The spin gap evolution upon Ca doping in Sr14-xCaxCu24O41 was systematically investigated using inelastic neutron scattering. We discover that the singlet-triplet spin gap excitation survives in this series with x up to 13, indicating the singlet dimer ground state in these compounds. This observation corrects the previous speculation that the spin gap collapses at x~13 by the NMR technique. The strong intensity modulation along QH in x=0 gradually evolves into a Q-independent feature in x>11. This could be attributed to the localized Cu moment magnetism developing into an itinerant magnetism with increasing x. It is a surprise that the spin gap persists in the normal state of this spin ladder system with metallic behaviour, which evidences the possibility of magnetically-mediated carrier pairing mechanism in a two-leg spin ladder lattice.
@article{arxiv.1308.1831,
title = {Spin gap evolution upon Ca doping in the spin ladder series $Sr_{14-x}Ca_xCu_{24}O_{41}$ by inelastic neutron scattering},
author = {Guochu Deng and N. Tsyrulin and P. Bourges and D. Lamago and H. Ronnow and M. Kenzelmann and S. Danilkin and E. Pomjakushina and K. Conder},
journal= {arXiv preprint arXiv:1308.1831},
year = {2013}
}