Spin Liquids and Antiferromagnetic Order in the Shastry-Sutherland-Lattice Yb₂Pt₂Pb

We present measurements of the magnetic susceptibility $\chi$ and the magnetization M of single crystals of metallic \ypp, where localized Yb moments lie on the geometrically frustrated Shastry-Sutherland Lattice (SSL). Strong magnetic frustration is found in this quasi-two dimensional system, which orders antiferromagnetically (AF) at T$_{N}$=2.02 \emph{K} from a paramagnetic liquid of Yb-dimers, having a gap $\Delta$=4.6 \emph{K} between the singlet ground state and the triplet excited states. Magnetic fields suppress the AF order, which vanishes at a 1.25 \emph{T} quantum critical point. The spin gap $\Delta$ persists to 1.5 \emph{T}, indicating that the AF degenerates into a liquid of dimer triplets at T=0. Quantized steps are observed in M(B) within the AF state, a signature of SSL systems. Our results show that \ypp is unique, both as a metallic SSL system that is close to an AF quantum critical point, and as a heavy fermion compound where geometrical frustration plays a decisive role.