We present local probe results on the honeycomb lattice antiferromagnet ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$. Muon spin relaxation measurements in a zero field down to 20 mK show unequivocally that there is a total absence of spin freezing in the ground state. Sb NMR measurements allow us to track the intrinsic susceptibility of the lattice, which shows a maximum at around 55 K and drops to zero in the low-temperature limit. The spin-lattice relaxation rate shows two characteristic energy scales, including a field-dependent crossover to exponential low-temperature behavior, implying gapped magnetic excitations.

• Received 8 June 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.117203