Magnetic excitations in the quasi-one-dimensional magnet Sr_0.73CuO₂: Coexistence of the spin gap and long-range magnetic order

We report on spin dynamics in the highly hole doped quasi-one-dimensional cuprate Sr0.73CuO2 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 E25meV. 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 E25meV is consistent with an excitation continuum. The spin excitation gap Δ10meV remains present below the long-range magnetic ordering temperature TAF10K, 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 CuGeO3. © 1999 The American Physical Society.