Spin Dynamics in Iron-Based Layered Superconductor (La_0.87Ca_0.13)FePO Revealed by ³¹P and ¹³⁹La NMR Studies

We report ³¹P and ¹³⁹La NMR studies of (La_0.87Ca_0.13)FePO, which is a family member of the recently discovered superconductor LaFeAs(O_1-xF_x). In the normal state, Knight shift and nuclear spin-lattice relaxation rate divided by T (1/T₁T) show that a Fermi-liquid state with moderate ferromagnetic fluctuations emerges below 30 K. From 1/T₁T of ³¹P and ¹³⁹La, a quasi-two- dimensional electronic structure is suggested, in which the FeP layer is more conductive than the LaO layer. In the superconducting (SC) state, although a clear Meissner signal was observed, 1/T₁T increases below T_c, in contrast to a decrease of 1/T₁T due to the opening of a SC gap, suggesting that novel low-energy spin dynamics develop in the SC state.