Magnetic excitations and ordering in the heavy-electron superconductor URu2Si2.

Abstract

Neutron scattering demonstrates the coexistence of antiferromagnetic order and superconductivity below 1 K in the heavy-electron system ${\mathrm{URu}}_{2}$${\mathrm{Si}}_{2}$. It is found that the upper (17.5 K) transition is to an antiferromagnetic phase with a (100) modulation wave vector and spins along the tetragonal c-axis. The ordered moment is unusually small, (0.03\ifmmode\pm\else\textpm\fi{}0.01)${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$. However, spin waves develop from damped, finite-gap excitations above ${\mathrm{T}}_{\mathrm{N}}$, and they are intense, propagating, and longitudinal with a zone centre gap of 1.8 meV.