Manifestation of Spin-Charge Separation in the Dynamic Dielectric Response of One-Dimensional ${\mathrm{Sr}}_2\mathrm{Cu}{O}_3$

We have determined the dynamical dielectric response of a one-dimensional, correlated insulator by carrying out electron energy-loss spectroscopy on ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$ single crystals. The observed momentum and energy dependence of the low-energy features, which correspond to collective transitions across the gap, are well described by an extended one-band Hubbard model with moderate nearest neighbor Coulomb interaction strength. An excitonlike peak appears with increasing momentum transfer. These observations provide experimental evidence for spin-charge separation in the relevant excitations of this compound, as theoretically expected for the one-dimensional Hubbard model.