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Register a new userEvolution of band structure from optimally doped to heavily overdoped Co-substituted NaFeAs
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Using angle-resolved photoemission spectroscopy, we studied the electronic structure of NaFe$_{1-x}$Co$_x$As from an optimally doped superconducting compound ($x=0.028$) to a heavily overdoped non-superconducting one ($x=0.109$). Similar to the case of 122 type iron pnictides, our data suggest that Co dopant in NaFe$_{1-x}$Co$_x$As supplies extra charge carriers and shifts the Fermi level accordingly. In the $x=0.109$ compound, the hole-like bands around the zone center $Gamma$ move to deeper binding energies and an electron pocket appears instead. The overall band renormalization remains basically the same throughout the doping range we studied, suggesting that the local magnetic/electronic correlations are not affected by carrier doping. We speculate that a balance between itinerant properties of mobile carriers and local interactions may play an important role for the superconductivity.
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We have performed high-resolution angle-resolved photoemission spectroscopy on heavily electron-doped non-superconducting (SC) BaFe$_{1.7}$Co$_{0.3}$As$_2$. We find that the two hole Fermi surface pockets at the zone center observed in the hole-doped superconducting Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ are absent or very small in this compound, while the two electron pockets at the M point significantly expand due to electron doping by the Co substitution. Comparison of the Fermi surface between non-SC and SC samples indicates that the coexistence of hole and electron pockets connected via the antiferromagnetic wave vector is essential in realizing the mechanism of superconductivity in the iron-based superconductors.
Heavily electron-doped iron-selenide (HEDIS) high-transition-temperature (high-$T_{rm{c}}$) superconductors, which have no hole Fermi pockets, but have a notably high $T_{rm{c}}$, have challenged the prevailing $s$$_pm$ pairing scenario originally proposed for iron pnictides containing both electron and hole pockets. The microscopic mechanism underlying the enhanced superconductivity in HEDIS remains unclear. Here, we used neutron scattering to study the spin excitations of the HEDIS material Li$_{0.8}$Fe$_{0.2}$ODFeSe ($T_{rm{c}}$ = 41 K). Our data revealed nearly ring-shaped magnetic resonant excitations surrounding ($pi$, $pi$) at $sim$ 21 meV. As the energy increased, the spin excitations assumed a diamond shape, and they dispersed outward until the energy reached $sim$ 60 meV and then inward at higher energies. The observed energy-dependent momentum structure and twisted dispersion of spin excitations near ($pi$, $pi$) are analogous to those of hole-doped cuprates in several aspects, thus implying that such spin excitations are essential for the remarkably high $T_{rm{c}}$ in these materials.
We report an x-ray emission spectroscopy (XES) study of the local fluctuating magnetic moment ($mu_{bare}$) in $mathrm{NaFe_{1-x}Co_{x}As}$ and $mathrm{NaFe_{1-x}Cu_{x}As}$. In NaFeAs, the reduced height of the As ions induces a local magnetic moment higher than $mathrm{Ba_2As_2}$, despite lower T$_N$ and ordered magnetic moment. As NaFeAs is doped with Co $mu_{bare}$ is slightly reduced, whereas Cu doping leaves it unaffected, indicating a different doping mechanism: based on electron counting for Co whereas impurity scattering dominates in the case of Cu. Finally, we observe an increase of $mu_{bare}$ with temperature in all samples as observed in electron- and hole-doped $mathrm{BaFe_2As_2}$. Since both Co and Cu doping display superconductivity, our findings demonstrate that the formation of Cooper pairs is not connected with the complete loss of fluctuating paramagnetic moments.
We present a numerical study of the doping dependence of the spectral function of the n-type cuprates. Using a variational cluster-perturbation theory approach based upon the self-energy-functional theory, the spectral function of the electron-doped two-dimensional Hubbard model is calculated. The model includes the next-nearest neighbor electronic hopping amplitude $t$ and a fixed on-site interaction $U=8t$ at half filling and doping levels ranging from $x=0.077$ to $x=0.20$. Our results support the fact that a comprehensive description of the single-particle spectrum of electron-doped cuprates requires a proper treatment of strong electronic correlations. In contrast to previous weak-coupling approaches, we obtain a consistent description of the ARPES experiments without the need to introduce a doping-dependent on-site interaction $U$.
We have performed high-resolution angle-resolved photoemission spectroscopy on heavily overdoped KFe_2As_2 (transition temperature (Tc = 3 K). We observed several renormalized bands near the Fermi level with a renormalization factor of 2-4. While the Fermi surface (FS) around the Brillouin-zone center is qualitatively similar to that of optimally-doped Ba_{1-x}K_xFe_2As_2 (x = 0.4; Tc = 37 K), the FS topology around the zone corner (M point) is markedly different: the two electron FS pockets are completely absent due to excess of hole doping. This result indicates that the electronic states around the M point play an important role in the high-Tc superconductivity of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ and suggests that the interband scattering via the antiferromagnetic wave vector essentially controls the Tc value in the overdoped region.
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