Ultrasonic attenuation in a LiIO3 crystal

The temperature dependences of the attenuation of longitudinal acoustic waves along the x axis of a LiIO₃ crystal are studied experimentally by the method of Bragg diffraction of light by ultrasound. The measurements are performed in the temperature range from 215 to 335 K at frequencies of 400–690 MHz. Above 260 K, the attenuation is found to exhibit a frequency-independent growth of a relaxational character, which is used to calculate the enthalpy of the activation of ionic motion in the direction normal to the hexagonal axis. In addition, resonance attenuation peaks are observed; as the frequency increases, the peaks are shifted toward lower temperatures. A phenomenological model is proposed to explain the origin of the resonance absorption.     

Channeling radiation and coherent bremsstrahlung

We show that calculations based on the use of Bloch functions in at least two-dimensions predict both coherent brems-strahlung peaks (k_CB) and channeling radiation peaks (k_CR) in the spectrum of photons emitted by electrons traversing a crystal when moving nearly parallel to a crystal plane; and that furthermore, “sidebands” (e.g., k_CB ± k_CR) are predicted when the electrons move nearly parallel to two intersecting crystal planes.     

The crystal-field dependency of sound attenuation in the spin-3/2 ising model

The effects of crystal-field (D) on sound attenuation are considered for the spin-3/2 Ising model by using Onsager theory of irreversible thermodynamics. It is assumed that the sound wave couples to the order-parameter fluctuations, therefore, it decays mainly via order-parameter relaxation process. The order-parameters, magnetization and quadrupole moment, are defined in terms of exact recursion relations (ERR) on Bethe lattice (BL). After our analysis, two relaxation times are obtained and they are used to calculate the sound attenuation coefficient (α). Consequently, the critical behaviors of sound attenuation coefficient are investigated in terms of frequency (w) and Onsager coefficient (γ) for the coordination numbers q=3, 4 ad 6 near the second-order (T_c) and first-order (T_t) phase transition temperatures in the ferromagnetic phase regions for the negative and positive D values and the results are presented on the (k_BT/J, α) planes. It is found that the peaks about T_t’s are observed at the same temperature, but the peaks about T_c’s are observed shifted to lower and higher temperatures in increasing (w)’s and (γ)’s, respectively. In addition, the peaks are also obtained near the tricritical points for all q.     

Brillouin and raman scattering studies of the melting transition in salol

Brillouin and Raman scattering studies of salol from room temperature to within 5 mK of the melting transition at T_m = 40.97°C are reported. Changes in the Brillouin shifts and linewidths were accurately determined by nonlinear least-squares fitting and deconvolution. A marked increase in the deconvoluted Brillouin linewidth (~400%) and a gradual softening (~20%) of the transverse acoustic modes were observed very close to T_m. The increase of the Brillouin linewidths was analyzed by a simple dislocation model assuming the hypersonic attenuation to be proportional to the concentration of thermally generated defects near T_m. The defect formation energy E_D(T) was computed from the temperature-dependent linewidth data, and was found to decrease significantly (~60%) near T_m, suggesting a cooperative effect producing a catastrophic growth of defects which brings about melting by destroying the long range order of the crystal. The conclusion that melting is mediated by a sudden growth of defect concentration near T_m was further strengthened by Raman scattering experiments in which 13 new Raman modes appeared close to T_m. These new modes are believed to be defect activated through breaking of the local symmetry of the crystal. A slight softening of the Raman modes (~5%) was observed close to the melting point.     

Critical behaviour in the shear elasticity of adamantane in the plastic phase

The elastic constants of the molecular crystal adamantane (C₁₀H₁₆) have been measured at 10 MHz as a function of temperature in the cubic plastic phase, with special attention given to the region just above the order-disorder transition into the tetragonal rigid phase. Both the velocity and attenuation of the C′ shear wave exhibit incipient critical behavior near this transition. The C′ stiffness also shows a pretransitional softening when this transition is approached by increasing the pressure along several isotherms.     

Ultrasonic attenuation study of erbium in a magnetic field

The ultrasonic attenuation coefficient of longitudinal waves propagating along the c-axis of a single crystal of erbium was measured over a temperature range which covers the transition temperature from the paramagnetic to the sinusiodal phase T₆ = 83.8 K and the transition temperature from the sinusoidal to elliptical phase T_⊥ = 53.9 K. These measurements were performed as a function of temperature in the presence of constant magnetic fields and at constant temperatures as a function of applied magnetic fields. A magnetic field parallel to the c-axis shifted T₆ to lower temperatures and reduced the peak attenuation. A broad new maximum emerged on the high temperature side of the field-shifted T₆. Application of H perpendicular to the c-axis produced little change in either T₆ or T_⊥. A model introduced by Tachiki and Mackawa is used to discuss the results.     

Longitudinal ultrasonic attenuation in the spin spiral state of single crystal holmium

Ultrasonic attenuation experiments have been performed using 15–255 MHz longitudinal sound waves along the c-axis of single crystal holmium. Without an external magnetic field, the attenuation increases in the spin-spiral state. This anomalous increase originates, at least in part, from the spin-phonon relaxation mechanism we have proposed, which states that the attenuation coefficient Δα₁ ∞ ω²/(1+ω²τ²) where τ is the longitudinal spin phonon relaxation time. The frequency dependence of the attenuation varies from ω^1·0 to ω^1·5 which suggests a frequency-dependent character for the spin-phonon relaxation time τ. A broad longitudinal attenuation maximum, which presumably is due to the competing processes between $\overline{S}$, J and τ, is observed in the spin-spiral state, where $\overline{S}$ is the thermal average of the spin angular momentum per trivalent ion and J the Fourier transform of the exchange integral. An anomalous suppression of the longitudinal attenuation spike at T_N for frequencies higher than 165 MHz is unexplainable at present. The longitudinal attenuation spike at T_N for frequencies higher than 165 MHz is unexplainable at present. The longitudinal spin-phonon relaxation time for Ho has been determined using experimental data. It has a T^?3 temperature dependence. In the presence of an external magnetic filed in the basal plane the attenuation is decreased and a new peak and a plateau appear at the intermediate phase transitions.     

Raman scattering of NiS2

Raman scattering measurements of NiS₂ is done and five optical phonon peaks are found. These peaks are assigned to A_g, E_g and 3T_g phonons which are all of the Raman active optical phonons in pyrite-type crystal. The results are compared with those of FeS₂ and MnS₂.     

Variation of electron energy loss spectra of Ni (100) with increasing primary energy

Electron energy loss spectra of clean Ni(1 0 0) show for the first time a 17 eV peak, which is attributed to an interband transtiion. All the observed peaks are shifted to higher energies as the primary electron energy E_p increases from 102 to 2045 eV. This shift is explained by a continuous decay in energy of the primary electrons inside the crystal. At E_p ? 700 eV, the decay takes place in the surface region of the crystal, while at E_p > 700 eV it takes place mainly in the bulk. The rate of decay increases with increasing temperature of the crystal between 300 and 900 K.     

Ultrasonic study of dysprosium in a magnetic field

The ultrasonic attenuation of longitudinal waves propagating along the c axis of single crystal dyprosium is reported, as a function of the applied basal plane field in the paramagnetic region, and as a function of the temperature, at constant applied basal plane field, in the spin-spiral region. In the paramagnetic region, anomalous attenuation behavior is explained on the basis of competing spin-polarization and spin-fluctuation effects. Two anomalous maxima in the temperature dependence of the attenuation were observed: one near T_N is attributed to spin fluctuations associated with short range ferromagnetic ordering; another one at 130 K is attributed to a magnetic phase transition from a fanstructure phase, intermediate between the spin-spiral and ferromagnetically ordered phase     

Thermodynamic and magnetic properties of HoRh2Si2: A comparison between experiments and calculations in a crystal field model

The origin of two peaks observed in the magnetization and the heat capacity curves of HoRh₂Si₂ is investigated. The thermodynamic and the magnetic properties of this compound are calculated using a Hamiltonian including crystal field and isotropic exchange interaction terms which are comparable in magnitude. Two components (parallel and perpendicular to the c-axis) of the magnetization are evaluated by the molecular field approximation. The two peaks in the magnetization and the heat capacity can be reproduced by choosing appropriate values for crystal field parameters and molecular field constant. The lower transition temperature corresponds to the temperature at which the perpendicular component of the magnetization disappears, and the higher one, to that at which the parallel component of the magnetization disappears.     

Raman scattering in Mg2Sn and Mg2Si

The Raman spectra of single crystal Mg₂Sn and Mg₂Si have been studied at room temperature. In Mg₂Sn we observe a single relatively strong line produced by the degenerate Raman active F_2g modes, seen previously by neutron inelastic scattering. Two weaker peaks are seen, one occuring at the F_2g overtone; the assignment for the other is uncertain. In Mg₂Si we find a strong line having a shift comparable to the value calculated by previous workers.     

Anisotropie der Plasmaschwingungen in Graphit

Energy loss spectra of electrons at different incidence angles α against the crystal axis were measured in transmission of thin graphite single crystals. The spectra show that the energy and the intensity of the loss peaks are dependent on the angle between the transferred pulse?k _p and the crystal axisc.     

Spin effects on oscillatory magnetoresistances in n-Pb1−xSnxTe

Shubnikov-de Haas oscillations in n-Pb_1?xSn_xTe have been measured in the magnetic field parallel to the [100] crystal direction at 1.5 K. In the longitudinal magnetoresistances, the one-side peaks of spin-splitting pair series are completely missing. This anomaly is well explained by the selection rules, theoretically derived in the study on a similar effect in Hg_1?xCd_xTe. Landau sublevel-crossings are also discussed.     

Phase transitions and the structure of the C60 crystal doped with lithium by electrodiffusion

The structure and phase transitions of C₆₀ crystals doped with lithium by injecting metal ions from the superionic crystal-C₆₀ single crystal heterojunction under electrodiffusion conditions are reported. The sample experienced irreversible transitions resulting in the virtually complete disappearance of EPR signals and MW conduction in the temperature range 320–370 K. In this temperature interval, a new C₆₀ phase was formed; the phase contained polymeric chains of C₆₀ molecules along the crystallographic c axis and lithium clusters. The structure of this phase was determined. Annealing at 620 K restored the EPR signal and, according to the X-ray data, the initial cubic structure of pure C₆₀. The X-ray pattern, however, contained additional diffraction peaks, which was evidence of the presence of one more phase with a structure yet unknown.     

Effect of local environment and Sm-codoping on the luminescence properties in the Eu-doped potassium tungstate phosphor for white LEDS

The luminescence properties of Eu³⁺- and Sm³⁺-doped potassium tungstate phosphors were investigated. The K_4−3(x+y) (WO₄)₂:Eu_x³⁺,Sm_y³⁺ phosphor was produced by solid-state reactions, followed by re-firing with a flux. The phosphor showed a strong absorption in the near-UV to green region due to 4f-4f electron transitions of the Eu³⁺ and Sm³⁺ ions, generating a red emission. The excitation spectrum could be adjusted by Sm³⁺-codoping. A small amount of Sm³⁺, acting as a sensitizer, increased the energy absorption peak at 405 nm. The crystal structure and local environment around the Eu³⁺ ions were determined using the Rietveld method. The crystal structure of this phosphor was determined to be monoclinic with a space group of C2/c. The small Eu-0 distance in the crystal led to high energy-level splitting at the ⁵D₀→⁷F₂ transition of the Eu³⁺ ions, resulting in more emission peaks.     

Impact of dynamical scattering on quantitative contrast for aberration-corrected transmission electron microscope images

Aberration-corrected transmission electron microscope images taken under optimum-defocus conditions or processed offline can correctly reflect the projected crystal structure with atomic resolution. However, dynamical scattering, which will seriously influence image contrast, is still unavoidable. Here, the multislice image simulation approach was used to quantify the impact of dynamical scattering on the contrast of aberration-corrected images for a 3C-SiC specimen with changes in atomic occupancy and thickness. Optimum-defocus images with different spherical aberration (C_S) coefficients, and structure images restored by deconvolution processing, were studied. The results show that atomic-column positions and the atomic occupancy for SiC ‘dumbbells’ can be determined by analysis of image contrast profiles only below a certain thickness limit. This limit is larger for optimum-defocus and restored structure images with negative C_S coefficient than those with positive C_S coefficient. The image contrast of C (or Si) atomic columns with specific atomic occupancy changes differently with increasing crystal thickness. Furthermore, contrast peaks for C atomic columns overlapping with neighboring peaks of Si atomic columns with varied Si atomic occupancy, which is enhanced with increasing crystal thickness, can be neglected in restored structure images, but the effect is substantial in optimum-defocus images.     

Magnetic and transport properties of Pr2Pd3Si5

We have investigated the magnetic and transport properties of a new ternary intermetallic compound Pr₂Pd₃Si₅ which forms in U₂Co₃Si₅-type orthorhombic structure (space group Ibam). At low field (0.01 T) magnetic susceptibility exhibits an abrupt increase below 7 K and peaks at 5 K, revealing a magnetic phase transition. The onset of magnetic order is also confirmed by well defined anomalies in the specific heat and electrical resistivity data. Apart from the sharp λ-type anomaly, magnetic part of specific heat also shows a broad Schottky-type hump due to crystal field effect. Magnetoresistance data as a function of temperature exhibits a pronounced peak in paramagnetic state which could be interpreted in terms of crystal field effect and short-range ferromagnetic correlations.     

Pressure effects on the absorption spectra of Na doped in solid Ar

1 Introduction Impelled by the research project for the High Energy Density Matter (HEDM) ofUSA, researchers have achieved fruitful results on the energetic species in cryogenicenvironments[1]. That the chemical performance of solid hydrogen fuel can be greatlyenhanced when being doped with a small amount of light metal is confirmed both theo-retically and experimentally[2]. For understanding the mechanism, similar systems haveattracted much interest. The system of rare gas solid doped w…