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

The precise momentum dependence of the superconducting gap in the iron-arsenide superconductor Ba 1−x K x Fe 2 As 2 ͑BKFA͒ with T c = 32 K was determined from angle-resolved photoemission spectroscopy ͑ARPES͒ via fitting the distribution of the quasiparticle density to a model. The model incorporates finite lifetime and experimental resolution effects, as well as accounts for peculiarities of BKFA electronic structure. We have found that the value of the superconducting gap is practically the same for the inner ⌫ barrel, X pocket, and "blade" pocket, and equals 9 meV, while the gap on the outer ⌫ barrel is estimated to be less than 4 meV, resulting in 2⌬ / k B T c = 6.8 for the large gap and 2⌬ / k B T c Ͻ 3 for the small gap. A large ͑77Ϯ 3 %͒ nonsuperconducting component in the photoemission signal is observed below T c . Details of gap extraction from ARPES data are discussed in Appendixes A and B.

Low energy polarized electronic Raman scattering of the electron-doped superconductor Nd2-x Ce x CuO4 ( x = 0.15, T(c) = 22 K) has revealed a nonmonotonic d(x(2)-y(2)) superconducting order parameter. It has a maximum gap of 4.4k(B)T(c) at Fermi surface intersections with an antiferromagnetic Brillouin zone (the "hot spots") and a smaller gap of 3.3k(B)T(c) at fermionic Brillouin zone boundaries. The gap enhancement in the vicinity of the hot spots emphasizes the role of antiferromagnetic fluctuations and the similarity in the origin of superconductivity for electron- and hole-doped cuprates.

Performing an in-depth analysis of the photoemission spectra along the nodal direction of the high temperature superconductor Bi-2212 we have developed a procedure to determine the underlying electronic structure and established a precise relation of the measured quantities to the real and imaginary parts of the self-energy of electronic excitations. The self-consistency of the procedure with respect to the Kramers-Kronig transformation allows us to draw conclusions on the applicability of the spectral function analysis and on the existence of well defined quasiparticles along the nodal direction even for the underdoped Bi-2212 in the pseudogap state.PACS numbers: 74.25.Jb, 74.72.Hs, 71.18.+y With modern angle-resolved photoemission spectroscopy (ARPES) [1,2] one gets a direct snapshot of the density of low energy electronic excited states in the momentum-energy space of 2D solids [3,4,5,6]. All the interactions of the electrons which are responsible for the unusual normal and superconducting properties of cuprates are encapsulated in such pictures, but are still hard to decipher. One way to take into account these interactions is to consider electronic excitations as quasiparticles which, compared to the non-interacting electrons, are characterized by an additional complex selfenergy [7]. Extraction of the self-energy from experiment is thus of great importance to check the validity of the quasiparticle concept and understand the nature of interactions involved, but appears to be problematic since the underlying band structure of the bare electrons is a priori unknown.One can evaluate the interaction parameters taking the bare band dispersion from band structure calculations [4], however this unavoidably increases the uncertainty of any conclusions on the strength and nature of the interactions involved. A direct determination of the bare band structure from experiment would be much more attractive in this sense. Previously, the bare band dispersion has been assigned to the high binding energy part of the experimental dispersion [8]. In Refs. 9, 10 we have discussed that the bare Fermi velocity estimated from the nodal ARPES spectra using the Kramers-Kronig (KK) transformation is in reasonable agreement with band structure calculations [11,12] and with an analysis of the anisotropic plasmon dispersion [13], although it has been pointed out that in order to quantify interaction parameters such as coupling strength [14] or self-energy [15] a precise and reliable approach of bare band determination is needed.In this Letter we introduce an approach to directly extract the bare band dispersion and the self-energy functions from ARPES spectra. We show that the approach is self-consistent within the highest experimental accuracy available today, and, applying the procedure to the spectra from the underdoped Bi-2212, we demonstrate the validity of the quasiparticle concept even in the underdoped regime and in the pseudogap state. The detailed description of the procedure as well as its application to other samples o...

Here we present a calculation of the temperature-dependent London penetration depth, λ(T ), in Ba1−xKxFe2As2 (BKFA) on the basis of the electronic band structure [1, 2] and momentumdependent superconducting gap [3] extracted from angle-resolved photoemission spectroscopy (ARPES) data. The results are compared to the direct measurements of λ(T ) by muon spin rotation (µSR) [4]. The value of λ(T = 0), calculated with no adjustable parameters, equals 270 nm, while the directly measured one is 320 nm; the temperature dependence λ(T ) is also easily reproduced. Such agreement between the two completely different approaches allows us to conclude that ARPES studies of BKFA are bulk-representative. Our review of the available experimental studies of the superconducting gap in the new iron-based superconductors in general allows us to state that all hole-doped of them bear two nearly isotropic gaps with coupling constants 2∆/kBTc = 2.5 ± 1.5 and 7 ± 2.