ar X iv : c on d - m at / 9 70 61 70 v 1 1 7 Ju n 19 97 Disappearance of the Spin Gap in a Zn doped 2 - Leg Ladder Compound

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. 75.10.Jm, 75.25.+z, 75.50.Ee Typeset using REVTEX