the polymer matrix experiences increased chemical degradation and morphological roughening which are accompanied by increased hardness and declining elasticity. At α doses >10¹⁰ Rad the polymer matrix suffers severe chemical degradation and material loss. Chemical degradation is observed in ToF-SIMS by detection of ions that are indicative of fragmentation, unsaturation, and functionalization of molecules in the PTFE matrix. The mass spectra also expose the subtle trends of crosslinking within the α-irradiated polymer matrix. ToF-SIMS images support the assertion that chemical degradation is the result of α particle irradiation and show morphological roughening of the sample with increased α dose. High resolution SEM images more clearly illustrate the morphological roughening and the mass loss that accompanies high doses of α particles. RGA confirms the supposition that the outcome of chemical degradation in the PTFE matrix with continuing irradiation is evolution of volatile species resulting in morphological roughening and mass loss. Finally, we reveal and discuss relationships between chemical structure and mechanical properties such as hardness and elastic modulus.     

Stabilizing the triclinic structure of Mn2V2O7 via isovalent cationic substitution

The thermal expansion of Mn₂V₂O₇ in the temperature range of ?190 to 1030°C is studied. The volumetric thermal expansion coefficients for triclinic (α) and monoclinic (β) modifications are 2.57 × 10^?5 and 3.86 × 10^?5 1/deg, respectively. It is shown that as the Ni²⁺ concentration in Mn_2?2x Ni_2x V₂O₇ rises, the point of the α → β phase transition point moves from room temperature for Mn₂V₂O₇ to 155 ° 5°C for Mn_1.46Ni_0.54V₂O₇ (27 mol % Ni₂V₂O₇).     

Study of 170,171,173Ta isotopes at high spinṡ

High-spin data are presented for ¹⁷⁰Ta and ¹⁷¹Ta for the first time, and the known level scheme of ¹⁷³Ta is extended to higher spins. Anomalies found in the crossing frequencies for the alignment of a pair of neutrons are considered. At high spin a large and constant value of J⁽²⁾, with J⁽²⁾ > J⁽¹⁾, is found for two ¹⁷¹Ta bands and the implications of this are discussed. The πi_13/2 [660]1/2⁺, α = + 1/2 orbital is identified as it crosses the rotational band built on the [402]5/2⁺, α = + 1/2 orbital in ¹⁷¹Ta. Magnetic transition probabilities obtained from measured branching ratios show the effects of the aligning pair of neutrons and the spectator orbital. The data are interpreted using cranked shell-model calculations and a semiclassical vector-coupling scheme developed by Donau and Frauendorf.