Magnetic excitations in the quasi-one-dimensional magnetSr0.73CuO2:Coexistence of the spin gap and long-range magnetic order

We report on spin dynamics in the highly hole doped quasi-one-dimensional cuprate ${\mathrm{Sr}}_{0.73}{\mathrm{CuO}}_2$ measured by inelastic neutron scattering. An excitation mode with peaks at $E=10\mathrm{}$ and 17.5 meV is observed in the spectrum. Additional magnetic scattering remains present up to $E\sim 25\mathrm{meV}.$ These features are due to excitations of dimerized antiferromagnetic chains; the two peaks are associated with the dispersion of the lowest-lying magnon branch, whereas the scattering in the region up to $E\sim 25\mathrm{meV}$ is consistent with an excitation continuum. The spin excitation gap $\Delta \sim 10\mathrm{meV}$ remains present below the long-range magnetic ordering temperature $T_{\mathrm{AF}}\sim 10\mathrm{K},$ indicating coexistence of the quantum-mechanical singlet ground state and the classical Néel ordered state. This result shows that such coexistence is not restricted to the doped spin-Peierls system ${\mathrm{CuGeO}}_3.$