Surface adatom conductance filtering in scanning tunneling spectroscopy of co-doped BaFe2As2 iron pnictide superconductors.

We establish in a combination of ab initio theory and experiments that the tunneling process in scanning tunneling microscopy or spectroscopy on the A-122 iron pnictide superconductors-in this case BaFe(2-x)Co(x)As(2)-involves a strong adatom filtering of the differential conductance from the near-E(F) Fe-3d states, which in turn originates from the topmost subsurface Fe layer of the crystal. The calculations show that the dominance of surface Ba-related tunneling pathways leaves fingerprints found in the experimental differential conductance data, including large particle-hole asymmetry and energy-dependent contrast inversion in conductance maps.

Magnetic domain fluctuations in an antiferromagnetic film observed with coherent resonant soft x-ray scattering.

We report the direct observation of slow fluctuations of helical antiferromagnetic domains in an ultrathin holmium film using coherent resonant magnetic x-ray scattering. We observe a gradual increase of the fluctuations in the speckle pattern with increasing temperature, while at the same time a static contribution to the speckle pattern remains. This finding indicates that domain-wall fluctuations occur over a large range of time scales. We ascribe this nonergodic behavior to the strong dependence of the fluctuation rate on the local thickness of the film.

Soft X-ray Emission Spectroscopy at ESRF Beamline 26 Based on a Helical Undulator.

A new soft X-ray spectrograph for X-ray emission spectroscopy excited by synchrotron radiation is presented. The apparatus is now installed on beamline 26 at the ESRF in Grenoble. A brief description of the beamline is given and then several components of the spectrograph are covered in more detail. Results of experiments performed both with direct non-monochromated undulator radiation and with monochromated radiation are reported.

Magnetization profile of ultrathin FePd films.

The method of circular dichroism in X-ray resonant magnetic scattering is presented which allows a straightforward determination of the magnetization profile of magnetic patterns in ultrathin films. Application to single crystalline FePd layers shows unambiguously the presence of magnetic flux closure domains whose thickness can constitute a significant fraction ( approximately 25%) of the total film.