Electromagnetic properties and electronic structure of the iron-based layered superconductor LaFePO

Structural, electronic, and magnetic properties of undoped and aliovalent-ion (Ca, F)-doped LaFePO, which undergo superconducting transitions at transition temperatures (Tc) 4--7 K [Y. Kamihara et al., J. Am. Chem. Soc. 128, 10012 (2006)], were investigated. Tc of the samples varied from 2.4 to 5.5 K in the undoped samples and was increased up to ~7 K by Ca and F doping. The Tc increases are correlated with a decrease in the lattice volume. LaFePO exhibits paramagnetism in the normal conducting state. Photoemission spectroscopy combined with first-principle band calculations clarified that Fe Fe 3d (dz2+(dxz,dyz)) orbitals hybridized with P 3p to form a Fermi surface. The band calculations also suggest that the $3d$ electron of the Fe in LaFePO is basically in the low-spin configuration, and that the spin moment of LaFePO is almost quenched, leading to the paramagnetism of the itinerant electrons.