Generalized BCS Equations and the Iron-Pnictide Superconductors

A detailed quantitative study of the pnictide composite superconductor (CS) Ba0.6K0.4Fe2As2 is presented in the frame-work of the recently derived set of generalized BCS equations. Invoking multiple Debye temperatures to take into account anisotropy of the CS, we address the current experimental data on its Tc and the (not so clear-cut) gap-values via different theoretical scenarios that attempt to identify the ion species responsible for pairing in it. This is done with the aid of the Bogoliubov’s restriction on the BCS dimensionless electron-phonon coupling constant. Significantly, our study sheds light on the gaps which have recently been observed in different iron-pnictide CSs as nodes or line-nodes on the Fermi surface and have evinced considerable interest.

Variational Monte Carlo study of the nematic state in iron-pnictide superconductors with a five-orbital model

We perform a variational Monte Carlo study of the nematic state in iron-pnictide superconductors within a realistic five-orbital model. Our numerical results show that the nematic state, formed by introducing an anisotropic hopping order into the projected wave function, is not stable unless the off-site Coulomb interaction V exceeds a critical value. This demonstrates that V plays a key role in forming the nematic state in iron-pnictide superconductors. In the nematic state,the orbital order and the anisotropic spin correlations are consistent with the experimental observations. We argue that the experimentally observed anisotropic magnetic couplings and structural transition are associated with the nematic state and can be understood in a unified framework.

Coexistence of Polaronic States and Superconductivity in Iron-Pnictide Compound Ba_2Ti_2Fe_2As_4O

The electronic structure of iron-pnictide compound superconductor Ba_2Ti_2Fe_2As_4O, which has metallic intermediate Ti_2O layers, is studied using angle-resolved photoemission spectroscopy. The Ti-related bands show a‘peak-dip-hump＇ line shape with two branches of dispersion associated with the polaronic states at temperatures below around 120 K. This change in the spectra occurs along with the resistivity anomaly that was not clearly understood in a previous study. Moreover, an energy gap induced by the superconducting proximity effect opens in the polaronic bands at temperatures below T_c（~21 K）. Our study provides the spectroscopic evidence that superconductivity coexists with polarons in the same bands near the Fermi level, which provides a suitable platform to study interactions between charge, lattice and spin freedoms in a correlated system.

Hole density dependent magnetic structure and anisotropy in Fe-pnictide superconductor

The competition between different magnetic structures in hole-doped Fe-pnicitides is explored based on an extended five-orbital Hubbard model including long-range Coulomb interactions.Our results show that the stabilized magnetic structure evolves with increasing hole doping level.Namely,the stripe antiferromagnetic phase dominates at zero doping,while magnetic structures with more antiferromagnetic linking numbers such as the staggered tetramer,staggered trimer,and staggered dimer phases become energetically favorable as the hole density increases.At a certain doping level,energy degeneracy of different magnetic structures appears,indicating strong magnetic frustration and magnetic fluctuations in the system.We suggest that the magnetic competition induced by the hole doping may explain the fast decrease of the Neel temperature TNand the moderately suppressed magnetic moment in the hole doped Fe-pnicitides.Moreover,our results show a sign reversal of the kinetic energy anisotropy as the magnetic ground state evolves,which may be the mechanism behind the puzzling sign reversal of the in-plane resistivity anisotropy in hole-doped Fe-pnicitides.

Cobalt-doping effects in single crystalline and polycrystalline EuFe_(2-x)Co_xAs_2 compounds

A series of Co-doped EuFe2-xCoxAs2 compounds were prepared in both of single crystalline and polycrystalline forms.The Co-doping effects on the crystal structure,electrical resistivity and magnetic susceptibility were systematically studied.Superconductivity was found in polycrystalline Co-doped samples from zero resistivity effects,with the highest onset superconducting transition temperature at 26 K in the optimum doped EuFe1.84Co0.16As2 compound.While due to the stronger competition between the superconducting order and the Eu2+ magnetic order,the zero resistivity effect is absent in the Co-doped single crystal samples.