Spin Fluctuations in the Underdoped High-T_c Cuprate La_1.93Sr_0.07CuO₄

We performed magnetic inelastic neutron-scattering experiments on La_1.93Sr_0.07CuO₄ over a wide range of ω and T; 2 ≤ ω ≤ 44 meV and 1.5 ≤ T < 300 K. The dynamic susceptibility cursive chi″ (q, ω) of this underdoped high-T_c superconductor (T_c = 17K) is characterized by broad, incommensurate peaks. Here, the incommensurate wavevector δ_ω is approximately 0.07 reciprocal lattice units at low T and ω. The superconducting phase does not possess an observable gap in the spin excitation spectrum down to at least 2 meV. Scaling behavior is demonstrated for the Q-integrated energy spectrum cursive chi″(ω)_T with respect to (ω/k_BT). This scaling establishes a connnection between the magnetic excitations of the compositions on either side of the insulator-superconductor boundary in the La_{2-cursive greek chi}Sr_{cursive greek chi}CuO₄ phase diagram. We note a possible cross-over from an incommensurate to a commensurate response for ω > 20 meV or T > 300 K.