Neutron scattering studies of RENi{sub 2}B{sub 2}C: Magnetic structures and lattice dynamics

Abstract

Neutron scattering techniques have been used to study the magnetic structure and lattice dynamical properties of various members of the recently discovered RENi2B2C family, were RE stands for a rare-earth element. The magnetic structures of superconducting DyNi2B2C) E T N~~B~C , HoNi2B2C) and non superconducting TbNi2B2C have been determined as a function of temperature, in the 2 -300 K temperature range. In the superconducting compounds we have long range magnetic order coexisting with superconductivity, and we have discovered some very interesting magnetic structures. In particular, sinusoidally modulated magnetic structures with similar wavevectors along the a* direction are present in all of the above compounds with the exception of DyNi2B2C. In the Tb compound we have, in addition to the incommensurate antiferromagnetic structure, the development of a small ferromagnetic component which is probably responsible for the absence of superconductivity.The low-lying phonon dispersion curves of superconducting LuNi2B2C (Tc M 16.6 K) and HoNi2B2C (Tc M 8.1 IC) have also been determined along the [E,O,O] and [0, 0, E] high symmetry directions, using inelastic neutron scattering techniques.The most interesting result of these studies is the discovery of pronounced soft phonon behavior for the acoustic and first optical A4 [E, 0, 01 branches at wavevectors close to 2 the zone-boundary point GI. This shows that the electron-phonon interaction is very strong and causes an incipient lattice instability, a behavior typical of strongly coupled conventional superconductors. Both the incommensurate magnetic structures and the soft phonon behavior seem to be influenced by common nesting features of the Fermi surface of the rare-earth nickel boride carbides, as band theoretical calculations suggest.