Effect of surface morphology and magnetic impurities on the electronic structure in cobalt-doped BaFe2As2 superconductors (original) (raw)
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Probing Local Variations of Superconductivity on the Surface of Ba (Fe1-xCox) 2As2 Single Crystals
2010
The spatially resolved electrical transport properties have been studied on the surface of optimally-doped superconducting Ba(Fe 1-x Co x ) 2 As 2 single crystal by using a four-probe scanning tunneling microscopy. While some non-uniform contrast appears near the edge of the cleaved crystal, the scanning electron microscopy (SEM) reveals mostly uniform contrast. For the regions that showed uniform SEM contrast, a sharp superconducting transition at T C = 22.1 K has been observed with a transition width ΔT C = 0.2 K. In the non-uniform contrast region, T C is found to vary between 19.6 and 22.2 K with ΔT C from 0.3 to 3.2 K. The wavelength dispersive x-ray spectroscopy reveals that Co concentration remains 7.72% in the uniform region, but changes between 7.38% and 7.62% in the non-* Corresponding author. apli@ornl.gov 2 uniform region. Thus the variations of superconductivity are associated with local compositional change.
2016
We observed the anisotropic superconducting-gap (SC-gap) structure of a slightly overdoped superconductor, Ba(Fe1−xCox)2As2 (x = 0.1), using three-dimensional (3D) angle-resolved photoemission spectroscopy. Two hole Fermi surfaces (FSs) observed at the Brillouin zone center and an inner electron FS at the zone corner showed a nearly isotropic SC gap in 3D momentum space. However, the outer electron FS showed an anisotropic SC gap with nodes or gap minima around the M and A points. The different anisotropies obtained the SC gap between the outer and inner electron FSs cannot be expected from all theoretical predictions with spin fluctuation, orbital fluctuation, and both competition. Our results provide a new insight into the SC mechanisms of iron pnictide superconductors. Iron pnictide superconductors1 have the second-highest superconducting transition tem-perature (Tc) and a wide variety of superconducting (SC) states, particularly as SC-gap struc-ture owing to multiple Fermi surfa...
Physical Review B, 2019
Doping dependence of the superconducting state structure and spin-fluctuation pairing mechanism in the Ba(Fe1-xCox)2As2 family is studied. BCS-like analysis of experimental data shows that in the overdoped regime, away from the AFM transition, the spin-fluctuation interaction between the electron and hole gaps is weak, and Ba(Fe1-xCox)2As2 is characterized by three essentially different gaps. In the three-gap state an anisotropic (nodeless) electron gap ∆e(x,φ) has an intermediate value between the dominant inner Δ2h(x) and outer Δ1h(x) hole gaps. Close to the AFM transition the electron gap ∆e(x, φ) increases sharply and becomes closer in magnitude to the dominant inner hole gap Δ2h(x). The same two-gap state with close electron and inner hole gaps Δ2h(x) ≈ ∆e(x, φ) is also preserved in the phase of coexisting antiferromagnetism and superconductivity. The doping dependence of the electron gap ∆e(x, φ) is associated with the strong doping dependence of the spin-fluctuation interaction in the AFM transition region. In contrast to the electron gap ∆e(x, φ), the doping dependence of the hole gaps Δ1,2h(x) and the critical temperature Tc(x), both before and after the AFM transition, are associated with a change of the density of states γnh(x) and the intraband electron-phonon interaction in the hole bands. The non-phonon spin-fluctuation interaction in the hole bands in the entire Co concentration range is small compared with the intraband electron-phonon interaction and is not dominant in the Ba(Fe1-xCox)2As2 family. The high-Tc iron-based superconductors (FeSCs) are multiband quasi-two-dimensional compounds with strongly anisotropic Fermi surface and low carrier density in the hole-like and electron-like bands [1]. The Fermi surface (FS) of these compounds consists of hole-like (h) pockets at the Γ point and electron-like (e) pockets centered at the X = (π, 0) and Y = (0, π) points of the Brillouin zone. Compared to strongly correlated high-Tc cuprates, which are similar in their basic characteristics, electron-electron correlations in FeSCs are not large (see, for example, reviews [2, 3]).The parent orthorhombic (Ort) Fe-based compounds are antiferromagnetic (AFM) metals of spin-density wave (SDW) type with the magnetic ordering vectors Q = (π, 0), (0, π). Unlike dielectric parent high-Tc cuprates, they have free electronic states at the FS that are not associated with magnetism but can, in principle, be involved in superconducting (SC) pairing. The electronic structure of these compounds is very sensitive to small changes in doping, pressure, and degree of disorder. When in parent compounds the magnetic atoms Fe (3d 6) in the a-b plane are replaced by atoms with larger number of d electrons (electron doping) or the non-magnetic atoms out of this plane are replaced by atoms with smaller valence (hole doping), antiferromagnetism is gradually suppressed which leads to the onset of superconductivity. In this regime, the AFM and SC gaps coexist at the Fermi surface
Physical Review B, 2009
The spatially resolved electrical transport properties have been studied on the surface of optimally-doped superconducting Ba(Fe 1-x Co x ) 2 As 2 single crystal by using a four-probe scanning tunneling microscopy. While some non-uniform contrast appears near the edge of the cleaved crystal, the scanning electron microscopy (SEM) reveals mostly uniform contrast. For the regions that showed uniform SEM contrast, a sharp superconducting transition at T C = 22.1 K has been observed with a transition width ΔT C = 0.2 K. In the non-uniform contrast region, T C is found to vary between 19.6 and 22.2 K with ΔT C from 0.3 to 3.2 K. The wavelength dispersive x-ray spectroscopy reveals that Co concentration remains 7.72% in the uniform region, but changes between 7.38% and 7.62% in the non-* Corresponding author. apli@ornl.gov 2 uniform region. Thus the variations of superconductivity are associated with local compositional change.
59Co and 75As NMR Investigation of Electron-Doped High Tc Superconductor BaFe1.8Co0.2As2 (Tc = 22 K)
Journal of the Physical Society of Japan, 2008
We report an NMR investigation of the superconductivity in BaFe2As2 induced by Co doping (Tc = 22 K). We demonstrate that Co atoms form an alloy with Fe atoms and donate carriers without creating localized moments. Our finding strongly suggests that the underlying physics of iron-pnictide superconductors is quite different from the widely accepted physical picture of high Tc cuprates as doped Mott insulators. We also show a crossover of electronic properties into a low temperature pseudo-gap phase with a pseudo-gap ∆P G/kB ∼ 560 K, where χspin ∼ constant and resisitivty ρ ∝ T. The NMR Knight shift below Tc decreases for both along the c-axis and ab-plane, and is consistent with the singlet pairing scenario.
Superconducting properties in heavily overdoped Ba(Fe0.86Co0.14)2As2 single crystals
Solid State Communications, 2015
In this work we report the influence of intrinsic superconducting parameters on the vortex dynamics in an overdoped Ba(Fe 1-x Co x) 2 As 2 (x=0.14) single crystal. We find a superconducting critical temperature of 13.5 K, magnetic penetration depth λ ab (0) = 660 ± 50 nm, coherence length ξ ab (0) = 5 nm, and the upper critical field anisotropy γ T→Tc ≈ 3.7. In fact, the Ginzburg-Landau model may explain the angular dependent H c2 for this anisotropic three-dimensional superconductor. The vortex phase diagram, in comparison with the optimally doped compound, presents a narrow collective creep regime. In addition, we found no sign of correlated pinning along the c axis. Our results show that vortex core to defect size ratio and λ play an important role in the resulting vortex dynamics in materials with similar intrinsic thermal fluctuations.
Impurity effects on the Fe-based superconductor A(Fe1−yCoy)2As2 (A=Ba and Sr)
Solid State Communications, 2012
In this study, we address the superconductivity of the Zn-and Mn-doped Fe-based superconductors Ba(Fe 1−x−y M x Co y ) 2 As 2 (M = Zn and Mn) and Sr(Fe 0.9−x Zn x Co 0.1 ) 2 As 2 . The impurity is successfully doped into the lattice of the superconductors under a high-pressure heating condition, resulting in a continuous decrease in T c toward zero. Not only the optimally doped, but also the under-and over-doped superconducting states show a noticeable decrease in T c as a function of the Zn concentration. The rate of decrease, however, is not as large as the theoretical value expected from the s ± -wave model, suggesting that this model is unlikely to be applied to the superconductivity of the present 122 system.