Candidate Spin-Liquid on the Honeycomb-Lattice

Since P.W. Anderson proposed the theoretical existence of the quantum spin liquid ground state in 1973, antiferromagnetic spin-1/2 systems exhibiting large quantum fluctuations have attracted significant interest as spin liquid candidates.  The experimental verification of the spin liquid ground state would not only advance fundamental condensed matter physics, but also provide a path forward to discovering new, exotic spin system ground states.  If only nearest neighbor interactions are considered, the spin-1/2 Heisenberg antiferromagnet on a honeycomb lattice should order in a Néel ground state. However, sufficiently strong next nearest neighbor interactions combined with a small coordination number and low dimensionality, which increase the role of quantum fluctuations, such as those exhibited in Ba₃Co₂O₆(CO₃)_0.7, could result in a quantum spin liquid ground state. For this reason, we have investigated the low temperature specific heat and magnetization of Ba₃Co₂O₆(CO₃)_0.7, a nonstoichiometric compound with the CoO₆ chains forming a honeycomb lattice. The magnetization exhibits an anomalous decrease in slope with increasing field beyond 10 T when the field is applied parallel to the c axis of the sample, indicating that randomness plays an important role in Ba₃Co₂O₆(CO₃)_0.7. Specific heat measurements failed to show any sign of ordering down to 160 mK in fields between zero and 18 T. This corroborates earlier measurements made at higher temperatures over a narrower field range by Igarashi et al. Further, the specific heat behaves linearly at temperatures below 700 mK. Interestingly, Ba₃Co₂O₆(CO₃)_0.7 has a very large Wilson ratio. Although not a theoretical prediction of any quantum spin liquid models, all known spin liquid candidates exhibit Wilson ratios greater than 1, often times greater by an order of magnitude. Herein we discuss our motivation, experiments, and results which advance strong evidence that Ba₃Co₂O₆(CO₃)_0.7 hosts a spin liquid ground state.