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Register a new userElectronic structure of heavily electron-doped BaFe$_{1.7}$Co$_{0.3}$As$_2$ studied by angle-resolved photoemission
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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.
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We present a comprehensive study of the low-energy band structure and Fermi surface (FS) topology of $A$Co$_2$As$_2$ ($A=$ Ca, Sr, Ba, Eu) using high-resolution angle-resolved photoemission spectroscopy. The experimental FS topology and band dispersion data are compared with theoretical full-potential linearized augmented-plane-wave (FP-LAPW) calculations, which yielded reasonably good agreement. We demonstrate that the FS maps of $A$Co$_2$As$_2$ are significantly different from those of the parent compounds of Fe-based high-temperature superconductors. Further, the FSs of CaCo$_2$As$_2$ do not show significant changes across its antiferromagnetic transition temperature. The band dispersions extracted in different momentum $(k_{it x}, k_{it y})$ directions show a small electron pocket at the center and a large electron pocket at the corner of the Brillouin zone (BZ). The absence of the hole FS in these compounds does not allow nesting between pockets at the Fermi energy ({it E}$_{rm F}$), which is in contrast to $A$Fe$_2$As$_2$-type parent compounds of the iron-based superconductors. Interestingly, we find that the hole bands are moved 300--400~meV below $E_{rm F}$ depending on the $A$ element. Moreover, the existence of nearly flat bands in the vicinity of $E_{rm F}$ are consistent with the large density of states at $E_{rm F}$. These results are important to understand the physical properties as well as the possibility of the emergence of superconductivity in related materials.
We report high resolution angle-resolved photoemission spectroscopy (ARPES) studies of the electronic structure of BaFe$_2$As$_2$, which is one of the parent compounds of the Fe-pnictide superconductors. ARPES measurements have been performed at 20 K and 300 K, corresponding to the orthorhombic antiferromagnetic phase and the tetragonal paramagnetic phase, respectively. Photon energies between 30 and 175 eV and polarizations parallel and perpendicular to the scattering plane have been used. Measurements of the Fermi surface yield two hole pockets at the $Gamma$-point and an electron pocket at each of the X-points. The topology of the pockets has been concluded from the dispersion of the spectral weight as a function of binding energy. Changes in the spectral weight at the Fermi level upon variation of the polarization of the incident photons yield important information on the orbital character of the states near the Fermi level. No differences in the electronic structure between 20 and 300 K could be resolved. The results are compared with density functional theory band structure calculations for the tetragonal paramagnetic phase.
We performed polarization- and photon-energy-dependent angle-resolved photoemission spectroscopy of a slightly overdoped iron pnictide superconductor, BaFe$_{1.8}$Co$_{0.2}$As$_{2}$, to clarify the three-dimensional electronic structure including its orbital characters at the Brillouin zone center. Two hole Fermi surfaces (FSs) with $d_{xz/yz}$ and $d_{xy/x^2-y^2}$ orbitals were observed but $d_{z^2}$ hole FS, which has nodes according to a theory of the spin-fluctuation superconductivity mechanism, did not appear. These results suggest that no node will appear at hole FSs at the zone center.
The electronic structure of LaOFeAs, a parent compound of iron-arsenic superconductors, is studied by angleresolved photoemission spectroscopy. By examining its dependence on photon energy, polarization, sodium dosing and the counting of Fermi surface volume, both the bulk and the surface contributions are identified. We find that a bulk band moves toward high binding energies below structural transition, and shifts smoothly across the spin density wave transition by about 25 meV. Our data suggest the band reconstruction may play a crucial role in the spin density wave transition, and the structural transition is driven by the short range magnetic order. For the surface states, both the LaO-terminated and FeAs-terminated components are revealed. Certain small band shifts are verified for the FeAs-terminated surface states in the spin density wave state, which is a reflection of the bulk electronic structure reconstruction. Moreover, sharp quasiparticle peaks quickly rise at low temperatures, indicating of drastic reduction of the scattering rate. A kink structure in one of the surface band is shown to be possibly related to the electron-phonon interactions.
One of central issues in iron-based superconductors is the role of structural change to the superconducting transition temperature (T_c). It was found in FeSe that the lattice strain leads to a drastic increase in T_c, accompanied by suppression of nematic order. By angle-resolved photoemission spectroscopy on tensile- or compressive-strained and strain-free FeSe, we experimentally show that the in-plane strain causes a marked change in the energy overlap (DeltaE_{h-e}) between the hole and electron pockets in the normal state. The change in DeltaE_{h-e} modifies the Fermi-surface volume, leading to a change in T_c. Furthermore, the strength of nematicity is also found to be characterized by DeltaE_{h-e}. These results suggest that the key to understanding the phase diagram is the fermiology and interactions linked to the semimetallic band overlap.
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