Absence of BCS–BEC crossover in FeSe_(0.45)Te_(0.55) superconductor

In iron-based superconductor Fe(Se,Te), a flat band-like feature near the Fermi level was observed around the Brillouin zone center in the superconducting state. It is under debate whether this is the evidence on the presence of the BCS–BEC[Bardeen–Cooper–Schrieffer(BCS), Bose–Einstein condensation(BEC)] crossover in the superconductor. High-resolution laser-based angle-resolved photoemission measurements are carried out on high quality single crystals of FeSe_(0.45)Te_(0.55) superconductor to address the issue. By employing different polarization geometries, we have resolved and isolated the dyz band and the topological surface band, making it possible to study their superconducting behaviors separately. The dyz band alone does not form a flat band-like feature in the superconducting state and the measured dispersion can be well described by the BCS picture. We find that the flat band-like feature is formed from the combination of the dyz band and the topological surface state band in the superconducting state. These results reveal the origin of the flat band-like feature and rule out the presence of BCS-BEC crossover in Fe(Se,Te) superconductor.

Intrinsic electronic structure and nodeless superconducting gap of YBa_(2)Cu_(3)O_(7)-σ observed by spatially-resolved laser-based angle resolved photoemission spectroscopy

The spatially-resolved laser-based high-resolution angle resolved photoemission spectroscopy(ARPES) measurements have been performed on the optimally-doped YBa_(2)Cu_(3)O_(7)-σ(Y123) superconductor. For the first time, we found the region from the cleaved surface that reveals clear bulk electronic properties. The intrinsic Fermi surface and band structures of Y123 were observed. The Fermi surface-dependent and momentum-dependent superconducting gap was determined which is nodeless and consistent with the d+is gap form.

Common (π,π) Band Folding and Surface Reconstruction in Fe As-Based Superconductors

High resolution angle-resolved photoemission spectroscopy(ARPES)measurements are carried out on CaKFe_4 As_4,KCa_2 Fe_4 As_4 F_2 and(Ba_(0.6)K_(0.4))Fe_2 As_2 superconductors.Clear evidence of band folding between the Brillouin zone center and corners with a(π,π)wave vector has been found from the measured Fermi surface and band structures in all the three kinds of superconductors.A dominant √2×√2 surface reconstruction is observed on the cleaved surface of CaKFe_4As_4 by scanning tunneling microscopy(STM)measurements.We propose that the commonly observed √2×√2 reconstruction in the FeAs-based superconductors provides a general scenario to understand the origin of the(π,π)band folding.Our observations provide new insights in understanding the electronic structure and superconductivity mechanism in iron-based superconductors.

Genuine electronic structure and superconducting gap structure in (Ba_(0.6)K_(0.4)Fe_(2)As_(2)FeAs superconductor

The electronic structure and superconducting gap structure are prerequisites to establish microscopic theories in understanding the superconductivity mechanism of iron-based superconductors.However,even for the most extensively studied optimally-doped Ba_(0.6)K_(0.4)Fe_(2)As_(2),there remain outstanding controversies on its electronic structure and superconducting gap structure.Here we resolve these issues by carrying out high-resolution angle-resolved photoemission spectroscopy(ARPES)measurements on the optimally-doped Ba_(0.6)K_(0.4)Fe_(2)As_(2)superconductor using both Helium lamp and laser light sources.Our results indicate the‘‘flat band"feature observed around the Brillouin zone center in the superconducting state originates from the combined effect of the superconductivity-induced band back-bending and the folding of a band from the zone corner to the center.We found direct evidence of the band folding between the zone corner and the center in both the normal and superconducting state.Our resolution of the origin of the flat band makes it possible to assign the three hole-like bands around the zone center and determine their superconducting gap correctly.Around the zone corner,we observe a tiny electronlike band and an M-shaped band simultaneously in both the normal and superconducting states.The obtained gap size for the bands around the zone corner(~5.5 meV)is significantly smaller than all the previous ARPES measurements.Our results establish a new superconducting gap structure around the zone corner and resolve a number of prominent controversies concerning the electronic structure and superconducting gap structure in the optimally-doped Ba_(0.6)K_(0.4)Fe_(2)As_(2).They provide new insights in examining and establishing theories in understanding superconductivity mechanism in iron-based superconductors.