(π, π) electronic order in iron arsenide superconductors

Abstract

The distribution of valence electrons in metals usually follows the symmetry of an ionic lattice. Modulations of this distribution often occur when those electrons are not stable with respect to a new electronic order, such as spin or charge density waves. Electron Calculations of the electronic structure of the new pnictide superconductors unanimously predict a Fermi surface (FS) consisting of hole-like pocket in the centre (Γ point) of the Brillouin zone (BZ) and electron-like ones at the corners (X point) of the BZ. A shift by the (π, π) vector would result in a significant overlap of these FSs. Such an electronic structure is highly unstable since any interaction allowing an electron to gain a (π, π) momentum would favour a density wave order, which then results in aforementioned shift and a concomitant opening of the gaps, thus strongly reducing the electronic kinetic energy. It is surprising that ARPES data are reported to be in general, and sometimes in very detailed [9], agreement with the calculations giving a potentially unstable solution [5,6,7]. Even in the parent compound, where the spin density wave transition is clearly seen by other techniques [16,17], no evidence for the expected energy gap has been detected by photoemission experiments [7,8]. In fact, no consensus exists regarding the overall FS topology. According to Refs. 6 and 5, there is a single electron-like FS pocket around the X point, while Ref. 18 reports two intensity spots without any discernible signature for the electron pocket in the normal state. Intensity spots near the X point can also be found in Refs. 6, 7 and 9, but those are interpreted as parts of electron-like pockets. Obviously, such substantial variations in the photoemission signal preclude unambiguous assignment of the observed features to the calculated FS, leaving the electronic structure of the arsenides unclear.In Fig. 1 we show experimental FS map of Ba 1−x K x Fe 2 As 2 (BKFA) measured in superconducting state. To eliminate possible effects of photoemission matrix elements, as well as to cut the electronic structure at different k z values, we have done measurements at several excitation energies (Fig. 1a-b) and polarizations ( Fig. 1c-d). Although there are obvious changes in the intensities of the features, no signatures indicating k z dispersion can be concluded. With this in mind, the apparently different intensity distributions at neighboring Γ points appear unusual. While in the first BZ the two concentric contours are broadly consistent with

Probing the pairing symmetry in the over-doped Fe-based superconductorBa0.35Rb0.65Fe2As2as a function of hydrostatic pressure

“…The above results provide clear evidence that the SC gap symmetry as well as the pressure effects on T c and on λ(0) strongly depends on doping level x. Note that in the optimally doped '122'-system Ba 1−x K x Fe 2 As 2 several bands cross the Fermi surface (FS) 9,75,76 . They consist of inner (α) and outer (β) hole-like bands, both centered at the zone center Γ, and an electron-like band (γ) centered at the M point.…”

Fermi-surface topology and low-lying electronic structure of the iron-based superconductor Ca10(Pt3As8)(Fe

“…Not only the in-plane lattice parameter a 0 but also the shapes, sizes and Fermi velocities of the Γ and X 0 Fermi pockets show very little difference with those of the 122 parent compounds. [21][22][23][24][25][26][27][28][29] This indicates that a universal electronic structure capturing the underlying superconducting mechanism may exist for different sub-families of the Fe-based superconductors (except for the K α Fe 2−β Se 2 series, where electron pockets instead of hole pockets are observed around the Γ point 30,31 ).…”

Iron-based superconductors: Magnetism, superconductivity, and electronic structure (Review Article)

“…5 (left) [94]. This said, [62] (left) and an optimally doped Ba 1-x K x Fe 2 As 2 (BKFA) [40] (right).…”