Fermi Surface Instability and Symmetry Breaking in Heavy-Fermion Compound YbBiPt

We report resistivity measurements on the cubic heavy-fermion compound YbBiPt at ambient and hydrostatic pressures to ≅6 kbar and in magnetic fields to 1 T. Resistivity anisotropy sets in below the phase-transition temperature $T_c=0.4\mathrm{}$ K. We interpret a rise of resistivity below 0.4 K as due to partial gapping of the Fermi surface with the weak coupling energy gap of $\frac{{\Delta }_0}{k_B{T}_c}=1.65\mathrm{}\pm 0.15$. Effects of hydrostatic pressure and magnetic field on the phase transition and heat capacity data are consistent with a spin density wave formation in a very heavy electron band at $T=0.4\mathrm{}$ K.