TDPAC characterization of tin oxides using181Ta

In connection with a general study of the evolution of tin-oxygen thin films, we report here on the hyperfine interactions of¹⁸¹Ta substitutionally replacting tin in the isolated phases SnO and SnO₂. For this purpose, pure SnO pressed powder and a thin SnO₂ film were implanted with¹⁸¹Hf. In both cases, unique quadrupole frequencies were found after thermal annealing treatments. The results indicate that the following hyperfine parameters: υ _Q = 740.6(2.1) MHz, η=0.07(2) and υ _Q = 971.5(1.9) MHz, η=0.72(1) characterize¹⁸¹Ta in SnO and SnO₂, respectively.     

Orientation of an oxygen-induced non-axially symmetric electric field gradient at Ta in Nb

The interaction of dilute Hf in Nb with interstitially dissolved oxygen has been investigated via the --TDPAC-technique applied to the 482 keV state of¹⁸¹Ta. After melting the parent radioactivity¹⁸¹Hf with Nb the trapping of oxygen at the probe atoms leads to a quadrupole interaction frequency _Q =268(1) MHz with an asymmetry parameter of =0.68(1). The temperature dependence of this frequency between 17 K and 293 K was found to be very weak whereas varies considerably. In a single crystal experiment the orientation of the three principal axes of the electric field gradient tensor leading to _Q has been determined. TheV _zz -axis points into a 110, theV _yy -axis into a 100 and theV _x -axis into a 110 direction. The results can be explained in the point charge model by assuming that one oxygen atom is trapped on the second nearest neighbour octahedral position next to the substitutional atom and the covalent bonds between the daughter Ta and the oxygen lead to a localization of charge near the probe atom.     

The quadrupole interaction at181Ta in Zn

The quadrupole interaction (QI) in hexagonal close packed zinc lattice was measured using the 482 keV, 10.6 ns probe state of¹⁸¹Ta employing the time-differential perturbed angular correlation technique. The electric field gradient (EFG) at¹⁸¹Ta in Zn was derived from the measured quadrupole interaction frequency at room temperature asV _zz =12.202×10¹⁷ V/cm². The quadrupole interaction measured at various temperatures displayed normal temperature dependence similar to that seen by this probe in many non-cubic hosts.     

The mapon technique and recent developments

Typical linewidths observed in NMRON on dilute impurities in ferromagnetic metals are of order 1 MHz, making difficult the observation of structure in the resonance with splitting Δv much less than this value. The technique of Modulated Adiabatic Passage on Oriented Nuclei (MAPON) was recently developed as a means of measuring the weak electric quadrupole splitting Δv_Q of the nuclear hyperfine interaction due to an electric field gradient V_zz. MAPON has successfully been applied to measure Δv_Q as low as 4 kHz, i.e. less than 0.5% of the inhomogeneously broadened NMRON CWFM resonance line. The isotopes⁵⁶Co,⁵⁷Co,⁵⁸Co and⁶⁰Co have been studied in iron single crystal hosts, yielding Δv_Q consistent with known and estimated quadrupole moments. In addition the results to date give striking confirmation of analyses based on the single impurity relaxation model. Following a brief summary of the theoretical development of MAPON a review of experimental data is given for the CoFe<100> system. The variation of Δv_Q with direction of magnetization, measured in⁵⁸CoFe and⁶⁰CoFe single crystal samples, is also described. Further MAPON measurements are described for a⁵⁶CoFe polycrystalline sample, for which the most probable value and width of the distribution of V_zz can be described simply in terms of the single crystal principal axis results. The application of the MAPON technique to the measurement of nuclear electric quadrupole moments in implanted and diffused samples is discussed.     

The magnetic hyperfine field and the electric field gradient at181Ta impurities in Gd metal

The magnetic hyperfine fieldH _hf and the electric field gradientV _zz at¹⁸¹Ta impurties in metallic Gd were determined by time differential perturbed angular correlation measurements with the 133 keV K-conversion electron 482 keV -cascade of¹⁸¹Ta. The sources for these measurements were prepared by implantation of radioactive¹⁸¹Hf ions into Gd. The results are: |H _hf(TaGd; 77 K)|=285(14)kG, and |V _zz(TaGd; 330 K)|=5.32(15)·10¹⁷V/cm². The value ofH _hf fits well into the systematics for 5d impurities in Gd and indicates a positive core polarisation contribution, which is expected if the conduction electrons of Gd have to a large extent d-character. The electric field gradients of the 5d impurities in Gd are not consistent with a proportionality between the ionic and the electronic contribution.     

Measurement of isolated photon production in deep-inelastic scattering at HERA

The production of isolated photons in deep-inelastic scattering ep→eγX is measured with the H1 detector at HERA. The measurement is performed in the kinematic range of negative four-momentum transfer squared 4<Q²<150 GeV² and a mass of the hadronic system W_X>50 GeV. The analysis is based on a total integrated luminosity of 227 pb^-1. The production cross section of isolated photons with a transverse energy in the range 3<E_T ^γ<10 GeV and pseudorapidity range -1.2<η^γ<1.8 is measured as a function of E_T ^γ, η^γ and Q². Isolated photon cross sections are also measured for events with no jets or at least one hadronic jet. The measurements are compared with predictions from Monte Carlo generators modelling the photon radiation from the quark and the electron lines, as well as with calculations at leading and next to leading order in the strong coupling. The predictions significantly underestimate the measured cross sections.     

Time-dependent perturbed angular correlation study of181Ta in Mg

The quadrupole interaction of¹⁸¹Ta in Mg metal has been investigated at room temperature by time differential perturbed angular correlation method yieldingv _Q=127±7 MHz. The observed EFG is calculated to be 20.8×10¹⁶ V/cm².     

TDPAC studies of the intercalation compound Li x TiS2 in the region 0≤x≤1

The electrochemical intercalation of Li⁺ into hafnium-doped TiS₂ was studied using time-differential perturbed angular correlations (TDPAC). The¹⁸¹Ta nuclear quadrupole interaction was monitored as a function of the charge transfer,n _F. For low uptakes, 0≤n _F≤0.03, a two-phase region was found with an empty host interaction, characterized byV _q=458(4) MHz, η=0.19(3) and δ-0.083(5), and a second interaction corresponding to lithiated material withv _q=597(5) MHz, η=0.13(4) and δ=0.061(5). No evidence was found for intermediate phases.     

Hydrogen and deuterium trapping at nitrogen in tantalum studied by PAC

PAC experiments using¹⁸¹Hf as probe atom were performed in Ta samples containing 1.1 at% of N and loaded with 0.2 at% of H or of 1.6 at% of D. At room temperature only the known frequency due to N is present. Below 100K or 140K, for H and D respectively, the amplitude of this interaction decreases and two new interaction frequencies characterized by V₀=240MHz, η=0.80 and V₀=250 MHz, η=0.40 appear. These frequencies are attributed to H(D) trapped by N in the vicinity of the probe atom¹⁸¹Hf. The disappearance of these interactions at 100K and 140K is interpreted as a release of H (D) from Hf but not from N. From the data the jump rates of H(D) around N in Ta are derived.     

NMR study of the electric field gradient in the paramagnetic phase of M<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript> (M = Co,Ni) compounds

The NMR spectra and the decay of a spin echo signal from ⁵¹V nuclei in Kagome-staircase Co₃V₂O₈ (CVO) and Ni₃V₂O₈ (NVO) single crystals are measured in the temperature range 30–300 K and a magnetic field H ₀ = 20 kOe. The orientation dependences of the ⁵¹V NMR line shape are used to determine the electric field gradient (EFG) parameters, namely, quadrupole frequency ν _Q and asymmetry parameter η. These parameters for NVO and CVO are ν _Q = 180(10) kHz, η = 0.5(1) and ν _Q = 130(10) kHz, η = 0.6(1), respectively. A comparison of the results of calculating EFG tensors with a point charge model and the NMR data indicates that the crystallographically equivalent vanadium atoms in the Ni₃V₂O₈ and Co₃V₂O₈ compounds differ in the EFG axis orientation. M₃V₂O₈ crystals are found to have vanadium positions (V1, V2) with different orientations of the z axis, which specifies the direction of the principal value of EFG (V _zz ): these orientations lie in the bc plane and make an angle of either +51(5)° (V1) or −51(5)° (V2) with axis c. In the temperature range 30–300 K, the EFG tensor components and the local symmetry of the charge surrounding of the vanadium positions in NVO and CVO oxides are found to change insignificantly.     

The nuclear quadrupole interaction at 111Cd and 181Ta sites in anatase and rutile TiO2: A TDPAC study

The nuclear quadrupole interaction of the I=5/2 state of the nuclear probes ¹¹¹Cd and ¹⁸¹Ta in the anatase and rutile polymorphs of bulk TiO₂ was studied using the time differential perturbed angular correlation (TDPAC) method. The fast–slow coincidence setup is based on CAMAC electronics. For anatase, the asymmetry parameter of the electric field gradient was η=0.22(1) and a quadrupole interaction frequency of ω_Q=44.01(3) Mrad/s was obtained for ¹⁸¹Ta. For rutile, the respective values are η=0.56(1) and ω_Q=130.07(9) Mrad/s. The values for rutile match closely with the literature values. In case of the ¹¹¹Cd probe produced from the beta decay of ¹¹¹Ag, the quadrupole interaction frequency for anatase was negligibly small as indicated by an unperturbed angular correlation in anatase. On the other hand for rutile the quadrupole frequency is ω_Q=61.74(2) Mrad/s and the asymmetry parameter η=0.23(1) for the ¹¹¹Cd probe. The results are interpreted in terms of the surrounding atom positions in the lattice and the charge state of the probe nucleus.     

Determination of the spectral parameters of nuclear quadrupole resonance from nutation spectra of powdered samples

An analytical expression I(ω) is obtained for a normalized function of the shape of an idealized nuclear quandrupole resonance nutation line of a powdered sample for spins I=3/2 (η≠0). Calculations are made of the initial moments <ωⁿ> of the nutation spectrum of the powder in the form of functions of ω₀=γB₁ and the asymmetry parameter η of the electric field gradient tensor. A method is proposed for determining the spectral parameters η and eQq_zz from the experimentally measured values of <ω>, <ω²>, <ω⁴>, and ω_1/2 of the nuclear quadrupole resonance nutation spectrum of the powder. State University, Kaliningrad. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 3–8, July, 1997.     

Hf3Al2 and Zr3Al2 Isostructural Aluminides Studied by PAC with 181Ta and 111Cd Probes

The electric quadrupole hyperfine interactions for ¹⁸¹Hf/¹⁸¹Ta and for ¹¹¹In/¹¹¹Cd probes in polycrystalline Zr₃Al₂ and Hf₃Al₂ compounds were measured in the temperature range 24–1100 K. The hyperfine quadrupole interaction parameters were determined after different doping techniques and heat treatments of the samples.¹⁸¹Hf/¹⁸¹Ta was found to substitute the Hf/Zr sites and the ¹¹¹In/¹¹¹Cd impurities appear to substitute both the 8(j) Al sites and the three nonequivalent Hf/Zr sites.     

Incorporation of the transition metal Hf into GaN

The perturbed angular correlation (PAC) technique was applied to study the incorporation of the transition metal Hf into GaN after implantation. To this end the PAC probe ¹⁸¹Hf(¹⁸¹Ta) was implanted into epitaxial Wurtzite GaN layers (1.3 μm on sapphire) with an energy of 160 keV and doses of 7× 10¹² at/cm². PAC spectra were recorded during an isochronal annealing programme, using rapid thermal annealing (RTA) and furnace annealing, in the 300–1000oC temperature range. After implantation the spectra show a damped oscillation corresponding to a quadrupole interaction frequency (QIF) of ν_Q= 340 MHz for 30% of the probe nuclei. Annealing up to 600oC reduces the damping of this frequency without an increase of the probe atom fraction f_s in these sites. Above 600oC f_s grows rapidly until after the 900oC RTA step more than 80% of the Hf probes experience a well defined QIF due to the incorporation of Hf on undisturbed sites of the hexagonal GaN wurtzite lattice. An interaction frequency of ν_Q= 340 MHz is derived. RTA and furnace annealing yield similar results for annealing up to 800oC, where the undisturbed fraction reaches about 60%. Then RTA at higher temperatures increases this fraction, while furnace annealing leads to a decrease down to 22% after annealing at 1000oC. To our knowledge this is the first time that a transition metal probe like Hf is incorporated to such a large extent into a semiconductor lattice. This revised version was published online in August 2006 with corrections to the Cover Date.     

PAC studies of hyperfine interactions of181Ta in YBa2Cu3O7-δ and of140Ce in La2CuO4 and La1.85Sr0.15CuO4

Results of TDPAC studies of hyperfine interaction in high temperature superconducting ceramics are reported. The γ-ray cascade of 329–487 keV in¹⁴⁰Ce excited in the decay of¹⁴⁰La in La_2-xSr_xCuO₄ samples (x=0 and 0.15), and 133–482 keV cascade in¹⁸¹Ta excited in the decay of¹⁸¹Hf in YBa₂Cu₃O_7-δ samples were used. The procedure of introducing radioactive¹⁸¹Hf into the ceramics is described and indirect evidence for the occupation of Cu sites by the¹⁸¹Hf-¹⁸¹Ta probe is presented.     

Shape-driving effect of the proton 1/2[541] band in 171Ta

High-spin states in ¹⁷¹Ta were populated through the heavy-ion fusion-evaporation reaction ¹⁵⁷Gd (¹⁹F, 5n)¹⁷¹Ta at 105MeV beam energy. Lifetimes of the levels of the πh_9/21/2[541] band have been measured by using the Doppler shift attenuation method. The transition quadrupole moments ( Q_t) and the quadrupole deformation (β₂) have been extracted. Both β₂ and Q_t values decrease slightly with increasing rotational frequency. The average β₂ value of 0.26 is 18% larger than that of the πh_11/2 9/2[514] band. Total Routhian Surface calculations have been performed with the non-axial deformed Woods-Saxon potential and the predicted values of the quadrupole deformation β₂ are in good agreement with that deduced from our lifetime measurement. The shape-driving effect is discussed.     

Magnetic and quadrupole interaction studies at 111Cd in GdMn2 and TbMn2

The temperature dependence of the electric quadrupole interaction and the magnetic hyperfine field at ¹¹¹Cd probe sites in RMn₂ (R=Gd, Tb) has been studied by TDPAC method. In the paramagnetic region the quadrupole interaction frequency ν_Q in both compounds varies linearly with temperature. Below the Neel temperature, we find an abrupt decrease in the magnitude of ν_Q which is consistent with an expansion of the unit cell. As an important feature, the data near T_N shows the coexistence of localized and itinerant magnetism of Mn atoms. This revised version was published online in August 2006 with corrections to the Cover Date.     

Perturbed angular correlation studies of Ta hyperfine interactions in Hf-Ni and Zr-Ni compounds

The hyperfine interaction experienced by ¹⁸¹Ta nuclei in the intermetallic compounds ZrNi₅, HfNi₅, and Hf₂Ni₇ has been investigated by perturbed angular correlation (PAC) spectroscopy. At temperatures T≥15 K the ¹⁸¹Ta angular correlation of appropriately annealed ZrNi₅ and HfNi₅ is unperturbed, indicating the absence of a magnetic hyperfine interaction. This observation rules out the possibility of spontaneous magnetic order of ZrNi₅ and HfNi₅ recently proposed in the literature. The temperature dependence of the electric quadrupole interaction of ¹⁸¹Ta in Hf₂Ni₇ suggests the existence of a reversible phase transformation at T≥500 K.     

Pac studies of the electric quadrupole interaction of181Ta in yttrium and copper oxides and in super conducting ceramics YBa2Cu3O7−δ

The electric quadrupole interaction of¹⁸¹Ta in Y₂O₃, CuO, and in the YBa₂Cu₃O_7−δ high temperature superconductors, doped with¹⁸¹Hf, has been studied by the time-differential perturbed angular correlation method (TDPAC). In Y₂O₃, two distinct fractions of the probe nuclei were detected, which were attributed to the substitutional positions at the two cation sites in the lattice. In CuO, and in the ceramic samples, broad distributions of quadrupole frequencies were observed. Possible configurations of the probe nuclei in these substances are discussed.     

Hyperfine and Zeeman studies of low-lying atomic levels of181Ta and the nuclear electric quadrupole moment

The hyperfine structure of the atomic levels 5d ³ 6s ^{2 4} F _{7/2, 9/2} and⁴ P _1/2 in¹⁸¹Ta has been studied by atomic-beam magnetic-resonance. Using intermediate coupling wave functions derived for the configurations (5d+6s)⁵ the hyperfine structure data of six low-lying metastable states have been analyzed with respect to the effective operator formalism. The effective radial parameters for the magnetic dipole and electric quadrupole interactions are determined from these measurements and compared with relativistic calculations. The value obtained for the spectroscopic quadrupole moment of the¹⁸¹Ta nuclear ground state is Q_hfs=3.44(17) barn (uncorrected for configuration interaction effects).