Emergence of Coherent Magnetic Excitations in the High Temperature Underdoped La2-xSrxCuO4 Superconductor at Low Temperatures

We use inelastic neutron scattering to measure the magnetic excitations in underdoped La2-xSrxCuO4 (x=0.085, T-c=22 K) for large energy (5 < E < 200 meV) and temperature (5 < T < 300 K) ranges. At low T, the response is highly structured in E and q, with peaks in the local susceptibility at 15 and 50 meV and a four-peaked structure in q for E approximate to 185 meV. Raising T from 30 K greater than or similar to T-c to 300 K causes the structure in chi('')(q,omega) present for E < 70 meV to disappear. It is replaced with a strongly damped response. We attribute this change to the disappearance of the pseudogap with increasing temperature.