Spinon continuum in the Heisenberg quantum chain compound Sr_2V_3O_9

Magnetic excitations in the spin chain candidate Sr_2V_3O_9 have been investigated by inelastic neutron scattering on a single crystal sample. A spinon continuum with a bandwidth of ∼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∼ 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_2V_3O_9 as a host of weakly coupled S = 1/2 chains dominated by antiferromagnetic intrachain interactions of ∼7.1(1) meV.