J.B. Goedkoop

Magnetic moment of Mn in the ferromagnetic semiconductor (Ga0.98Mn0.02)As

We have studied the quasibinary ferromagnetic semiconductor (Ga0.98Mn0.02)As by magnetic circular dichroism in x-ray absorption. We find a richly structured Mn absorption spectrum typical for localized 3d electrons. An analysis of the magnetization-averaged and dichroism line shapes shows a local Mn moment of 4.6μB, which is close to the Hund’s rule moment for the half-filled 3d shell. The magnitude of the dichroism reveals that only about 1/7 of the Mn atoms participate in the ferromagnetic order. Our experiment does not show a distinction between the ferro- and paramagnetic Mn atoms.

Instrumentation development for ESRF beamlines

Abstract We report on instrumentation developments concerning a subset of ESRF beamlines: BL6 & 26 (linear, circular dichroism); BL8 (energy dispersive XAFS); BL18 (multipurpose energy scanned XAFS studies). We review in more detail the instrumentation of BL6 & 26 which are now under commissioning. A 1 4 wave plate was successfully used at 2.8 keV in the direct undulator beam. High-quality CMXD spectra have already been recorded in the fluorescence detection mode.

Nanoscale superconducting-gap variations and lack of phase separation in optimally doped BaFe1.86Co0.14As2

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.

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.

Lattice of microtraps for ultracold atoms based on patterned magnetic films

We have realized a two-dimensional permanent magnetic lattice of Ioffe-Pritchard microtraps for ultracold atoms. The lattice is formed by a single 300-nm magnetized layer of FePt, patterned using optical lithography. Our magnetic lattice consists of more than 15 000 tightly confining microtraps with a density of 1250 traps/mm{sup 2}. Simple analytical approximations for the magnetic fields produced by the lattice are used to derive relevant trap parameters. We load ultracold atoms into at least 30 lattice sites at a distance of approximately 10 {mu}m from the film surface. The present result is an important first step toward quantum information processing with neutral atoms in magnetic lattice potentials.

Existence, character and origin of surface-related bands in the high temperature iron pnictide superconductor BaFe2-xCoxAs2

Low energy electron diffraction (LEED) experiments, LEED simulations, and finite slab density functional calculations are combined to study the cleavage surface of Co doped BaFe(2-x)Co(x)As2 (x = 0.1,0.17). We demonstrate that the energy dependence of the LEED data can only be understood from a terminating 1/2 Ba layer accompanied by distortions of the underlying As-Fe2-As block. As a result, surface-related Fe 3d states are present in the electronic structure, which we identify in angle resolved photoemission spectroscopy (ARPES) experiments. The close proximity of the surface-related states to the bulk bands inevitably leads to broadening of the ARPES signals, which excludes the use of the BaFe(2-x)Co(x)As2 system for accurate determination of self-energies using ARPES.

Cleavage surfaces of the BaFe2−xCoxAs2 and FeySe1−xTex superconductors: A combined STM plus LEED study

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 Ba122 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.

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.

High-resolution, hard x-ray photoemission investigation of BaFe2As2: Moderate influence of the surface and evidence for a low degree of Fe 3d-As 4p hybridization of electronic states near the Fermi energy

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.

Magnetic properties of chromium doped rare earth manganites Ln0.5Ca0.5Mn1−xCrxO3 (Ln=Pr, Nd, Sm and 0.05⩽x⩽0.10) by soft x-ray magnetic circular dichroism

Soft-x-ray magnetic circular dichroism (SXMCD) at Mn, Cr L2,3 and Ln M4,5 edges of Ln0.5Ca0.5Mn1−xCrxO3 (Ln=Pr, Nd, Sm and 0.05⩽x⩽0.10) bulk polycrystalline samples have been performed at T=20 K below the ferromagnetic Curie temperature. We show the existence of magnetic sublattice on each of the probed cations. Chromium cations order at low temperature antiparallel to the manganese subnetwork and rare earth cations likely exhibit a sperimagnetic ordering. These results are compared with magnetization measurements and a tentative correlation with magnetoresistance properties is discussed. This work also demonstrates that SXMCD can probe element and site specific magnetic properties of multicomponent systems.