We have studied the electronic structure of the nonmagnetic LiFeAs (T(c)∼18  K) superconductor using angle-resolved photoemission spectroscopy. We find a notable absence of the Fermi surface nesting, strong renormalization of the conduction bands by a factor of 3, high density of states at the Fermi level caused by a van Hove singularity, and no evidence for either a static or a fluctuating order except superconductivity with in-plane isotropic energy gaps. Our observations suggest that these electronic properties capture the majority of ingredients necessary for the superconductivity in iron pnictides.

Using angle-resolved photoemission spectroscopy, we report on the direct observation of the energy gap in 2H-NbSe2 caused by the charge-density waves (CDW). The gap opens in the regions of the momentum space connected by the CDW vectors, which implies a nesting mechanism of CDW formation. In remarkable analogy with the pseudogap in cuprates, the detected energy gap also exists in the normal state (T>T0) where it breaks the Fermi surface into "arcs," it is nonmonotonic as a function of temperature with a local minimum at the CDW transition temperature (T0), and it forestalls the superconducting gap by excluding the nested portions of the Fermi surface from participating in superconductivity.

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

Using angle-resolved photoemission spectroscopy we demonstrate that a normal-state pseudogap exists above T(N-IC) in one of the most studied two-dimensional charge-density wave (CDW) dichalcogenides 2H-TaSe(2). The initial formation of the incommensurate CDW is confirmed as being driven by a conventional nesting instability, which is marked by a pseudogap. The magnitude, character, and anisotropy of the 2D-CDW pseudogap bear considerable resemblance to those seen in superconducting cuprates.

The onset of superconductivity at the transition temperature is marked by the onset of order, which is characterized by an energy gap. Most models of the iron-based superconductors find a sign-changing (s±) order parameter [1–6], with the physical implication that pairing is driven by spin fluctuations. Recent work, however, has indicated that LiFeAs has a simple isotropic order parameter [7–9] and spin fluctuations are not necessary [7,10], contrary to the models [1–6]. The strength of the spin fluctuations has been controversial [11,12], meaning that the mechanism of superconductivity cannot as yet be determined. We report the momentum dependence of the superconducting energy gap, where we find an anisotropy that rules out coupling through spin fluctuations and the sign change. The results instead suggest that orbital fluctuations assisted by phonons [13,14] are the best explanation for superconductivity