  The spin gap material IPA-CuCl3 has been extensively studied as a
ferromagnetic-antiferromagnetic bondalternating S = 1/2 chain. This description
of the system was derived from structural considerations and bulk measurements.
New inelastic neutron scattering experiments reveal a totally different
picture: IPA-CuCl3 consists of weakly coupled spin ladders with
antiferromagnetic legs and ferromagnetic rungs. The ladders run perpendicular
to the originally supposed bondalternating chain direction. The ferromagnetic
rungs make this system equivalent to a Haldane S = 1 antiferromagnet. With a
gap energy of 1.17(1) meV, a zone-boundary energy of 4.1(1) meV, and almost no
magnetic anisotropy, IPA-CuCl3 may the best Haldane-gap material yet, in terms
of suitability for neutron scattering studies in high magnetic fields.
