Local nuclear orientation of125I in tin and graphite using defect induced electric field gradients

Defect-induced field electric field gradients, generated by ion implantation of¹²⁵I in α- and β-Sn and in graphite, were used to obtain local nuclear orientation at low temperatures. The resulting nuclear orientation at the parent¹²⁵I state was detected by¹²⁵Te Mössbaner spectroscopy. From the measured nuclear quadrupole interaction strengths at the parent state, the electric field gradient and the cooling behavior of implanted atoms were studied.     

Local nuclear orientation using defect-induced electric field gradients in cubic lattices

It is demonstrated that the detection of low temperature nuclear orientation by Mössbauer spectroscopy, rather than by the observation of the angular distribution of the emitted radiation, allows to determine nuclear quadrupole interaction strengths generated by randomly oriented defect-induced electric field gradients in cubic lattices. Also the cooling behaviour of radioactive nuclei implanted in semiconductors is studied.     

Magnetically induced electric field gradients

Several investigations of the Mössbauer spectrum of Fe₂As have yielded results which do not agree with those obtained by neutron diffraction studies. We report here a new investigation, and we show that the earlier disagreement is caused by a lack of appreciation of several factors. These include fluctuating hyperfine magnetic fields (relaxation) and an asymmetry of the electric field gradient produced by the spontaneous magnetization.On leave of absence at Laboratoire de Spectroscopie Hertzienne de l'Ecole Normale Superieure, 24 Rue Lhomond, 75005 Paris, France     

Geometrical significance of the orientation of defect-induced electric field gradients

One of the features of defect-induced axial symmetric electric field gradients(efg) is the orientation of their symmetry axis with respect to the crystal lattice. Especially in the case of simple trapped defects(mono-, di-vacancies, single interstitials or single impurity atoms) this orientation should follow the symmetry of the geometrical probe atom defect arrangement. This will be illustrated by PAC measurements at¹¹¹In/¹¹¹Cd for the case of trapped impurity atoms in Cu.This work was supported by the Bundesminister für Forschung und Technologie.     

Systematics of electric field gradients in metals

The intensive study of electric field gradients (EFG) at the site of atomic nuclei in noncubic metals has revealed several systematic trends, e.g. in the relation of the total EFG to the so called lattice EFG as well as in its variation with temperature. Numerous investigations have been carried out in order to test these systematics. The results will be reviewed and compared to the known trends. Progress in theoretical calculations of the EFG in pure and impurity — host systems will be discussed and compared to the latest available experimental data. Recent measurements of the EFG at metal surfaces and new calculations of the EFG at host sites in impure cubic metals may contribute to the understanding of the EFG in metals.     

Optimal crystal geometry and orientation in electric field sensing using electro-optic sensors

For optimal sensitivity in electric field measurements, electro-optic (EO) crystals are typically selected based on their EO coefficients and dielectric constants. However, the conventional figure of merit yields sensitivity predictions regarding EO materials that are inconsistent with experimental data. In this Letter, we demonstrate that depolarization effects, which are often ignored, can dramatically enhance responsivity depending on the shape and orientation of the EO crystal. For optimal sensitivity, these effects are best exploited in longitudinal EO sensors, where they yield an optical modulation depth that increases quadratically with crystal length.     

Systematics of electric field gradients in Cu alloys

The electric field gradients (EFG) at the first two near neighbours (nn's) in Cu alloys are investigated. The valence EFG q^V is evaluated using an analytic solution, valid at all distances from the impurity. In the evaluation of the size EFG q^S, an oscillatory form of the nn displacement is used. Without any floating parameter, good agreement is obtained between theory and experiment for all the systems at all nn's.     

Nuclear spin conversion in CH F induced by an alternating electric field

The nuclear spin conversion in CH₃F molecules subjected to an alternating electric field was investigated experimentally. The conversion rate was found to be almost unaffected by low electric fields ( V/cm) but sharply increased tenfold when the electric field amplitude exceeds the values ( V/cm) sufficiently high to produce crossings of the ortho and para states of the molecule. A theoretical model for the molecular conversion in alternating electric field was developed. The results of the experiment were found to be in a good agreement with the theory. Received 23 July 1999 and Received in final form 7 September 1999     

Nuclear orientation of124I in Fe

¹²⁴I has been implanted into iron foils using the recoil technique with the¹²³Sb(,3n) reaction. The temperature dependence of the anisotropies was followed down to 16 mK. Using the lattice site occupations known from mass separator implanted IFe sources a magnetic moment ¦¦=1.14±0.08 nm was derived for the 2 ground state of¹²⁴I. This value can be understood if the three g7/2 protons above Z=50 are coupled to spin 9/2, and the odd neutron is in the h11/2 shell.Anisotropies of 18 transitions in¹²⁴Te were determined, resulting in 5 new unique spin assignments; multipole mixing ratios were extracted. Of these (E2/M1;723)=–2.0±0.4 conflicts with the value adopted from angular correlation experiments. By comparison of () and NO anisotropies, a better choice between the solutions for (E2/M1;1489) could be made. The new value is =+0.13±0.10.This work was performed as part of the research program of the Stichting voor Fundamenteel Onderzoek der Materie (F.O.M.), which is financially supported by the Nederlandse Organisatie voor Zuiver Wetenschappelijk Onderzoek (Z.W.O.).     

Lattice sum of electric field gradients in tetragonal crystals

A new method to calculate the lattice contribution to electric field gradients at a nuclear site in tetragonal crystals is developed. The crystal is regarded as an assembly of positive ions at lattice points embedded in a uniform background of negative charge (point charge model). The method uses Euler-Maclaurin formula and makes the plane-wise summation in the direct crystal space unlike most of the previous methods utilising Fourier transform to reciprocal space. The numerical values obtained using the above approach agree well with previous results.     

Spin orientation of two-dimensional electrons in electric field

The Rashba Hamiltonian is used to analyze the spin orientation of two-dimensional electrons in electric field. The mean electron spin is found to be oriented in the sample plane perpendicularly to the electric field. In the limit of weak spin-orbit interaction, the spin orientation factor may increase.     

Mechanisms for electric field gradients in ferromagnetic cubic metals

Conclusions The calculation of the spin-orbit and the Casimir contributions are in progress, from our preliminary results we have observed the Casimir effect to be at least as large if not larger than the spin-orbit effect. This corresponds well with the atomic situation^6,7. The calculation itself requires very careful analysis involving the summations over , the symmetry¹², and the use of the excited states, in order to arrive at definite conclusions regarding the relative importance of the three mechanisms carried over from the atom and any additional mechanisms that may be characteristic only for solid-state. It is hoped that conclusions drawn from our calculations on pure iron will be useful in applying these mechanisms to more complicated systems such as heavy atom alloys.Supported by National Science Foundation.     

First-principles calculations of electric field gradients of Sn compounds

The Discrete Variational (DV) LCAO Molecular Orbital method in the local density approximation was employed to obtain the electronic structure of clusters representing three compounds of Sn(II), namely SnF₂, SnO and SnS and two of Sn(IV): SnF₄ and SnO₂. The electric field gradients at the Sn nucleus were calculated and a value for the nuclear quadrupole moment Q was determined.     

Instability of nanostructures patterned in polystyrene under high electric field gradients

A nanolithography technique based on the lateral displacement of electrically biased AFM tip was developed for nanostructures formation of 30-100 nm in width and 1-10 nm in height in the polystyrene (PS) films. It was demonstrated that the nanostructures patterned in annealed PS films (90K Mw) show slow exponential relaxation between 55 and 265 h depending on their size. Relaxation of the nanostructures in non-annealed films usually occurred in minutes. It was observed that in the annealed samples a negative electric charge accumulated in the areas where the nanostructures formed while in the non-annealed samples only the positive charge in exposed areas was detected using the electric force microscopy. After 320 h of monitoring under the humidity maintained between 25 and 27% it was suggested that slow dynamical changes of the nanostructures can be attributed to the negative electric charge dissipation in the annealed samples.     

Lattice locations and electric field gradients of boron implanted into vanadium

Polarised emitting¹²B nuclei were recoil implanted into single crystalline vanadium atT=305 K. radiation detected nuclear magnetic resonance measurements showed that two implanation sites occur simultaneously which were identified as the substitutional site and an interstitial site with axial symmetry. Extensive cross-relaxation measurements were performed on¹²B in both lattice positions. By this means the interstitial site could be identified as the octahedral one. The impurity induced electric field gradients at nearest and at next nearest⁵¹V neighbours about¹²B could be determined for both implantation sites.     

Crystal field theory and electric field gradients at 49Ti nuclei sites in LaTiO3

The energy level diagram and the wave functions for the Ti³⁺ ions (3d ¹) in LaTiO₃ are calculated using modern crystal-field theory. The relative orbital ordering of these ions in the ground state is obtained. It turns out that the states of the ground triplet are considerably split and therefore the effect of the electronic-vibrational interaction is suppressed despite the fact that the distortions of the TiO₆ building block seem to be small. The components of the electric field gradient tensor at the Ti³⁺ nuclei sites are calculated using the wave functions of the ground states obtained. The calculated asymmetry parameter agrees well with the experimental values, which demonstrates the adequacy of the proposed orbital-ordering pattern of the Ti³⁺ ions in the ground state.     

Systematically weak temperature dependence of electric field gradients in distorted cubic metals

The temperature dependence of the electric field gradient for radiation-damaged copper and for the dilute alloy Zn$\text{Ag}$ has been measured using the perturbed angular correlation technique. A systematically weak temperature dependence is found for distorted cubic metals compared to non-cubic metals. The strong temperature dependence for the Zn$\text{Ag}$, In$\text{Ag}$ and Sn$\text{Ag}$ alloys as reported in literature has not been confirmed in the present experiment.