The Electric Field Gradient in Noncubic Metals and Alloys
- PDF / 738,222 Bytes
- 6 Pages / 415.8 x 635.4 pts Page_size
- 84 Downloads / 190 Views
397
THE ELECTRIC FIELD GRADIENT IN NONCUBIC METALS AND ALLOYS
W. WITTHUHN, U. DE, W. ENGEL, S. HOTH, R. KEITEL, W. KLINGER, and R. SEEBOCK Physikalisches Institut der Universitlt Erlangen-Nirnberg, 852 Erlangen, Germany
ABSTRACT The electric field gradient (efg) present in noncubic solids causes an nnergy-splitting of the nuclear levels via the quadrupole hyperfine interaction. During the last few years the peiturbed angular correlation method has proved a unique experimental tool for investigating this interaction especially in metals. The basic principles of the method are discussed. Recent experimental results are given for pure metals and highly diluted systems as well as for alloys and intermetallics. The last section deals with theoretical aspects of the temperature dependence of the efg in pure metals.
INTRODUCTION The knowledge of the charge distribution in a solid is of interest for the Lunderstanding of its basic properties and of great importance for many applications in material science. In noncubic solids this charge distribution creates an electric field gradient (efg), which causes an energy-splitting of the nuclear levels due to the quadrupole hyperfine interaction (QI) between the nuclear quadrupole moment Q and the efg at the site of the nucleus. This interaction can be studied by a variety of experimental methods. In metals, however, they are limited in general to a few favourable cases or are restricted to low temperatures. Stimulated by the work of Raghavan [1] the time-differential perturbed angular correlation (distribution) methods (TDPAC TDPAD) have been demonstrated during the last years to be a unique experimental tool for investigating this interaction. Here no limitations due to skin effect exist as in classical NMR or NQR measurements and no low temperatures in the mK range are needed as required for specific heat and nuclear orientation experiments. Thus, the full temperature range from mK up to the melting points of the metals or alloys can be covered. In this respect the method is superior to the Mbssbauer-effect where the signal depends on a Debye-Waller-factor and decreases with increasing temperatures. A recent comprehensive review of the electric field gradients in metals has been given by Kaufmann and Vianden [2] . Here a detailed comparison of the experimental methods as well as a survey of the available experimental data and present status of the theoretical understanding of the origin of the efg can be found. METHOD The perturbed angular correlation methods have been described highly detailed in Ref. [31 , here only the principles will be mentioned. The detection of the quadrupole interaction is based on the observation of the anisotropic y-radiation patterns emitted from an isomeric nuclear state with a nonrandom nuclear spin orientation. The useful lifetimes of the excited levels range from a few ns up to some 4s, in favourable cases up to a few ms, the limits being given by the interaction strength or the time resolution of the apparatus and the statistics or measurin
Data Loading...
