(p, p) electronic order in iron arsenide superconductors

The distribution of valence electrons in metals usually follows the symmetry of the underlying 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 density waves have been observed in many families of superconductors, and are often considered to be essential for superconductivity to exist. Recent measurements seem to show that the properties of the iron pnictides are in good agreement with band structure calculations that do not include additional ordering, implying no relation between density waves and superconductivity in these materials. Here we report that the electronic structure of Ba12xKxFe2As2 is in sharp disagreement with those band structure calculations, and instead reveals a reconstruction characterized by a (p, p) wavevector. This electronic order coexists with superconductivity and persists up to room temperature (300 K). Calculations of the electronic structure of the new pnictide superconductors unanimously predict a Fermi surface consisting of a holelike pocket at the centre (Cpoint) of the Brillouin zone and electron-like ones at the corners (X points) of the Brillouin zone. A shift by a (p,p) vector would result in a significant overlap of these Fermi surfaces. Such an electronic structure is highly unstable because any interaction allowing an electron to gain a (p,p) momentum would favour a density-wave order, resulting in a shift of the aforementioned type and a concomitant opening of the gaps, thus strongly reducing the electronic kinetic energy. It is surprising that angle-resolved photoemission spectroscopy (ARPES) data are reported to be in general, and sometimes in very detailed, agreement with calculations that give a potentially unstable solution. Even in the parent compound, where the spin-density-wave transition is clearly seen using other techniques, no evidence for the expected energy gap has been detected in photoemission experiments. In fact, no consensus exists regarding the overall Fermi surface topology. According to refs 5 and 6, there is a single electron-like Fermi surface pocket around X, whereas ref. 18 reported two intensity spots without any discernible signature for the electron pocket in the normal state. Intensity spots near X were also reported in refs. 6, 7 and 9, but those are interpreted as parts of electron-like pockets. Such substantial variations in the photoemission signal preclude unambiguous assignment of the observed features to the calculated Fermi surface, leaving the electronic structure of the arsenides unclear. In Fig. 1, we show the experimental Fermi surface map of Ba12xKxFe2As2 (BKFA) measured in the superconducting state. To eliminate possible effects of photoemission matrix elements, as well as to cut the electronic structure at different values of momentum, kz, we made 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 kz dispersion can be conclusively identified. With this in mind, the apparently different intensity distributions at neighbouring C points appear unusual. In the first Brillouin zone, the two concentric contours are broadly consistent with band structure calculations, but the ‘design wheels’ centred at the G points (Fig. 1a, b) in the second Brillouin zone are at variance with predicted hole-like circles. The major discrepancy with theoretical calculations and ARPES data is observed near X, where, according to the calculations, a sizeable double-walled electron pocket is expected. Instead, we observe a propeller-shaped structure consisting of five small Fermi surface sheets: a pocket, situated directly at X, and four ‘blades’ surrounding it. Fig 1c, d shows that these Fermi surfaces are not only well separated but also have different symmetries. To examine the topology of these five pockets, we look at the momentum distribution of intensity below and above the Fermi level. As can be seen in Fig. 2a–c, the size of the X-centred pocket