The electric field gradient of111Cd in an α-Al2O3 single crystal

Radioactive¹¹¹In⁺ ions were implanted into an α-Al₂O₃ single crystal. The hyperfine parameters of¹¹¹Cd at substitutional Al lattice sites were identified by measuring the perturbed angular correlation for different sample orientations. The electric field gradientV _zz =1.04(17)·10²²V/m² was obtained from the quadrupole coupling constant. This result is compared with the efg values of²⁷Al in α-alumina and¹¹¹Cd in α-Fe₂O₃, which also has the corundum structure. Two additional fractions with broad frequency distributions were observed, one of which is attributed to¹¹¹Cd atoms in a strongly distorted Al₂O₃-lattice.     

The effect of temperature and concentration on the electric field gradient in binary indium alloys

The concentration and temperature dependence of the quadrupole hyperfine interaction of¹¹¹Cd in InTl (hcp),InPb (fct),InTl (fct) andInCd (fct and fcc) alloys were studied using the perturbed angular correlation technique. The change in the observed quadrupole interaction frequency with concentration can be described by a linear dependence on the axial ratioc/a in all cases. In the alloys with identical crystal structures the strength of thec/a dependence is independent of the solute, in contrast to the strength of the concentration dependence. In all cases where no phase transition occurs, the change in the electric field gradient with temperature follows the empirical relationV _zz (T)=V _zz(0) · (1–B·T ^3/2), where the coefficientB depends on the lattice structure, on the solute-solvent combination and on the concentration. The phase transitions ofInCd alloys at 293 K could clearly be seen as discontinuities in the temperature curves. A similar series of discontinuities observed around 116 K suggests the existence of a cubic low temperature phase.     

Hyperfine interaction of ZrO2 — Tetragonal phase

The quadrupole hyperfine interaction of the tetragonal phase of the zirconium oxide has been measured at 1523 K using the time-differential perturbed angular correlation technique. The electric field gradient at zirconium sites was determined to beV _zz =(17.5±0.4)·10¹⁷ Vcm⁻² and axially symmetric.     

TDPAC study of amorphous alloys Pd0.8Si0.2 and Pd0.75Si0.20Ag0.05

The amorphous alloys Pd_0.8Si_0.2 and Pd_0.75Si_0.20Ag_0.05 have been studied by the time differential perturbed angular correlation method. A broad EFG distribution with a relative width of 0.48–0.51 was sensed by the probe nuclei¹¹¹Cd for both the amorphous alloys. The unique quadrupole interaction frequencies were observed after annealing above the phase transition temperature. The observed broad distribution of the EFG implies that the crystallographic structure of the amorphous alloys is characterized by short-range order and long-range disorder, which is in accord with the continuous random model.     

Hyperfine interactions of111Cd impurities in Cr2O3

The electric and (antiferro-)magnetic hyperfine interaction of¹¹¹Cd in α-Cr₂O₃ after¹¹¹In implantation was studied via PAC. Two fractions with axially symmetric electric field gradients were observed having antishielding factors of V_zz/V_zz ^pc=79(12) and 106(16), respectively. From the line broadening of the Fourier components of the first fraction its supertransferred magnetic field was estimated as B_loc (20 K)≤0.41 T, about two orders of magnitude smaller than for substitutional¹¹¹Cd in NiO and CoO.     

Magnetic and electric hyperfine interactions of181Ta implanted in thulium

The magnetic and electric hyperfine interaction at the site of dilute¹⁸¹Ta impurities in the rare earth metal Tm has been investigated as a function of temperature by TDPAC measurements. The samples were prepared by ion implantation of radioactive¹⁸¹Hf. In the paramagnetic phase between 100 K and 700 K the electric fieldgradient is a linear function of temperature: V_zz(T)=V_zz(O)·(1-A·T) with A=4.6·10^?4K^?1 and V_zz(293K)=6.4 (4)·10¹⁷v/cm². The TDPAC spectrum observed at 4.2 K reflects the 4 magnetically non-equivalent sites for an impurity in magnetically ordered Tm. The relative values and amplitudes of the corresponding 4 magnetic hyperfine fields are consistent with the predictions of the RKKY theory.     

Third neighbour hyperfine field shift in a diluteNiAl alloy

In the binary Ni_98.5Al_1.5 alloy we observed well resolved satellite hyperfine fields at¹¹¹Cd nuclear probes. The hyperfine shifts derivated are −4.0(3); −8.5(3) and ⃛14.0(3)% relative to the pure nickel value. From the relative intensity of the four components as well as from the quadrupole interaction data we identify the satellites as impurity-probe configurations containing zero, one, two or three Al impurities in the third neighbour shell only. The third neighbour contribution to the hyperfine field at Cd in nickel equals ΔH ₃= +4.5kG/μ _B-atom, allowing an estimate ΔH ₁≃ −17.8 kG/μ _B-atom for the first shell contribution, confirmed by a similar experiment on Ni_98.5Pd_1.5.     

X-ray determination of thermal expansion of cadmium fluoride and lead fluoride

The lattice parameters of CdF₂ andβ-PbF₂ have been determined over the temperature range 300–670 K. The coefficient of expansion at room temperature is 21·3 × 10⁻⁶ K⁻¹ and 25·4 × 10⁻⁶ K⁻¹ for CdF₂ and PbF₂ respectively and it increases linearly with temperature over the range of temperature covered. The Grüneisen parameter decreases with temperature in both the crystals.     

Hypervirial perturbation theory for eigenenergies for two types of atomic potentials

Eigenenergies are calculated for the potentialsV ₁(r)=−(a/r)[1+(1+br)e^−2br ] andV ₂(r)=−(v/r)[1 −λr(1−Z ⁻¹)(1+λr)⁻¹], using renormalized series technique. Accurate results produced here for various eigenstates agree with those available in the literature.     

Studies of weak electric quadrupole interaction in57,60CoFe using modulated adiabatic passage on oriented nuclei

The electric quadrupole interactions at⁵⁷Co and⁶⁰Co in co-diffused^57,60CoFe single crystal have been measured using Modulated Adiabatic Passage on Oriented Nuclei (MAPON). The quadrupole splittings Δν_Q<100> are + 18(2) kHz for⁵⁷Co and +9(2) kHz for⁶⁰Co, corresponding to a principal electric field gradient (efg) tensorV _zz =2.0(5)×10¹⁹ Vm⁻² and 2.5(6)×10¹⁹ Vm⁻², respectively, in broad agreement with previous MAPON results for CoFe. The distributions of the efg’s are very similar for the two isotopes, verifying that previously reported differences in⁵⁸CoFe and⁶⁰CoFe could be attributed to different host preparations. The measurement of such weak efg’s which are not spectroscopically resolved, allows determination of new nuclear electric quadrupole moments, not accessible by other techniques. Applications to other systems are discussed.     

Thermal expansion of thin C60 films

The thermal expansion coefficient a and structure of C₆₀ films with thickness t∼3–10 nm were investigated in the temperature interval from room to liquid-nitrogen temperature by electron-optical methods. The thermal expansion coefficient was determined from the temperature shift of the diffraction maxima in the electron diffraction patterns. The objects of investigation were epitaxial C₆₀ films condensed in vacuum on a (100) NaCl cleavage surface and oriented in the (111) plane. A surface-induced size effect in the thermal expansion coefficient was observed. It was established that as t decreases α _f increases and is described well by the relation α _f=17·10⁻⁶ K⁻¹+8.3·10⁻⁵ nm K⁻¹ t ⁻¹. This relation was used to estimate the linear expansion coefficient α _s of the C₆₀ surface in the (111) plane as α _s=60·10⁻⁶K⁻¹, which is several times larger than the bulk value. The experimental results agree satisfactorily with the theoretical calculations of the mean-square displacements of molecules located in a region near the surface. Zh. éksp. Teor. Fiz. 114, 1868–1875 (November 1998)     

High Temperature Diffusion of 6Li Adsorbed on a Ru(001) Single Crystal Surface as Seen by Pulse NMR

Diffusion measurements on lithium atoms adsorbed on a ruthenium single crystal were performed in the high temperature regime (1100–1200 K). Pulsed NMR techniques were utilized to produce and observe the decay of magnetization patterns from which the diffusion coefficient was extracted. The observed temperature dependence could be described by D = (10 ± 7) cm²/s · exp (−(0.46 ± 0.07) eV/kT). The extremely high diffusion coefficient and prefactor are understood by a gas like adsorbate behavior. The electric field gradient has been measured with ⁷Li: V _zz = −5.0 ± 0.1 10¹⁵ V/cm² with an inhomogeneity of less then 1% as judged by the width of the satellite transitions.     

High Pressure Mössbauer Studies on FCC Fe53Ni47 Alloy

Perturbed angular correlation measurements of the hyperfine interaction of ¹¹¹In in sapphire show, that after implantation and annealing at 1000°C, the fraction of undisturbed probe atoms exhibiting a unique quadrupole interaction with ν _Q = 219(1) MHz (η = 0) varies between 50% at 4 K, 5% at 100 K and 80% at 973 K in a reversible manner. A possible explanation for this surprising behaviour is the influence of so-called ‘after effects’ following the EC-decay of ¹¹¹In to ¹¹¹Cd. Immediately after the decay the ¹¹¹Cd is in an ionized state, then collects electrons from its surroundings and reaches the ground state. The different electronic configurations that arise during this relaxation process affect the amplitude (f _u) and the damping (δ _u) of the unique quadrupole interaction.     

The electric quadrupole interaction of 111Cd in ZrZn2 and Zn in the samples prepared at 8 GPa

The perturbed angular correlation (PAC) measurements with the ¹¹¹In-¹¹¹Cd nuclear probe embedded into the lattice of the cubic (C15) Laves compound ZrZn₂ showed that ¹¹¹Cd nuclei experienced an axially symmetric electric quadrupole interaction with a frequency ν _Q  = 132.4 MHz at room temperature. The samples were synthesized and doped with the probe at a pressure 8 GPa. The temperature dependence of ν _Q was shown to be linear: ν _Q (T) = 147(1 − 0.033 T) MHz. Since the value of ν _Q is very close to that known for ¹¹¹Cd in the lattice of Zn, we have checked if it could be assigned to residual Zn metal in the sample. For the Zn sample melted and doped with ¹¹¹In at 8 GPa we have obtained ν _Q  = 117.3 MHz at 300 K and 127 MHz at 80 K – both values considerably lower than that for ¹¹¹In doped Zn samples prepared at an ambient pressure. These data, and the fact that ν _Q (T) in Zn is known to follow the T ^3/2 law, allow to attribute the ν _Q value quoted above to ¹¹¹Cd nuclei at the substitutional sites with tetrahedral symmetry in the Zn sublattice of ZrZn₂.     

Single-Crystal EPR Identification of a Low Hyperfine Value and Interstitial Position of Copper Impurity in Diaquabis[malonato(1-)-κ2O,O′] Zinc(II)

Electron paramagnetic resonance studies have been carried out at 300 K on the Cu(II)-doped [Zn(C₃H₂O₄)₂(H₂O)₂] system in single-crystal and powder forms in order to rationalize the low parallel ⁶³Cu hyperfine value. Angular variation of the hyperfine resonances in the three orthogonal planes shows the presence of only one magnetic site with g and A values equal to g _zz = 2.455, g _yy = 2.121, g _xx = 2.105 and A _zz = 160.9 · 10⁻⁴ cm⁻¹, A _yy = 12.5 · 10⁻⁴ cm⁻¹, A _xx = 7.35 · 10⁻⁴ cm⁻¹. The crystal structure of the host lattice is isostructural with the corresponding cobalt complex and contains two molecules per unit cell. The low magnitude of A _zz value for the complex is rationalized in terms of an admixture of the ground state with the excited state and delocalization of the unpaired spin density onto the ligands. In addition, the highest hyperfine value obtained from the single-crystal data (160.9 · 10⁻⁴ cm⁻¹) is considerably larger than that obtained from the powder spectrum (138 · 10⁻⁴ cm⁻¹). Authors' address: P. Sambasiva Rao, Department of Chemistry, Pondicherry University, Pondicherry 605014, India     

Search for higher twists in meson resonance production in π− N interactions atPT≥2 GeV/c

We study the production of meson resonance atpT≥2 GeV/c inπ ⁻ Be interactions at two beam energies (150 and 300 GeV/c). The motivation is to look for the higher twist (HT) production mechanism, expected to exist besides standard hadron production through the fragmentation process of scattered quarks and gluons, the so-called leading twist (LT) mechanism. We show that the φ, K*⁰ (892) andK*⁰ (892) are found free from HT effects with a good accuracy, as expected from HT estimates and QCD sum rules. The magnitudes of thef ₂(1270) andρ ⁰ mesons signals are found larger than expected from the standard Lund (LT) Monte Carlo and from information provided bye ⁺ e ⁻ annihilation experiments. The possible connection of these excesses with HT physics is discussed. We also report the first evidence for the production off ₀(975) mesons in hadronic experiments at largep _T.     

Electroluminescence in the phosphor anthracene doped by 9-vinylanthracene

Electroluminescence has been excited in the phosphor anthracene doped by 9-vinylanthracene (10⁻⁴ mole %) at room temperature (300°K). Variation of light output with voltage is governed by the relationB=B ₀ exp [−(C/V ^1/2)] as in the case of inorganic phosphors. HereB ₀ andC are constants andV is the rms value of the voltage applied to the phosphor. Frequency dependence of the electroluminescence has also been studied at different voltages of the applied electric field.     

Static electric quadrupole interaction of Ta- and Hf-ions in barium and lead titanate

The static electric quadrupole interaction of¹⁸¹Ta and¹⁷⁸Hf in polycrystalline barium and lead titanate at the site of titanium has been measured using time differential PAC and the Mössbauer effect. The electric field gradients (EFG) at room temperature at the¹⁸¹Ta nucleus are ¦V _zz¦=(3.6±0.2)·10¹⁷V/cm² in BaTiO₃ and ¦V _zz¦=(14.6±0.6)·10¹⁷ V/cm² in PbTiO₃. The temperature dependence of the quadrupole interaction has been studied giving the following EFG values: ¦V _zz¦=(2.4±0.2)·10¹⁷ V/cm² in the monoclinic and ¦V _zz¦=(1.1±0.3)·10¹⁷ V/cm² in the rhomboedral phase of BaTiO₃, and ¦V _zz¦=(15.7±0.6)·10¹⁷ V/cm² for¹⁸¹Ta/PbTiO ₃ at 77 °K. The EFG of¹⁷⁸Hf in PbTiO₃ has been derived from a Mössbauer effect experiment to beV _zz=+(10.7±0.5)·10¹⁷ V/cm². The results are compared with EFG's calculated in a point charge model and with experimental EFG's measured at⁴⁴Sc and⁵⁷Fe in the same titanates by other authors. Contributions of covalent bonds to the effective EFG's in perovskit crystals are discussed.     

NMR-ON measurements of187WFe,182,183,186ReNi,186ReFe and203PbFe

The magnetic hyperfine splitting frequencies of¹⁸⁷WFe,¹⁸²Re(j ^π=2⁺)Ni,¹⁸³ReNi,¹⁸⁶ReNi,¹⁸⁶ReFe and²⁰³PbFe in a zero external magnetic field have been determined by the NMR-ON method at about 7 mK as 225.56(6), 130.9(1), 98.17(4), 136.6(4), 1007.0(3) and 58.43(3) MHz, respectively. With the knowng-factors ofg(¹⁸⁶Re, 1⁻)=1.739(3) andg(²⁰³Pb, 5/2⁻)=0.27456(20), the following hyperfine fields were deduced:B _HF(¹⁸⁶ReNi)=−103.05(35) kG;B _HF(¹⁸⁶ReFe)=−759.7(13) kG;B _HF(²⁰³PbFe)=+279.18(25) kG. Taking hyperfine anomalies into account, theg-factor of¹⁸³Re was deduced as |g(¹⁸³Re, 5/2⁺)|=1.267(6). With the assumption of Knight shift factorK=0, theg-factors of¹⁸²Re and¹⁸⁷W and the hyperfine field of¹⁸⁷WFe were determined as |g(¹⁸²Re, 2⁺)|=1.63(5), |g(¹⁸⁷W, 3/2⁻)|=0.414(10) andB _HF(¹⁸⁷WFe) =−714(18) kG. The large hyperfine anomaly was deduced to be¹⁸³W Δ¹⁸⁷W =−0.124(22).     

Effect of Mg2+ on the Magnetic Compensation of Lithium–Chromium Ferrite

PAC measurements on ¹¹¹In(¹¹¹Cd) implanted and thermally treated α-Fe have shown an indication for a cubic defect with the ¹¹¹Cd probe in the centre of it. The measured room temperature (R. T.) magnetic hyperfine fields are B _hf1 = −38.4(8) T for substitution and B _hf2 = +11.5(3) T for the cubic defect. Additionally, probes with pure quadrupole frequency distributions were observed, which are incorporated in surface contaminations.