Magnetic excitations in the quasi-one-dimensional magnet Sr0.73CuO2: Coexistence of the spin gap and long-range magnetic order

We report on spin dynamics in the highly hole doped quasi-one-dimensional c uprate Sr0.73CuO2 measured by inelastic neutron scattering. An excitation m ode with peaks at E=10 and 17.5 meV is observed in the spectrum. Additional magnetic scattering remains present up to E similar to 25 meV. These featu res are due to excitations of dimerized antiferromagnetic chains; the two p eaks are associated with the dispersion of the lowest-lying magnon branch, whereas the scattering in the region up to E similar to 25 meV is consisten t with an excitation continuum. The spin excitation gap Delta similar to 10 meV remains present below the long-range magnetic ordering temperature T-A F similar to 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 CuGeO3. [S0163-1829(99)03838-2].