Inhomogeneous spin excitations in weakly coupled spin- 12 chains

Abstract

We present a systematic inelastic neutron scattering and neutron diffraction study on the magnetic structure of the quasi-one-dimensional spin-$\frac{1}{2}$ magnet ${\mathrm{SrCo}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$, where the interchain coupling in the N\'eel-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 N\'eel order gets partially destabilized at ${\ensuremath{\mu}}_{0}{H}^{★}=2.0\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ and completely suppressed at ${\ensuremath{\mu}}_{0}{H}_{\mathrm{p}}=3.9\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, above which a quantum disordered Tomonaga--Luttinger liquid (TLL) prevails. The low-energy spin excitations between ${\ensuremath{\mu}}_{0}{H}^{★}$ and ${\ensuremath{\mu}}_{0}{H}_{\mathrm{p}}$ are not homogeneous, containing the dispersionless (or weakly dispersive) spinon bound states excited in the N\'eel 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 N\'eel domains, the underlying spin chains enter the TLL state via a local quantum phase transition at ${\ensuremath{\mu}}_{0}{H}^{★}$, making the N\'eel/TLL coexistence a stable configuration until the excitation gap in the N\'eel domains closes at ${\ensuremath{\mu}}_{0}{H}_{\mathrm{p}}$.