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Register a new userSurface terminations and layer-resolved spectroscopy in 122 iron pnictide superconductors
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The surface terminations of 122-type alkaline earth metal iron pnictides AEFe2As2 (AE = Ca, Ba) are investigated with scanning tunneling microscopy/spectroscopy (STM/STS). Cleaving these crystals at a cryogenic temperature yields a large majority of terminations with atomically resolved square-root-two (rt2) or 1*2 lattice, as well as the very rare terminations with 1*1 symmetry. By means of lattice alignment and chemical marking, we identify these terminations as rt2-AE, 1*2-As, and rt2-Fe surfaces, respectively. Layer-resolved spectroscopy on these terminating surfaces reveals a well-defined superconducting gap on the As terminations, while the gap features become weaker and absent on AE and Fe terminations respectively. The local gap features are hardly affected by the surface reconstruction on As or AE surface, whereas a suppression of them along with the in-gap states can be induced by As vacancies. The emergence of two impurity resonance peaks at +-2 meV is consistent with the sign-reversal pairing symmetry. The definite identification of surface terminations and their spectroscopic signatures shall provide a more comprehensive understanding of the high-temperature superconductivity in multilayered iron pnictides.
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The upper critical fields ($H_{c2}$) of the single crystals $rm(Sr,Na)Fe_2As_2$ and $rm Ba_{0.55}K_{0.45}Fe_2As_2$ were determined by means of measuring the electrical resistivity, $ rho_{xx}(mu_0H)$, using the facilities of pulsed magnetic field at Los Alamos. In general, these compounds possess a very large upper critical field ($H_{c2}(0)$) with a weak anisotropic effect. The detailed curvature of $H_{c2}(T_c)$ may depend on the magnetic field orientation and the sample compositions. We argue that such a difference mainly results from the multi-band effect, which might be modified via doping.
We establish in a combination of ab initio theory and experiments that the tunneling process in scanning tunneling microscopy/spectroscopy on the A-122 iron pnictide superconductors - in this case BaFe$_{2-x}$Co$_x$As$_2$ - involve a strong adatom filtering of the differential conductance from the near-EF Fe3d states, which in turn originates from the top-most sub-surface Fe layer of the crystal. The calculations show that the dominance of surface Ba-related tunneling pathways leaves fingerprints found in the experimental differential conductance data, including large particle-hole asymmetry and an energy-dependent contrast inversion.
Insight into the electronic structure of the pnictide family of superconductors is obtained from quantum oscillation measurements. Here we review experimental quantum oscillation data that reveal a transformation from large quasi-two dimensional electron and hole cylinders in the paramagnetic overdoped members of the pnictide family to significantly smaller three-dimensional Fermi surface sections in the antiferromagnetic parent members, via a potential quantum critical point at which an effective mass enhancement is observed. Similarities with the Fermi surface evolution from the overdoped to the underdoped normal state of the cuprate superconducting family are discussed, along with the enhancement in antiferromagnetic correlations in both these classes of materials, and the potential implications for superconductivity.
Detailed measurements of the in-plane resistivity were performed in a high-quality Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ ($x=0.065$) single crystal, in magnetic fields up to 9 T and with different orientations $theta$ relative to the crystal $c$ axis. A significant $rho(T)_{H,theta}$ rounding is observed just above the superconducting critical temperature $T_c$ due to Cooper pairs created by superconducting fluctuations. These data are analyzed in terms of a generalization of the Aslamazov-Larkin approach, that extends its applicability to high reduced-temperatures and magnetic fields. This method allows us to carry out a criterion-independent determination of the angular dependence of the upper critical field, $H_{c2}(theta)$. In spite of the relatively small anisotropy of this compound, it is found that $H_{c2}(theta)$ presents a significant deviation from the single-band 3D anisotropic Ginzburg-Landau (3D-aGL) approach, particularly for large $theta$ (typically above $sim60^o$). These results are interpreted in terms of the multiband nature of these materials, in contrast with other proposals for similar $H_{c2}(theta)$ anomalies. Our results are also consistent with an effective anisotropy factor almost temperature independent near $T_c$, a result that differs from the ones obtained by using a single-band model.
We show that the zero field normal-state resistivity above Tc for various levels of electron doping - both for LaO1-xFxFeAs (La-1111) and SmO1-xFxFeAs (Sm-1111) members of the 1111-iron-pnictide superconductor family - can be scaled in a broad temperature range from 20 to 300 K onto single curves for underdoped La-1111 (x=0.05-0.075), for optimally and overdoped La-1111 (x=0.1-0.2) and for underdoped Sm-1111 (x=0.06-0.1) compounds. The scaling was performed using the energy scale {Delta}, the resistivity {rho}_{Delta} and the residual resistivity {rho}_0 as scaling parameters as well as by applying a recently proposed model-independent scaling method (H. G. Luo, Y. H. Su, and T. Xiang, Phys. Rev. B 77, 014529 (2008)). The scaling parameters have been calculated and the compositional variation of {Delta} has been determined. The observed scaling behaviour for {rho}(T) is interpreted as an indication of a common mechanism which dominates the scattering of the charge carriers in underdoped La-1111, in optimally and overdoped La-1111 and in underdoped Sm-1111 compounds..
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