Panel IIIShort summary

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Submitted on 1 Jan 1979

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Panel IIIShort summary

Lander, G., Brooks, M.S.S., Cooper, B.R., Schoenes, J., Sinha, S.K.

To cite this version:

Lander, G., Brooks, M.S.S., Cooper, et al.. Panel IIIShort summary. Journal de Physique Colloques, 1979, 40 (C4), pp.C4-238-C4-239. �10.1051/jphyscol:1979476�. �jpa-00218872�

JOUKNAI. DE PHYSIQUE Colloque C4, supplement au no 4, Tome 40, avril 1979, page C4-238

Panel

Short summary

Discussion Leader : Lander, G.

Panelists : Brooks, M. S . S., Cooper, B. R., Schoenes, J.. and Sinha, S. K.

Occasionally a conference in a certain field marks the emergence of some new aspect of experimental or theoretical work. The Actinide Conference of

1978 was such a conference ; for the first time a large number of contributions arose from detailed work on single crystals of uranium compounds. Out of a total of 28 papers (30 % of the total) 12 were devoted t o work on single crystals and a further 6 on theoretically aspects treating properties that require single crystals for their measurement. The reason for this large increase in popularity lies mainly with the availability of single crystals but also with their simple structure, varied properties, and the possibili- ty of comparing their microscopic behaviour with the well-studied analogous rare-earth series.

I n the panel discussion B. R. Cooper (West Virgi- nia University, U.S.A.) led off with a discussion of the magnetization results. These exhibit essentially infinite anisotropy, but, in some cases, a first-order transition occurs t o a ferrimagnetic state. The aniso- tropy also gives rise t o unusual domain effects, which must be suppressed before reliable high-field susceptibilities can be obtained. Phase transitions have also been studied by thermal expansion, a s a function of pressure, and searches made for changes in symmetry associated with the magnetic ordering.

T h e papers o n photon spectroscopy were summa- rized by J. Schoenes (ETH, Zurich, Switzerland).

These include reflectivity, X-ray photoemission, and spin-polarized XPS measurements. From the results on USb, for example, emerges a picture of a narrow Sf band pinned a t E , and a wide 6d band just above E,, but which is probably hybridized with the Sf electrons. The spin-polarized photoemission results on UTe address the question of the conduction- electron polarization observed in actinide ferroma- gnets by comparing neutron and magnetization mea- surements, and show that these electrons are predo- minantly 6d in character.

S. K. Sinha (Argonne Laboratory, U.S.A.) then discussed the neutron work being done in this field.

He pointed out the relevance of the measurements of the critical scattering on both U N and USb t o the arguments about bonding between the metal Sf elec- trons and the anion p states. T h e unexpected absen- ce of the transverse fluctuations in the critical regi- me, which defines the Ising-like nature of these

systems, also occurs in the excitation spectra (measured by inelastic neutron scattering) where only longitudinal excitations have been seen. Fur- thermore no crystal-field like excitations can b e readily identified in these spectra, showing that the analogy with the rare-earth systems, a t least for dynamical behaviour, is very difficult t o make.

J. Rossat-Mignod (CEN-Grenoble) then raised the question of whether multi-axial structures can exist in the antiferromagnetic binary compounds, a s has been found in the pseudo-binary USb,,Te,, . Neu- tron measurements are often ambiguous in that they represent the Fourier transform of components of the magnetic structure and combining these together requires information about the phase, which cannot be obtained from diffraction. However, the pseudo- binary systems are known t o have complex structu- res, whereas there is little evidence a s yet t o suggest the pnictides are s o complicated.

The theoretical questions were then addressed by M. S. S. Brooks (Euratom, Karlsruhe, FRG). The problem of performing self-consistent relativistic band structure calculations in actinide systems pre- sents considerable problems, and clearly a number of approximations have t o be made. Experimental evidence, however, can often be important in deli- neating the approximations that can b e made, and we are seeing the beginning of this. Of particular interest is t o examine the types of bonding that can occur, and how they vary with lattice parameter. For example, one has USf-U6d hybridization between neighbouring uranium sites ; but, in addition, f-d hybridization may occur with the p electrons of the anion.

In the general discussion the questions of bonding, and how it might effect other kinds of physical properties, was raised again. One obvious parameter that has not been changed u p t o now in actinide systems is the interatomic spacing via pressure measurements. Some of these are beginning on uranium systems and if our general ideas are correct one would hope for some changes in physical pro- perties, provided the compressibilities are large enough.

I n summary, new measurements with a number of different techniques on single crystals have clearly focused much attention on the NaCl actinide sys-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979476

SHORT SUMMARY C4-239

tems. The field owes a particular note of thanks to Cf and Bk compounds. The future of the NaCl 0. Vogt of ETH, Zurich for his efforts in growing subgroup in actinide research thus appears assured these crystals. Although this panel was dominated and we can look forward to a deeper level of by work on uranium systems, there was reference to understanding and a more quantitative dialogue the preparation and lattice parameter of Am mono- between theorists and experimentalists at future chalcogenides and efforts are underway to prepare meetings.