Rückstreuung von Neutronen an perfekten Silizium-Einkristallen

A neutron backscattering diffractometer is described. Using this instrument, we measured the Bragg reflections of neutrons which are backscattered from the (111)-and the (333)-lattice planes of silicon single crystals. The relative line widths of the (111)-reflex (Δa/a)₁₁₁=3.3 · 10^?5 and that of the (333)-reflex (Δa/a)₃₃₃=6.8 · 10^?6 are broader than the theoretical values of (Δa/a)₁₁₁=3.0 · 10^?5 and (Δa/a)₃₃₃=3.2 · 10^?6. The additional broadening of 3 · 10^?6 is due to experimental inaccuracies. The measured integrated reflectivities for both reflexes agreed with the theory.     

ESR study of Fe3+ and Mn2+ ions on trigonal sites in ilmenite structure MgTiO3

Electron spin resonance has been observed for Fe³⁺ and Mn²⁺ ions occupying sites with trigonal symmetry in undoped and doped Verneuil-grown crystals of the ilmenite type compound MgTiO₃. At 300 K, the fine structure parameters in the spin Hamiltonian are (in 10^?4cm^?1) D = +844 (± 1), (a? F) = +118 (± 1), a = 69 (± 7) for Fe³⁺ and D = +164 (± 1), (a ? F) = +10.2 (± l), a = 7.0 (± 1) for Mn²⁺. These values are compared with literature data for Fe³⁺ and Mn²⁺ in other oxides, especially Al₂o₃, with particular reference to the recent “superposition” theory of the effect of a trigonal distortion. From the orientation of the axes of cubic pseudosymmetry of the spin Hamiltonian, and with the assumption that a has the same sign for both ions, it is proposed that Fe³⁺ and Mn²⁺ occupy the same octahedral site, namely the Mg²⁺ site. Anomalous line splittings observed for one sample were attributed to twinning on (0001) or {1120} planes.     

The anomalous magnetic moment of positive and negative muons

The anomalous g-factor a ≡ (g?2)/2 has been measured for muons of both charges in the Muon Storage Ring at CERN. The two results, a_μ⁺ = 1165910(12) × 10^?9 and a_μ^? = 1165936(12) × 10^?9, are in good agreement with each other, and combine to give a mean a_μ = 1165922(9) × 10^?9, which is very close to the most recent theoretical prediction 1165921(10) × 10^?9. For the experimental results, the total statistical and systematic error is given. The measurements thus confirm the remarkable QED calculation plus hadronic contribution, and serve as a precise verification of the CPT theorem for muons.     

Ultrasonic determination of electron-phonon scattering rates in copper

Electron-phonon scattering rates in ultrapure single crystals of copper have been determined from the temperature dependence of the magnetoacoustic oscillation amplitudes for various orbits on the Fermi surface using both longitudinal and transverse waves. The central belly orbit seattering rate is found to be (6.0±0.3)×10⁶ T ³ sec^?1. Additionally, a rate of (2.9±0.2)×10⁶ T ³ sec^?1 is found which is attributed to belly orbits displaced from the zone center by about 1.25/a ₀, wherea ₀ is the lattice constant. Geometric oscillations associated with the [111]—directed open orbit are observed at low fields forq‖ [113] and the rate for this orbit is found to be (4.8±0.3)×10⁶ T ³ sec^?1. Geometric oscillations for the dog's bone and neck orbits are observable but rates for these orbits are believed to be unreliable. Our measured rates are compared with those of other workers.     

Influence of the defect state of samples on the luminescence spectra of CdS single crystals irradiated by electrons

The photoluminescence spectra of CdS single crystals irradiated by electrons (E = 1.2 MeV, Φ = 2×10¹⁷ cm^?2) are investigated in the visible and near-infrared regions of electromagnetic radiation. Some samples of the CdS single crystals are preliminarily irradiated by neutrons (E = 2 MeV, Φ = 2 × 10¹⁸ cm^?2) with the aim of increasing the concentration of initial structural defects. From analyzing the peak intensities of photoluminescence in the irradiated single crystals at the wavelengths λ_m = 0.720, 1.030, and 0.605 μm, it is concluded that the CdS samples with a low concentration of structural defects in the initial state possess the highest resistance to electron radiation. It is assumed that the observed transformation of the photoluminescence spectra of the imperfect CdS single crystals subjected to electron irradiation is determined by either the mechanisms of subthreshold defect formation or the transformation of the defect complexes in elastic and electric fields near the large structural damages of the crystal lattice.     

Study of sensitization of Cr4+ ions by Yb3+ ions in yttrium-aluminum garnet crystals

The possibility of using ytterbium ions as sensitizers of luminescence of tetravalent chromium in yttrium-aluminum garnet single crystals is studied. It is found that average values of the microparameters of electronic energy transfer between ytterbium and chromium ions are C _DA=7.05×10^?36 cm⁶/s and C _DD=4.55×10^?43 cm⁶/s.     

Electron paramagnetic resonance of Dy3+ ions in lead thiogallate single crystals

The results of electron paramagnetic resonance (EPR) studies of Dy³⁺ ions in lead thiogallate PbGa₂S₄ single crystals have been presented. It has been shown that the ground state of these ions corresponds to the lowest Stark sublevel Γ₆ of the term ⁶ H _15/2. The spectra are well described by the axially symmetric spin Hamiltonian with the effective spin S = 1/2 with the factors g _‖ = 15.06 and g _⊥ = 2.47. The Dy³⁺ ions substitute Pb²⁺ ions in the crystal lattice of PbGa₂S₄. The observed hyperfine structure has allowed to unambiguously interpret the EPR spectra. The hyperfine interaction constants of two odd isotopes of dysprosium in lead thiogallate single crystals have been found to be A _‖ = 675 × 10^?4 cm^?1 and A _⊥ = 111 × 10^?4 cm^?1 for ¹⁶³Dy and A _‖ = 472 × 10^?4 cm^?1 and A _⊥ = 77 × 10^?4 cm^?1 for ¹⁶¹Dy.     

Charge transfer over localized states in a TlS single crystal

It is revealed that TlS single crystals exhibit a variable range hopping conduction along a normal to their natural layers at temperatures T ≤ 230 K in a dc electric field and a nonactivated hopping conduction at low temperatures in strong electric fields. Estimates are made for the density of states near the Fermi level (N _F = 2.8 × 10²⁰ eV^?1 cm^?3 and their energy spread (ΔW = 0.02 eV), the localization radius (a = 33 Å), the average jump distance in the region of activated (R _av(T) = 40 Å) and nonactivated (R _av(F) = 78 Å) hopping conduction, and also the drop in the charge carrier potential energy along the jump distance in an electric field F: eFR = 0.006 and 0.009 eV at F = 7.50 × 10³ and 1.25 × 10⁴ V/cm, respectively.     

Electron paramagnetic resonance in deuterated nickel fluosilicate

Electron paramagnetic resonance measurements in single crystals of NiSiF₆. 6D₂O were made at K, Ku and Ka bands at 4.2 K and between 77 K and 300 K. The measured g values were in the range 2.23–2.26, while the zero-field splitting parameter D varied from ?(0.185 ± 0.005) cm^?1 at 4.2 K to ?(0.53 ± 0.01) cm^?1 at 298 K. The parameters of the trimolecular hexagonal unit cell were determined to be approximately a = 9.28 Å, c = 9.58 Å from powder X-ray diffraction measurements at room temperature.     

Magnetization behaviour of DyAl2 single crystals

We report the theoretical interpretation of the magnetization and the magnetocrystalline anisotropy of ferromagnetic DyAl₂ single crystals between 4.2 and 60 K and magnetic fields up to 15 T. Good agreement between theory and experiment is obtained by using three temperature independent parameters: the two crystal field parameters B₄ = (?0.50 ± 0.05) × 10^?4 meV, B₆ = ? (0.51 ± 0.05) × 10^?6 meV and the Curie temperature T_c = (62 ± 2) K.     

New evaluation of muon (g − 2) hadronic anomaly

The hadronic part a_H of the muon g-factor anomaly $\text{a ≡}\text{(g ? 2)}\text{2}$ is evaluated from latest data on σ(e⁺e^? → hadrons). For a p-wave ππ scattering length of a₁ = 0.04±0.005 we calculate a_H = (66±10) × 10^?9, compared to a(experiment) ? a(QED) = (60±29) × 10^?9. Half of the uncertainty on a_H is associated with the energy interval $\text{0.92 <}\text{s}\text{< 2}\text{GeV}$.     

ESR of V3+ in zinc oxide single crystals

Electron spin resonance has been investigated in zinc oxide single crystals containing vanadium. Several groups of ordinary and forbidden transitions can be observed. The experimental results are interpreted with the aid of the spin Hamiltonian, for which the following parameters were determined:g∥=1.945; ⊥=1.937; ¦D¦=750×10^?4 cm^?1, ¦A¦=68 × 10^?4 cm^?1; ¦B ¦=93×10^?4 cm^?1; ¦A?P¦=65×10^?4cm^?1.     

Dark-current relaxation in EuGa2S4 single crystals

The results of the investigation of dark-current relaxation in EuGa₂S₄ single crystals are reported. The depth and concentration of the traps are found to be E_t = 0.79 eV and N_t = 1.64 × 10¹⁴ cm^?3, respectively.The charge accumulation region (d_c = 3.3 × 10^?5 cm) and contact capacitance (C_c = 1.23 × 10^?10 F) are also estimated.     

Absorption of Dy3+ and Nd3+ ions in BaR 2F8 single crystals

The Dy³⁺ absorption and excitation spectra of BaY₂F₈ and BaYb₂F₈ single crystals are investigated in the ultraviolet, vacuum ultraviolet, and visible ranges at a temperature of 300 K. These crystals exhibit intense broad absorption bands due to the spin-allowed 4f-5d transitions in the range (56–78) × 10^?3 cm^?1 and less intense absorption bands that correspond to the spin-forbidden transitions in the range (50–56) × 10^?3 cm^?1. The Nd³⁺ absorption spectra of BaY₂F₈ single crystals are studied in the range (34–82) × 10^?3 cm^?1 at 300 K for different crystal orientations.     

New measurement of (g−2) of the Muon

The g-factor anomaly, a≡(g?2)/2, has been measured for μ⁺ in the new Muon Storage Ring at CERN. The result is a = (1 165 895 ± 27) × 10^?9. This is (13 ± 29) × 10^?9 below the theoretical value which includes sixth-order QED terms and a hadronic contribution of (73 ± 10) × 10^?9.     

Lattice parameter of sodium close to the melting point

We report high precision lattice parameter measurements (Δa/a < 10^?6) of high purity sodium single crystals close to the melting transition. Data were taken by a neutron-double crystal-back-scattering device. Just below the melting point a decrease in the lattice parameter is observed. This effect is discussed in terms of the defect structure and the limit of stability in the crystalline solid state.     

Anomalous shear ultrasonic attenuation peak at 95.5°K in holmium2

An anomalous attenuation peak at 95.5°K has been observed for shear ultrasonic waves transmitted along the c-axis of two holmium single crystals. The velocity change across this temperature is on the order of 10^?4. In the presence of an external magnetic field, the peak moves toward high temperatures for both H∥a and H∥c. A rotation of the field in the basal plane produces a two-fold symmetry pattern in the attenuation. A double peak structure centered at the c-plane has also been observed when the field is rotated away from the a-axis. A considerable fraction of the attenuation peak appears to go as sin² θ where θ is the angle between the shear wave polarization and the external field.     

Specific features of anion transfer in HoF<Subscript>3</Subscript> crystals at high temperatures

The anionic conductivity of HoF₃ single crystals with a β-YF₃ structure (orthorhombic crystal system, space group Pnma) is investigated over a wide range of temperatures (323–1073 K). The unit cell parameters of HoF₃ crystals are as follows: a=0.6384±0.0009 nm, b=0.6844±0.0009 nm, and c=0.4356±0.0005 nm. It is revealed that the conductivity anisotropy of the HoF₃ crystals is insignificant over the entire temperature range covered. The crossover from one mechanism of ion transfer to another mechanism is observed near the critical temperature T_c≈620 K. The activation enthalpy of electrical conduction is found to be ΔH₁=0.744 eV at T<T_c and ΔH₂=0.43 eV at T>T_c. The fluorine vacancies are the most probable charge carriers in HoF₃ crystals. The fluorine ionic conductivities at temperatures of 323, 500, and 1073 K are equal to 5×10^?10, 5×10^?6, and 2×10^?3 S cm^?1, respectively.     

Optical absorption edge in SnS

Optical absorption in single crystals of tin sulfide has been studied at many temperatures between 100 and 300 °K, in the wavelength range 2·2–0·8 μ. From the interference fringe patterns the absorption coefficient, reflection coefficient and index of refraction as a function of wavelength were determined for two light polarizations (ε∥a and ε∥b). From an analysis of the data, indirect band gaps of 1·142 and 1·095 eV were found for the two directions of polarization. Also it was found that the phonon assisted transitions required the participation of two phonons at different energy thresholds with energies 0·033 or 0·038 eV and 0·082 or 0·113 eV, with reference to the two axis. The temperature dependence of the indirect band gap for each direction of light polarization is linear with a slope ?4·05 × 10^?3eV and ?4·37 × 10^?3 eV respectively.     

Frequency dispersion of dielectric coefficients of layered TlGaS<Subscript>2</Subscript> single crystals

Frequency dependence of the dissipation factor tanδ, the permittivity ɛ, and the ac conductivity σ_ac across the layers in the frequency range f=5×10⁴−3×10⁷ Hz was studied in layered TlGaS₂ single crystals. A significant dispersion in tanδ was observed in the frequency range 10⁶−3×10⁷ Hz. In the range of frequencies studied, the permittivity of TlGaS₂ samples varied from 26 to 30. In the frequency range 5×10⁴−10⁶ Hz, the ac conductivity obeyed the f ^0.8 law, whereas for f>10⁶ Hz σ_ac was proportional to f ². It was established that the mechanism of the ac charge transport across the layers in TlGaS₂ single crystals in the frequency range 5×10⁴−10⁶ Hz is hopping over localized states near the Fermi level. Estimations yielded the following values of the parameters: the density of states at the Fermi level N _F=2.1×10¹⁸ eV⁻¹ cm⁻³, the average time of charge carrier hopping between localized states τ=2 μs, and the average hopping distance R=103 ?. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 6, 2004, pp. 979–981. Original Russian Text Copyright ? 2004 by Mustafaeva.