Cooperative paramagnetism in the geometrically frustrated pyrochlore antiferromagnet Tb2Ti2O7

Neutron scattering, muon spin relaxation, and de susceptibility studies hav e been carried out on polycrystalline Tb2Ti2O7, a pyrochlore antiferromagne t in which the Tb3+ moments reside on a network of corner-sharing tetrahedr a. Unlike other geometrically frustrated systems, Tb2Ti2O7 remains paramagn etic down to similar to 0.07 K. rather than ordering into a conventional Ne el or spin-glass-like state, despite the fact that short-range antiferromag netic correlations (AFC) develop at similar to 50 K. At the first AFC wave vector, its low-lying, relatively flat magnetic excitation spectrum softens partially below 30 K.