Magnetic excitations in the heavy-fermion superconductorURu2Si2

Abstract

Antiferromagnetic order and fluctuations in the heavy-fermion superconductor ${\mathrm{URu}}_2$ ${\mathrm{Si}}_2$ have been studied by magnetic neutron scattering. Below ${\mathit{T}}_{\mathit{N}}$=17.5 K, ${\mathrm{URu}}_2$ ${\mathrm{Si}}_2$ is a type-I antiferromagnet with an anomalously small ordered moment of (0.04±0.01)${\mathrm{\mu }}_{\mathit{B}}$ polarized along the tetragonal c axis. Dispersive resonant excitations exist in the ordered state with a zone-center gap of 0.43 THz. The excitations are polarized along the ordered moment and have a large dipolar matrix element, which suggests that they are coupled transitions between singlet crystal-field-like states. For energy transfer above 3 THz, peaks have not been identified in the magnetic excitation spectra, but instead a continuous spectrum of scattering peaked around the ordering wave vector indicates the presence of overdamped antiferromagnetically correlated spin fluctuations. Upon heating above ${\mathit{T}}_{\mathit{N}}$, the resonant excitations abruptly become heavily damped but the magnetic scattering at higher energies does not change at ${\mathit{T}}_{\mathit{N}}$. Instead, the disappearance of the antiferromagnetic modulation of the higher-energy scattering coincides with the maximum in the resistivity of ${\mathrm{URu}}_2$ ${\mathrm{Si}}_2$.