Role of Antiferromagnetic Ordering in the (1×2) Surface Reconstruction ofCa(Fe1−x

Abstract

Low energy electron diffraction, scanning tunneling microscopy and spectroscopy, and first-principles spin-dependent density functional theory are utilized to investigate the geometric, electronic, and magnetic structures of the stripe-ordered (1×2) surface of Ca(Fe1-xCox)2As2 (x=0, 0.075). The surface is terminated with a 50% Ca layer. Compared to the bulk, the surface Ca layer has a large inward relaxation (∼0.5 Å), and the underneath As-Fe2-As layer displays a significant buckling. First-principles calculations show that the (1×2) phase is stabilized by the bulk antiferromagnetic spin ordering through the spin-charge-lattice coupling. Strikingly, a superconducting gap (∼7 meV at 7.4 K) is observed to spatially coexist with the (1×2) phase (x=0.075 compound). This implies the coexistence of both superconductivity and AFM ordering at the surface.

Interrogating the superconductor Ca10(Pt4As8)(Fe2−xPtxAs2)5 Layer-by-layer

“…2d is ~0.7 Å shorter than the corresponding bulk spacing between Ca2 and Pt1 (1.67 Å). This STM result is consistent with the behavior reported at the surface of CaFe 2 As 2 with 0.5 ML Ca coverage20, where Ca atoms are pulled down by ~0.5 Å (~30%) determined by low energy electron diffraction (LEED). In addition, the calculated charge states using the Bader scheme in ref.…”

“…5b) exhibits a finite zero bias conductance (ZBC). Such a feature has been observed previously in other Fe-based superconductors20212223242526, even when probed at much lower temperatures than their T c , indicating the ZBC is not due to the thermal broadening effect. It was also shown that spectral features can vary on different reconstructed surfaces in BaFe 2−x Co x As 2 26 suggesting reconstructions may be closely related to the observed ZBC in Fe-based superconductors.…”

“…We recall that superconducting coherence peaks have been observed in spectra taken on the surfaces of other Fe-based superconductors202327, in which the surfaces are ordered. For example, the cleaved surface of Ca(Fe 0.925 Co 0.075 ) 2 As 2 has an ordered stripe phase formed by a half monolayer Ca on the Fe 2 As 2 layer20. Moreover, superconducting features are enhanced in the Ca2- (Fig.…”

“…3c, which is obtained from the as-cleaved Ca/Fe 2−x Pt x As 2 surface. For comparison, the surface of CaFe 2 As 2 has a (1 × 2) structure formed by 0.5 ML Ca on the top of Fe 2 As 2 layer20. For Ca 10 Pt 4 As 8 (Fe 2−x Pt x As 2 ) 5 , the absence of ordered Ca/Fe 2−x Pt x As 2 surface is undoubtedly due to the asymmetric interlayer spacing: the Ca layer is closer to the Fe 2−x Pt x As 2 layer than to the Pt 4 As 8 layer.…”

Enhanced surface superconductivity in Ba(Fe0.95Co0.05)2As2

“…As a result the region adjacent to the cleaved surface, the selvedge 4 , differs both chemically and structurally from the bulk. The remaining surface Ba atoms surfaces 12 . Either an uneven distribution of the Ba adatoms, or structural distortions in the surface lattice (or a combination of the two) may lead to a modified doping and changes in the superconductivity in the selvedge.…”