CeFePO: f-d Hybridization and Quenching of Superconductivity

The unusual superconducting properties of the novel Febased oxopnictides with transition temperatures (Tc) up to 55 K have attracted considerable attention [1‐4]. While pure RFeAsO (R: rare-earth elements) compounds reveal metallic properties, doping by F on O sites leads to superconductivity. The proximity of the superconducting state to spin-density wave formation gave rise to speculations that the underlying pairing mechanism is based on magnetic fluctuations [5]. Superconductivity without doping, although at reduced Tc with respect to the arsenides, is found in the isoelectronic phos phides, except for R=Ce [6, 7]. In CeFeAsO both Fe and Ce order antiferromagnetically below a Neel temperature of 140 K [8] and 3.7 K [9], respectively. A gradual replacement of As by P leads first to the vanishing of the Fe magnetism, coupled with a change of the Ce order to ferromagnetism [10]. For further P doping the Ce order is suppressed, resulting in a paramagnetic heavy-fermion compound [11]. This wide variation of properties is a consequence of a strong sensitivity of the valence-band (VB) structure to th e lattice parameters and to interaction with localized f states. Close to the Fermi level (EF ) the electronic structure of RFePnO (Pn: phosphorus or arsenic) materials is dominated by five energy bands that have predominantly Fe 3d character [12, 13]. Small variations of the lattice parameters aff ect particularly two of these bands, namely those containing dxy and d3z2 r2 orbitals. Increasing the distance of the pnictogen ions to the Fe plane shifts the dxy-derived band towards lower and the d3z2 r2 -derived bands towards higher binding energies (BE) leading to a transition from 3D to 2D behavior of the Fermi surface (FS). As discussed in Ref. [13], superconductivity delicately depends on nesting conditions between the FS sheets generated by the above mentioned bands around the point and those located around the M point in the Brillouin zone (BZ). The nesting conditions may be affected by variations of the lattice parameters or interaction with 4f states. Purpose of the present work is to study the electronic structure of CeFePO by means of angle-resolved photoemission (ARPES) in order to understand possible reasons for the quenching of superconductivity. We find that closely below EF both the position and the dispersion of the valence bands are strongly changed with respect to the ones in LaFePO what is at least partly due to interactions with the Ce 4f states. Hybridization of the Fe 3d-derived VBs and the Ce 4f states ¯