Quadrupole Interaction Frequency Research Articles

ELECTRIC FIELD GRADIENTS AT THE 111IN SITE IN THE FERROELECTRIC AND PARAELECTRIC PHASES OF LITAO3

The temperature dependence of the electric-field gradient of 111Cd in single crystalline LiTaO3 was studied from room temperature to 1040 K in the ferroelectric and paraelectric phases. The data taken at room temperature show unambiguously the presence of two quadrupole interaction frequencies, νQ1=230 MHz and νQ2=242 MHz, with nonzero asymmetry parameters, while above the Curie temperature (TC=878 K) the data are well described by a unique frequency. The electric field gradient shows a usual temperature dependence, increasing aproximately in a linear fashion until TC and then decreasing faster. The initial increase is explained mostly by the lattice expansion, while above TC it is necessary to consider Li and O displacements.

Cavities in silicon investigated with the PAC-probe 111 In

The perturbed angular correlation technique (PAC) was used to study the creation and development of He-induced cavities. In order to investigate the interaction of Indium atoms with cavities in Silicon the Bonn isotope separator was used to implant overlapping profiles of He (10 keV) and radioactive 111In (160 keV) into undoped FZ-silicon. To get insight into the cavity formation mechanism samples were prepared with various He-doses (0.6, 2 and 6× 1016 ions/cm2). The samples were measured directly after implantation and after different annealing steps (thold= 10 min, T = 500–1100oC). Further, different implantation and annealing sequences were used. At higher He doses (2 and 6× 1016 ions/cm2) we find a large fraction of 111In probe atoms subjected to an electric field gradient (EFG) corresponding to a quadrupole interaction frequency (QIF) of νQ= 411(6) MHz with η= 0.25(4). The corresponding defect configuration is formed most effectively after He implantation into annealed, 111In doped Si. This and the affinity of In to vacancies leads us to the assumption that, similarly to the situation in metals, the Indium atoms act as nucleation centres for vacancy clusters (cavities) and are situated on the inner walls of the cavities.

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 νQ in both compounds varies linearly with temperature. Below the Neel temperature, we find an abrupt decrease in the magnitude of νQ which is consistent with an expansion of the unit cell. As an important feature, the data near TN shows the coexistence of localized and itinerant magnetism of Mn atoms.

Study of the static quadrupole moment of the K = 35/2 isomer in

The quadrupole interaction frequency of the high-K five-quasiparticle isomer in implanted in a Tl host at a temperature of 473 K has been determined using the LEMS method. A value of 65.7(7.5) MHz has been found.

Electric Quadrupole Interaction at 181Ta in Isostructural Orthorhombic Cu8Hf3 and Cu8Zr3 Compounds

Abstract The quadrupole interaction of 181Ta probes in isostructural Cu8Hf3 and Cu8Zr3 intermetallic compounds was studied in the temperature range 24 K -1100 K with the time-differential perturbed angular correlation technique. Two nonaxial electric field gradients corresponding to two nonequiv-alent hafnium or zirconium sites in the investigated structure were found. A linear decrease in the quadrupole interaction frequency with increase of temperature for the 8(d) sites was evidenced, while the vQ(T) dependence for the 4(c) sites is weaker and has a T3/2 character.

Evaluation of the 57mFe Quadrupole Moment from Hartree-Fock Calculations

Abstract Two theoretical evaluations of 57mFe quadrupole moment (Q), based on different formalisms, namely the Hartree-Fock theory and the Linearized Augmented Plane Wave method have yielded results differing by a factor of two. In both cases, Q was obtained from experimental quadrupole interaction frequencies through investigation of the Electric Field Gradients at the nuclear site of the 57mFe probe. It is the purpose of the present work to reexamine the earlier Hartree-Fock approach. In particular, the earlier model is extended through a more realistic description of the environment of 57mFe in the respective experiments, as well as through inclusion of electron correlation effects.

Drastic Increase of the Nuclear Deformation in Bromine betweenA=79and 77

The electric quadrupole interaction frequencies eQVzzyh of 77Br (I ­ 3y2; T1y2 ­ 57 h) and 82Br (I ­ 5; T1y2 ­ 35 h) in hcp Co were measured with modulated adiabatic fast passage on oriented nuclei. The electric quadrupole moment of 77Br is deduced to be Q ­ 10.530s22d b, which, on the condition of strong coupling, implies a nuclear deformation of b2 ­ 10.354s15d. This manifests a drastic increase of the nuclear deformation between 79Br (b2 , 10.22) and 77Br. [S0031-9007(98)06361-3]

Calculation of quadrupole interactions in the high pressure phase of $$\beta {\text{ - }} $$ gallium metal

Electric field gradient q and quadrupole interaction frequency \(\nu \) calculated at 256.7 K in the high pressure phase (orthorhombic) of \(\beta {\text{ - }} \) Ga metal are reported. The results are: q=+0.251 atomic units (au), \(\nu \)=5.479 MHz. These are compared with results from experiment and previous calculation available for the monoclinic phase of \(\beta {\text{ - }} \) Ga metal at normal pressure. The results from the previous calculation at 248 K are: q=-0.250 au and \(\nu \)=5.318 MHz. The result from experiment extrapolated to 256.7 K is: \(\nu \)=4.871 MHz. The sign reversal of the calculated q is attributed mainly to the change of point symmetry of the lattice from the orthorhombic to monoclinic. That the interaction frequency in high pressure phase is higher than experiment may be partly due to the increase of pressure and partly to the structural phase change.

Hyperfine interactions of111Cd inCuGeO3studied by perturbed angular correlations

The electric quadrupole interaction of ${}^{111}$Cd in poly- and single-crystalline CuGeO${}_{3}$ has been measured in the temperature range from 4.2 to 556 K, using perturbed angular correlation spectroscopy. Two electric field gradients have been observed: a well-defined one having a quadrupole interaction frequency of ${\ensuremath{\nu}}_{q}=407$ MHz and an asymmetry parameter of $\ensuremath{\eta}=0.34$ at room temperature, and a broadly distributed one. Possible sites for the implanted ${}^{111}$In probes are discussed on the basis of the point charge model and perturbed angular correlation (PAC) results in CuO and other binary and ternary oxides. No anomalous change of the hyperfine interaction parameters has been detected at the spin-Peierls transition temperature ${T}_{\mathrm{SP}}.$ We suggest that the absence of an anomaly at ${T}_{\mathrm{SP}}$ is caused by the doping of CuGeO${}_{3}$ with the ${}^{111}$Cd PAC impurity, which prevents lattice distortions and spin dimerization at the nearest-neighbor Cu sites in Cu-O-Cu chains.

Structural Phase Transition ofBaRu2/3M1/3O3(M=Ca, Cd, and Sr) Studied by117In(←117Cd) Time-Differential Perturbed-Angular-Correlation

Electric field gradients at 117 In(← 117 Cd) nuclei in BaRu 2/3 (M 0.97 116 Cd 0.03 ) 1/3 O 3 (M=Ca, Cd, and Sr) were measured in the temperature range between 4 and 773 K by time-differential perturbed-angular-correlation of the 90-345 keV cascade γ -rays. For each oxide, the quadrupole interaction frequency markedly increases below a specific temperature; the temperatures are between 4 and 77 K for M=Ca, and about 300 and 600 K for M=Cd and Sr, respectively. Combined with the results of the X-ray powder diffraction measurements, it is concluded that a hexagonal-monoclinic structural phase transition, associated with a deformation of MO 6 octahedra, takes place at the temperature.

Perturbed angular correlation measurements at 181Ta in Hf2Pd and Zr2Pd tetragonal MoSi2-type phases

The electric quadrupole interaction at 181Ta in Hf2Pd and Zr2Pd compounds was studied by the perturbed angular correlation method in a broad temperature range. The powder X-ray analysis verified a single-phase C11b structure of the samples. The least-squares fits of the perturbation factor to the experimental spectra measured for Hf2Pd and Zr2Pd compounds yielded an electric interaction with a well-defined single quadrupole frequency (vQ = 305(2) MHz for Hf2Pd and 291(4) MHz for Zr2Pd) and asymmetry parameter η = 0, reflecting the existence of one axially symmetric probe site. A decrease in the quadrupole interaction frequency with an increase of temperature is very rapid for both Hf2Pd and Zr2Pd compounds.

Extension of the level mixing resonance (LMR) method to study the alignment and the quadrupole moment of light exotic projectile fragments

The level mixing resonance (LMR) method has proven to be a very powerful method to measure the quadrupole interaction frequency of long-lived low-spin nuclear states which decay through γ-radiation [R. Coussement et al., Hyperfine Interactions 23 (1985) 273, G. Scheveneels et al., ibid., 52 (1989) 257, 179]. In this paper the concept of LMR has been extended for β-decaying nuclei produced in a fragmentation reaction at a recoil mass spectrometer. The idea is to apply the LMR method to study some features of light exotic nuclei (e.g. initial alignment, radiation parameter, quadrupole moment, magnetic moment, spin, etc.). It is found that in a level mixing resonance, an appreciable amount of initial alignment is transferred to polarization due to a combined electromagnetic interaction. This transfer of alignment to polarization induces a resonant change in the β-anisotropy as a function of the applied magnetic field. From the amplitude of the resonance, the initial alignment of the projectile fragments can be derived immediately if the radiation parameter A 1 is known (and vice versa). The position of the resonance as a function of an externally applied magnetic field is extremely sensitive to the quadrupole interaction frequency of the nucleus in a host. The quadrupole frequency can be derived if the magnetic moment is known. If not, the ratio μ ν Q is determined. The LMR method applied to recoil separated exotic nuclei produced in a fragmentation reaction provides thus a very powerful tool to study several aspects of the recoil fragments.

Quadrupole Interaction at Surfaces -Nuclear Methods

Abstract The measurement of the electric quadrupole interaction with nuclear techniques allows the study of monolayer coverages or even isolated atoms at surfaces and interfaces due to the inherent sensitivity of these methods. Recently electric field gradients at surface atoms have been investigated with Mößbauer Spectroscopy, Polarized Beam NMR , and Perturbed Angular Correlation (PAC). The highly sensitive PAC technique with 111In and 111mCd sources, in particular, has been applied to several metal and semiconductor surfaces. The use of flat as well as stepped surfaces has allowed the assignment of the measured quadrupole interaction frequencies to specific surface sites. For the Pd(111) surface a particularly complete description is possible, since for five different sites (terrace adatoms, ledge adatoms, kink adatoms, ledge and terrace atoms) the field gradients and their orientation could be determined. They represent a challenging problem for understanding the electronic structure of metallic surfaces. Through the study of the temperature dependence of the different components the diffusion of the radioactive probe atoms can be followed. The data from 57Fe Mößbauer spectroscopy on iron surfaces with monolayer coverage and NMR surface studies of 7Li and 23Na at high temperatures are reviewed. An outlook for further developments of the nuclear techniques is presented, particularly in view of the variety of radioactive isotope beams available from on-line isotope separators like the ISOLDE facility at CERN.

Defect formation and defect-impurity interaction in the hexagonal transition metals Re and Lu

The formation of defects and their interaction with impurities in the hexagonal transition metals is largely unknown. We have employed the γ-γ perturbed angular correlation (PAC) technique to study the defects produced by the implantation of heavy ions into the transition metals rhenium and lutetium via the hyperfine interaction they produce at the lattice site of the PAC probe 111In. In rhenium, during an isochronous annealing program the formation of four different defect complexes at the implanted indium impurities was observed. It is concluded that the complexes are formed by the trapping of one or more vacancies at the In probe. In lutetium, for the first time, it was possible to introduce the In probe nuclei onto substitutional sites of the hexagonal Lu lattice. The corresponding quadrupole interaction frequency is v O = 8.52(3) MHz, η = 0. Further the formation of two different defect configurations at the In probe was observed. One is attributed to the trapping of an implantation induced single vacancy, whereas the second is due to the formation of In-H complexes with residual H dissolved in the sample. In both systems the temperature dependence of the interaction frequencies was measured.

TDPAC studies of Hf doped YBCO

Time differential perturbed angular correlation measurements of the 133–482 keVγ-γ cascade of181Ta in Hf-doped YBa2Cu3O7−x are presented. The181Hf precurser nuclei are incorporated into the sample by thermal neutron irradiation. Two quadrupole interaction frequencies are observed in the as-irradiated sample:v Q1=161±10 MHz with intensityf 1=75%, asymmetry parameterη 1=0.32 and damping parameter Λ1=0.42, andv Q2=1108±40 MHz withf 2=25%,η 2=0.62, and Λ2=0.60. On annealing the sample in air at various temperaturesT a and quenching to room temperature,f 1 remained nearly constant forT a<600°C andv Q1 for all annealing temperatures indicating that these are insensitive to oxygen stoichiometry. This frequency is interpreted to be due to181Hf substitutingY sites. BeyondT a=600°C,f 1 increased and reached a constant value of 90% forT a=800°C. The value ofv Q2 showed a slight variation between 1086 and 1160 MHz, whilef 2 remained nearly constant at 25% forT a<600°C. This component is identified to be due to181Hf substituting Cu 1 sites in the Cu-O chains of YBCO. Above 600°Cv Q2 decreased and reached a value of 808 MHz beyond 750°C.

Measurements of quadrupole interaction frequencies of long-lived isomers with the level mixing spectroscopy (LEMS) method

The level mixing spectroscopy (LEMS) method has proven to be a very useful method to determine the quadrupole interaction frequency of an isomer in a solid host. Especially in the “difficult” cases, e.g. when the isomeric lifetime is very long or its spin is very high, the method yields valuable information which is not accessible with other methods (such as TDPAD). Since the development of the method some years ago, many experiments have been performed on high spin isomers in the lead region. The static quadrupole moment of isomers with lifetimes ranging from 20 ns up to 13 ms andspins up to 65/2ħ have been determined in neutron deficient isotopes of Bi, At, Fr andRa.

Muonium acoustic resonance in alpha-quartz

The effect of acoustic oscillations of the α-quartz crystalline lattice on the behaviour of muonium polarization has been studied experimentally. It is shown that when the frequency of the standing ultrasound wave coincides with one of the muonium quadrupole interaction frequencies a specific broadening of the muonium line occurs; that is a muonium acoustic resonance is observed. The possibilities of using this effect for investigating properties of solids by the μSR method are discussed.

TDPAC measurement of quadrupole interaction at111Cd in the nearly heavy fermion system USn2.9

Quadrupole interaction at111Cd probe has been studied in the nearly heavy fermion system USn3−δ using the TDPAC method. We observe a large quadrupole interaction frequency withvQ=89.9(10) MHz at 300 K which is much higher than the earlier reported result. The observed frequency shows a linear temperature dependence in the range 25–300 K.

Nuclear level mixing resonances on non-axially symmetric systems

For several years, nuclear Level Mixing Resonances (LMR) have been well studied, both theoretically [1,2] and experimentally [3,4]. The experimental results show that LMR is a very powerful technique to determine the static quadrupole interaction frequencies of long living isomers (10 ns up to 100 ms). The LMR formalism has been developed for isomers implanted in a solid host, providing an axially symmetric electric field gradient (EFG). In this paper, the theory has been elaborated to non-axially symmetric systems (η≠0). It will be shown that these systems have some special features, for example the possibility to derive the asymmetry η of the EFG if its orientation (s, γ) in the LAB system is known.

Quadrupole moments of low-energy isomers in 210Po

The quadrupole interaction frequencies of the 8 +, 11 −, 13 − and 16 + isomers in 210Po have been measured in a Bi single crystal by the DPAD method with the 209Bi(t, 2n) 210Po reaction. The results for the quadrupole moments, normalized to Q(8 + 1) = −55.2 (20)fm 2, are | Q(11 −)| = 86 (11)fm 2, | Q(13−)| = 90(7)fm 2 and | Q(16 +)| = 129.7 (20) fm 2. The quadrupole moment of the 0 i 13 2 proton orbital was deduced, and found to be surprisingly small. The results are compared with shell model calculations using the surface delta interaction as residual interaction.