X-ray resonant scattering determination of the antiferroquadrupolar ordering inUPd3 at low temperatures

Abstract

We have used x-ray resonant scattering techniques to make a comprehensive study of the quadrupolarorder in UPd3. New measurements reveal that the order parameter describing the in-phase stacking along thec-axis of the quadrupole moments of the uranium ions on the quasi-cubic sites is primarilyQxy in the two lowest temperature phases. These results are discussedwithin the context of a three-level crystal field model for the uranium5f2 electrons, the allowed quadrupolar operators, and the constraints deducedfrom the analysis of heat capacity measurements. We have now deduced, andsummarize, the order parameters for each of the four antiferroquadrupolar phases ofUPd3.