A recently discovered material, $6H\mathrm{\text{−}}\mathrm{B}\mathrm{\text{−}}{\mathrm{Ba}}_3{\mathrm{NiSb}}_2{\mathrm{O}}_9$ was found to display unusual low-temperature phenomenology, interpreted as a quantum spin liquid with spin $S=1$ on a triangular lattice. We study a spin $S=1$ exchange model on an $AB$ stacked triangular lattice near its quantum paramagnet-to-spiral transition, driven by easy-plane single-ion anisotropy. We demonstrate that the frustrated inter- and intralayer exchanges induce contour lines of low-energy excitations that lead to a broad crossover regime of linear-temperature dependence of the specific heat. Based on this and various other predictions, we argue that the observed phenomenology can be understood in terms of a conventional picture of a proximity to this frustrated critical point.

• Received 13 January 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.016402