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Employing β-NMR on 8 Li adsorbed on Ru(0 0 1) the local (at the Li nucleus) density of states at the Fermi energy has been determined as function of Li coverage. It is independent of coverage below 0.2 monolayer (ML) and saturates at a twice as large plateau value beyond 1 ML. Calculations employing all electron density functional theory reproduce qualitatively the trend of the data. At low and high coverages, agreement between theory and experiment is almost quantitative. In addition, new terrace diffusion parameters for Li on Ru(0 0 1) have been obtained.

We have investigated epitaxial YBCO films by Perturbed γγ-Angular Correlation Spectroscopy (PAC) using 111In probe atoms. The probes substitute on the yttrium lattice site and hyperfine parameters have been determined unambiguously. Excellent agreement is achieved with full potential electric field gradient (efg) calculations. Good reproducibility has been achieved by our preparation method allowing us to study the influence of oxygen motion on the structural orthorhombic to tetragonal phase transition.

Oxygen-deficient, tetragonal thin films of YBa2Cu3O6+x with x ≈ 0.25, quenched from the deposition temperature, change to the oxygenated, orthorhombic phase with x ≈ 1, between 200 °C and 400 °C in flowing oxygen. The transition is not reversible in flowing oxygen, and cannot be completely reversed by cooling in flowing argon. We do not observe a transition of the orthorhombic films to the tetragonal phase up to 800 °C in flowing oxygen. We observe that the major impurity phases to appear under nonoptimal annealing conditions are oriented phases of YcuO2 and BaCu2O2, with Y2BaCu2O5 and Y2Cu2O5 conspicuously absent. These conclusions have been drawn from a study that uses perturbed angular correlation spectroscopy to probe the local microstructure of the films.

Indium-doped zinc oxide powders have been prepared which show room-temperature electrical conductivities as high as 30 Ω−1cm−1. The indium doping apparently occurs as Zn1−xInxO, Zn1−yInyO1+y/2, or a combination of these. Optimum conductivity occurs for Zn1−xInxO where the maximum value ofxobtained was about 0.5 at%. This substitution results in a lattice volume expansion of 0.4%. The degrees of sample reduction were determined by iodimetric titration. Time differential perturbed angular correlation (TDPAC) spectroscopy on indium doped zinc oxide is consistent with indium substituting at normal zinc sites in the ZnO lattice. TDPAC studies on zinc oxide annealed under zinc vapors show a second environment for the111In probe. In this case, there is an unusually high temperature dependence of the electric field gradient which may be caused by a nearby zinc interstitial. An important conclusion of this work is that zinc interstitials are not ionized and do not therefore contribute significantly to the increased conductivity of reduced zinc oxide.

Very dilute (10{sub 12} cm{sup {minus}3}) indium donors in CdTe and ZnSe powders and in CdTe single crystals were investigated using {sup 111}In Perturbed angular correlation spectroscopy. Most indium atoms are in uncomplexed sites but can form weakly-bound complexes with native defects in very defective material. The only complex observed in CdTe is an indium-Cd vacancy pair. The CdTe in which these pairs occur is apparently n-type, most Cd vacancies are free and doubly-charged, and the binding energy with indium is 0.15 eV. In ZnSe, indium can pair with a Zn vacancy or with some other presently unidentified defect. These complexes form in ZnSe containing large concentrations of both free Zn vacancies and complexes of Zn vacancies with other defects. In CdTe, the pair formation equilibration time constant is two days at 15C,an implication that Cd vacancies are mobile at room temperature. Lattice relaxation around a Cd vacancy in CdTe was probed by single crystal PAC experiments.

The present status of surface, interface andthin film studies with perturbed γγ-angular correlation (PAC) spectroscopy is reviewed. Applications include adsorbate diffusion andstructural phase transitions on stepped metal surfaces, surface andinterface magnetism as well as compound formation at reactive metal/metal interfaces. First applications to semiconductor surfaces andmetal/semiconductor interfaces are discussed.

PAC spectroscopy has been used to study the compound formation at the Pd(100)/In interface. Indium films of various thicknesses have been deposited onto Pd(100) single crystals. The111In PAC probes are either deposited on the Pd(100) surface before covering it with In or introduced into the In metal before producing the interface. Pd/In compounds with polycrystalline structure of various stoichiometry are identified from the detection of significant electric field gradient tensors after isochronal as well as isothemal annealing. In particular we find an ordered structure at the interface directly after In deposition.

Growth and melting behaviour of thin indium films on Ge(100) have been investigated by Auger-electron spectroscopy (AES), atomic force microscopy (AFM) and perturbed γγ angular correlation (PAC) spectroscopy, respectively. At room temperature inidium is found to grow in three-dimensional islands even at submonolayer coverages. A very rough film surface is observed for thicknesses up to 230 ML. The melting behaviour of such films has been studied by PAC. A reduction of the melting temperature T m as well as a strong supercooling of the films is observed. The electric field gradient for 111In(111Cd) in the indium islands is determined as a function of temperature and is used to monitor the local crystalline order of the films up to temperatures just below the melting point.

The magnetic behavior of the bordering Ni and Cu monolayers at the interfaceof ultrathin Cu films on Ni(111) has been investigated. 111 In probes have been used, utilizing perturbed γγ angular correlations, to study strength, orientation, and temperature dependence of the magnetic hyperfine field at the probe nuclear site. This magnetic hyperfine field is strongly reduced to about 50% in the first Ni layer covered by Cu; it vanishes in the adjacent Cu layer. The results are compared to hyperfine fields at free Ni(111) surfaces, to magnetometry measurements, and to theoretical predictions

Magnetic and electric hyperfine fields were measured at 111 In( 111 Cd) probe atoms in the topmost atomic layer of Ni(111) and Ni(100) single-crystal surfaces as well as in ultrathin Ni films epitaxially grown on Cu(100), utilizing the perturbed γγ-angular correlation (PAC) method. The behaviour of the magnetic hyperfine fields as a function of temperature were studied for both Ni surfaces as well as for different Ni(100) films with thicknesses ranging from 2 up to 10 monolayers. It was found that strength and orientation of the magnetic hyperfine fields are quite different for these systems. The critical temperatures are strongly reduced for thin nickel films and approach the bulk value with increasing film thickness, whereas at the surfaces no change in the Curie temperatures was detected.

Interface compound formation in Ni/In film couples has been studied by means of the PAC method using radioactive111In probe atoms. Subsequent occurrence of the compounds Ni10In27, Ni2In3, NiIn and Ni3In has been observed after isochronal annealing. Interdiffusion was found to start at temperatures about 250K whereby Ni propagates into the In film.

Applications of hyperfine-interaction techniques, like NMR, PAC and Mosbauer spectroscopy, to well-characterized surfaces are discussed and the present knowledge of surface hyperfine fields is reviewed. Measurements of nuclear spin relaxation permit to extract the local density of electron states at the Fermi level of adsorbed alkali atoms. From the observed electric-field-gradient properties surface probe sites and diffusion processes can be inferred; the experimentally determined magnetic hyperfine fields give access to the electron-spin behaviour at magnetic surfaces.

The magnetic hyperfine field and the electric-field gradient of free single-crystal Ni(111) surfaces have been investigated with the perturbed yy angular correlation (PAC) method using radioactive 111 In probe atoms with a concentration of 10 −4 ML. The hyperfine fields have been measured at probe atoms in the top-most surface monolayer, in the 3.5th monolayer and in bulk over a temperature range from 80 K up to the Curie temperature.

Diffusion processes on vicinally cut Ag(100) and Ag(111) surfaces have been studied utilizing the perturbed-angular-correlation (PAC) spectroscopy with 111 In probes at very low concentration (10 −4 ML). The occupation of step and terrace sites by PAC probes at the Ag surfaces is detected via the electric-field-gradient tensor and is observed as a function of annealing temperature. From the thermal conversion between the different 111 In surface sites activation energies for microscopic diffusion steps have been estimated.

The diffusion behaviour of indium atoms on various vicinal silver and copper surfaces has been investigated by the perturbed γγ-angular correlation (PAC) technique. The electric-field-gradient tensor, acting as a ‘fingerprint’ for 111In probe sites on the surface, was used to study diffusion on an atomic scale. The results can be interpreted within a microscopic diffusion model. From the conversion temperatures, activation energies for the different diffusion steps are estimated.

The appearance of a reversible order-disorder transition at Cu(115) and Cu(117) surfaces has been investigated using perturbed angular correlation spectroscopy. The local atomic arrangement at 111 In probes is detected via nuclear quadrupole hyperfine interaction. A broadening of the electric field gradient distribution is measured for elevated temperatures due to structural changes around the probes. The onset of disorder on the atomic scale is found to be well above the roughening transition temperatures measured by various scattering techniques

The perturbed γγ angular correlation method has been applied to study compound formation in Ni/In thin film couples. During an isochronal anneal cycle a variety of electric field gradient situations has been observed at 111In/111Cd probe nuclei, corresponding to the formation of different Ni/In compounds. Structure and stoichiometry of the compounds has been determined by comparison with bulk samples of the stable Ni/In alloys.

The PAC method was applied to study interface compound formation in thin In/Sb film couples. Above annealing temperatures of 220 K, kept for 10 min, the compound InSb is growing out of the interface. The average thickness of InSb increases with the square root of time at the beginning, indicating a diffusion controlled start of interface compound growth.

The electric field gradient at111In probe atoms on Cu (100) surfaces was studied. At clean surfaces all probes are exposed to a well-defined surface field gradient. This is used to investigate indium surface diffusion, where the applied PAC method allows to observe diffusion steps on an atomistic scale. The jump rate for indium on Cu (100) was found to be in the order of 10−3 Hz at 200 K.

The perturbed γγ angular correlation (PAC) method has been applied to study isolate 111In probe atoms on vicinal Cu(111) surfaces. The properties of the observed electric-field gradients and the annealing behaviour suggest the existence of indium as step adatoms up to 140 K, above this temperature these adatoms convert into substitutional step atoms. At 220 K the indium probes finally start to diffuse into substitutional terrace sites. Desorption and diffusion into bulk was also detected. From the thermal conversion between the different sites activation energies have been estimated.

The perturbed gamma gamma angular correlation (PAC) method has been applied to study isolated 111In atoms on low index silver surfaces. Strong, axial-symmetric electric field gradients are observed on the highly symmetric (111) and (100) surfaces, whereas a non-axial-symmetric field gradient is found for the (110) orientation. Strength and symmetry of the electric field gradients have been studied as a function of measurement temperature. The influence of impurities on the field gradient distributions is discussed. Finally, a comprehensive comparison is given to results reported previously for 111In on the respective copper surfaces. The observed behaviour reflects well known features of the bulk metals.

III In hyperfine probe atoms with a concentration of about 10 −4 ML have been utilized as nuclear observers on various copper surfaces. These isolated III In 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 III In 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.