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Register a new userLocal correlation effects on the s+-- and s++-wave superconductivities mediated by magnetic and orbital fluctuations in the 5-orbital Hubbard model for iron pnictides
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We investigate the electronic state and the superconductivity in the 5-orbital Hubbard model for iron pnictides by using the dynamical mean-field theory in conjunction with the Eliashberg equation. The renormalization factor exhibits significant orbital dependence resulting in the large change in the band dispersion as observed in recent ARPES experiments. The critical interactions towards the magnetic, orbital and superconducting instabilities are suppressed as compared with those from the random phase approximation (RPA) due to local correlation effects. Remarkably, the s++-pairing phase due to the orbital fluctuation is largely expanded relative to the RPA result, while the s+--pairing phase due to the magnetic fluctuation is reduced.
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A growing list of experiments show orthorhombic electronic anisotropy in the iron pnictides, in some cases at temperatures well above the spin density wave transition. These experiments include neutron scattering, resistivity and magnetoresistance measurements, and a variety of spectroscopies. We explore the idea that these anisotropies stem from a common underlying cause: orbital order manifest in an unequal occupation of $d_{xz}$ and $d_{yz}$ orbitals, arising from the coupled spin-orbital degrees of freedom. We emphasize the distinction between the total orbital occupation (the integrated density of states), where the order parameter may be small, and the orbital polarization near the Fermi level which can be more pronounced. We also discuss light-polarization studies of angle-resolved photoemission, and demonstrate how x-ray absorption linear dichroism may be used as a method to detect an orbital order parameter.
The experimental consequences of different order parameters in iron-based superconductors are theoretically analyzed. We consider both nodeless and nodal order parameters, with an emphasis on the $cos(k_x)cdot cos(k_y)$ nodeless order parameter recently derived by two of us. We analyze the effect of this order parameter on the spectral function, density of states, tunneling differential conductance, penetration depth, and the NMR spin relaxation time. This extended s-wave symmetry has line-zeroes in between the electron and hole pockets, but they do not intersect the two Fermi surfaces for moderate doping, and the superconductor is fully gapped. However, this suggests several quantitative tests: the exponential decay of the penetration depth weakens and the density of states reveals a smaller gap upon electron or hole doping. Moreover, the $cos(k_x) cdot cos(k_y)$ superconducting gap is largest on the smallest (hole) Fermi surface. For the $1/T_1$ NMR spin relaxation rate, the inter-band contribution is consistent with the current experimental results, including a (non-universal) $T^{3}$ behavior and the absence of a coherence peak. However, the intra-band contribution is considerably larger than the inter-band contributions and still exhibits a small enhancement in the NMR spin relaxation rate right below $T_c$ in the clean limit.
Using an RPA approximation, we have calculated the strengths of the singlet and triplet pairing interactions which arise from the exchange of spin and orbital fluctuations for a 2-orbital model of the Fe-pnictide superconductors. When the system is doped with F, the electron pockets become dominant and we find that the strongest pairing occurs in the singlet d-wave pairing and the triplet p-wave pairing channels, which compete closely. The pairing structure in the singlet d-wave channel corresponds to a superposition of near neighbor intra-orbital singlets with a minus sign phase difference between the $d_{xz}$ and $d_{yz}$ pairs. The leading pairing configuration in the triplet channel also involves a nearest neighbor intra-orbital pairing. We find that the strengths of both the singlet and triplet pairing grow, with the singlet pairing growing faster, as the onsite Coulomb interaction approaches the value where the S=1 particle-hole susceptibility diverges.
We use point contact spectroscopy to probe $rm{AEFe_2As_2}$ ($rm{AE=Ca, Sr, Ba}$) and $rm{Fe_{1+y}Te}$. For $rm{AE=Sr, Ba}$ we detect orbital fluctuations above $T_S$ while for AE=Ca these fluctuations start below $T_S$. Co doping preserves the orbital fluctuations while K doping suppresses it. The fluctuations are only seen at those dopings and temperatures where an in-plane resistive anisotropy is known to exist. We predict an in-plane resistive anisotropy of $rm{Fe_{1+y}Te}$ above $T_S$. Our data are examined in light of the recent work by W.-C. Lee and P. Phillips (arXiv:1110.5917v2). We also study how joule heating in the PCS junctions impacts the spectra. Spectroscopic information is only obtained from those PCS junctions that are free of heating effects while those PCS junctions that are in the thermal regime display bulk resistivity phenomenon.
The electronic structure of BaFe$_2X_3$ ($X$ = S and Se) and CsFe$_2$Se$_3$ in which two-leg ladders are formed by the Fe sites are studied by means of x-ray absorption and resonant inelastic x-ray scattering spectroscopy. The x-ray absorption spectra at the Fe L edges for BaFe$_2X_3$ exhibit two components, indicating that itinerant and localized Fe 3$d$ sites coexist. Substantial x-ray linear dichroism (XLD) is observed in polarization dependent spectra, indicating the existence of orbital order or fluctuation in the Fe-ladder even above the N{e}el temperature $T_N$. Direct exchange interaction along the legs of the Fe-ladder stabilizes the orbital and antiferromagnetic orders in BaFe$_2$S$_3$, while the ferromagnetic molecular orbitals are realized between the rungs in CsFe$_2$Se$_3$.
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