Working Group Electron Spectroscopy

X-Ray Diffraction (M. Merz)

For the characterization of samples of crystalline materials, IQMT operates an in-house x-ray laboratory with a number of x-ray diffractometers :

• An imaging plate single-crystal diffractometer STOE IPDS 2T allows for a very accurate and detailed determination of lattice parameters, atomic positions, bond lengths or structural/nematic phase transitions in the temperature range between 80 K and 400 K.
• In a further imaging plate system equipped with a Heatstream, single crystals can be investigated at temperatures up to 1100 K. The system is operated in collaboration with INT.
• A fully motorized PHOTONIC SCIENCE Laue diffraction system with backscattering geometry is used for high-precision adjustment (~ 0.1°) of single crystals.
• For transmission measurements in pseudo-Guinier geometry with pure Mo-K_α,1 radiation, a STOE STADI P powder diffractometer with a Ge(111) monochromator on the primary side and an area-sensitive detector is available.
• Moreover, a modular rotating anode system RIGAKU Smartlab 9 kW can be operated with Cu as well as Mo radiation for the measurement of powder and film samples. For high-resolution measurements in Bragg-Brentano geometry the system is equipped with a Ge(111) Johansson monochromator. Furthermore, all optical components (parabolic as well as elliptical x-ray mirrors and double monochromators) for operation in high-resolution parallel-beam or focusing geometry are available. Flat samples can be measured down to temperatures of ~ 10 K, flat and capillary samples up to ~ 1500 K. A sample changer allows for the measurement of up to 10 samples in a row. An Eulerian cradle can be used for the measurement of thin films. Depending on the application an area-sensitive Si-HPC- or a Si-strip detector is employed.

Upper left: STOE Imaging Plate System IPDS 2T for the measurement of single crystals between 80 K and 400 K. Lower left: PHOTONIC SCIENCE Laue device for high-precision alignment of single crystals. Right: Diffraction patterns recorded by these devices.

High-resolution RIGAKU Smartlab 9 kW for the measurement of powder samples in the temperature range ~ 10 to 1500 K with superimposed recorded diffraction pattern. Lower left: STOE Stadi-P for transmission measurement of powder samples.

Cobaltates

For many transition-metal oxides, the intricate interplay between charge, spin (up or down), orbital, and lattice degrees of freedom leads to interesting and unusual electronic and magnetic phenomena such as high-temperature superconductivity, colossal magnetoresistance, and complex magnetic orbital ordering. In the case of the cobaltates, the spin state is an additional degree of freedom depending on the delicate balance between the crystal-field splitting, i.e., the energetic splitting between t_2g and e_g orbitals, and the exchange interaction associated with Hund’s rule coupling.

Comparison of the experimental Co L2,3 NEXAFS spectra of La2CoO4, La1.5Ca0.5CoO4, LaCaCoO4, and La0.5Ca1.5CoO4 taken at 300 K (left) with the results of Co 2p XAS multiplet calculations of Co for different valence and spin states (lower curves in the right panel; LS: low spin, IS: intermediate spin, HS: high spin): Co3+ LS, Co3+ HS, Co3+ IS, Co2+ HS, and Co4+ HS. Simulated spectra of La2CoO4, La1.5Ca0.5CoO4, LaCaCoO4, and La0.5Ca1.5CoO4 are shown in the upper part of the right panel. For clarity, the spectra for the different configurations are offset vertically.

Manganites

The charge ordering (CO) phenomena in colossal magneto resistance (CMR) materials occurs in perovskite manganese oxides in which the on-site Coulomb interaction is stronger than the kinetic energy of the charge carriers. The antiferromagnetic insulating (AFMI)-ferromagnetic metallic (FMM) transition induced by charge carrier doping in many of the CMR compounds is suppressed by the formation of CO, especially when the carrier concentration equals a commensurate fraction such as 1/8, 1/3 or 1/2. This CO state is intimately related to the CMR properties.

Fe-based Superconductors

Superconductivity in iron-based materials emerges - as in heavy-fermion systems and high-T_c cuprates - in the vicinity of a magnetic instability. Edge-sharing Fe(As/Se)₄ tetrahedra are the structural key ingredient. Many studies strongly suggest that distinct nesting properties of the Fermi surfaces are important for the magnetic properties (development of a spin density wave and antiferromagnetic order at low temperature) as well as for the superconducting characteristics.
Our investigation shows that charge carrier doping of the Fe 3d states is not crucial for superconductivity in Co-doped Fe pnictides. Rather, the change of the topology of the Fermi surface induced by the Co-substitution seems to be the key parameter.

Comparison of the (a) normal- and (b) grazing-incidence Fe L2,3 NEXAFS spectra of Sr(Fe1-xCox)2As2 (x = 0, 0.05, 0.11, 0.17, and 0.38; for clarity, the spectra are vertically offset) recorded at 300 K. The spectral shape of both edges is unaffected upon doping. The multiplet calculations show that the spectra can be described reasonably well for tetrahedrally coordinated Fe2+. In addition, the spectrum of a deteriorated iron-oxide-containing sample is included in (a) (topmost panel), thereby exhibiting the respective peak positions of the iron oxide.

Collaborations with External Groups

Within the allocation of ANKA beam time to external users we collaborate with many external groups who perform experiments at WERA. Many projects in the field of magnetism and superconductivity are of common interest.