RESUMO
We present a high-pressure NMR study of the overdoped iron pnictide superconductor NaFe0.94Co0.06As. The low-energy antiferromagnetic spin fluctuations in the normal state, manifest as the Curie-Weiss upturn in the spin-lattice relaxation rate 1/(75)T1T, first increase strongly with pressure but fall again at P>Popt=2.2 GPa. Neither long-ranged magnetic order nor a structural phase transition is encountered up to 2.5 GPa. The superconducting transition temperature Tc shows a pressure dependence identical to the spin fluctuations. Our observations demonstrate that magnetic correlations and superconductivity are optimized simultaneously as a function of the electronic structure, thereby supporting very strongly a magnetic origin of superconductivity.
RESUMO
We use (75)As nuclear magnetic resonance to investigate the local electronic properties of Ba(Fe(1-x)Ru(x))(2)As(2) (x = 0.23). We find two phase transitions: to antiferromagnetism at T(N) ≈ 60 K and to superconductivity at T(C) ≈ 15 K. Below T(N), our data show that the system is fully magnetic, with a commensurate antiferromagnetic structure and a moment of 0.4µ(B)/Fe. The spin-lattice relaxation rate 1/(75)T(1) is large in the magnetic state, indicating a high density of itinerant electrons induced by Ru doping. On cooling below T(C), 1/(75)T(1) on the magnetic sites falls sharply, providing unambiguous evidence for the microscopic coexistence of antiferromagnetism and superconductivity.
