Field-induced transition within the superconducting state of CeRh2As2.

Materials with multiple superconducting phases are rare. Here, we report the discovery of two-phase unconventional superconductivity in CeRh2As2 Using thermodynamic probes, we establish that the superconducting critical field of its high-field phase is as high as 14 tesla, even though the transition temperature is only 0.26 kelvin. Furthermore, a transition between two different superconducting phases is observed in a c axis magnetic field. Local inversion-symmetry breaking at the cerium sites enables Rashba spin-orbit coupling alternating between the cerium sublayers. The staggered Rashba coupling introduces a layer degree of freedom to which the field-induced transition and high critical field seen in experiment are likely related.

EuTM(2)Ga(8) (TM = Co, Rh, Ir) - a contribution to the chemistry of the CeFe(2)Al(8)-type compounds.

The isostructural compounds EuTM(2)Ga(8) (TM = Co, Rh, Ir) were prepared by direct reaction of the elements by high-frequency thermal treatment. All three phases are isotypic with CeFe(2)Al(8) (space group Pbam, Pearson symbol oP44, Z = 4). The crystal structure was established from single-crystal X-ray diffraction data: a = 12.4322(7) A, b = 14.3814(9) A, and c = 4.0378(2) A for EuCo(2)Ga(8); a = 12.6001(6) A, b = 14.6757(7) A, and c = 4.1172(2) A for EuRh(2)Ga(8); and a = 12.6237(7) A, b = 14.6978(8) A, and c = 4.1486(2) A for EuIr(2)Ga(8), respectively. Analysis of the chemical bonding in EuRh(2)Ga(8) with the electron localizability tools reveals formation of the 3D [Rh(2)Ga(8)] polyanion build by polar covalent bonds. Europium interacts in two ways with the polyanion: mainly as a cation by charge transfer and additionally covalently by means of the electrons of the inner shells. Magnetic susceptibility measurements show Curie-Weiss paramagnetic behavior above 40 K with effective magnetic moments of 7.81, 8.05, and 8.27 micro(B)/f.u. for EuTM(2)Ga(8) (TM = Co, Rh, Ir). Antiferromagnetic ordering of Eu moments is observed in all three compounds below 20 K. Independently on the chemical composition of the coordination sphere, magnetic behavior and, especially, X-ray absorption spectra indicate predominantly the 4f(7) electronic configuration of europium with small admixture of the 4f(6) state.

Structural stability and thermoelectric performance of high quality synthetic and natural pyrites (FeS2).

Synthetic bulk and natural pyrite from the hydrothermal mine in Schönbrunn (Saxony, Germany) are confirmed to be stoichiometric FeS2 compounds and stable (for thermoelectric applications) up to ∼600 K by combined thermal, chemical, spectroscopic and X-ray diffraction analyses. Natural pyrite with a small amount (<0.6 wt%) of well-defined transition metal carbonates revealed characteristics of a nondegenerate semiconductor and is suitable as a model system for the investigation of thermoelectric performance. In the temperature range 50-600 K both natural and synthetic high quality bulk FeS2 samples show electrical resistivity and Seebeck coefficients varying within 220-5 × 10-3 Ω m and 4 - (-450) µV K-1, respectively. The large thermal conductivity (∼40 W m-1 K-1 at 300 K) is exclusively due to phononic contribution, showing a well pronounced maximum centered at ∼75 K for natural pyrite (grain size ≤5 mm). It becomes almost completely suppressed in the sintered bulk samples due to the increase of point defect concentration and additional scattering on the grain boundaries (grain size ≤100 µm). The thermoelectric performance of pure pyrite with ZT ∼ 10-6 at 600 K is indeed by a factor of ∼1000 worse than those reported earlier for some minerals and synthetic samples.

Disorder in the composite crystal structure of the manganese `disilicide' MnSi1.73 from powder X-ray diffraction data.

The crystal structure of the higher manganese silicide MnSi1.7 (known in the literature as HMS) is investigated in samples with different compositions obtained by different techniques at temperatures not higher than 1273 K. Powder X-ray diffraction was applied. The crystal structure is described as incommensurate composite. In addition to the ordered model already known in the literature, the partial disorder in the silicon substructure was detected and described introducing an additional atomic site with a different modulation function.

Structural investigations on YbRh2Si2: from the atomic to the macroscopic length scale.

YbRh2Si2 has advanced to a prototype material for investigating physics related to the Kondo effect. An optimization of the synthesis resulted in single crystals of extraordinary crystalline quality. At the atomic scale, we utilize scanning tunneling microscopy to study the topography of cleaved single crystals. A structural and chemical analysis was conducted by highly accurate x-ray diffraction and wavelength dispersive x-ray spectroscopy measurements. The latter indicate a homogeneity range of the YbRh2Si2 phase between approximately 40.0­40.2 at.% Rh. For our high-quality samples the number of defects found on the atomic scale (of the order of 0.3% of the visible lattice sites) is in quantitative agreement with a very small off-stoichiometry within this homogeneity range. Comparing our results for these samples allows an assignment of the structural defects observed at the cleaved surfaces to Rh occupying Si sites and, even less numerous Si in Rh sites. Such an analysis is hampered for samples of lesser quality, but there seem to be numerous empty Si-sites. Based on these observations the results of scanning tunneling spectroscopy can be analyzed in further detail and provide insight into the Kondo physics.