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 $\sim 22$ 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 $S=1\mathrm{/}2$ chains dominated by antiferromagnetic intrachain interactions of $\sim 7.1$(1) meV.