Spin excitations and the electronic specific heat ofURu2Si2

We have calculated the mass enhancement due to the interaction of conduction electrons with spin fluctuations, using the generalized spin susceptibility determined from neutron-scattering measurements, for the heavy-fermion superconductor ${\mathrm{URu}}_2$ ${\mathrm{Si}}_2$. Reasonable coupling between the spin excitations and conduction electrons yields ${\mathit{m}}^{\mathrm{*}}$/${\mathit{m}}_{\mathit{b}}$≊7.7 for T<${\mathit{T}}_{\mathit{N}}$ and ${\mathit{m}}^{\mathrm{*}}$/${\mathit{m}}_{\mathit{b}}$≊11.4 for T>${\mathit{T}}_{\mathit{N}}$ which is sufficient to account for the difference between band-structure calculations and the measured electronic specific heat. In addition, inclusion of the temperature dependence of the spin excitations as ${\mathit{T}}_{\mathit{N}}$ is approached from below reproduces, qualitatively, the peak observed in the specific heat at ${\mathit{T}}_{\mathit{N}}$. The peak arises from a gap in the spin, not charge spectrum below ${\mathit{T}}_{\mathit{N}}$.