Spin band gap and emerging low energy excitations in a doped cobaltate perovskite

We investigated spin dynamics in half doped cobaltate La$_{1.5}$Sr$_{0.5}$CoO$_{4}$, using inelastic neutron scattering. At high temperatures we observe a broad band of continuum antiferromagnetic dynamic spin correlations, extending to energies above 20 meV. As temperature decreases below spin ordering transition (T$_{SO}$ = 30K), a single coherent magnon mode emerges at low energies (bandwith ~ 15 meV), separated by an energy band gap from a broad continuum scattering residing at high energies $20 \le E \le 30$ meV. This points to an emerging 2D-XY behavior of spins, consistent with the strong planar anisotropy of this material.