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Using the angle-resolved photoemission spectroscopy and band structure calculations we study the electronic structure of KFeCoAs2, which is isoelectronic to the parent material of 122 series of iron-based superconductors BaFe2As2. Although band structure calculations predict nearly identical dispersions of the electronic states in both compounds, experiment reveals drastic differences in both the global renormalization and Fermi surfaces. On the basis of the comparison of electronic structures of these two isoelectronic compounds, we demonstrate local magnetic correlations as a vital role for the peculiar low-energy electron dynamics of iron-based superconductors.

We report a comprehensive study of the tridimensional nature and orbital characters of the low-energy electronic structure in KCo2Se2, using polarization- and photon energy-dependent angle-resolved photoemission spectroscopy. We observed one electron-like Fermi surface (FS) at the Brillouin zone (BZ) center, four electron-like FSs centered at the BZ corner, and one hole-like FS at the BZ boundary. The FSs show weak dispersion along the kz direction, indicating the near-two-dimensional nature of FSs in KCo2Se2. In combination with the local-density approximation calculations, we determined the orbital characters of the low-energy electronic bands, which are mainly derived from the Co 3d orbital, mixed with part of the Se 4p states. The [Formula: see text] orbital gives a significant contribution to the band crossing the Fermi level. A band renormalization of about 1.6 is needed to capture the essential dispersive features, which suggests that electronic correlations are much weaker than that in KyFe2-xSe2.

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 (Zabolotnyy et al 2009 Nature 457 569, Zabolotnyy et al 2009 Physica C 469 448) and momentum-dependent superconducting gap (Evtushinsky et al 2009 Phys. Rev. B 79 054517) extracted from angle-resolved photoemission spectroscopy (ARPES) data. The results are compared to the direct measurements of λ(T) by muon spin rotation (μSR) (Khasanov et al 2009 Phys. Rev. Lett. 102 187005). 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 most of them bear two nearly isotropic gaps with coupling constants 2Δ/kBTc=2.5±1.5 and 7±2., New Journal of Physics, 11 (5), ISSN:1367-2630

By means of high-resolution angle resolved photoelectron spectroscopy (ARPES) we have studied the fermiology of 2H transition metal dichalcogenide polytypes TaSe2, NbSe2, and Cu0.2NbS2. The tight-binding model of the electronic structure, extracted from ARPES spectra for all three compounds, was used to calculate the Lindhard function (bare spin susceptibility), which reflects the propensity to charge density wave (CDW) instabilities observed in TaSe2 and NbSe2. We show that though the Fermi surfaces of all three compounds possess an incommensurate nesting vector in the close vicinity of the CDW wave vector, the nesting and ordering wave vectors do not exactly coincide, and there is no direct relationship between the magnitude of the susceptibility at the nesting vector and the CDW transition temperature. The nesting vector persists across the incommensurate CDW transition in TaSe2 as a function of temperature despite the observable variations of the Fermi surface geometry in this temperature range. In Cu0.2NbS2 the nesting vector is present despite different doping level, which lets us expect a possible enhancement of the CDW instability with Cu-intercalation in the CuxNbS2 family of materials., Accepted to New J. Phys