Novel Field-Induced Quantum Phase Transition of the Kagome-Lattice Antiferromagnet

The magnetization process of the S = 1/2 kagome-lattice quantum antiferromagnet is investigated using the numerical exact diagonalization up to 39-site clusters. The finite-size scaling analysis with the rohmbic clusters indicated the "magnetization ramp" as a novel field-induced quantum phase transition at 1/3 of the saturation magnetization. The magnetization ramp is characterized by the following properties; (i) the field derivative dm/dH is divergent at the lower-field side, (ii) dm/dH is vanishing at the higher-field side, and (iii) there is a single critical field H-c (no plateau). Applying the critical exponent analysis for the non-rohmbic clusters, as well as the rohmbic ones, we confirmed the above properties of the magnetization at 1/3 of the saturation for the kagome lattice antiferromagnet.