Inhomogeneous spin excitations in weakly coupled spin-1/2 chains

We present a systematic inelastic neutron scattering and neutron diffraction study on the magnetic structure of the quasi-one-dimensional spin-1/2 magnet SrCo2V2O8, where the interchain coupling in the Neel-type antiferromagnetic ground state breaks the static spin lattice into two independent domains. At zero magnetic field, we have observed two new spin excitations with small spectral weights inside the gapped region defined by the spinon bound states. In an external magnetic field along the chain axis, the Neel order gets partially destabilized at mu H-0*=2.0T and completely suppressed at mu(0)Hp=3.9T, above which a quantum disordered Tomonaga-Luttinger liquid (TLL) prevails. The low-energy spin excitations between mu H-0* and mu H-0(p) are not homogeneous, containing the dispersionless (or weakly dispersive) spinon bound states excited in the Neel phase and the highly dispersive psinon-antipsinon mode characteristic of a TLL. We propose that the two new modes at zero field are spinon excitations inside the domain walls. Since they have a smaller gap than those excited in the Neel domains, the underlying spin chains enter the TLL state via a local quantum phase transition at mu H-0*, making the Neel/TLL coexistence a stable configuration until the excitation gap in the Neel domains closes at mu H-0(p).