Superconductivity without doping in ThFeAsN
Unlike the widely studied ReFeAsO series, the newly discovered iron-based superconductor ThFeAsN exhibits a remarkably high critical temperature of 30 K, without chemical doping or external pressure. Here we investigate in detail its magnetic and superconducting properties via muon-spin rotation/relaxation and nuclear magnetic resonance techniques and show that ThFeAsN exhibits strong magnetic fluctuations, suppressed below ~35 K, but no magnetic order. This contrasts strongly with the ReFeAsO series, where stoichiometric parent materials order antiferromagnetically and superconductivity appears only upon doping. The ThFeAsN case indicates that Fermi-surface modifications due to structural distortions and correlation effects are as important as doping in inducing superconductivity. The direct competition between antiferromagnetism and superconductivity, which in ThFeAsN (as in LiFeAs) occurs at already zero doping, may indicate a significant deviation of the s-wave superconducting gap in this compound from the standard s± scenario.
To understand how pressure affects its electronic properties, we carried out microscopic investigations up to 3 GPa via magnetization, nuclear magnetic resonance, and muon-spin rotation experiments. The temperature dependence of the 75AsKnight shift, the spin-lattice relaxation rates, and the magnetic penetration depth suggest a multiband s±-wave gap symmetry in the dirty limit, whereas the gap-to-Tc ratio Δ/kBTc hints at a strong-coupling scenario. Pressure modulates the geometrical parameters, thus reducing Tc as well as Tm, the temperature where magnetic-relaxation rates are maximized, both at the same rate of approximately −1.1K/GPa. This decrease in Tc with pressure is consistent with band-structure calculations, which relate it to the deformation of the Fe 3dz2 orbitals.