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Register a new userCore Level and Valence Band Study of LaO$_{0.9}$F$_{0.1}$FeAs
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Using angle-integrated photoemission spectroscopy we have probed the novel LaO$_{0.9}$F$_{0.1}$FeAs superconductor over a wide range of photon energies and temperatures. We have provided the first full characterization of the orbital character of the VB DOS and of the magnitude of the d-p hybridization energy. Finally, we have identified two characteristic temperatures: 90K where a pseudogap-like feature appears to close and 120K where a sudden change in the DOS near E$_F$ occurs. We associate these phenomena with the SDW magnetic ordering and the structural transition seen in the parent compound, respectively. These results suggest the important role of electron correlation, spin physics and structural distortion in the physics of Fe-based superconductors.
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We have performed 75As Nuclear Magnetic Resonance (NMR) measurements on aligned powders of the new LaO0.9F0.1FeAs superconductor. In the normal state, we find a strong temperature dependence of the spin shift and Korringa behavior of the spin lattice relaxation rate. In the superconducting state, we find evidence for line nodes in the superconducting gap and spin-singlet pairing. Our measurements reveal a strong anisotropy of the spin lattice relaxation rate, which suggest that superconducting vortices contribute to the relaxation rate when the field is parallel to the c-axis but not for the perpendicular direction.
We have employed a new route to synthesize single phase F-doped LaOFeAs compound and confirmed the superconductivity above 20 K in this Fe-based system. We show that the new superconductor has a rather high upper critical field of about 54 T. A clear signature of superconducting gap opening below T$_c$ was observed in the far-infrared reflectance spectra, with 2$Delta/textit{k}T_capprox$3.5-4.2. Furthermore, we show that the new superconductor has electron-type conducting carrier with a rather low carrier density.
We present point-contact spectroscopy data for junctions between a normal metal and the newly discovered F-doped superconductor LaO$_{0.9}$F$_{0.1-delta}$FeAs (F-LaOFeAs). A zero-bias conductance peak was observed and its shape and magnitude suggests the presence of Andreev bound states at the surface of F-LaOFeAs, which provides a possible evidence of an unconventional pairing symmetry with a nodal gap function. The maximum gap value $Delta_0approx3.9pm0.7$meV was determined from the measured spectra, in good agreement with the recent experiments on specific heat and lower critical field.
We have studied the newly found superconductor compound LaO$_{1-x}$F$_x$FeAs through the first-principles density functional theory calculations. We find that the parent compound LaOFeAs is a quasi-2-dimensional antiferromgnetic semimetal with most carriers being electrons and with a magnetic moment of $2.3mu_B$ located around each Fe atom on the Fe-Fe square lattice. Furthermore this is a commensurate antiferromagnetic spin density wave due to the Fermi surface nesting, which is robust against the F-doping. The observed superconduction happens on the Fe-Fe antiferromagnetic layer, suggesting a new superconductivity mechanism, mediated by the spin fluctuations. An abrupt change on the Hall measurement is further predicted for the parent compound LaOFeAs.
We have re-examined the valence-band (VB) and core-level electronic structure of NiO by means of hard and soft x-ray photoemission spectroscopy (PES). The spectral weight of the lowest energy state found to be enhanced in the bulk sensitive Ni 2p core-level PES. A configuration-interaction model including the bound state screening has shown significant agreement with the core-level spectra, and the off and on-resonance VB spectra. These results identify the lowest energy state in core-level and VB-PES as the Zhang-Rice doublet bound state, consistent with the spin-fermion model and recent ab initio calculation with dynamical mean-field theory (LDA + DMFT).
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