Quantum spin fluid behaviors of the kagome- and triangular-lattice antiferromagnets

The S=1/2 kagome-lattice antiferromagnet is one of interesting frustrated quantum spin systems. The systems exhibit the quantum spin fluid behavior, which was proposed as an origin of the high-Tc superconductivity. The spin gap is an important physical quantity to characterize the spin fluid behavior. Whether the S=1/2 kagome-lattice antiferromagnet is gapless or has a finite spin gap, is still unsolved issue. Because any recently developped numerical calculation methods are not enough to determine it in the thermodynamic limit. Our large-scale numerical diazonalization up to 42-spin clusters and a finite-size scaling analysis indicated that the S=1/2 kagome-lattice antiferromagnet is gapless in the thremodynamic limit[1-3]. It is consistent with the U(1) Dirac spin liquid theory of the kagome-lattice antiferromagnet[4,5]. On the other hand, the density matrix renormalization group calculations supported the gapped Z2 topological spin liquid theory[6,7]. We propose one of better methods to determine whether the spin excitation is gapless or gapped, based on the finite-size scaling analysis of the spin susceptibility calculated by the numerical diagonalization. The present work indicates that the kagome-lattice antiferromagnet is gapless, as well as the triangular-lattice one.28th International Conference on Low-Temperature Physics (LT28