S. Thirupathaiah

Electronic structure studies of BaFe2As2 by angle-resolved photoemission spectroscopy

We report high resolution angle-resolved photoemission spectroscopy (ARPES) studies of the electronic structure of BaFe2As2, which is one of the parent compounds of the Fe-pnictide superconductors. ARPES measurements have been performed at 20 and 300 K, corresponding to the orthorhombic antiferromagnetic phase and the tetragonal paramagnetic phase, respectively. Photon energies between 30 and 175 eV and polarizations parallel and perpendicular to the scattering plane have been used. Measurements of the Fermi surface yield two hole pockets at the Γ point and an electron pocket at each of the X points. The topology of the pockets has been concluded from the dispersion of the spectral weight as a function of binding energy. Changes in the spectral weight at the Fermi level upon variation in the polarization of the incident photons yield important information on the orbital character of the states near the Fermi level. No differences in the electronic structure between 20 and 300 K could be resolved. The results are compared with density functional theory band structure calculations for the tetragonal paramagnetic phase.

Unusual band renormalization in the simplest iron-based superconductor FeSe 1 − x

The electronic structure of the iron chalcogenide superconductor FeSe_{1-x} was investigated by high- resolution angle-resolved photoemission spectroscopy (ARPES). The results were compared to DFT calculations showing some significant differences between the experimental electronic structure of FeSe_{1-x}, DFT calculations and existing data on FeSe_{x}Te_{1-x}. The bands undergo a pronounced orbital dependent renormalization, different from what was observed for FeSe_{x}Te_{1-x} and any other pnictides.

Ultrafast Momentum-Dependent Response of Electrons in Antiferromagnetic EuFe2As2 Driven by Optical Excitation

Employing the momentum-sensitivity of time- and angle-resolved photoemission spectroscopy we demonstrate the analysis of ultrafast single- and many-particle dynamics in antiferromagnetic EuFe2As2. Their separation is based on a temperature-dependent difference of photo-excited hole and electron relaxation times probing the single particle band and the spin density wave gap, respectively. Reformation of the magnetic order occurs at 800 fs, which is four times slower compared to electron-phonon equilibration due to a smaller spin-dependent relaxation phase space.

Orbital character variation of the Fermi surface and doping dependent changes of the dimensionality in BaFe2-xCoxAs2 from angle-resolved photoemission spectroscopy

From a combination of high resolution angle-resolved photoemission spectroscopy and density functional calculations, we derive information on the dimensionality and the orbital character of the electronic states of BaFe2−xCoxAs2. Upon increasing Co doping, the electronic states in the vicinity of the Fermi level take on increasingly three-dimensional character. Both the orbital variation with kz and the more three-dimensional nature of the doped compounds have important consequences for the nesting conditions and thus possibly also for the appearance of antiferromagnetic and superconducting phases.

Electronic structure studies of BaFe 2 As 2 by angle-resolved photoemission spectroscopy

We report high resolution angle-resolved photoemission spectroscopy (ARPES) studies of the electronic structure of BaFe2As2, which is one of the parent compounds of the Fe-pnictide superconductors. ARPES measurements have been performed at 20 and 300 K, corresponding to the orthorhombic antiferromagnetic phase and the tetragonal paramagnetic phase, respectively. Photon energies between 30 and 175 eV and polarizations parallel and perpendicular to the scattering plane have been used. Measurements of the Fermi surface yield two hole pockets at the Γ point and an electron pocket at each of the X points. The topology of the pockets has been concluded from the dispersion of the spectral weight as a function of binding energy. Changes in the spectral weight at the Fermi level upon variation in the polarization of the incident photons yield important information on the orbital character of the states near the Fermi level. No differences in the electronic structure between 20 and 300 K could be resolved. The results are compared with density functional theory band structure calculations for the tetragonal paramagnetic phase.

Observation of an isotropic superconducting gap at the Brillouin zone centre of Tl0.63K0.37Fe1.78Se2

We performed a high-resolution angle-resolved photoemission spectroscopy study on superconducting (SC) Tl0.63K0.37Fe1.78Se2 (T-c = 29 K) in the whole Brillouin zone (BZ). In addition to a nearly isotropic similar to 8.2 meV 2-dimensional (2D) SC gap (2 Delta/k(B)T(c) similar to 7) on quasi-2D electron Fermi surfaces (FSs) located around M(pi, 0, 0)-A(pi, 0, pi), we observe a similar to 6.2 meV isotropic SC gap (2 Delta/k(B)T(c) similar to 5) on the Z-centred electron FS that rules out any d-wave pairing symmetry and rather favors an s-wave symmetry. All isotropic SC gap amplitudes can be fit by a single-gap function derived from a local strong-coupling approach suggesting an enhancement of the next-nearest neighbor exchange interaction in the ferrochalcogenide superconductors. Copyright (C) EPLA, 2012

Interaction-induced singular Fermi surface in a high-temperature oxypnictide superconductor

In the family of iron-based superconductors, LaFeAsO-type materials possess the simplest electronic structure due to their pronounced two-dimensionality. And yet they host superconductivity with the highest transition temperature Tc ≈ 55K. Early theoretical predictions of their electronic structure revealed multiple large circular portions of the Fermi surface with a very good geometrical overlap (nesting), believed to enhance the pairing interaction and thus superconductivity. The prevalence of such large circular features in the Fermi surface has since been associated with many other iron-based compounds and has grown to be generally accepted in the field. In this work we show that a prototypical compound of the 1111-type, SmFe0.92Co0.08AsO , is at odds with this description and possesses a distinctly different Fermi surface, which consists of two singular constructs formed by the edges of several bands, pulled to the Fermi level from the depths of the theoretically predicted band structure by strong electronic interactions. Such singularities dramatically affect the low-energy electronic properties of the material, including superconductivity. We further argue that occurrence of these singularities correlates with the maximum superconducting transition temperature attainable in each material class over the entire family of iron-based superconductors.

WhyTcof (CaFeAs)10Pt3.58As8is twice as high as (CaFe0.95Pt0.05As)10Pt3As8

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Electronic band structure and momentum dependence of the superconducting gap in Ca1−xNaxFe2As2 from angle-resolved photoemission spectroscopy

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Droplet-like Fermi surfaces in the anti-ferromagnetic phase of EuFe2As2, an Fe-pnictide superconductor parent compound

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