  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.
