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Anisotropic Superconducting Properties of Optimally Doped BaFe$_2$(As$_{0.65}$P$_{0.35}$)$_2$ under Pressure

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 Added by Swee Goh
 Publication date 2010
  fields Physics
and research's language is English




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Magnetic measurements on optimally doped single crystals of BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$ ($xapprox0.35$) with magnetic fields applied along different crystallographic axes were performed under pressure, enabling the pressure evolution of coherence lengths and the anisotropy factor to be followed. Despite a decrease in the superconducting critical temperature, our studies reveal that the superconducting properties become more anisotropic under pressure. With appropriate scaling, we directly compare these properties with the values obtained for BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$ as a function of phosphorus content.



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We report anisotropic dc magnetic susceptibility $chi(T)$, electrical resistivity $rho(T)$, and heat capacity $C(T)$ measurements on the single crystals of CaFe$_{2-x}$Co$_x$As$_2$ for $x$ = 0 and 0.06. Large sized single crystals were grown by the high temperature solution method with Sn as the solvent. For the pure compound with $x$ = 0, a high temperature transition at 170 K is observed which is attributed to a combined spin density wave (SDW) ordering and a structural phase transition. On the other hand, for the Co-doped samples for $x$ = 0.06, the SDW transition is suppressed while superconductivity is observed at $simeq$17 K. The superconducting transition has been confirmed from the magnetization and electrical resistivity studies. The $^{57}$Fe Mossbauer spectrum in CaFe$_2$As$_2$ indicates that the SDW ordering is incommensurate. In the Co-doped sample, a prominent paramagnetic line at 4.2 K is observed indicating a weakening of the SDW state.
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159 - Y.-M. Xu , Y.-B. Huang , X.-Y. Cui 2010
The iron-pnictide superconductors have a layered structureformed by stacks of FeAs planes from which the superconductivity originates. Given the multiband and quasi three-dimensional cite{3D_SC} (3D) electronic structure of these high-temperature superconductors, knowledge of the quasi-3D superconducting (SC) gap is essential for understanding the superconducting mechanism. By using the KZ-capability of angle-resolved photoemission, we completely determined the SC gap on all five Fermi surfaces (FSs) in three dimensions on BKFAOP samples. We found a marked KZ dispersion of the SC gap, which can derive only from interlayer pairing. Remarkably, the SC energy gaps can be described by a single 3D gap function with two energy scales characterizing the strengths of intralayer $Delta_1$ and interlayer $Delta_2$ pairing. The anisotropy ratio $Delta_2/Delta_1$, determined from the gap function, is close to the c-axis anisotropy ratio of the magnetic exchange coupling $J_c/J_{ab}$ in the parent compound cite{NeutronParent}. The ubiquitous gap function for all the 3D FSs reveals that pairing is short-ranged and strongly constrain the possible pairing force in the pnictides. A suitable candidate could arise from short-range antiferromagnetic fluctuations.
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