Magnetic excitations in the longitudinally polarized antiferromagnetic phase of praseodymium

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Abstract
In the ordered phase of Pr, which can be induced by the application of a uniaxial pressure, the length of the moments is modulated sinusoidally with a period incommensurate with the lattice. A comprehensive theory has been developed to describe the ground-state properties and the excitation spectrum of this antiferromagnetic phase. Numerical results, based on parameters determined from paramagnetic Pr, are compared with neutron scattering experiments. In order to account for linewidth effects several mechanisms which broaden the excitations are considered. Contributions due to the incommensurate modulation are included but are found to be small. To first order in the high-density (1/z) expansion, the effects of the single-site fluctuations vanish exponentially in the zero-temperature limit. The second-order contributions, derived in the present paper, are then the leading-order terms at low temperatures and are found to be important. Additionally, the scattering against electron-hole pair excitations of the conduction electrons is treated by incorporating it directly in the 1/z expansion. The theory yields a reasonably realistic description of the neutron scattering results. It can be concluded that the phason mode, near the ordering wave vector, is a damped diffusive mode. The principal remaining puzzle, the quasielastic peak not centered at the ordering wave vector, is tentatively ascribed to heavy-fermion behavior in Pr.