E. C. Svensson

Ground‐ and excited‐state spin waves in PrAl2

The magnetic excitations in ferromagnetic PrAl2 have been studied by means of neutron inelastic scattering. Branches of excitations corresponding to four of the six allowed magnetic‐dipole transitions have been observed. At the magnetic zone boundary, where the excitation spectrum most closely approximates a single‐ion spectrum, the observed frequencies are 1.54±0.04, 2.05±0.05 and 3.76±0.06 THz. The results are analyzed in terms of exchange and crystal‐field parameters using the pseudoboson theory for all states of the J=4 manifold. The crystal‐field‐only ground state is found to be a non‐magnetic doublet but the exchange interaction is strong enough to induce almost the full moment at low temperatures. At elevated temperatures in the ordered phase, excited‐state spin waves have been observed at 0.55±0.10 THz. These correspond to a transition between the upper member of the exchange‐split ground doublet, and the second member of the exchange‐split triplet state.

13. Solid and Liquid Helium

Publisher Summary This chapter discusses that the condensed phases of the stable isotopes of helium, 3 He, and 4 He continue to be of high interest to scientists in general and to neutron scatterers in particular. This interest stems from the fact that these substances form the prototype examples in the nature of quantum solids and liquids of interacting bosons and fermions. A very meaningful comparison of the results of first-principles theoretical calculations, with experiment, is in fact possible for these materials, because the interatomic potential is reasonably simple and well known. Different regions of this potential can readily be probed in the experiments by varying the pressure and temperature to vary the interatomic distance, an extremely important advantage. The chapter highlights some of the most important features of the structure and excitations of the condensed phases of 3 He and 4 He; concentrating on the results from neutron scattering measurements, and on those theoretical developments that have direct consequences for neutron studies.

Spin Waves and Central Modes in Crystal Field Systems

For magnetically ordered intermetallic compounds such as PrAl2, Pr3Tl and TbSb neutron inelastic scattering experiments show that the spin-wave spectrum consists of several branches of excitations. These correspond at T=0 to allowed transitions between the ground state and excited states of the lowest multiplet of the rare-earth ion in the exchange field existing in the material. A good description of the spin-wave spectrum can be obtained within the random phase approximation with crystal field terms and interionic interactions. At elevated temperatures excited-state spin waves are observed which correspond to transitions between excited states of the multiplet. Close to the ordering temperature and ordering wavevector the spectrum is dominated by elastic scattering with less important contributions from inelastic transitions. Generalization of the spin-wave theory to finite temperatures leads to a qualitatively correct description of these effects and insight into the nature of the ‘central’ mode.