Structural, electronic, and dynamical properties of the tetragonal and collapsed tetragonal phases of KFe2As2

Abstract

Compounds with a tetragonal ThCr2Si2-type structure are characterized by the possibility of the isostructural phase transition from the tetragonal phase to the collapsed tetragonal phase induced by the external pressure. An example of a compound with such a phase transition is KFe2As2, which belongs to the family of the iron-based superconductors. In this paper, we investigate the effects of the phase transition on the structural, electronic, and dynamical properties of this compound. Performing the ab initio calculations, we reproduce the dependence of the lattice constants on pressure and analyze the changes of the inter-atomic distances in the tetragonal and collapsed tetragonal phases. Using the tight binding model with maximally localized Wannier orbitals, we calculate and discuss the influence of pressure on the electronic band structure as well as on the shape of the Fermi surface. We found a precursor of the phase transition in the form of enhancement of overlapping between two Wannier orbitals of As atoms. In order to better understand the superconducting properties of KFe2As2, we study the orbital-projected Cooper pairs susceptibility as a function of pressure. We found a decrease of susceptibility with the increasing pressure in a good qualitative agreement with experimental observation. The structural transition also influences the phonon spectrum of KFe2As2, which exhibits pronounced changes induced by pressure. Some phonon modes related with the vibrations of Fe and As atoms show an anomalous, nonmonotonic dependence on pressure close to the phase transition. *

Superconductivity of KFe2As2 Under Pressure: Ab Initio Study of Tetragonal and Collapsed Tetragonal Phases

“…The spectra measured by the angle resolved photoemission spectroscopy (ARPES) showed the existence of three hole and four electron pockets centered at the Γ and the X point, respectively [20,[47][48][49]. To achieve a consistency of these results with our DFT calculations, it is necessary to shift the theoretical value of the Fermi level of approximately 75 meV [46]. Such a small shift corresponds to the change of the average number of particles by δn ≈ 0.075 [50], what does not change qualitatively the obtained electronic structure.…”

“…The image was rendered using XCRYSDEN software [40] overlap of orbitals is possible. When it happens, the p z orbitals of the As atoms cover partially the p z orbitals of the As atoms from neighboring FeAs layers [46]. It means that the initially separated isosurfaces bring together creating "hourglass" shapes as it is shown in Fig.…”

“…On panel (a), Γ -and X-points of the Brillouin zone are indicated. The image was rendered using FERMISURFER software [41] is induced by the pressure, which leads to the strong modification of the band structure around p c [46]. The FS shape is also strongly changed as it is shown in Fig.…”

Electronic and Magnetic Anisotropies in FeSe Family of Iron-Based Superconductors

“…, the alkali iron selenide family) [16][17][18]. The 122 family, especially the electron-and hole-doped BaFe 2 As 2 , are the most intensively studied materials [19][20][21][22] because superconductivity can be induced by doping a large variety of chemicals including K/Na on Ba sites (hole-doping) [23,24], Co/Ni on Fe sites (electron-doping) [25,26] and P on As sites (isovalent doping) [27], and large-sized single crystals can be grown by self-flux methods in most cases [28]. In the phase diagram of Co-doped BaFe 2 As 2 , the static AF order is gradually suppressed and separated from the structural transition by Co-doping in the underdoped region [12,29].…”

Microstructure and Flux Pinning of Reacted-and-Pressed, Polycrystalline Ba0.6K0.4Fe2As2 Powders

“…These advantages were already manifested in several efforts to produce 122-based wires/tapes (see, e.g., the recent review of Yao et al [4]), and trapped fields above 1 T were already recorded in bulk, HIP (hot isostatic pressed)-processed 122 samples [5]. On the other hand, the 122 material demonstrates several unique properties, e.g., it undergoes an isostructural transition under pressure [6,7,8,9], from which important information on the nature of superconductivity can be obtained.…”