The spin gap in a quantum antiferromagnet on the kagome lattice

A new spin S = 1/2 antiferromagnet on the kagome lattice, [Cu-3(titmb)(2) (OCOCH3)(6)].H2O (titmb = 1 3, 5-tris(imidazol-1-ylmethyl)-2, 4, 6 trimethylbenzene), has been grown and its magnetic properties have been studied. We have found for the first time a two-peak feature in the temperature dependence of the heat capacity. The higher-temperature peak is explained as due to a short-range magnetic ordering in two dimensions. We analysed the lower-temperature peak on the basis of an energy level scheme with a singlet ground state and a spin gap to the first excited triplet. The analysis reproduces the experiment fairly well, suggesting that the ground state is a disordered quantum liquid.