Li-induced spin and charge excitations in a spin ladder

A lithium dopant in a cuprate spin ladder acts as a vacant (nonmagnetic) site accompanied by an extra hole bound by a Coulomb potential. We find that, although the undoped ladder spin gap is not essentially altered by Li doping, a dopant-magnon bound state appears just below the gap. Contrary to Zn-doped ladders, the spin liquid character of a Li-doped ladder should be robust against antiferromagnetism. We give a possible explanation for the similarity of Zn and Li dopants in underdoped cuprates and propose experimental tests.