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Photoemission data taken with hard x-ray radiation on cleaved single crystals of the barium parent compound of the MFe2As2 pnictide high-temperature superconductor family are presented. Making use of the increased bulk sensitivity upon hard x-ray excitation, and comparing the results to data taken at conventional vacuum ultraviolet photoemission excitation energies, it is shown that the BaFe2As2 cleavage surface provides an electrostatic environment that is slightly different to the bulk, most likely in the form of a modified Madelung potential. However, as the data argue against a different surface doping level, and the surface-related features in the spectra are by no means as dominating as seen in systems such as YBa2Cu3Ox, we can conclude that the itinerant, near-EF electronic states are almost unaffected by the existence of the cleavage surface. Furthermore, exploiting the strong changes in photoionization cross section between the Fe and As states across the wide photon energy range employed, it is shown that the degree of energetic overlap between the iron 3d and arsenic 4p valence bands is particularly small at the Fermi level, which can only mean a very low degree of hybridization between the Fe 3d and As 4p states near and at EF. Consequently, this means that the itinerancy of the charge carriers in this group of materials involves mainly the Fe 3d-Fe 3d overlap integrals with at best a minor role for the Fe 3d-As 4p hopping parameters and that the states which support superconductivity upon doping are essentially of Fe 3d character.

We present tunneling data from superconducting BaFe1.86Co0.14As2 and its parent compound, BaFe2As2. In the superconductor, clear coherencelike peaks are seen across the whole field of view, and their analysis reveals nanoscale variations in the superconducting gap value, Δ. The average peak-to-peak separation gives a 2Δ of ∼7.4kBTc, which exceeds the BCS weak coupling value for either s- or d-wave superconductivity. The characteristic length scales of the deviations from the average gap value and of anticorrelations observed between the gap magnitude and both the zero-bias conductance and coherence peak strength match well with the average separation between the Co dopant ions in the superconducting FeAs planes.

Photoemission data taken with hard x-ray radiation on cleaved single crystals of the bilayered, colossal magnetoresistant manganite La2−2xSr1+2xMn2O7 (LSMO) with 0.30

We elucidate the termination surface of cleaved single crystals of the BaFe2−xCoxAs2 and FeySe1−xTex families of the high-temperature iron-based superconductors. By combining scanning tunneling microscopic data with low-energy electron diffraction we prove that the termination layer of the BaFe2As2 systems is a remnant of the Ba layer, which exhibits a complex diversity of ordered and disordered structures. The observed surface topographies and their accompanying superstructure reflections in electron diffraction depend on the cleavage temperature. In stark contrast, FeySe1−xTex possesses only a single termination structure—that of the tetragonally ordered Se1−xTex layer.

We report the measurement of the local Ni 3d spin polarization, not only below but also above the Curie temperature (T-C), using the newly developed spin-resolved circularly polarized 2p (L-3) resonant photoemission technique. The experiment identifies the presence of 3d(8) singlets at high energies and 3d(8) triplets at low energies extending all the way to the Fermi energy, both below and above T-C, showing that it is the orbital degeneracy of the 3d band and the Hund's rule splitting which is of utmost importance to understand Ni and other 3d ferromagnets.

Contains fulltext : mmubn000001_086249649.pdf (Publisher’s version ) (Open Access) Promotores : J. Fuggle en G. Sawatzky 157 p.