Spin and lattice excitations in the heavy-fermion superconductorUNi2Al3

Abstract

Inelastic neutron scattering measurements have been carried out on the heavy fermion superconductor UNi 2 Al 3 . This hexagonal material orders magnetically into an incommensurate structure, characterized by the ordering wave vector Q o r d =(½′τ,0,½) with τ∼0.11, below T N =4.6 K and then superconducts below T C =1.2 K. For energies above 2 meV, we observe quasielastic magnetic neutron scattering both near (0, 0, n/2) and in ridges along H of the form (H, 0, n/2), with n odd. This scattering has a characteristic energy width of ∼6 meV, and evolves on a temperature scale of ∼80 K, which is close to the coherence temperature of this heavy fermion metal. High energy resolution measurements, below 2 meV, show the spectral weight of these fluctuations to evolve from the commensurate wavevector (0, 0, n/2), with n odd, to the material's incommensurate ordering wave vector as the energy of the fluctuations decreases. This observation is particularly interesting in light of the fact that UNi 2 Al 3 's isostructural sister heavy fermion superconductor, UPd 2 Al 3 , orders into a commensurate antiferromagnetic structure with this same (0, 0, n/2) wave vector. A search for inelastic magnetic scattering associated with the superconducting phase transition, as has been observed in UPd 2 Al 3 , revealed no additional scattering at the incommensurate ordering wavevector and energies above 0.10 meV. Measurements of the low-lying acoustic phonons were also performed, and show zone boundary energies lying in the range 10-13 meV.