Frank Steckel

Ba 2 YIrO 6 : A cubic double perovskite material with Ir 5 + ions

Materials with a 5d4 electronic configuration are generally considered to have a nonmagnetic ground state (J=0). Interestingly, Sr2YIrO6 (Ir5+ having 5d4 electronic configuration) was recently reported to exhibit long-range magnetic order at low temperature and the distorted IrO6 octahedra were discussed to cause the magnetism in this material. Hence, a comparison of structurally distorted Sr2YIrO6 with cubic Ba2YIrO6 may shed light on the source of magnetism in such Ir5+ materials with 5d4 configuration. Besides, Ir5+ materials having 5d4 are also interesting in the context of recently predicted excitonic types of magnetism. Here we report a single-crystal-based analysis of the structural, magnetic, and thermodynamic properties of Ba2YIrO6. We observe that in Ba2YIrO6 for temperatures down to 0.4 K, long-range magnetic order is absent but at the same time correlated magnetic moments are present. We show that these moments are absent in fully relativistic ab initio band-structure calculations; hence, their origin is presently unclear.

Spin density wave order and fluctuations in Mn3Si: a transport study

We present a comprehensive transport investigation of the itinerant antiferromagnet Mn3Si which undergoes a spin density wave (SDW) order below TN � 21.3 K. The electrical resistivity, the Hall-, Seebeck and Nernst effects exhibit pronounced anomalies at the SDW transition, while the heat conductivity is phonon dominated and therefore is insensitive to the intrinsic electronic ordering in this compound. At temperatures higher than TN our data provide strong evidence for a large fluctuation regime which extends up to � 200 K in the resistivity, the Seebeck effect and the Nernst effect. From the comparison of our results with other prototype SDW materials, viz. LaFeAsO and Chromium, we conclude that many of the observed features are of generic character.

Combined resistivity and Hall effect study on NaFe1−xRhxAs single crystals

Electrical transport measurements are used to study the Rh-doped NaFeAs superconductor series with a focus on the tetragonal phase. The resistivity curvature has an anomalous temperature dependence evidencing in the phase diagram two crossover regions of changes in the scattering rate, the effective mass as well as of the charge carrier density. The first crossover region is directly connected to the structural transition and resembles the onset of resistivity anisotropy. The second crossover region can as well be deduced from the temperature dependent Hall coefficient. A comparison to literature NMR data suggests this region to be connected with nematic fluctuations far above the tetragonal to orthorhombic phase transition.

Crystal growth and the electronic phase diagram of the 4

Single crystals of Na

d

_{1-\delta}

doped Na

Fe

_{1-\delta}

_{1-x}

Fe

T

_{1-x}

_x

Rh

As with T = Co, Rh have been grown using a self-flux technique. The crystals were thoroughly characterized by powder X-ray diffraction, magnetic susceptibility and electronic transport with particular focus on the Rh-doped samples. Measurements of the specific heat and ARPES were conducted exemplarily for the optimally doped compositions. The spin-density wave transition (SDW) observed for samples with low Rh concentration (