Dual structure of low-energy spin fluctuations in La_1.80Sr _0.14Ce_0.06CuO₄

We performed inelastic neutron scattering measurements on La _1.94-xSr_xCe_0.06CuO₄(LSCCO; x - 0:14). Both temperature and energy dependences of low-energy magnetic excitation spectra strongly suggest the coexistence of two distinct components in the spin excitations; one is a gapped excitation and the other is an in-gap one. Reducing the corrugation of CuO₂planes by a co-doping of Sr and Ce ions revealed that the former is robust against both lattice distortion and chemical disorder, while the latter is found to be sensitive to lattice distortion. Through these features in LSCCO, we succeeded in determining the size of the spin gap in the underdoped region and so far reported spin gap energies (E_sg) for La-based single layered cuprates is summarized. The obtained common dome-shaped hole concentration dependence of E _sg=T_c(T_cis the optimal superconducting temperature in the system) is similar to that of T_c, suggesting an intimate relation between the spin gap and the superconductivity.