Low energy spin excitations in Sr 14− xCa xCu 24O 41

Abstract

We report the results of nuclear spin lattice relaxation rate (1/ T 1) measurements in the hole doped two-leg ladder cuprates Sr 14− x Ca x Cu 24O 41 at Cu and two distinct oxygen sites. For the heavily doped metallic compound with x = 12, a peak in 1/ T 1 was observed near 180 K at Cu sites but not at oxygen sites. The rung oxygen sites show steeper increase of 1/ T 1 with temperature from 60 K to 200 K than the leg oxygen sites. This implies that the singlet correlation along the rung is effectively reduced while the antiferromagnetic correlation is enhanced above 60 K, at which the resistivity along the leg shows a minimum. We propose that dissociation of bound hole pairs gradually turns a two-leg ladder at low temperatures into nearly decoupled two chains at high temperatures.