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Register a new userElectronic structure and Fermi surface of new K intercalated iron selenide superconductor KxFe2Se2
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Using the ab initio FLAPW-GGA method we examine the electronic band structure, densities of states, and the Fermi surface topology for a very recently synthesized ThCr2Si2-type potassium intercalated iron selenide superconductor KxFe2Se2. We found that the electronic state of the stoichiometric KFe2Se2 is far from that of the isostructural iron pnictide superconductors. Thus the main factor responsible for experimentally observed superconductivity for this material is the deficiency of potassium, i.e. the hole doping effect. On the other hand, based on the results obtained, we conclude that the tuning of the electronic system of the new KxFe2Se2 superconductor in the presence of K vacancies is achieved by joint effect owing to structural relaxations and hole doping, where the structural factor is responsible for the modification of the band topology, whereas the doping level determines their filling.
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The electronic structure of the vacancy-ordered K$_{0.5}$Fe$_{1.75}$Se$_2$ iron-selenide compound (278 phase) is studied using the first-principles density functional method. The ground state of the 278 phase is stripe-like antiferromagnetic, and its bare electron susceptibility shows a large peak around $(pi, pi)$ in the folded Brillouin zone. Near Fermi level, the density of states are dominated by the Fe-3d orbitals, and both electron-like and hole-like Fermi surfaces appear in the Brillouin zone. Unfolded band structure shows limited similarities to a hole doped 122 phase. With 0.1e electron doping, the susceptibility peak is quickly suppressed and broadened; while the two-dimensionality of the electron-like Fermi surfaces are greatly enhanced, resulting in a better nesting behavior. Our study should be relevant to the recently reported superconducting phase K$_{0.5+x}$Fe$_{1.75+y}$Se$_2$ with both $x$ and $y$ very tiny.
By means of the first-principles calculations, we have studied in details the structural, elastic and electronic properties of the new tetragonal CaBe2Ge2-type 5.2K superconductor SrPt2As2 in comparison with two hypothetical SrPt2As2 polymorphs with ThCr2Si2-type structures which differ by atomic configurations of [Pt2As2] (or [Pt2As2]) blocks. We have found that CaBe2Ge2-type SrPt2As2 is a quite unique system with complicated 2D-3D character of near-Fermi bands, and the intermediate type of the Fermi surface, which consists of electronic pockets having cylinder-like (2D) topology (typical for 122 FeAs phases) together with 3D-like electronic and hole pockets, which are characteristic for ThCr2Si2-like iron-free low-Tc superconductors. Our analysis reveals that against ThCr2Si2-like 122 phases, the other features for CaBe2Ge2-like SrPt2As2 are: (1). The essential differences of contributions of states from [Pt2As2] and [Pt2As2] blocks into near-Fermi region when the conduction is expected to be anisotropic and happening mainly in [Pt2As2] blocks; (2). The formation of the 3D system of strong covalent Pt-As bonds (inside and between of [Pt2As2]/[As2Pt2] blocks) which is responsible for enhanced stability of this polymorph, and (3). the essential charge anisotropy between the adjacent [Pt2As2] and [As2Pt2] blocks. We have predicted also that CaBe2Ge2-like SrPt2As2 is mechanically stable, relatively soft material with high compressibility and will behave in a ductile manner. On the contrary the ThCr2Si2-type SrPt2As2 polymorphs which contain only [Pt2As2] or [As2Pt2] blocks, are less stable, their Fermi surfaces adopt a multi-sheet three-dimensional type - similar to ThCr2Si2-like iron-free 122 phases, and these polymorphs will be ductile materials with high elastic anisotropy.
Alkali-doped iron selenide is the latest member of high Tc superconductor family, and its peculiar characters have immediately attracted extensive attention. We prepared high-quality potassium-doped iron selenide (KxFe2-ySe2) thin films by molecular beam epitaxy and unambiguously demonstrated the existence of phase separation, which is currently under debate, in this material using scanning tunneling microscopy and spectroscopy. The stoichiometric superconducting phase KFe2Se2 contains no iron vacancies, while the insulating phase has a surd5timessurd5 vacancy order. The iron vacancies are shown always destructive to superconductivity in KFe2Se2. Our study on the subgap bound states induced by the iron vacancies further reveals a magnetically-related bipartite order in the superconducting phase. These findings not only solve the existing controversies in the atomic and electronic structures in KxFe2-ySe2, but also provide valuable information on understanding the superconductivity and its interplay with magnetism in iron-based superconductors.
One of the ultimate goals of the study of iron-based superconductors is to identify the common feature that produces the high critical temperature (Tc). In the early days, based on a weak-coupling viewpoint, the nesting between hole- and electron-like Fermi surfaces (FSs) leading to the so-called $spm$ state was considered to be one such key feature. However, this theory has faced a serious challenge ever since the discovery of alkali-metal-doped FeSe (AFS) superconductors, in which only electron-like FSs with a nodeless superconducting gap are observed. Several theories have been proposed, but a consistent understanding is yet to be achieved. Here we show experimentally that a hole-like band exists in KxFe2-ySe2, which presumably forms a hole-like Fermi surface. The present study suggests that AFS can be categorized in the same group as iron arsenides with both hole- and electron-like FSs present. This result provides a foundation for a comprehensive understanding of the superconductivity in iron-based superconductors.
In this article we review our studies of the K0.80Fe1.76Se2 superconductor, with an attempt to elucidate the crystal growth details and basic physical properties over a wide range of temperatures and applied magnetic field, including anisotropic magnetic and electrical transport properties, thermodynamic, London penetration depth, magneto-optical imaging and Mossbauer measurements. We find that: (i) Single crystals of similar stoichiometry can be grown both by furnace-cooled and decanted methods; (ii) Single crystalline K0.80Fe1.76Se2 shows moderate anisotropy in both magnetic susceptibility and electrical resistivity and a small modulation of stoichiometry of the crystal, which gives rise to broadened transitions; (iii) The upper critical field, Hc2(T) is ~ 55 T at 2 K for H||c, manifesting a temperature dependent anisotropy that peaks near 3.6 at 27 K and drops to 2.5 by 18 K; (iv) Mossbauer measurements reveal that the iron sublattice in K0.80Fe1.76Se2 clearly exhibits magnetic order, probably of the first order, from well below Tc to its Neel temperature of Tn = 532 +/- 2 K. It is very important to note that, although, at first glance there is an apparent dilemma posed by these data: high Tc superconductivity in a near insulating, large ordered moment material, analysis indicates that the sample may well consist of two phases with the minority superconducting phase (that does not exhibit magnetic order) being finely distributed, but connected with in an antiferromagnetic, poorly conducting, matrix, essentially making a superconducting aerogel.
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