Sheet Dependence on Superconducting Gap in Oxygen-Deficient Iron-Based Oxypnictide Superconductors NdFeAsO_0.85

Abstract

Although angle-integrated photoemission spectroscopy (PES) directly allows us to measure the electronic structure and the energy gap, the electronic contributions from those two types of FS sheets are merged in the final states when we used the conventional photon source like a helium discharge lamp. On the other hand, the low-energy tunable photons are very useful for separating out those contributions. With decreasing the photon energy extremely, the photoelectrons are to be observable only from the limited momentum space centered at the Γ point as shown in Fig. 1. Such low-energy high-resolution photoemission measurements have recently performed using a laser giving the photon energy of about 7 eV [4,5]. These studies found a clear pseudogap but did not obtain any sign for the superconducting gap opening.Considering the probing area provided by 7 eV photons, there is no superconducting gap for the hole-like FS sheets around the Γ point. It is thus essential to elucidate the characteristic electronic structure due to the electron-like FS sheets around the M point, in order to understand the pairing mechanism and origin of the superconductivity.In this letter, we report high-resolution photoemission spectroscopy with low-energy tunable photons (LEPES) on oxygen-deficient iron-based oxypnictide superconductors NdFeAsO0.85. We have found a clear shift of the leading-edge away from EF in the superconducting state using 9.5 eV photons. On the other hand, a clear Fermi cutoff with little leading-edge shift can be observed in the case of 6 eV, which is qualitatively consistent with previous laser based photoemission results [4,5]. Present LEPES results demonstrate that the superconducting gap opens on the electron-like FS sheets around the Μ point, but not on the hole-like ones around the Γ point, suggesting that different type of Fermi surface sheets may have a different role in the superconducting states.Polycrystalline samples of NdFeAsO1− δ were synthesized at high pressure and high temperature using a cubic-anvil high-pressure apparatus. Details of synthesis method are given in [6]. Although the nominal oxygen deficiency in the NdFeAsO1− δ samples is 0.35 estimated from the starting materials, recent powder neutron diffraction revealed that the samples are largely shift towards higher oxidation [7].Judging from the relationship between the lattice parameters and the oxygen deficient content, the actual oxygen content used in this experiment is estimated to be 0.85. The

Temperature-dependent pseudogap in the oxypnictidesLaFeAsO1−xFxandLaFePOIshida1, Shimojima2, Ishizaka3 et al. 2009Phys. Rev. B240120View full textAdd to dashboardCite

“…Through investigating the T dependence of the angleintegrated spectra recorded by a laser-PES system, 21 we find that even the low-T c P05 exhibits a pseudogap feature similar to those of the high-T c FeAs system. [13][14][15][16][17] We also find a large T-dependent pseudogap feature specific to high-T c As26 manifesting at high T's. Our study shows that pseudogaps do exist in the iron oxypnictides, but they are qualitatively different from the doping-dependent pseudogaps of the cuprates.…”

“…Angleintegrated PES studies on FeAs-based superconductors have revealed pseudogap features with energies ranging from 15 to 40 meV at T Շ 100 K. [13][14][15][16][17] However, subsequent angleresolved PES studies on iron oxypnictides 18,19 showed that the Fermi surfaces in the normal state were fully preserved, in contrast to the doping-dependent pseudogaps of the cuprates that cause the Fermi-surface sectors to vanish. 20 Herein, we present a comparative electronic-structure study of optimally F-doped LaFeAsO:F ͑As26͒ ͑Ref.…”

“…One can see that the spectral DOS of As26 is depressed with decreasing T below 70 K in the energy range of Ϯ ϳ 20 meV, and we identify it to the 15-40 meV pseudogaps reported in the angle-integrated PES studies on the FeAs system. [13][14][15][16][17] Surprisingly, a T dependence similar to the ϳ20-meV pseudogap of As26 is present even in the spectral DOS of low-T c P05 ͓Fig. 1͑e͔͒.…”

Opening of the superconducting gap in the hole pockets of Ba(Fe1−xCox)2Mansart1, Papalazarou2, Brouet3 et al. 2012Phys. Rev. B5040View full textAdd to dashboardCite

“…Experimentally, Angle-Resolved PhotoElectron Spectroscopy (ARPES) has been widely used in this search, thanks to its capability of resolving in k-space the electronic structure of materials [6]. This technique has been employed to measure the amplitude of the superconducting gap (SC gap) in all the existing ironbased compound families: the 1111 (RE(O 1−x F x )FePn, RE being a rare-earth and Pn a pnictogen) [7,8], 11 (FeSe 1−x , FeSe 1−x Te x and FeTe 1−x S x ) [9], 111 (LiFeAs and CaFeAs) [10,11], in the iron selenides A x Fe 2 Se 2 (A=Cs, K) [12] and in the 122 compounds (doped AEFe 2 As 2 , AE being an alcaline earth metal).…”

Unusual Pseudogap Features Observed in Iron Oxypnictide Superconductors

“…Temperature-dependent angle-integrated PES studies by other groups have also shown evidence for pseudogaps in the iron-oxypnictide compounds. [13][14][15][16][17] Most of the re- * E-mail: ishida@spring8.or.jp ports showed a pseudogap having an energy scale of ∼20 meV, [13][14][15][16][17] indicating that this feature occurs commonly in the layered iron oxypnictides and is insensitive to T c or to doping. Through a wide-temperature-range study, Ou et al also showed T -dependent pseudogap features in SmFeAsO:F 15) similar to that in As26, 12) although there are rooms for speculation concerning its origin.…”