Common Fermi-surface topology and nodeless superconducting gap ofK0.68Fe1.79

Abstract

We carried out high resolution angle-resolved photoemission measurements on the electronic structure and superconducting gap of K0.68Fe1.79Se2 (Tc=32 K) and (Tl0.45K0.34)Fe1.84Se2 (Tc=28 K) superconductors. In addition to the electron-like Fermi surface near M(π,π), two electronlike Fermi pockets are revealed around the zone center Γ(0,0) in K0.68Fe1.79Se2. This observation makes the Fermi surface topology of K0.68Fe1.79Se2 consistent with that of (Tl,Rb)xFe2−ySe2 and (Tl,K)xFe2−ySe2 compounds. A nearly isotropic superconducting gap (∆) is observed along the electron-like Fermi pocket near the M point in K0.68Fe1.79Se2 (∆∼ 9 meV) and (Tl0.45K0.34)Fe1.84Se2 (∆∼ 8 meV). The establishment of a universal picture on the Fermi surface topology and superconducting gap in the AxFe2−ySe2 (A=K, Tl, Cs, Rb and etc.) superconductors will provide important information in understanding the superconductivity mechanism of the iron-based superconductors.

Spin fluctuations and pairing symmetry in AxFe2−ySe2: dual effect of the itinerant and the localized nature of electrons

“…The gap of the d wave symmetry for J 2 = 0 [ Fig. 4 (b)] has no node on the FS, this is consistent with the ARPES results of a nodeless SC gap on the large Fermi pockets around the zone corner [5][6][7]14,15 . But, it has line nodes along the diagonal direction in the Brillouin zone.…”

“…Khodas et al 13 suggest an s +− pairing symmetry, in which the gap changes sign between the hybridized pockets, by including the interpocket pairing. Experimentally, the ARPES measurements have reported a nodeless superconducting (SC) gap on the large Fermi pockets around the zone corner in these materials [5][6][7]14,15 . In particular, the recent measurements further find an isotropic SC gap distribution on the small electron Fermi pocket around the Z point 16,17 , which favors the s-wave pairing symmetry.…”

Crossover from weak to strong pairing in unconventional superconductors

“…A further increase of the 2∆/k B T c ratio towards ∼ 10, close to the strong-coupling limit of the Eliashberg theory, 70 is observed in Hg-1223 (T c = 130 K) and Hg-1201 (T c = 96 K) cuprates, 36 suggesting that the positive correlation between this ratio and T c , similar to the one we found for Febased compounds, could be universal for all unconventional superconductors, including cuprates. Gap ratios in the most recently discovered iron-selenide superconductors [71][72][73] (T c, max ≈ 33 K) also conform to this general trend [74][75][76][77][78][79] and are similar to those of optimally-doped BKFA.…”

Large local disorder in superconducting K0.8Fe1.6Se2studied by extended x-ray absorption fine structure

“…Thus, the Fermi surface of K x Fe 2−y Se 2 is made by only the electron pockets around the M points [15,14], and hence the scenario based on the nesting between electron and hole pockets, usually argued to understand the iron pnictides, looses its ground [16]. On the other hand, although isostructural to the 122-type BaFe 2 As 2 pnictides, the K x Fe 2−y Se 2 has distinct microstructure, characterized by an iron-vacancy order and a phase separation [17,18,19,20].…”