One-dimensional spin-liquid without magnon excitations

It is shown that a sufficiently strong four-spin interaction in the spin-1/2 spin ladder can cause dimerization. Such interaction can be generated either by phonons or (in the doped state) by the conventional Coulomb repulsion between the holes. The dimerized phases are thermodynamically undistinguishable from the Haldane phase, but have dramatically different correlation functions: the dynamical magnetic susceptibility, instead of displaying a sharp single magnon peak near q = π, shows only a two-particle threshold separated from the ground state by a gap. PACS numbers: 75.10 Jm, 75.40 Gb. The qualitative difference between the universal properties of one-dimensional Heisenberg antiferromagnets with half-integer and integer spin, originally predicted by Haldane [1], is now well understood. Typical examples are the S = 1/2 and S = 1 chains. The spin S = 1/2 chain has a quasi-ordered singlet ground state with algebraically decaying spin correlations. Its spectrum is gapless, and the elementary excitations (spinons), carrying spin 1/2, in all physical states with integer total spin appear only in pairs [2]. Deconfinement of the spinons implies that conventional S = 1 magnons fail to be stable quasiparticles: the spectral density of the staggered magnetization n(x) ∼ (−1)Sn shows a purely incoherent background.