Spin liquids and antiferromagnetic order in the Shastry-Sutherland-lattice compound Yb2Pt2Pb.

We present measurements of the magnetic susceptibility χ and the magnetization M of single crystals of metallic Yb(2)Pt(2)Pb, where localized Yb moments lie on the dimerized and frustrated Shastry-Sutherland lattice (SSL). Strong magnetic frustration is found in this quasi-two-dimensional system, which orders antiferromagnetically at T(N) = 2.02 K from a paramagnetic liquid of Yb dimers, having a gap Δ = 4.6 K between the singlet ground state and the triplet excited states. Magnetic fields suppress the antiferromagnetic (AF) order, which vanishes at a 1.23 T quantum critical point. The spin gap Δ persists to 1.5 T, indicating that dimer singlets survive the collapse of the B = 0 AF state. Quantized steps are observed in M(B) within the AF state, a signature of SSL systems. Our results show that Yb(2) Pt(2)Pb is unique, both as a metallic SSL system that is close to an AF quantum critical point, and as a heavy fermion compound where quantum frustration plays a decisive role.