Hidden phase in parent Fe-pnictide superconductors

TL;DR

This study uses ARPES to uncover a hidden collapsed tetragonal phase in CaFe2As2 at ambient conditions, revealing its influence on Fermi surface topology and potential role in exotic superconducting properties.

Contribution

It provides evidence of a hidden cT phase in Fe-pnictides at ambient conditions and links its evolution to Fermi surface topology changes, advancing understanding of their electronic behavior.

Findings

Discovery of a hidden cT phase at ambient conditions.

Fermi surface topology changes are not directly driven by structural transitions.

Identification of energy bands related to the cT phase with Fe 3d character.

Abstract

We investigate the origin of exoticity in Fe-based systems via studying the Fermiology of CaFe2As2 employing Angle Resolved Photoemission spectroscopy (ARPES). While the Fermi surfaces (FSs) at 200 K and 31 K are observed to exhibit two dimensional (2D) and three dimensional (3D) topology, respectively, the FSs at intermediate temperatures reveal emergence of the 3D topology at much lower temperature than the structural & magnetic phase transition temperature (170 K, for the sample under scrutiny). This leads to the conclusion that the evolution of FS topology is not directly driven by the structural transition. In addition, we discover the existence in ambient conditions of energy bands related to the collapsed tetragonal (cT) phase. These bands are distinctly resolved in the high-photon energy spectra exhibiting strong Fe 3d character. They gradually move to higher binding energies due to thermal compression with cooling, leading to the emergence of 3D topology in the Fermi surface. These results reveal the so-far hidden existence of a cT phase in ambient conditions, which is argued to lead to quantum fluctuations responsible for the exotic electronic properties in Fe-pnictide superconductors.