  An inelastic neutron scattering study was performed on a Zn-substituted
spin-1/2 Heisenberg 2-leg ladder compound Sr(Cu1-xZnx)2O3 (x < 0.04) to
investigate nonmagnetic impurity effects on the quantum spin system with a
large spin gap of about 400 K. The magnitude of the spin gap was found to be
independent of Zn concentration, 33 meV, while the integrated magnetic
scattering which corresponds to the singlet-triplet excitation decreased
monotonically with increasing x. On account of the total-sum rule, this result
supports the existence of a finite in-gap density of states at E = 0 which was
suggested from the T-linear magnetic specific heat.
