Magnetic excitations in the quasi-one-dimensional magnet Sr(0.73)CuO(2); Coexistence of spin gap and long-range magnetic order

We report on spin dynamics in the highly hole-doped quasi-one-dimensional cuprate Sr(0.73)CuO(2) measured by inelastic neutron scattering. An excitation mode with peaks at E =10 and 17.5 meV is observed in the spectrum. Additional magnetic scattering remains present up to E ~ 25 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 ~ 25 meV is consistent with an excitation continuum. The wave-vector dependence of the scattering intensity gives convincing evidence that the dimers are formed between next-nearest-neighbor Cu atoms. The spin excitation gap delta ~ 10 meV remains present below the long-range magnetic ordering temperature T(AF) ~ 10 K, indicating coexistence of the quantum-mechanical singlet ground state and the classical Neel ordered state. This result shows that such coexistence is not restricted to the doped spin-Peierls system CuGeO(3).