NMR Study of Carrier Doping Effects on Spin Gaps in the Spin Ladder Sr14-xAxCu24O41 (A = Ca, Y, and La)

Carrier doping effects on spin dynamics in Sr14-xAxCu24O41 ( A = Ca, Y, and La) are investigated by Cu NMR and NQR. The energy gaps in the spin excitation spectra (spin gap) are confirmed to be Δ = 140 K for the CuO2 chain and Δ = 470 K for the Cu2O3 ladder in Sr14Cu24O41. The spin gap for the ladder increases with the Y and La substitutions for Sr (up to x = 3), while it decreases with increasing x in Sr14-xCaxCu24O41 and seems to collapse around x = 13 of the Ca substitution. Hole-doping effects on the spin gaps for the ladder configuration are interpreted in terms of magnon and hole-depairing excitations for the spin S = 1/2 ladder