Gapless quantum spin liquid ground state in the spin-1 antiferromagnet 6HB-Ba3NiSb2O9

We present an in-depth study of the magnetic properties of the spin-1 antiferromagnet 6HB-Ba3NiSb2O9. mu SR measurements demonstrate that this material shows no static magnetism down to temperatures as low as 20 mK, making it a likely candidate for a quantum spin liquid state. Sb-121 NMR shift measurements show that the local, intrinsic susceptibility levels off at temperatures below similar to 60 K. The NMR spin-lattice relaxation rate 1/T-1 is essentially constant in temperature and the muon relaxation rate exhibits a low-temperature relaxation plateau, all indications of gapless spin excitations. Our local probe measurements are discussed in the context of several theories proposed for this material.