Lifetime spectra of positrons in ionic compounds

Lifetime spectra of some sulphides, AlN, and SiC consist of two components. Temperature dependences of the spectra of Sb₂S₃ were investigated from 296–723 K and analyzed with a non-trapping model. It was estimated that cation vacancies due to nonstoichiometry were responsible for the τ₂ component. Paper C8 presented at 3rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973).     

Recovery of deformed iron studied by positrons

The isochronal annealing of two plastically deformed irons of 99.998% and 99.86% purities is studied by positron lifetime and annihilation line shape measurements. The results show that trapping of positrons is caused by dislocations. At around 300° C a recovery process attributed to a rearrangement of dislocation structure is observed in the pure but not in the impure iron. The positron lifetime in deformed and recrystallized state is found to be 167 ps and 117 ps, respectively.     

Ferromagnetic properties of cyclically deformed Fe3Ge and Ni3Ge

The magnetisation behaviour of cyclically deformed and non-deformed Fe₃Ge and Ni₃Ge is examined at sufficiently low temperatures below the Curie point. Despite these two intermetallics having the same L1₂ structure, they are found to show quite different behaviour in their ferromagnetic properties; the spontaneous magnetisation (M _S) remains unaffected in the former whereas it decreases notably in the latter after cyclic deformation. The origin of the difference is investigated and attributed to the difference in operative shear planes. These are mainly on {001} planes without the introduction of notable amounts of anti-phase boundary (APB) tubes in Fe₃Ge and mainly on {111} planes with the introduction of a high density of APB tubes composed of {111} APBs in Ni₃Ge. The effects of cyclic deformation on the high-field susceptibility?χ?and the coercive force (H _C) are also discussed by taking into account the dislocation distributions introduced by the {001} and {111} slips.     

Hadronic spectra from deformed AdS backgrounds

Because of the presence of modified warp factors in metric tensors,we use deformed AdS_5 spaces to apply the AdS/CFT correspondence to calculate the spectra for even and odd glueballs,scalar and vector mesons,and baryons with different spins.For the glueball cases,we derive their Regge trajectories and compare them with those related to the pomeron and the odderon.For the scalar and vector mesons,as well as baryons,the determined masses are compatible with the PDG.In particular,for these hadrons we found Regge trajectories compatible with another holographic approach as well as with the hadronic spectroscopy,which present an universal Regge slope of approximately 1.1 GeV~2.     

Secondary electron spectra of gold under bombardment by very low-energy positrons

Measurements of the secondary electron energy spectra resulting from very low-energy positron bombardment of a polycrystalline Au surface are presented. The low-energy part of the secondary spectra contain significant contributions from two processes: (1) annihilation-induced Auger electrons that have lost energy before leaving the surface and (2) secondary electrons resulting from direct energy exchange with an incident positron. Our data indicate that the second process (direct energy exchange with the primary positron) is still important at and below 3 eV incident beam energy. Since energy conservation precludes secondary electron generation below an incident beam energy equal to the difference between the electron and positron work functions (∼3 eV), the fact that we still observe significant secondary electron emission at energies at or below this value provides strong evidence that the incident positrons are falling directly into the surface state and transferring all of the energy difference to an outgoing secondary electron. These measurements were also used to obtain the first experimentally determined upper limit on the intensity of the spectrum of Auger-induced secondary electrons.     

Drag thermoelectric power of uniaxially deformed n-type Ge in quantizing magnetic fields

An investigation is made into the drag thermoelectric power of pure n-type Ge deformed strongly along [111] in quantizing magnetic fields for the phonon-phonon relaxation mechanisms of Herring and Simons. Calculations are made for a wide range of magnetic fields from classical strong fields to maximal quantizing fields, including the region of so-called weak quantization. An experiment is suggested that could indicate the mechanism of phonon-phonon relaxation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 66–70, October, 1980.We thank Professor A. G. Samoilovich for assistance in the work.     

On the trapping rate of positrons in deformed lead over the temperature range 4–100 K

The trapping of positrons in annealed and plastically deformed samples of lead has been studied over the temperature range 4–100 K. The rate of trapping by defects has been found to be independent of temperature — a finding that supports the golden rule positron trapping model.     

Lifetime measurements of spherical and deformed states in 1f7/2 nuclei

Lifetimes have been determined using the Doppler Shift Attenuation Method in the N ≈ Z nuclei ⁴⁶V and ⁴⁸V, populated with the reaction ²⁸Si on ²⁴Mg at 115 MeV and ²⁴Mg on ²⁸Si at 100 MeV using Au and Pb backed targets. The coexistence of spherical and deformed states in the middle of the 1f_7/2 shell is discussed. The B(E2) and B(M1) reduced rates agree very well with large scale shell model predictions.     

Spin-orbit splitting in theN-Δ spectra and the deformed baryon model

The absence of appreciable spin-orbit splitting in the low-lying even and odd parity states of the nucleon and delta is puzzling in conventional quark models. A constitutent quark model, in which the quarks interact through gluon as well as pion exchange, and the baryon is allowed to deform in the excited states, may provide a solution to the problem.     

Gamma spectra resulting from the annihilation of positrons with electrons in a single core level

The first gamma spectra associated with the annihilation of positrons with individual core levels (Cu 3p and Ag 4p) are presented. The spectra were obtained by measuring the energy of gamma rays time coincident with Auger electrons emitted as a result of positrons annihilating with a selected core level. Relativistic calculations show good agreement with experiment over a limited range of momenta. However, statistically significant differences indicate that the measurements can provide an impetus to new calculations of many body effects in positron-core electrons annihilation.     

Optical absorption spectra of Si with Ge quantum dots

The results of calculations of optical absorption spectra of silicon containing Ge nanoclusters of spherical shape and different size are reported. The optical transitions from the Ge cluster levels to the silicon bulk energy band states are analyzed.     

On the projected spectra from a deformed correlated intrinsic state

It is argued that the residual interaction being relatively of long range, should produce Random Phase Approximation (RPA) type of correlations in the Hartree-Fock (HF) intrinsic state. A model is described to take these correlations inti account in the intrinsic state. A comparison of the projected spectra from this state with the exact shell model diagonalization for a model problem bears out this point. An application of the model is distinguishing two almost degenerate HF solutions for the 2s-1d shell nuclei is mentioned.     

Phonon localization in Ge nanoislands and its manifestation in Raman spectra

Multilayer structures with germanium nanoislands that are formed on the silicon (111) surface upon submonolayer deposition by molecular-beam epitaxy have been investigated using Raman spectroscopy. To interpret the experimental Raman spectra, numerical calculations of the spectra have been performed for nanoislands containing from several to several hundred germanium atoms. The calculations demonstrate that the in-plane sizes of nanoislands (with sizes less than 2–3 nm) substantially affect the frequencies of phonons localized in these nanoislands. The experimental Raman spectra confirm the occurrence of the quantum size effect.     

Motion of positrons in metals

In this review we discuss the different aspects of positron annihilation in metals that involve the dynamics of positron motion before annihilation. The emphasis is on the theory, but also some experimental evidence is quoted. The topics covered are: slowing down and thermalization, effective mass, temperature dependence of positron vacancy trapping, positron channeling, and escape of low energy positrons from metal surfaces.     

Above the band gap structures in Ge nanoparticles: optical-absorption spectra

Summary We report absorption measurements on Ge nanoparticles in the range 0.6–6 eV. By reducing the dot average radius to about 13 ?, one observes a relevant blue-shift of theE ₁+Δ ₁ andE ₂ peaks and a change of the oscillator strength of the two main structures. Simulations using effective-medium theories have been carried out and the embedding matrix contribution to the measured properties has been evaluated. The different behaviour of the absorption peaksE ₁,E ₁+Δ ₁ andE ₂ under quantum confinement has been related to their specific nature within the whole Brillouin zone. The authors of this paper have agreed to not receive the proofs for correction.     

Mobility of positrons in silicon

The drift velocity v₊ of positrons in Si has been measured by observing the Doppler shift of the annihilation γ's. The electric field dependence of v₊ yields the positron mobility μ₊: at 80 K μ₊=460±20 cm²V^-1 sec^-1 and at 184 K μ₊=173±15 cm²V^-1 sec^-1.     

Comparison of Si and Ge low-loss spectra to interpret the Ge contrast in EFTEM images of Si(1-x) Ge(x) nanostructures

Recently, an EFTEM imaging method, exploiting the inelastically scattered electrons in the 60-90eV energy range, was proposed to visualise Ge in SiGe alloys [Pantel, R., Jullian, S., Delille, D., Dutartre, D., Chantre, A., Kermarrec, O., Campidelli, Y., Kwakman, L.F.T.Z., 2003. Inelastic electron scattering observation using Energy Filtered Transmission Electron Microscopy for silicon-germanium nanostructures imaging. Micron 34, 239-247]. This method was proven to be highly more efficient in terms of noise, drift and exposure time than the imaging of the weak and delayed ionization GeL2,3 edge at 1236eV. However, the physical phenomenon behind this Ge contrast was not clearly identified. In this work, we explain the origin of this Ge contrast, by comparing in details EELS low-loss spectra (<100eV) recorded from pure Si and Ge crystals. High resolved low-loss experiments are performed using analytical Field Emission Gun Transmission Electron Microscopes fitted or not with a monochromator. Low-loss spectra (LLS) are then deconvoluted from elastic/quasi-elastic and plural scattering effects. The deconvolution procedure is established from Si spectra recorded with the monochromated machine. The absence of second plasmon and the measurement of a band gap (1.12eV) on the Si single scattering distribution (SSD) spectrum allowed us to control the accuracy of the deconvolution procedure at high and low energy and to state that it could be reliably applied to Ge spectra. We show that the Ge-M4,5 ionisation edge located at 29eV, which is shadowed by the high second plasmon in the unprocessed Ge spectrum, can be clearly separated in the single scattering spectrum. We also show that the front edge of Ge-M4,5 is rather sharp which generates a high intensity post edge tail on several tens of eV. Due to this tail, the Si and Ge EELS signals in the 60 to 100eV energy window are very different and the monitoring of this signal gives information about the Ge concentration inside SiGe alloys. It is now evident that the EFTEM imaging technique proposed to quantify Ge (90eV/60eV image ratio) in Si-Ge nanostructures is valid and is a relevant way of exploiting the Ge-M4-5 ionisation edge.     

73Ge NMR spectra in germanium single crystals with different isotopic composition

We have studied the influence of isotopic disorder on the local deformations in Ge single crystals from both experimental and calculation points of view. The nuclear magnetic resonance (NMR) spectra of⁷³Ge nuclei (the nuclear spin equals 9/2) in perfect single crystals of germanium with different isotopic content were measured at temperatures 80, 300 and 450 K. Abnormal broadening of the spectrum was found to occur when the magnetic field was aligned along the [111] axis of a crystal. The observed specific angular dependence of the quadrupole broadening was attributed to isotopic disorder among atoms of germanium sited around the⁷³Ge NMR probe. Local lattice deformations in germanium crystal lattice due to isotopic impurity atoms were calculated in the framework of the adiabatic bond charge model. The results obtained were applied to study random noncubic crystal field interactions with the nuclear quadrupole moments and corresponding effects in NMR spectra. Simulated second and fourth moments of resonance frequency distributions caused by the magnetic dipole-dipole and electric quadrupole interactions are used to analyze the lineshapes, theoretical predictions agree qualitatively with the experimental data.