Complex hybridization physics and evidence of structural anomaly to be a bulk property in an exotic Fe-based compound, CaFe2As2

TL;DR

This study uses bulk-sensitive hard x-ray photoemission spectroscopy to investigate the electronic and structural properties of CaFe2As2, revealing that certain anomalies are inherent bulk features rather than surface effects, with implications for understanding exotic Fe-based compounds.

Contribution

The paper demonstrates that structural anomalies in CaFe2As2 are bulk properties, using core level spectra analysis to distinguish surface and bulk electronic structures in an exotic Fe-based material.

Findings

Core level spectra reveal significant hybridization in Ca 2p levels.

Bulk spectra show additional structural phases not seen at the surface.

Surface electronic structure aligns with phase transitions, bulk shows structural anomalies.

Abstract

Surface of quantum materials often exhibits significantly different behavior than the bulk due to changed topologies and symmetry protections. The outstanding problem is to find out if the exoticity of a material is linked to the changed topology at the surface or it is a bulk property. Hard x-ray photoemission spectroscopy (HAXPES) is a significantly bulk sensitive technique (escape depth of valence electrons is about 40 \AA\ for 6 keV photon energy) and the probing depth can be tuned by changing the electron emission angle. Therefore, HAXPES is often used to reveal the surface-bulk differences in a material. Here, we show that the delineation of surface-bulk differences in the valence band spectral functions using this method is highly non-trivial due to the complexity arising from linear dichroic effect in addition to the change in surface sensitivity. We show that core level spectra can be used to reveal the surface-bulk differences in the electronic structure. The Ca 2p spectra exhibit evidence of significant hybridization with the conduction electrons revealing their importance in the electronic properties of the system as also found for the charge reservoir layers in cuprate superconductors. The Fe 2p core level spectra as a function of bulk sensitivity and temperature reveals an unusual scenario; while the surface electronic structure corroborates well with the observed phase transitions of the system, the bulk spectra exhibit signature of additional structural phases providing a rare evidence of structural anomaly to be a bulk property.