Emergence of well-screened states in a superconducting material of the CaFe2As2 family

Abstract

Coupling among conduction electrons (e.g., Zhang-Rice singlet) are often manifested in the core level spectra of exotic materials such as cuprate superconductors, manganites, etc. These states are believed to play key roles in the ground state properties and appear as low binding energy features. To explore such possibilities in the Fe-based systems, we study the core level spectra of a superconductor CaFe 1.9 Co 0.1 As 2 (CaCo122) in the CaFe 2 As 2 (Ca122) family employing high-resolution hard x-ray photoemission spectroscopy. While As core levels show almost no change with doping and cooling, the Ca 2p peak of CaCo122 shows reduced surface contribution relative to Ca122 and a gradual shift of the peak position towards lower binding energies with cooling. In addition, we discover the emergence of a feature at the lower binding energy side of the well-screened Fe 2p signal in CaCo122. The intensity of this feature grows with cooling and indicates additional channels to screen the core holes. The evolution of this feature in the superconducting composition and its absence in the parent compound suggests relevance of the underlying interactions in the ground state properties of this class of materials. These results reveal another dimension in the studies of Fe-based superconductors and the importance of such states in the unconventional superconductivity in general.

Giant spectral renormalization and complex hybridization physics in the Kondo lattice system CeCuSb2

“…Such multiplets are observed in the Ce 3d core-level spectra of other systems [25]. The observation of a distinct feature, A 1 (B 1 in the 3d 3/2 case), is interesting and presumably related to the screening by highly delocalized electrons as observed in other systems [26][27][28][29]. These are observed essentially in the bulk spectra due to higher itineracy in the bulk as expected.…”

Doping induced band renormalization in 122-type Fe-based superconductor

“…Thus, the increase in the energy difference between the bands observed above suggests significant increase in SOC strength with doping which is unusual as both Co and Fe atoms have very similar atomic weights. It is worth to note here that the changes observed in the near F electronic structure of the parent compound with doping will enhance the total density of states close to F , which explains the doping induced effects observed in earlier bulk electronic structure studies carried out using hard x-ray photoemission spectroscopy and x-ray absorption spectroscopy techniques [9,8]. Clearly, the evolution of the electronic structure with Co-doping at Fe-site is complex and calls for more theoretical and experimental studies.…”

“…No signature of magnetic or structure transition was observed in temperature dependent magnetic susceptibility and x-ray diffraction measurements. A sharp diamagnetic response was observed below 15 K indicating onset of the superconducting phase of the compound [9]. The crystallinity of the as grown samples were studied using Laue x-ray diffraction (XRD) technique.…”

“…The compound, CaCo122 shows SC phase with transition temperature of T c ∼15 K which is the highest in this class of materials [1]. The electronic structure studies of both the parent and doped compounds suggest weak doping induced effects in the momentum integrated spectral density of states [8,9]. Evidentally, the role of Co-doping at the Fe-sites leading to superconducting state is complex and subtle changes close to the Fermi level needs to be studied carefully.…”

Layer-resolved electronic behavior in a Kondo lattice system, CeAgAs2

“…In addition, there are low energy intense features A 1 and B 1 . Signature of such features are found in some systems [8,34,35], where the hole created in the conduction/ligand bands due to the hopping of an electron to the core hole site, forms a singlet with other holes and/or has highly itinerant character that allows the valence hole to propagate far away from the photoemission site to lower the energy further. The intensity of these features appear to be quite large in this material.…”