Calculated perturbed angular correlations for111In doped cubic single crystals

The perturbed angular correlation functions pertaining to the quadrupolar hyperfine interaction of¹¹¹In activity in cubic single crystals were calculated for various low-index crystallographic orientations of the principle axes of the electric field gradient (efg). The asymmetry of the efg was varied. The capacity and limitations of the method are discussed, especially for the case of non-axial efg.     

Defects in CdS: In detected by perturbed angular correlation spectroscopy (PAC)

The local lattice environment of the donor In in CdS is investigated measuring the electric-field gradient at the site of the radioactive probe atom ¹¹¹In by the perturbed γγ angular correlation technique. It is shown that implantation of In into CdS with subsequent annealing drives 100% of the In atoms to Cd lattice sites. Diffusion of In into CdS under S overpressure results in the formation of In_Cd-V_Cd pairs which seem to be responsible for the self-compensation of In donors in CdS.     

Structural lattice defects in silicon observed at111in by perturbed angular correlation

Probing of structural defects in silicon by the perturbed γγ angular correlation (PAC) technique is demonstrated between 77 K and 1300 K. The behaviour of radioactive¹¹¹ In probe atoms implanted at 295 K, is monitored during isochronal annealing in n-type, p-type and intrinsic Si. Trapping of defects, produced by the¹¹¹In implantation itself or by postirradiation is studied in P-doped crystals (10¹⁶/cm³-10¹⁷/cm³).     

In situ studies of solid state reactions by perturbed angular correlation

We report on three examples of in situ studies of solid state reactions by time differential perturbed angular correlation of¹⁸¹Ta: (i) the oxidation of hafnium metal and the doping of ZrS₂ with Hf during iodine vapour transport crystal growth; (ii) the observation of sublattice melting during polymorphic phase transitions in TaS₂; (iii) the electrointercalation of 2H-TaS₂ with silver.     

Hyperfine interactions of 111Cd in TiO2 (Rutile) studied by perturbed angular correlations

The time-differential perturbed angular correlation for ¹¹¹Cd nuclei has been measured after ¹¹¹In implantation into polycrystalline TiO₂. The observed perturbation functions are characterized by a well-defined electric field gradient with the quadrupole coupling constant ν_Q = 105(1) MHz and the asymmetry parameter η = 0.18(1). We attribute these hyperfine parameters to ¹¹¹Cd on the (distorted) substitutional cation site in rutile. The PAC results are compared with those in SnO₂ rutile as well as with X-ray diffraction, RBS channeling experiments and point charge model calculations including relaxation of the probe atom surrounding.     

Molybdate polymerisation studied by perturbed angular correlation

We determined the average nuclear quadrupole interaction (NQI) at⁹⁹tc in frozen aqueous molybdate solutions at 77K as a function of pH. The polymerisation shows up clearly in the titration curve at pH 6.5 and leads to a dramatic increase of the NQI.on leave from Tanta University, Egypt     

Characterization of Cd implanted and annealed GaAs and InP by perturbed angular correlation (PAC) spectroscopy

Investigations of ^111mCd implanted GaAs and InP crystals using the microscopically sensitive perturbed angular correlation technique show that the implanted Cd is incorporated on unperturbed substitutional lattice sites during rapid thermal annealing at significant lower temperatures than for electrical activation is required. In GaAs the higher implantation temperature at 473 K did not show any influence on this annealing stage, whereas as higher implantation dose hinders the annealing. We conclude that not only the local environment of the implant but also the long-range lattice perfection has to be restored for the electrical activation of implants in III-V compound semiconductors.     

High temperature study of yttrium--barium--copper oxides near the YBa2Cu3Ox composition by 111In/Cd time differential \gamma--\gamma perturbed angular correlation spectroscopy

The local atomic order around very dilute indium impurities in c-axis-oriented YBa₂Cu₃O₇ and YBa₂Cu₃O_6.25 films at room temperature and in YBa₂Cu₃O_x bulk powders held in air or oxygen over a wide temperature range were investigated using ¹¹¹In/Cd \gamma--\gamma perturbed angular correlation (PAC) spectroscopy. The probe atoms occupy a single substitutional lattice position in YBa₂Cu₃O_x, and evidence reported here strongly supports previous claims that this is the yttrium position. In YBa₂Cu₃O_x powders, the local atomic order changes continuously with temperature. At room temperature the electric field gradient measured by PAC in bulk YBa₂Cu₃O_x powder is indistinguishable from that in the fully oxygenated YBa₂Cu₃O₇ film. Near the decomposition temperature, the bulk YBa₂Cu₃O_x powder data are semiquantitatively similar to data in the room temperature YBa₂Cu₃O_6.25 film, with quantitative differences that we attribute to temperature averaging in the former. Other sites usually found in PAC spectra of YBa₂Cu₃O_x bulk samples arise from ¹¹¹In dissolved in Y₂BaCuO₅ and Y₂Cu₂O₅ impurity phases. The latter phase apparently arises because of significant barium loss during processing; the barium deficiency is clearly demonstrated by comparison of PAC data with the alloy phase diagram above the decomposition temperature. PAC data on pure Y₂Cu₂O₅ are reported here also. This revised version was published online in July 2006 with corrections to the Cover Date.     

Positron angular correlation in oriented single crystals of alkali halides

Precision measurements of the angular correlation of two-photon annihilation in polycrystalline and single crystals of NaCl, KCl and KBr are presented. These show a small but systematic variation with crystal orientation. The differences in the angular distributions of different orientations show marked oscillations which extend well beyond the first Brillouin zone. Using the Goldanskii and Prokopev model of polaron states for positions in ionic crystals, the experimental FWHM are used to calculate the effective masses of positions and anion effective charges for different crystallographic directions. The observed variations of effective charge must be taken as the direct experimental evidence for ionic distortions from spherical symmetry. Consequently polaron masses of positrons are different in different directions.     

Vacancy-induced hyperfine interaction in solids detected by perturbed angular distributions

Relaxation effects in time-differential perturbation factors of an implanted nucleus are studied when its environment stochastically changes by fluctuating electric field gradients due to vacancy jumps in the lattice. The average evolution operator can be calculated from easy matrix equations. Special attention is paid to the evolution of a non-equilibrium vacancy distribution around the impurity. Two examples show the validity of approximate solutions for extreme values of the jump frequency and give an insight into the influence of the important parameters. Qualitative agreement with experimental data is reached for the quasistatic case.This work was supported by the Bundesministerium für Technologie and is part of the research program of the Sonderforschungsbereich 161 der Deutschen Forschungsgemeinschaft.     

Temperature dependence of the hyperfine fields of 111In in sapphire (Al2O3) single crystals

Cr has been added to FeCo substituting 10 at.% of Co or Fe in the alloy. The alloys Fe₅₀Co₄₀Cr₁₀, Fe₄₀Co₅₀Cr₁₀ and Fe₅₀Co₅₀ were prepared by mechanical alloying for 2, 5, 10, 20, 40 and 60 h. The formation of the alloy and the incorporation of the elements have been followed by X-Ray Diffraction (XRD) and Mössbauer Spectroscopy. The kinetics of mixing occurs by incorporation of Co and Cr into the Fe structure. After prolonged milling it seems that Cr incorporates itself into both $\upalpha $ -Fe and $\upalpha $ -FeCo structures and a mixture of FeCoCr rich in Cr and FeCoCr rich in Co solid solutions is obtained.     

Perturbed angular correlation study of surface magnetization in iron single crystals

The behaviour of closure domains at the surface of iron single crystals in an external magnetic field was studied by DPAC on samples implanted with¹¹¹In. It is observed that the surface magnetization does not follow that of the bulk. The movement of both the 90 and 180 walls of the closure domains is blocked up to a certain starting value H_s of the external field that is related to the demagnetization factor D of the sample and also depends on the precise orientation of the crystal axes and on the implanted indium dose.On leave from the Institue of Nuclear Physics, Cracow, PolandOn leave from the Institue of Physics, Jagiellonian University, Cracow, PolandThis work forms part of the research program of the Foundation for Fundamental Research of Matter (FOM) subsidized through the Netherlands Organization for Pure Research (ZWO).     

Perturbed angular correlation experiments on111In in oxidized AgCd alloy

The hyperfine interaction of¹¹¹Cd probes in preoxidized and α-irradiated Ag_0.94Cd_0.06 alloy has been investigated by PAC measurements and compared with the results for pure Ag. Several probe atom sites were distinguished and characterized by their hyperfine parameters and temperature evolution. The different behavior of In-oxygen and In-oxygen-vacancy complexes in AgCd and Ag was found and discussed. A considerable influence of the irradiation temperature and sample thickness on the different In-oxygen-vacancy complexes formation was evidenced. The parameters of¹¹¹Cd quadrupole interaction measured during isochronal annealing studies in preoxidized and deuteron irradiated AgCd were very similar to the ones observed after α-irradiation.     

Indium-defect complexes in silicon studied by perturbed angular correlation spectroscopy

The formation of molecule-like complexes, consisting of a defect and a radioactive¹¹¹In atom, is studied using the perturbed angular correlation technique (PAC). The complexes are characterized by their defect specific electric field gradients which also contain information on the geometry of the formed complexes. Whereas the complex is formed with the¹¹¹In atom, its electric field gradient is measured after the decay of the radioactive¹¹¹In atom to¹¹¹Cd. Formation and dissolution of the molecule-like complexes is pursued for a variety of different conditions, such as sample temperature, dopant concentration and position of the Fermi level. In particular, the interaction of In atoms with the following defects in Si was investigated: Intrinsic defects, created by particle irradiation; substitutional donor atoms (P, As, Sb, Bi); and interstitial impurity atoms (Li, H, and an unidentified X defect); especially, the latter ones are known to passivate acceptor atoms in Si. Methodology and specific properties of the PAC technique will be illustrated with the help of these examples.     

Search for magnetic interaction in In doped AlN using perturbed angular correlation spectroscopy

The possible presence of a large magnetic field due to spin polarization of a Cd nucleus (decay product of ¹¹¹In) at an Al substitutional site in AlN is investigated with perturbed angular correlation (PAC) spectroscopy. The PAC spectra of ¹¹¹In/¹¹¹Cd in AlN show two probe environments: a weak quadrupole interaction (quadrupole interaction constant, $\nu _{\rm Q}^{\,\,\,\rm lattice} = 30$  MHz) due to ¹¹¹In probes at a defect free Al substitutional site and an unknown large interaction ( $\nu _{\rm Q}^{\,\,\,\rm complex} = 300$  MHz) tentatively attributed to a nearest neighbour pair between ¹¹¹In and a nitrogen vacancy (V_N) aligned along the c-axis. Surprisingly, in density functional theory (DFT) calculations, such a large electric field gradient (EFG) could not be reproduced. However, an inclusion of spin polarization in the calculations indicates a strong magnetic field at ~50 % of the ¹¹¹In/¹¹¹Cd site. An attempt to verify the presence of the strong magnetic field and to explain the origin of the strong interaction is made. Orientation measurements show, the large interaction is not characterised by a magnetic interaction and is predominantly due to the EFG. However, in the presence of an external magnetic field, the strong interaction probe environment becomes more uniform and the EFG increases by ~10 %. This definitely hints towards some sort of magnetic interaction at the strong interaction probe site.     

Ionic and electronic transport in In2S3 studied via perturbed angular correlation spectroscopy

We report on Perturbed Angular Correlation measurements in polycrystalline In₂S₃ samples in the temperature range from 8 K to 1000 K where two different crystallographic phases β and α occur. As probes, implanted ¹¹¹In nuclei have been used. The three observed EFGs are attributed to probes residing substitutionally in the different sulfur-octahedra and -tetrahedra of β-In₂S₃. A strong damping between 150 K and 300 K has been attributed to EFG fluctuations following the ¹¹¹In(EC)¹¹¹Cd decay. The α-phase (above 680 K) is characterized by a different dynamical damping of the perturbation functions, caused by mobile In atoms. Therefore, the semiconductor In₂S₃ shows, in two different temperature ranges, dynamical PAC-spectra which correspond to different types of mobile charge carriers. Since ¹¹¹In is a self atom in In₂S₃, this compound is an ideal substance to study the charge transport phenomena by the PAC technique. This revised version was published online in August 2006 with corrections to the Cover Date.     

Monitoring positional changes in bixbyite oxides with perturbed angular correlation spectroscopy

The refinement of atomic positions with perturbed angular correlation spectroscopy for several isostructural sesquioxides M₂O₃ (M=Sc, In, Y, Yb, Dy, Ho, Gd, Sm) is presented. The experimental electric field gradients of¹¹¹Cd residing at the two substitutional sites were reproduced by the simple point charge model with an accuracy of less than 1% by varying the coordinates (available from X-ray and neutron diffraction experiments) within their experimental uncertainty. In this way, for each oxide coordinates were obtained having an accuracy of 0.3 pm, which is typically five times better than that of available X-ray and neutron diffraction results.     

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 ε phases were investigated using the time-dependent perturbed angular correlation (TDPAC) technique, with¹⁸¹Ta as the nuclear probe. Two EFGs were found in the ε phase, indicating the existence of two major symmetries surrounding the Ta probe. The results indicate that the trend of the EFGs, in the ε phase, are due to the changes in the lattice parametersa _o,c _o 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.