Spinon continuum in the Heisenberg quantum chain compound Sr\(_2\)V\(_3\)O\(_9\)

Magnetic excitations in the spin chain candidate Sr\(_2\)V\(_3\)O\(_9\) have been investigated by inelastic neutron scattering on a single crystal sample. A spinon continuum with a bandwidth of \(\sim22\) meV is observed along the chain formed by alternating magnetic V\(^{4+}\) and nonmagnetic V\(^{5+}\) ions. Incipient magnetic Bragg peaks due to weak ferromagnetic interchain couplings emerge when approaching the magnetic transition at \(T_N\sim 5.3\) K while the excitations remain gapless within the instrumental resolution. Comparisons to the Bethe ansatz, density matrix renormalization group (DMRG) calculations, and effective field theories confirm Sr\(_2\)V\(_3\)O\(_9\) as a host of weakly coupled \(S = 1/2\) chains dominated by antiferromagnetic intrachain interactions of \(\sim7.1\)(1) meV.