The effects of concentration and temperature on the EFG generated by N.N. Pd impurities in a cubic Ag host

The time differential perturbed angular correlation (TDPAC) method is used to investigate the electric field gradients (EFG) generated by Pd impurity atoms alloyed into the cubic Ag host. As the concentration of the impurity is increased from 0.25 to 2.5 at.% we observed in detail how different near neighbour (n.n.) sites to the probe nuclei are populated, creating different EFG. The temperature dependence of the different EFG for a sample with 0.5 at.% of Pd showed that all follow theT ^3/2 law found previously for other doped cubic systems. In addition we observed that the n.n. population of impurities is strongly related to the thermal treatment of the sample.Work supported in part by Conselho Nacional de Desenvolvimento Científico e Tecnológico and Fianciadora de Estudos e Projectos.     

Decay of bcc-structure and of bond-orientational order in a fluid

The decay of an initially prepared bcc structure and the bond-orientational order (anisotropy of the first coordination shell) are studied in a non-equilibrium molecular dynamics simulation for a fluid of 1024 particles interacting with a repulsive r^-12 potential. Data are presented for pair-correlation functions and order parameters associated with an angle dependence described by cubic harmonics of the ranks 4, 6 and 8. These cubic pair-correlation functions and cubic order parameters are defined by the expansion of the pair-correlation function and of the bond orientational distribution function with respect to Cartesian tensors. The relaxation of the local anisotropy shows a pretransformational slowing down for densities approaching the freezing point.     

Perturbed angular correlations with111Cd in the α and δ phases of In-Sn

The influence on the TDPAC distribution of dissolved impurities (Sn in In) and (In in Sn) as a function of concentration has been studied at room temperature and at liquid-nitrogen temperature. Models for the observed distributions of the electric field gradients are discussed. The results are consistent with theoretical estimates of the screening charge distribution around the valency impurity.     

Static electric quadrupole interaction of Ce and Pb ions in lead titanate

The static electric quadrupole interaction of¹⁴⁰Ce and²⁰⁴Pb has been studied in polycrystalline PbTiO₃ at the lead site with the time-differential perturbed angular correlation (TDPAC) method. The interaction frequencies areω _Q(2083 keV;¹⁴⁰Ce)=2.11 (17) MHz andω _Q(1274keV;²⁰⁴Pb)=3.60(3) MHz. The results are compared with the electric field gradient calculated in a point charge model. Under the assumption that the covalent contributions for Ce and Pb are equal, one can derive the quadrupole moment of the 1274 keV state of²⁰⁴Pb to be ¦Q¦=0.68 b.     

Magnetostriction in LaAg single crystal with Tb impurities

We have made magnetostriction measurements on a La_0.99Tb_0.01Ag single crystal at low temperature. The shape magnetostriction dominates, and the 〈001〉 and 〈111〉 effects have opposite signs. The volume magnetostriction is positive but small. We analyse the results in a single-ion model, using the crystal-field parameters suggested by Hoenig et al., and we deduce the orbit-lattice coupling parameters V³₂ and V⁵₂. Results are compared with those in other cubic metallic systems containing rare earths.     

TDPAC measurements on phase transition and hydrogen up-take in C15 laves phases

Intermetallic compounds with C15 structure are studied by TDPAC in the search for changes in the electric field gradient (EFG) on¹⁸¹Ta probes due to martensitic phase transition and hydrogen absorption. The residual low EFG of the cubic phase of HfV₂, ZrV₂ and Hf₅Zr₅V₂ is distinctly increased in the low temperature modification. Hydrogen does not affect the distribution of probe sites, but influences the features of the martensitic transition. In TaV₂ the sites with low EFG are split into an unperturbed and a strongly perturbed fraction.     

Flow distribution and pressure of air due to ionic wind in pin-to-plate corona discharge system

An electrohydrodynamic investigation has been carried out in a pin-to-plate gas discharge system to clarify the mechanism of repulsive force generation between a pin and plate electrode at corona discharge. Numerical calculations have been conducted in two steps. First, the axi-cylindrical static corona discharge field was calculated with the finite-element method to deduce the Coulombic body force ρ E applied to the air, where ρ is the charge density and E is the electric field, and then the induced ionic wind was calculated with the finite differential method. The calculated pressure distribution on the plate electrode was on the order of 10 Pa which was in good agreement with the measured pressure distribution. The calculated air velocity at the center was several m/s and was confirmed by a time-of-flight experiment and the velocity distribution near the pin electrode also agreed with measurements using a laser Doppler velocimeter. Pressure and wind velocity were increased at high-applied voltage. These results confirm that the ionic wind is the cause of the repulsive force to the pin electrode at the corona discharge.     

High resolution181Hf TDPAC spectroscopy using fast BaF2-detectors

A high resolution TDPAC setup with four BaF₂ detectors was introduced for measurements in chemical and biological systems. High electric field gradients and large asymmetry parameters could be determined very precisely. The data are compared to previous NaI(Tl) measurements on HfF₄·HF·2H₂O, HfO₂, and¹⁸¹Hf-labelled transferrin.     

The PbPb collision

The reaction ²⁰⁸Pb on ²⁰⁸Pb was studied at bombarding energies of 7.0 and 7.57 MeV/u. One-particle inclusive measurements using a large-area position-sensitive ionisation chamber delivered the kinetic energy, charge and scattering angle of the reaction products. A precise calibration of the stopping power for very heavy ions in the detector gas was performed. The measured Wilczynski diagrams show, for increasing loss of kinetic energy, an increase of the mean scattering angle. It is attributed to the dominance of the repulsive Coulomb forces with respect to the attractive nuclear forces. The element distribution for the ²⁰⁸Pb on ²³⁸U reaction at 7.5 MeV/u was also measured and compared to the PbPb and UU reactions. Fission probabilities are derived as a function of charge and total kinetic energy loss. The most striking result is seen in the σ_z² versus TKEL correlation: the average rate of energy loss per nucleon exchange is abnormally large. It is shown that this behaviour is associated with the double magic closed shell character of the colliding nuclei. Nuclear structure information is extracted through a simple parametrisation.     

Electronic structure and term energy calculations for cubic and axial iron defects in SrTiO3

The spin-unrestricted Xα method is used to obtain the molecular orbital diagrams relevant to cubic Fe²⁺, Fe³⁺ and axial Fe⁺ ?V₀ to Fe³⁺ ?V₀ defects in SrTiO₃, where V₀ is an oxygen vacancy. The relative energies of the lower spin multiplets are determined through the transition-state model of Slater. The electronic structures and the term patterns allow to discuss the stability and the ground term of each center trapped in cubic SrTiO₃. The mechanisms of ligand-metal and intervalence charge transfers are examined and compared to the abundant experimental data provided by EPR and optical measurements. The different behaviours of ferroelectrics like BaTiO₃: Fe and LiNbO₃: Fe in regard to the mechanisms implied in the photorefractive effects are reviewed according to the theoretical electronic structures with a special emphasis to the Fe²⁺ ? Fe³⁺ charge transfer.     

Helium implanted CuHf as studied by TDPAC and positron lifetime measurements

¹⁸¹Ta time differential perturbed angular correlation (TDPAC) and positron lifetime measurements were carried out on homogeneously α-implanted CuHf samples. TDPAC measurements indicate the trapping of vacancy clusters and helium associated defect complexes by Hf atoms. The presence of helium-vacancy complexes and helium stabilised voids has been identified by positron lifetime measurements. Further the nucleation and growth stages of helium bubbles have been identified. TDPAC and positron lifetime measurements indicate that Hf atoms act as heterogeneous nucleating centers for helium bubbles. Hf atoms are found to suppress the bubble growth in CuHf as indicated by the results of positron lifetime measurements.     

PAC perturbation factor for spin 5/2 nuclei subject to a rapidly fluctuating EFG1

We report numerical computations of the PAC perturbation factor G₂(1) for spin 5/2 nuclei subject to a static EFG symmetric about the z-axis and an additional axially-symmetric EFG whose symmetry axis fluctuates randomly among the x, y, z directions. For sufficiently large fluctuation rates, the numerical results are described by the expression for the static interaction alone with the addition of relaxation terms. Results of applying this model to¹¹¹Cd TDPAC measurements on tetragonal ZrO₂ are described briefly. The model allows one to evaluate the probability that oxygen vacancies are trapped. the energy of association of vacancy-metal pairs, and the vacancy activation energy of motion. Supported in part by USDOE contract DE-FG06-85ER45191     

TDPAC studies of111mCd in calciproteins: Calmodulin and parvalbumin

TDPAC measurements of the 150–247 keV gamma-ray cascade in¹¹¹Cd have been performed at room temperature on Calmodulin (CaM) and Parvalbumin (Pa) labelled with radioactive^111mCd. The anisotropy of the coincidence counting rate shows a time-dependent behaviour, typical of two different nuclear quadrupole interactions (NQI) in CaM. The NQI parameters indicate the presence of two distinct metal-binding sites, presenting non-equivalent local electric charge structure. In Pa samples, only one NQI is observed and the measured electric field gradient (EFG) value agrees well with the large one obtained in CaM.     

Relaxation to a Perpetually Pulsating Equilibrium

Paper in honour of Freeman Dyson on the occasion of his 80th birthday. NormalN-body systems relax to equilibrium distributions in which classical kinetic energy components are 1/2kT, but, when inter-particle forces are an inverse cubic repulsion together with a linear (simple harmonic) attraction, the system pulsates for ever. In spite of this pulsation in scale,r(t), other degrees of freedom relax to an ever-changing Maxwellian distribution. With a new time, τ, defined so thatr ²d/dt = d/dτ it is shown that the remaining degrees of freedom evolve with an unchanging reduced Hamiltonian. The distribution predicted by equilibrium statistical mechanics applied to the reduced Hamiltonian is an ever-pulsating Maxwellian in which the temperature pulsates liker ^-2. Numerical simulation with 1000 particles demonstrate a rapid relaxation to this pulsating equilibrium.     

Ion-molecule collisions in gaseous111InCl and111InI systems

Using gaseous sources of¹¹¹InCl and¹¹¹InI with densities between 1.0·10¹⁷ cm^?3 and 4.4·10¹⁹ cm^?3 the perturbation of the 171.3–245.4 keVγ-γ cascade was measured as a function of time and density by the time-differential perturbed angular correlation (TDPAC) method. The anisotropy shows a strong dependence on the density. By means of an extended model based on a stochastic model of Bosch and Spehl collision cross sections for charge transfer and deorientation could be determined. The cross sections for charge transfer collisions were within the region from 2·10^?15 cm² to 26·10^?15 cm² and for deorientation collisions between 1·10^?15 cm² and 100·10^?15 cm².     

Screening of excitonic states by low-density 2D charge carriers in GaAs/AlGaAs quantum wells

Screening of excitonic states by a system of 2D electrons (or holes) in GaAs/AlGaAs single quantum wells is studied. With increasing concentration of 2D charge carriers, a threshold-type disappearance of excitonic states is observed in both luminescence and reflectance spectra. The higher the quality of the 2D system, the lower the corresponding threshold concentration. In the best systems, the collapse of excitonic states occurs at unexpectedly low electron densities n _e =5×10⁹ cm^?2, which correspond to the mean dimensionless distance between the particles r _s =8. This value far exceeds the threshold values observed for 3D systems (r _s ≈2), as well as the values obtained for quantum wells in previous studies. The problem of measuring the concentration of low-density 2D charge carriers in photoexcitation conditions is solved by applying the method of optical detection of the dimensional magnetoplasma resonance. This method provides reliable measurements of the density of a 2D system to the values about 10⁹ cm^?2.     

Suppression of the charge density waves in 1T-TaS2 by 70 MeV16O6+ bombardment

We report on the measurement of the nuclear quadrupole interaction by TDPAC on a single crystal of 1T-TaS₂ before and after bombardment with 70 MeV¹⁶O⁶⁺ ions. In 1T-TaS₂, two groups of inequivalent Ta-sites are observed at 298 K due to the presence of a nearly commensurate charge density wave (CDW) Upon¹⁶O⁶⁺ bombardment a single slightly broadened line is observed which resembles closely the incommensurate CDW state.     

Hyperfine interactions in Zr(Fe1−x Al x )2 measured forx≤0.20 by TDPAC and Mössbauer spectroscopy

Mössbauer (⁵⁷Fe) and TDPAC spectroscopy (¹⁸¹Hf) have been used to study quasibinary compounds Zr(Fe_1?x Al _x )₂ forx≤0.20. It has been found that the dependence of the mean values of the hyperfine magnetic field, quadrupole splitting and isomer shift on the Al concentrationx is strong. The dependence of the hyperfine magnetic field on the number of Al atoms as nearest and next-nearest neighbours of⁵⁷Fe has been established. The TDPAC results also indicate a dependence of the hyperfine field on¹⁸¹Ta on Al concentration.     

Integral representation for non-maxwell models and the approach to equilibrium

An integral representation is proved for the nonequilibrium distribution function of molecules interaction through potentials which are repulsive near r = 0 and diverge less rapidly than r^?4 as r→ 0. The asymptotic approach to the absolute-equilibrium distribution is studied for general initial states.     

The analysis on the rearrangement of charge density distribution in response to magnetic behavior in Mn doped SnO2 nanoparticles

An investigation on the rearrangement of charge density distribution in response to the magnetic behavior is done on the nanosized Sn_1−xMn_xO₂ systems prepared via sol-gel technique. The samples were analyzed using X-ray diffraction, SEM and VSM measurements. The charge ordering is found as a signature to its magnetic response from the reconstruction of the charge density in the unit cell derived from the MEM/Rietveld analysis of the X-ray structure factors. Quantitative investigation is done by analyzing the 3-dimensional, 2-dimensional and 1-dimensional charge density profiles drawn along the bonding directions. The observed para- to dia-magnetic transition with respect to doping concentration of Mn is explained using defect chemistry and the charge ordering. The MEM/Rietveld analysis is used for the first time to analyse the charge and magnetic ordering in the chosen system.