Nuclear <Superscript>111</Superscript>Cd probes detect a hidden symmetry change at the γ → α transition in cerium considered isostructural for 60 years

We use the time-differential perturbed angular correlation technique to study nuclear electric quadupole hyperfine interactions of probe ¹¹¹Cd nuclei in cerium lattice sites at room temperature under pressures up to 8 GPa. We have found that the well known γ → α phase transition in cerium is not isostructural. In α-Ce, the probe ¹¹¹Cd nuclei reveal a quadrupole electron charge density component that is absent in γ-Ce. The hidden spacial structure of electronic quadrupoles in α-Ce is triple-q antiferroquadrupolar, as was suggested in [14]. We relate our findings to the current understanding of the γ → α phase transition and also report on nuclear quadrupole interactions in other high-pressure phases of cerium: α″ (C2/m space symmetry) and α′ (α-U structure).     

Migration and agglomeration of heliumatoms in copper

The magration and agglomeration of Helium atoms in He irradiated Cu was studied between 10 K and 900 K by perturbed γγ angular correlation (PAC) measurements using the radioactive probe atom¹¹¹In. Trapping of interstitial and substitutional He atoms is observed already after irradiation at 10 K. Substitutional He (He-yacancy pair) is bound in the nearest neighbourhood to the¹¹¹In probe atom and seems to be stable up to 725 K (E _He1V1 ^b ≥2.1 eV). The onset of vacancy assisted He agglomeration in Cu is observed at 250 K.     

Magnetic Properties of Iron Clusters in Silver

Time differential perturbed γ-γ angular correlation technique was used to measure the magnetic hyperfine field (MHF) at Tb sites in the intermetallic compound Tb₃In₅ using the ¹⁴⁰La → ¹⁴⁰Ce nuclear probe. The measurements were carried out in the temperature range of 8 to 295 K. Two different temperature dependent magnetic frequencies were observed below 30 K, which were assigned as ¹⁴⁰Ce substituting the two inequivalent Tb sites in the orthorhombic structure of Tb₃In₅. The temperature dependence of MHF also shows a possible deviation from an expected Brillouin-like behavior for temperatures below 18 K. A Néel transition at 27 K was observed from magnetization measurements in the samples. The magnetization as a function of the applied magnetic field was measured at two temperatures, 5 and 40 K, and the results show antiferromagnetic and a typical paramagnetic behavior, respectively. In both cases it was not observed saturation under high magnetic field.     

Structural properties of the donor indium in nanocrystalline ZnO

The structural properties of the nanocrystalline semiconductor ZnO (nano-ZnO) doped with the donor Indium were investigated by perturbed γγ angular correlation spectroscopy (PAC) and extended X-ray absorption fine structure measurements (EXAFS). Up to an average concentration of one In atom per nanocrystallite, PAC measurements show that about 12% of the ¹¹¹In atoms are incorporated on substitutional Zn sites. At higher In concentrations, new In defect complexes are visible in the PAC spectra, which dominate the spectra if the average In concentration exceeds one In atoms per nanocrystallite. In addition, the local environment of Zn and In atoms in In doped nano-ZnO was investigated by EXAFS. The measurements at the K edge of Zn show that the crystal structure of nano-ZnO corresponds to bulk ZnO. In heavily In doped nano-ZnO the EXAFS experiments at the K edge of In exhibit an expansion of the first O shell about the In site. Since about four O atoms are detected in this first shell a substitutional incorporation of the In atoms in the ZnO lattice is suggested. The second shell to be occupied by Zn atoms as well as higher shells are almost invisible, which might have the same microscopic origin as the occurrence of defect complexes observed by PAC.     

Dynamics of defects in semiconductors

Hyperfine interaction techniques involving radioactive probe atoms like the perturbed angular correlation technique (PAC) and the Mössbauer effect have, due to their inherent sensitivity, successfully been applied to the study of defects in semiconductors. By probing the characteristic charge distribution around the probe atom interacting with a defect, they contributed to the microscopic understanding of the nature, structure and stability of complexes formed between radioactive dopant atoms and defects present in elemental and compound semiconductors. Moreover, dynamic effects can be studied by hyperfine interaction probe techniques. In this case, dynamics always means the fluctuation of a charge distribution resulting in a time dependent hyperfine interaction within the time scale defined by the lifetime of the isomeric nuclear state used for the measurement. Such fluctuations can either be caused by structural changes like local rearrangements of a defect complex or by electronic transitions in the semiconductor resulting in a change of the charge state of a defect complex. Examples using PAC to monitor such processes will be discussed for the semiconductor silicon.     

Laser surface-melting of metals studied by PAC

High purity Pt, Ni, Cu, Fe and Al metal foils were electroplated with carrier-free¹¹¹In probe atoms and melted at 293 K using 32 ns ruby laser pulses with energy densities in the range 1–10 J cm⁻². Three distinct lattice locations of the probe atoms were detected in the melted surface layers using perturbed γ-γ angular correlation spectroscopy: (i) defect-free substitutional sites, (ii) non-unique sites with broad distributions of quadrupole interactions, and (iii) a probemonovacancy complex (for Pt only). The defect-free fraction was found to approach 100% when Pt is irradiated at 9 J cm⁻². The fraction of probes on defect-free sites was found to increase with the energy density of the pulse and with the solubility of In in the metals. The data are consistent with the idea that laser surface-melting produces high concentrations of vacancies and lattice sinks, although no thermal trapping of point defects was detected. Supported in part by National Science Foundation grant DMR 86-19688 (Metallurgy Program).     

Residence sites for19F ions implanted into diamond

¹⁹F has been used to probe the structure of the diamond lattice. The measurements were performed via the time differential perturbed angular distribution (TDPAD) technique applied to the 197 keV state (τ=128 ns,Q=0.10 b) of¹⁹F. The nuclei were excited and recoil implanted into an oriented diamond using a (p,p′γ) reaction initiated by a pulsed 4 MeV proton beam. Model functions simulating a variety of residence sites for the probe ions in a single crystal were developed for the analysis of the data. The data implies the formation of two molecular types, or sites within the lattice, with possibly a third radiation damage component.     

Efficient loading of a Rb dark magneto-optical trap by controlling current through a getter source

We report a study on the loading of a Rb dark magneto-optical trap from a current-controlled getter source. The effect of changing the temperature and number density of the background atoms on the cold atomic population in the dark state has been investigated by passing current of different magnitudes through the getter source. We observe that the dark state collection rate of the cold atoms is maximized for an optimum value of the getter current used. In our experiments, cold atoms in the dark state have been collected with the maximum collection rate 3.6 × 10⁷ atoms/s and background atom collision rate γ = 1.9 ± 0.2 s⁻¹ for the getter current ∼4 A.     

Ionization of atoms in weak fields and the asymptotic behavior of higher-order perturbation theory

Using the imaginary time method, we study the structure of the perturbation series for the hydrogen atom in electric ℰ and magnetic ℋ fields. It is shown that there is a “critical” value of the ratio γ=ℋ|ℰ at which the perturbation series for the ground state changes from having a fixed sign (for γ<γ _c ) to having a variable sign (for γ>γ _c ). This conclusion is confirmed by direct higher-order perturbation calculations. The change in the asymptotic regime is explained by competition among the contributions of the various complex trajectories that describe the subbarrier motion of the electrons. Here the parameter γ _c depends on the angle θ between the electric and magnetic fields. Zh. éksp. Teor. Fiz. 113, 2047–2055 (June 1998)     

μ− SR in semiconductorsSR in semiconductors

μ⁻ SR experiments have been performed on Si between room temperature and 6 K. The amplitude of the muon spin precession signal in an applied magnetic field of 0.04 T decreased below 30 K. A zero-field measurement at 6 K revealed a μ⁻ spin precession frequency of 650 MHz. The muonic atom represents an aluminium acceptor in the silicon matrix, its electronic state is responsible for the μSR signal. A possible influence of the γ recoil produced by the X-ray cascade is discussed.     

Theoretical investigation of the ultrafast NeNePo spectroscopy of Au 4 and Ag 4 Clusters

Ultrafast ground state nuclear dynamics of Au₄ and Ag₄ is theoretically explored in the framework of negative ion - to neutral - to positive ion (NeNePo) pump-probe spectroscopy based on the ab initio Wigner distribution approach. This involves the preparation of a nonequilibrium neutral ensemble by pump induced photodetachment of a thermal anionic ground state distribution, gradient corrected DFT classical trajectory simulations “on the fly” on the neutral ground state, and detection of the relaxation process of the ensemble in the cationic ground state by a time-delayed probe pulse. In Au₄, the initially prepared linear structure is close to a local minimum of the neutral state giving rise to characteristic vibrations in the signals for probe wavelength near the initial Franck-Condon transition. A timescale of ∼1 ps for the structural relaxation towards the stable rhombic D_2h neutral isomer was determined by the increase of the signal for probe wavelength in vicinity of the vertical ionization energy of the rhombic structure. In contrast, the relaxation dynamics in Ag₄ is characterized by normal mode vibrations since both the initially prepared anionic ground state and the neutral ground state have rhombic minimum geometries. Thus, time-resolved oscillations of pump-probe signals are fingerprints of structural behaviour which can be used experimentally for the identification of particular isomers in the framework of NeNePo spectroscopy. Received 22 December 2000     

Thermal and acoustic effects in the γ irradiation of amorphous Ti−Ni−Cu alloys

The influence of γ radiation on the thermal and acoustic effects in amorphous Ti−Ni−Cu alloys is investigated. On the basis of the results obtained, a mechanism by which γ quanta affect the structure of amorphous metal alloys (AMA) is proposed. In the interaction of a γ flux with AMA, the energy is redistributed and accumulates at intercluster boundaries. Then, when restructuring of all the intercluster boundaries is complete, restructuring of locally regular clusters begins; this leads, in turn, to restructuring of the intercluster boundaries. Further increase in radiation dose again triggers the mechanism of intercluster-boundary restructuring, but at a different level. This is evident in the oscillatory fluctuation in the activation energy of relaxation processes and of phase transformation from amorphous to crystalline structure. Tomsk State Architectural and Building Academy. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 75–80, September, 1996.     

The influence of the anion vibrational temperature on the fs dynamics in a NeNePo experiment

A negative-neutral-positive (NeNePo) femtosecond charge-reversal experiment studying the temperature-dependent relaxation dynamics of linear Ag₃ is described. A wavepacket is prepared on the potential-energy surface of the electronic ground state of the neutral trimer by photodetachment from an anion ensemble held at different, well-defined temperatures between 20 K and 350 K. The wavepacket dynamics is probed by resonant two-photon ionization. The cooled octopole ion trap developed to prepare the cold anions is described. The relaxation dynamics of the initially linear Ag₃ from a saddle point of the potential-energy surface into the triangular equilibrium configuration shows a significant dependence on the anion temperature, which is rationalized in terms of a simple model. We demonstrate that a low anion vibrational temperature results in the generation of “narrower” wavepackets on the neutral potential-energy surface. This allows us to probe coherent effects more sensitively. Received: 10 January 2000 / Published online: 24 July 2000     

Paramagnetic labeling as a method for the soft spectroscopy of electronic states

A self-consistent microscopic theory of the relaxation of the crystal-field levels of an impurity ion in a state with an integer valence implanted in a normal metal is devised. A microscopic approach based on the Coqblin-Schrieffer-Cooper approach, rather than the formal model of the sf exchange interaction, makes it possible to take into account the specific details of both the crystal-field states of the impurity ion and the electronic band spectrum of the metal. A new method for the soft spectroscopy of electronic states based on measurements of the temperature dependence of the width Γ_MM′(T) of transitions between the crystal-field states |M〉 of a paramagnetic ion implanted in the compound being studied is proposed. To make specific use of this method in neutron and optical spectroscopy, a classification of the types of temperature dependence of the natural relaxation width γ _M (T) of the levels is devised, and procedures for possible experimental methods are proposed. A nonzero value of the natural relaxation width γ _G (T) of the crystal-field ground state | G〉 of an impurity ion at zero temperature is obtained within the proposed self-consistent model, but is beyond the scope of perturbation theory. It is shown that the widely accepted estimate of the characteristic temperature of Kondo systems T*=Γ _G(T=0)/2 from the quasielastic scattering width at zero temperature Γ _G (T=0)/2 is incorrect in the case of strong relaxation in a system with soft crystal fields. The proposed model is applied to the quantitative analysis of the relaxation of the crystal-field levels of paramagnetic Pr³⁺ ions implanted in CeAl₃ and LaAl₃. The results of the calculations are in quantitative agreement with the experimental data. Zh. éksp. Teor. Fiz. 113, 1843–1865 (May 1998)     

Probe spectrum of a four-level atom in a double-band photonic crystal

In this paper, the probe absorption spectrum of an atom in a double-band photonic crystal have been studied. In the modes, we assume that one of the two atomic transitions in a Λ-type atomic system is interacting with free vacuum modes, and another transition is interacting with free vacuum modes, isotropic photonic band gap (PBG) modes and anisotropic PBG modes, separately. The effects of the fine structure of the atomic lower levels on the probe absorption spectrum are investigated in detail in the three cases. The most interesting thing is that the two (four) transparencies at one (two) probe absorption peak(s), caused by the fine structure of the lower levels of an atom, are predicted in the case of isotropic PBG modes.     

Evidence of a new structural phase of manganese-iron oxide

Coprecipitation of manganese and iron followed by calcination at temperatures below 500°C results in a new structural phase for the Mn−Fe−O system: a non-stoichiometric spinel with, in every three unit cells, one metal atom missing. About 57% of the iron in this spinel has the composition γMnFeO₃ which is paramagnetic at room temperature. The remaining 43% of the iron has the composition Mn_yFe_2-yO₃ with y∼0.16 which is magnetic at room temperature.     

Point defects in NiAl near the equiatomic composition

Perturbed γ-γ angular correlation spectroscopy was applied to study the structure and properties of point defects near¹¹¹In probes in quenched and annealed NiAl samples with five compositions in the range c_Ni=48–52%. Quadrupole interaction signals are analysed in terms of near-neighbor Ni-vacancy and nextnear-neighbor Ni antisite atom defects. Large mono- and divacancy concentrations were observed in all samples after quenching. For c_Ni<50%, the vacancy defects could not be annealed out (structural defects). For C_Ni>50%, large quenched-in vacancy concentrations annealed out near 500°C. For c_Ni=50%, most vacancies annealed out near 500°C, but about 35% of the¹¹¹In probes retained a trapped monovacancy. This is attributed to a very low value of the formation enthalpy of a vacancy next to the In probe, estimated to be 0.16(2) eV. A lesser annelaing stage detected at 300°C is tentatively attributed to diffusion of Al-vacancies.     

Emergence and visualization of an interface state during contact formation with a single molecule

Contact formation dynamics and electronic perturbations arising from the interaction of a metallic probe and a single molecule (1,3 cyclohexadiene) bound on the Si (100) surface are examined using a series of plane wave, density functional theory calculations. The approach of the probe induces a relaxation of the molecule that ultimately leads to the formation of an interface state due to a specific interaction between the probe apex atom and the C=C bond of the molecule. The calculated interface state is located 0.2 eV above the Fermi energy, in agreement with low temperature scanning tunneling spectroscopy local density of states data (0.35 eV), and is responsible for the contrast observed in low bias empty-state STM images.     

Comparison of the dynamics of grid structures under thermal and radiation actions on amorphous Co-Ni-P alloys

The dynamics of the grid structures of Co-Ni-P alloys is investigated via transmission electron microscopy under thermal action and irradiation by γ quanta. Statistical processing of the electron-microscopic images of grid structures makes it possible to construct the empirical functions of size distribution of grid cells in the presence of different types of actions. Analysis of empirical distribution functions and their moments has been performed to study the statistics of distribution functions corresponding to different stages of structural relaxation during thermal and radiation actions and to classify structural relaxation processes. The presence of correlative interaction between structure components has been found in the initial state of an amorphous alloy; the type of correlation has been determined. The correlation behavior is analyzed under irradiation by γ quanta and thermal action. It has been revealed that grid cells exhibit different correlative interactions determined by the type of action.     

Application of thermal desorption methods in studies of catalysis: II. The oxidation of carbon monoxide on platinum

One of the major difficulties encountered in catalytic research has been the experimental characterization of the catalyst surface in its working state. Using thermal desorption techniques, we have developed a method capable of providing a quantitative measure of the concentrations of certain adsorbed reactants during catalysis. In a study of the oxidation of CO on polycrystalline platinum, three distinct reaction mechanisms have been distinguished: (1) the interaction of an oxygen molecule with a vacant Pt surface site and an adjacent adsorbed CO molecule forming a complex which dissociates to form a CO₂ molecule and an adsorbed oxygen atom; (2) the reaction between adsorbed CO and an adsorbed oxygen atom, a process in which surface transport is required to bring the reactants together; and (3) the reaction between a colliding CO molecule and an adsorbed oxygen atom. Rate constants for the reaction steps have been measured, and the active surface sites and reactive surface states on the Pt catalyst have been characterized in terms of the adsorption properties of CO.