Angular Correlation Studies Research Articles

Positron angular correlation studies on rare earth substituted 123 compounds

Positron angular correlation studies of REBa 2Cu 3O 7−x (RE=Y, Nd, Sm, Eu, Gd, Ho, Er) compounds were made. The ACPAR curves were fitted with parabolic and Gaussian components. The integrated deviation is directly related to T c.

The magnetic phase diagram of La2−x Sr x CuQ4+y observed via111In PAC

We report perturbed angular correlation studies of111In diffused into La2−xSrxCuO4+y. We determine the Neel temperature, TN, as a function of x for y ≈ 0 and provide an empirical calibration of the charge defect concentration via the PAC linewidth.

Electron-photon angular correlation study of electron impact excitation of the 4p5(2P3/2)5s3P1 state of krypton

Excitation of the 4p5(2P3/2)5s3P1 state of krypton by 30 eV incident electrons has been studied using the electron-photon angular correlation technique. The alignment and orientation of this state can be completely described using four independent parameters, three of which can in principle be obtained from the present measurements of angular correlations with two detector geometries. Difficulties encountered in the extraction of the spin-dependent parameters are discussed. There is generally good agreement between the experimentally measured angular correlations and distorted-wave Born approximation predictions.

Positron annihilation studies on nasicon analogues containing cation vacancies

Positron annihilation studies were carried out on the Nasicon analogue Na2(La, Al)Zr(PO4)3 compound for three different concentrations (2·2, 2·8 and 5·2 by wt.%) of ZrO2 in the nutrient. Angular correlation study of annihilated photons reveals that the defect concentration is maximum for 2·8 (wt.%) of ZrO2. Further, positron lifetime studies indicate that the positrons are trapped at cation vacancies. Application of a two-state trapping model to this system made it possible to evaluate the lifetime of positrons in the Bloch state and of positrons trapped at cation vacancies.

Time differential perturbed angular correlation studies of hafnium hydrides

The dependence of the electric field gradients (EFG) in Hafnium-Hydride systems as a function of the composition H/Hf in the cubic δ and tetragonal e phases were investigated using the time-dependent perturbed angular correlation (TDPAC) technique, with181Ta as the nuclear probe. Two EFGs were found in the e phase, indicating the existence of two major symmetries surrounding the Ta probe. The results indicate that the trend of the EFGs, in the e phase, are due to the changes in the lattice parametersao,co as hydrogen is added to the Hf-H system. In the δ phase, only one major symmetry was found. Both phases are characterized by broad frequency distributions and large anisotropies.

Positron Localisation in Metallic Glasses in Terms of the Crystalline Embryo Model

One-dimensional angular correlation studies of the Ni73TM5Si10B12 (TM=V, Cr, Mn, Fe and Co) metallic glasses have been performed. The analysis of the angular correlation data, consisting of a deconvolution into one Gaussian and two parabolic curves, indicates the possibility that two 'species' of the nearly free electrons are present in the particular materials. We have attempted to verify the physical reliability of this fitting procedure by assuming that the crystalline embryos tend to localise the thermalised positrons. It is shown that some of the estimated parameters describing the shape of the angular correlation curves systematically change with changing the atomic number of the TM admixtures. It allows the effect of TM on the thermal stability to be precisely recognised.

Perturbed angular correlation studies of the Hf—EDTA complex

The integral perturbed angular correlation (IPAC) technique has been used to determine the stability constant of the Hf—EDTA complex in 4.6 M HClO 4. The stability constant (log K) was found to be: 30.1±0.44. The time differential perturbed angular correlation (TDPAC) study of polycrystalline Hf—EDTA was carried out to estimate the quadrupole interaction frequency ( w Q) and the asymmetry of the EFG for the complex, before and after heating at 160°C. These parameters are very similar in these two cases and indicate the possibility of partial polymerization through OH bridging, whereby the immediate chemical surrounding of the hafnium nucleus is expected to remain unchanged.

A study of angular correlations of projectile and target fragments in 1.4 A GeV Kr+Ag(Br) collisions

A sample of high multiplicity quasi-central Kr84+Ag(Br) interactions has been'analysed in terms of angular correlations between projectile and target fragments. The azimuthal correlation of PFs and TFs suggests the presence of a weak collective (bounce-off) effect. Other observations like, target recoil velocity,F/B ratio of TFs with respect to direction of their source etc. are interpreted in terms of a simple two body quasi-elastic scattering and a modified Maxwell-Boltzmann distribution.

PAC study of magnetic critical behavior and line broadening in La2CuO4

We report perturbed angular correlations studies of 111In diffused into La2CuO4 and find a mean field value of β = 0.50(4) in material with a Néel temperature of TN = 317 K. We also provide a calibration of the linewidth in La2−xSrxCuO4 supporting argument that TN = 317 K material is near stoichiometric.

Surface physics with nuclear probes

A series of typical experiments using nuclear moments as microscopic probes for hyperfine fields on surfaces are discussed: Mossbauer spectroscopy, perturbed angular correlation studies, nuclear magnetic resonance, nuclear spin relaxation due to spin-lattice relaxation, and low-dimensional diffusion. Experiments are selected which deal mainly with well-characterized surfaces. Out of a wealth of data, particularly those that contribute to currently discussed topics in surface science are selected.

Perturbed angular correlation studies of dopant atom interactions in silicon

The perturbed γγ angular correlation (PAC) technique was used to study the interaction of implanted 111 In probe atoms with the donor atoms P, As, and Sb in Si. Nearest-neighbor pairs of In-P, In-As, and In-Sb atoms, characterized by νQ1 =179(1), 229(1), and 271(1) MHz, respectively, and having trigonal symmetry about a 〈111〉 axis (η1 =0), were observed after annealing the samples between 540 and 1170 K. These results indicate a strong interaction between acceptor and donor atoms in Si, thus explaining the present and earlier Rutherford backscattering-channeling results that the In atom solubility in Si was enhanced by the addition of As. The In-donor atom binding energy was about 0.5 eV. For increasing As concentrations, PAC data showed the appearance of small In-As atom clusters characterized by νQ2 (As)=238(1) MHz, η2 = 0.65(1); they were probably In-As2 complexes produced when mobile In atoms were trapped by As2 pairs.

Perturbed angular correlation study of impurity interactions in a cubic host metal: Hf in Nb.

{sup 181}Ta perturbed angular correlation experiments have been performed on Nb-Hf alloys containing 0.5--5.5 at. % Hf. A Maxwellian frequency distribution model is proposed to account for the electric-field-gradient distribution due to impurities in a cubic matrix. The model was used to analyze the Nb-Hf data. If an unperturbed fraction is included with the Maxwellian distribution, then agreement with experiment is excellent. The unperturbed site is apparently required to correct for underestimation of the actual distribution by the Maxwellian at low frequencies. The unperturbed fraction is large only for small impurity concentrations and corresponds approximately to sites more distant than five lattice spacings from the nearest impurity. The peak of the Maxwellian quadrupole frequency distribution is linear in Hf concentration.

Study of angular correlation by N.M.R. of magnetically aligned molecules

A theoretical description of angular correlation in N.M.R. of diamagnetically aligned molecules is presented. High field deuterium N.M.R. is employed to study solutions of benzene, naphthalene and anthracene in binary mixtures of cyclohexane and hexafluorobenzene. Strong orientational cross correlation between the aromatic hydrocarbons and hexafluorobenzene is detected.

Isolated indium atoms on copper surfaces: A perturbed γ-γ angular correlation study

Isolated indium atoms on copper surfaces: A perturbed γ-γ angular correlation study

Isolated indium atoms on copper surfaces: A perturbed γ-γ angular correlation study

IIIIn hyperfine probe atoms with a concentration of about 10 −4 ML have been utilized as nuclear observers on various copper surfaces. These isolated IIIIn probe atoms are highly sensitive to the local atomic structure via the electric-field-gradient tensor acting at the probe nuclear site; this quantity has been detected using the perturbed γ — γ angular correlation (PAC) method. The experiments deal with the localization of indium on substitutional terrace sites for Cu(100), Cu(111) and Cu(110) surfaces. Step sites have been observed in addition on vicinal Cu(111) and from thermal conversion between different sites diffusion data have been deduced. The electric field gradient has been investigated on Cu(110) for IIIIn deposited at various depths from the surface, revealing the extreme short-range character of the electric field gradient. Finally depositions (few percent of a ML) of natural indium have allowed to study formation and thermal decomposition of In-In configurations on copper surfaces.

Effect of cholesterol on the uptake and intracellular degradation of liposomes by liver and spleen; a combined biochemical and γ-ray perturbed angular correlation study

We investigated the effect of cholesterol on the uptake and intracellular degradation of liposomes by rat liver and spleen macrophages. Multilamellar vesicles (MLV) consisting of distearoylphosphatidylcholine/phosphatidylserine (molar ratio 9:1) or distearoylphosphatidylcholine/cholesterol/phosphatidylserine (molar ratio 4:5:1) were labeled with [ 3H]cholesteryl hexadecyl ether and/or cholesteryl [ 14C]oleate. After i.v. injection the cholesterol-containing liposomes were eliminated less rapidly from the bloodstream and taken up to a lesser extent by the liver (macrophages) than the cholesterol-free liposomes. Assessment of the 3H/ 14C ratios in liver and spleen cells revealed that the cholesterol-containing liposomes are substantially more resistant towards intracellular degradation than the cholesterol-free liposomes. These results could be confirmed by measuring the release of 111In from liposomes after uptake by liver and spleen by means of γ-ray perturbed angular correlation spectroscopy. Experiments with cultured Kupffer cells in monolayer also revealed that incorporation of cholesterol results in a decrease of the uptake and an increase of the intracellular stability of cholesteryl [ 14C]oleate-labeled liposomes. Finally, incubation of both types of liposomes with lysosomal fractions prepared from rat liver demonstrated a difference in susceptibility to lysosomal degradation: the cholesterol-free vesicles were much more sensitive to lysosomal esterase than the cholesterol-containing liposomes. These results may be relevant to the application of liposomes as a drug carrier system to liver and spleen (macrophages).

Perturbed Angular Correlation Study of Hydrogen Diffusion in Hydrides of Crystalline and Amorphous Zr2Ni*

Article Perturbed Angular Correlation Study of Hydrogen Diffusion in Hydrides of Crystalline and Amorphous Zr2Ni* was published on February 1, 1989 in the journal Zeitschrift für Physikalische Chemie (volume 163, issue 2).

Perturbed Angular Correlations Studies of Alloys

Perturbed Angular Correlations Studies of Alloys

Local atomic order and re-entrant magnetic behaviour in Au85Fe15

A perturbed angular correlation study on Au85Fe15 in different metallurgical states and over a wide temperature range is reported. The hyperfine field and its temperature variation reflect a cluster glass behaviour due to chemical clusters of different sizes rather than a magnetic double transition.

Perturbed angular correlation study of local diffusion in highly concentrated Zr 2Ni hydrides and deuterides

The diffusion of deuterium in crystalline Zr 2NiD 5 was observed between 210 K and 470 K by gamma-gamma perturbed angular correlation (PAC) spectroscopy on a 181Ta probe substituted for zirconium. The temperature dependence of the relaxation time of the nuclear spins confirms the existence of two diffusive processes as previously observed in Zr 2NiH 4.8, one of them being characterized by a low activation energy (≈ 0.1 eV) and an attempt frequency factor of ≈ 10 9 s −1. However, the absence of an important isotopic effect indicates that any significant contribution of a tunnelling mechanism should be discarded. It is suggested that this process could rather correspond to the diffusion of H(D) atoms localized in interstitial holes with a nickel-rich environment.