Electronic Identification of the Parental Phases and Mesoscopic Phase Separation ofKxFe2−ySe2Superconductors

Abstract

The nature of the parent compound of a high-temperature superconductor (HTS) often plays a pivotal role in determining its superconductivity. The parent compounds of the cuprate HTSs are antiferromagnetically ordered Mott insulators, while those of the iron-pnictide HTSs are metals with spin-density-wave order. Here we report the electronic identification of two insulating parental phases and one semiconducting parental phase of the newly discovered family of K x Fe 2Ày Se 2 superconductors. The two insulating phases exhibit Mott-insulator-like signatures, and one of the insulating phases is even present in the superconducting and semiconducting K x Fe 2Ày Se 2 compounds. However, it is mesoscopically phaseseparated from the superconducting or semiconducting phase. Moreover, we find that both the superconducting and semiconducting phases are free of the magnetic and vacancy orders present in the insulating phases, and that the electronic structure of the superconducting phase could be developed by doping the semiconducting phase with electrons. The rich electronic properties discovered in these parental phases of the K x Fe 2Ày Se 2 superconductors provide the foundation for studying the anomalous behavior in this new class of iron-based superconductors.

The orbital characters of low-energy electronic structure in iron-chalcogenide superconductor K x Fe2−y Se2

“…Previous ARPES studies on K x Fe 2−y Se 2 superconductors have observed an electron pocket around the zone corner [7,11,12], while two electron pockets could be resolved for other iron-based superconductors [ 16, 17, 23, 24, 28]. In the polarization-dependent photoemission data around the zone corner (Figure 4), we could observe a shallow electron-like band (δ) with band bottom near −50 meV in both s and p geometries, and a deep electron-like band (δ ) in the p geometry which quickly disperses to high binding energies in Figure 4(c) at the temperature of 35 K [33].…”

“…However, we have observed another two bands, κ and η, around the Z point. The κ band exhibits strong k z dependence and its Fermi surface surrounds the Z point [7,11]. It only emerges in the p geometry along both #1 and #2 directions, so it could be either d xz or d z 2 .…”

“…While the parent compounds of most iron-based superconductors are metallic [2][3][4], the parent compound of K x Fe 2−y Se 2 superconductor is found to be either insulating or semiconducting [5][6][7][8][9]. The insulating parental phase even exhibits certain Mott-insulator signatures, and it is phase separated from the superconducting phase in the K x Fe 2−y Se 2 superconductor at a mesoscopic scale [7,10].…”

“…The insulating parental phase even exhibits certain Mott-insulator signatures, and it is phase separated from the superconducting phase in the K x Fe 2−y Se 2 superconductor at a mesoscopic scale [7,10]. Moreover, angle-resolved photoemission spectroscopy (ARPES) studies have shown that its Fermi surface is consisted of electron pockets only [7,11,12], which is again distinct from most of the iron-based superconductors [13][14][15][16][17]. These unique properties of K x Fe 2−y Se 2 have raised a lot of interest.…”

“…While the parent compounds of most iron-based superconductors are metallic [2][3][4], the parent compound of K x Fe 2−y Se 2 superconductor is found to be either insulating or semiconducting [5][6][7][8][9]. The insulating parental phase even exhibits certain Mott-insulator signatures, and it is phase separated from the superconducting phase in the K x Fe 2−y Se 2 superconductor at a mesoscopic scale [7,10]. Moreover, angle-resolved photoemission spectroscopy (ARPES) studies have shown that its Fermi surface is consisted of electron pockets only [7,11,12], which is again distinct from most of the iron-based superconductors [13][14][15][16][17].…”