At low temperature, macroscopic properties of ${\mathrm{URu}}_2{\mathrm{Si}}_2$ display a characteristic energy scale ${\Delta }_0\left(B\right)$ which decreases when a magnetic field is applied, and eventually vanishes at an extrapolated value of the field of about 40 T. We have performed inelastic neutron scattering measurements of the magnetic dynamics of ${\mathrm{URu}}_2{\mathrm{Si}}_2$ in applied fields along the $c$ axis of intensities up to 12 T. We show that ${\Delta }_0\left(B\right)$ is not related to gaps in the magnetic fluctuations spectra. This provides direct evidence of the fact that two distinct energy scales govern the physics of this compound.

• Received 7 December 1999

DOI:https://doi.org/10.1103/PhysRevLett.85.654