Quantum spin liquid ground state in the Co-based triangular antiferromagnet Na2BaCo(PO4)2

A triangular lattice occupies a special position in the quantum spin-liquid (QSL) physics. Although the triangular-lattice Heisenberg antiferromagnet is known to evade a QSL, several ingredients including exchange randomness, spin-orbit coupling, anisotropic and biquadratic exchange interactions have been controversially discussed as a route to stabilize QSLs. Recently, the Co-based triangular antiferromagnet Na2BaCo(PO4)2 has emerged as a new candidate for QSL, The triangular layers of Co2+ (jeff = 1/2) ions are separated by a single layer of nonmagnetic BaO12 polyhedra and stacked along c-axis. Our preliminary results defied magnetic and structural phase transitions down to 50 mK. Rather, the large residual magnetic entropy is retained at low temperatures, indicating abundant low-lying magnetic excitations. With the aid of muSR, we will corroborate a truly dynamic ground state.