Numerical-diagonalization study of spin gap issue of the kagome lattice Heisenberg antiferromagnet

We study the system size dependence of the singlet-triplet excitation gap in the S=1/2 kagome-lattice Heisenberg antiferromagnet by numerical diagonalization. We successfully obtain a new result of a cluster of 42 sites. The two sequences of gaps of systems with even-number sites and that with odd-number sites are separately analyzed. Careful examination clarifies that there is no contradiction when we consider the system to be gapless. (author)