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Register a new userFermi surface shrinking and interband coupling in iron-based pnictides
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Added by
Emmanuele Cappelluti
Publication date
2009
fields
Physics
and research's language is
English
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Recent measurements of Fermi surface with de Haas-van Alphen oscillations in LaFePO showed a shrinking of the Fermi pockets with respect to first-principle LDA calculations, suggesting an energy shift of the hole and electrons bands with respect to LDA. We show that these shifts are a natural consequence of the strong particle-hole asymmetry of electronic bands in pnictides, and that they provide an indirect experimental evidence of a dominant interband scattering in these systems.
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The wave-vector q and doping (x,y) dependences of the magnetic energy, iron moment, and effective exchange interactions in LaFeAsO{1-x}F{x} and Ba{1-2y}K{2y}Fe2As2 are studied by self-consistent LSDA calculations for co-planar spin spirals. For the undoped compounds (x=0, y=0), the minimum of the calculated total energy, E(q), is for q corresponding to stripe antiferromagnetic order. Already at low levels of electron doping (x), this minimum becomes flat in LaFeAsO{1-x}F{x} and for x>=5, it shifts to an incommensurate q. In Ba{1-2y}K{2y}Fe2As2, stripe order remains stable for hole doping up to y=0.3. These results are explained in terms of the band structure. The magnetic interactions cannot be accurately described by a simple classical Heisenberg model and the effective exchange interactions fitted to E(q) depend strongly on doping. The doping dependence of the E(q) curves is compared with that of the noninteracting magnetic susceptibility for which similar trends are found.
Based on an effective two-band model and using the fluctuation-exchange (FLEX) approach, we explore spin fluctuations and unconventional superconducting pairing in Fe-based layer superconductors. It is elaborated that one type of interband antiferromagnetic (AF) spin fluctuation stems from the interband Coulomb repulsion, while the other type of intraband AF spin fluctuation originates from the intraband Coulomb repulsion. Due to the Fermi-surface topology, a spin-singlet extended s-wave superconducting state is more favorable than the nodal $d_{XY}$-wave state if the interband AF spin fluctuation is more significant than the intraband one, otherwise vice versa. It is also revealed that the effective interband coupling plays an important role in the intraband pairings, which is a distinct feature of the present two-band system.
Based on the minimum two-orbital model and the phase diagram recently proposed by Tai et al. (Europhys. Lett. textbf{103}, 67001(2013)) for both electron- and hole-doped 122 iron-based superconducting compounds, we use the Bogoliubov-de Gennes equations to perform a comprehensive investigation of the evolution of the Fermi surface (FS) topology in the presence of the collinear spin-density-wave (SDW) order as the doping is changed. In the parent compound, the ground state is the SDW order, where the FS is not completely gapped, and two types of Dirac cones, one electron-doped and the other hole-doped emerge in the magnetic Brillouin zone. Our findings are qualitatively consistent with recent angle-resolved photoemission spectroscopy and magneto-resistivity measurements. We also examine the FS evolution of both electron- and hole-doped cases and compare them with measurements, as well as with those obtained by other model Hamiltonians.
The discovery of high-temperature superconductivity in iron pnictides raised the possibility of an unconventional superconducting mechanism in multiband materials. The observation of Fermi-surface(FS)-dependent nodeless superconducting gaps suggested that inter-FS interactions may play a crucial role in superconducting pairing. In the optimally hole-doped Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$, the pairing strength is enhanced simultaneously (2$Delta$/Tc$sim$7) on the nearly nested FS pockets, i.e. the inner holelike ($alpha$) FS and the two hybridized electronlike FSs, while the pairing remains weak (2$Delta$/Tc$sim$3.6) in the poorly-nested outer hole-like ($beta$) FS. Here we report that in the electron-doped BaFe$_{1.85}$Co$_{0.15}$As$_2$ the FS nesting condition switches from the $alpha$ to the $beta$ FS due to the opposite size changes for hole- and electron-like FSs upon electron doping. The strong pairing strength (2$Delta$/Tc$sim$6) is also found to switch to the nested $beta$ FS, indicating an intimate connection between FS nesting and superconducting pairing, and strongly supporting the inter-FS pairing mechanism in the iron-based superconductors.
In a comprehensive study, we investigate the electronic scattering effects in EuFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ by using Fourier-transform infrared spectroscopy. In spite of the fact that Eu$^{2+}$ local moments order around $T_text{Eu} approx 20$,K, the overall optical response is strikingly similar to the one of the well-known Ba-122 pnictides. The main difference lies within the suppression of the lower spin-density-wave gap feature. By analysing our spectra with a multi-component model, we find that the high-energy feature around 0.7,eV -- often associated with Hunds rule coupling -- is highly sensitive to the spin-density-wave ordering, this further confirms its direct relationship to the dynamics of itinerant carriers. The same model is also used to investigate the in-plane anisotropy of magnetically detwinned EuFe$_{2}$As$_{2}$ in the antiferromagnetically ordered state, yielding a higher Drude weight and lower scattering rate along the crystallographic $a$-axis. Finally, we analyse the development of the room temperature spectra with isovalent phosphor substitution and highlight changes in the scattering rate of hole-like carriers induced by a Lifshitz transition.
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