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Register a new userLow work function in the 122-family of iron-based superconductors
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We determine the work functions of the iron arsenic compounds $A$Fe$_2$As$_2$ ($A=mathrm{Ca, Ba, Cs}$) using photoemission spectroscopy to be 2.7 eV for CaFe$_2$As$_2$, 1.8 eV for BaFe$_2$As$_2$, and 1.3 eV for CsFe$_2$As$_2$. The work functions of these 122 iron-based superconductors track those of the elementary metal $A$ but are substantially smaller. The most likely explanation of this observation is that the cleaving surface exposes only half an $A$-layer. The low work function and good photoemission cross section of BaFe$_2$As$_2$ and CsFe$_2$As$_2$ enable photoemission even from a common white LED light.
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The momentum dependence of the nematic order parameter is an important ingredient in the microscopic description of iron-based high-temperature superconductors. While recent reports on FeSe indicate that the nematic order parameter changes sign between electron and hole bands, detailed knowledge is still missing for other compounds. Combining angle-resolved photoemission spectroscopy (ARPES) with uniaxial strain tuning, we measure the nematic band splitting in both FeSe and BaFe$_2$As$_2$ without interference from either twinning or magnetic order. We find that the nematic order parameter exhibits the same momentum dependence in both compounds with a sign change between the Brillouin center and the corner. This suggests that the same microscopic mechanism drives the nematic order in spite of the very different phase diagrams.
We investigate the influence of itinerant carriers on dynamics and fluctuation of local moments in Fe-based superconductors, via linear spin-wave analysis of a spin-fermion model containing both itinerant and local degrees of freedom. Surprisingly against the common lore, instead of enhancing the ($pi$,0) order, itinerant carriers with well nested Fermi surfaces is found to induce significant amount of textit{spatial} and temporal quantum fluctuation that leads to the observed small ordered moment. Interestingly, the underlying mechanism is shown to be intra-pocket nesting-associated long-range coupling, rather than the previously believed ferromagnetic double-exchange effect. This challenges the validity of ferromagnetically compensated first-neighbor coupling reported from short-range fitting to the experimental dispersion, which turns out to result instead from the ferro-orbital order that is also found instrumental in stabilizing the magnetic order.
High temperature superconductivity in iron pnictides and chalcogenides emerges when a magnetic phase is suppressed. The multi-orbital character and the strength of correlations underlie this complex phenomenology, involving magnetic softness and anisotropies, with Hunds coupling playing an important role. We review here the different theoretical approaches used to describe the magnetic interactions in these systems. We show that taking into account the orbital degree of freedom allows us to unify in a single phase diagram the main mechanisms proposed to explain the (pi,0) order in iron pnictides: the nesting-driven, the exchange between localized spins, and the Hund induced magnetic state with orbital differentiation. Comparison of theoretical estimates and experimental results helps locate the Fe superconductors in the phase diagram. In addition, orbital physics is crucial to address the magnetic softness, the doping dependent properties, and the anisotropies.
We investigate a two-orbital model for iron-based superconductors to elucidate the effect of interplay between electron correlation and Jahn-Teller electron-phonon coupling by using the dynamical mean-field theory combined with the exact diagonalization method. When the intra- and inter-orbital Coulomb interactions, $U$ and $U$, increase with $U=U$, both the local spin and orbital susceptibilities, $chi_{s}$ and $chi_{o}$, increase with $chi_{s}=chi_{o}$ in the absence of the Hunds rule coupling $J$ and the electron-phonon coupling $g$. In the presence of $J$ and $g$, there are distinct two regimes: for $J stackrel{>}{_sim} 2g^2/omega_0$ with the phonon frequency $omega_0$, $chi_{s}$ is enhanced relative to $chi_{o}$ and shows a divergence at $J=J_c$ above which the system becomes Mott insulator, while for $J stackrel{<}{_sim} 2g^2/omega_0$, $chi_{o}$ is enhanced relative to $chi_{s}$ and shows a divergence at $g=g_c$ above which the system becomes bipolaronic insulator. In the former regime, the superconductivity is mediated by antiferromagnetic fluctuations enhanced due to Fermi-surface nesting and is found to be largely dependent on carrier doping. On the other hand, in the latter regime, the superconductivity is mediated by ferro-orbital fluctuations and is observed for wide doping region including heavily doped case without the Fermi-surface nesting.
A comprehensive study on the evolution of Stoner factor with doping concentration for various doped 122 systems (like BaFe$_2$As$_2$, SrFe$_2$As$_2$) of Fe-based superconductors is presented. Our first principles electronic structure calculations reveal that for Co/Ru (electron or iso-electronic) doping at Fe sites or P doping at As sites result in a reduction of Stoner factor with increasing doping concentration. On the contrary, in case of Na/K (hole) doping at the Ba sites, Stoner factor is enhanced for higher doping concentrations. This may be considered as an indicator of elevation of magnetic fluctuation in these systems. We find that the Stoner factor uniquely follows the variation of the pnictide height z$_{As}$/Fe-As bond length with various kinds of doping. Our calculated Fermi surfaces explicate the diversities in the behaviour of Stoner factors for various doped 122 systems ; larger degree of Fermi surface nesting, larger the value of Stoner factor and vice versa.
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