Droplet-like Fermi surfaces in the anti-ferromagnetic phase of EuFe ₂ As ₂ , an Fe-pnictide superconductor parent compound

Abstract

Using angle resolved photoemission it is shown that the low lying electronic states of the iron pnictide parent compound EuFe2As2 are strongly modified in the magnetically ordered, low temperature, orthorhombic state compared to the tetragonal, paramagnetic case above the spin density wave transition temperature. Back-folded bands, reflected in the orthorhombic/ anti-ferromagnetic Brillouin zone boundary hybridize strongly with the non-folded states, leading to the opening of energy gaps. As a direct consequence, the large Fermi surfaces of the tetragonal phase fragment, the low temperature Fermi surface being comprised of small droplets, built up of electron and hole-like sections. These high resolution ARPES data are therefore in keeping with quantum oscillation and optical data from other undoped pnictide parent compounds. The electronic structure and properties of the newly discovered iron-pnictide superconductors [1] are a focus of much research. A central theme is the understanding of the origins of the magnetic ordering and its possible interplay with superconductivity. The characteristics of the low-temperature, orthorhombic, antiferromagnetic (AFM) phase have been experimentally determined, for example for the undoped 122 -parent compounds, M Fe 2 As 2 (with M = Ca, Sr, Ba, Eu. . . ). A common proposition is that the magnetism is of an itinerant, spin density wave (SDW) type, connected to nesting of the warped cylindrical Fermi surfaces (FS's) centered at the (0, 0) (Γ) and (π, π) points (X) of the tetragonal Brillouin zone (BZ) [2]. Such an SDW would have dramatic consequences for the band structure and Fermi surface, leading to reconstruction, the opening of gaps and major Fermi surface depletion. Recently, quantum oscillation (QO) experiments have presented evidence for Fermi surfaces comprising small pocketsdue to the effects of the SDW order -in SrFe 2 As 2 [3] and BaFe 2 As 2 [4]. Both studies find the existence of three distinct orbits, with FS areas of only 0.3%, 0.6% and 1.5% of the tetragonal BZ (compared to a total FS area larger than 20% in the tetragonal phase). In addition, the opening of gaps as well as a dramatic reduction of the free charge carrier density upon entering the orthorhombic AFM state has been concluded from optical conductivity measurements [5,6]. Given that a FS-nesting-driven SDW is rooted in k-space, it is of great importance whether the SDW 'fingerprints' of reconstruction, gapping and FS depletion really take place in the E(k)-space hosting the electronic states of these materials. Angle-resolved photoemission (ARPES) is a powerful probe of such issues, and consequently there have been numerous studies of the parent compounds of the pnictide superconductors using ARPES. Early studies were unable to detect significant changes below the magnetic ordering temperature [7,8]. More recently, ARPES studies have shown the existence of small, additional FS pockets (either hole or electron-like) around the (0, 0) or (π, π) point [9,10,11,12], and a detailed temperature-dependent...

Quantum oscillations inEuFe2As2single crystals

“…For the parent compounds AFe 2 As 2 (A = Ba, Ca, Eu, Sr, ...), ARPES data usually display four Fe 3d FS sheets in the paramagnetic regime: two electron-like bands around the M -point and two hole-like bands around the Γ-point of the Brillouin zone (BZ). Furthermore, polarized ARPES measurements show the predominance of t 2g orbitals (xy, xz/yz) at the FS with the outer hole-pocket being mostly xy, in agreement with DFT + DMFT calculations 10,12,14,15 . Interestingly, a comparison between Ba122 and Eu122 in the paramagnetic state reveals that the outer hole xy pocket in Ba122 is half the size of Eu122 13 , suggesting an increase of the planar xy occupation in the Ba-rich end.…”

Pseudogap-less high-T c superconductivity in BaCo x Fe 2−x As 2

“…Since at 20 K the system is still well within the orthorhombic, magnetically ordered state, one might expect gap-like features in the spectra due to a depression of spectral weight as a result of gapping of parts of the Fermi surface [32][33][34]. Assuming a mean-field gap, such features would be typically at ∼10 mV.…”

Complex temperature evolution of the electronic structure of CaFe2As2

“…High pressure 5 , substitution of Fe by Co, Ni 6 and other dopants induces superconductivity in CaFe 2 As 2 (transition temperature, T c ∼ 15 K). The SDW transition is found to accompany a nesting of the Fermi surface 7,8 along with a transition from two dimensional (2D) to three dimensional (3D) Fermi surface associated with the structural transition [9][10][11] . Fe 3d states play a major role in the electronic properties of these systems, while the doped holes in cuprates possess dominant ligand 2p orbital character 1 .…”