Andreas Leithe-Jasper

Superconducting state in SrFe2-xCoxAs2 by internal doping of the iron arsenide layers.

In the strontium iron-cobalt arsenides SrFe2-xCoxAs2 (0.2 < or = x < or = 0.4) superconductivity with T_{c} up to 20 K is observed in magnetic susceptibility, electrical resistivity, and specific heat data. This first observation of bulk superconductivity induced by electron doping in this family of compounds-despite strong disorder in the Fe-As layer-favors an itinerant electronic theory in contrast to the strongly correlated cuprates and renders a p- or d-wave pairing unlikely. The magnetic ordering present in SrFe2As2 is rapidly suppressed by substitution of Fe by Co. Density functional theory calculations show that this is due to a rigid downshift of the Fe-3d_{x;{2}-y;{2}}-related band edge in the density of states.

Raman effect in icosahedral boron-rich solids

Abstract We present Raman spectra of numerous icosahedral boron-rich solids having the structure of α-rhombohedral, β-rhombohedral, α-tetragonal, β-tetragonal, YB66, orthorhombic or amorphous boron. The spectra were newly measured and, in some cases, compared with reported data and discussed. We emphasize the importance of a high signal-to-noise ratio in the Raman spectra for detecting weak effects evoked by the modification of compounds, accommodation of interstitial atoms and other structural defects. Vibrations of the icosahedra, occurring in all the spectra, are interpreted using the description of modes in α-rhombohedral boron by Beckel et al. The Raman spectrum of boron carbide is largely clarified. Relative intra- and inter-icosahedral bonding forces are estimated for the different structural groups and for vanadium-doped β-rhombohedral boron. The validity of Badgers rule is demonstrated for the force constants of inter-icosahedral B–B bonds, whereas the agreement is less satisfactory for the intra-icosahedral B–B bonds.

AFe2As2 (A = Ca, Sr, Ba, Eu) and SrFe2-xTMxAs2 (TM = Mn, Co, Ni): crystal structure, charge doping, magnetism and superconductivity

The electronic structure and physical properties of the pnictide compound families REOFeAs (RE = La, Ce, Pr, Nd, Sm), AFe2As2 (A = Ca, Sr, Ba, Eu), LiFeAs and FeSe are quite similar. Here, we focus on the members of the AFe2As2 family whose sample composition, quality and single-crystal growth are more controllable compared with the other systems. Using first-principles band structure calculations, we focus on understanding the relationship between the crystal structure, charge doping and magnetism in AFe2As2 systems. We will elaborate on the tetragonal to orthorhombic structural distortion along with the associated magnetic order and anisotropy, the influence of doping on the A site and the Fe site and the changes in the electronic structure as a function of pressure. Experimentally, we investigate the substitution of Fe in SrFe2 xTMxAs2 by other 3d transition metals, TM = Mn, Co or Ni. In contrast to a partial substitution of Fe by Co or Ni (electron doping), a corresponding Mn partial substitution does not lead to the suppression of the antiferromagnetic order or the appearance of superconductivity. Most of the calculated properties agree well with the measured properties, but several of them are sensitive to the As z position. For a microscopic understanding of the electronic structure of this new family of superconductors, this structural feature related to the Fe-As interaction is crucial, but its correct ab initio treatment still remains an open question.

Superconductivity in the Platinum Germanides MPt4Ge12 (M = Rare-Earth or Alkaline-Earth Metal) with Filled Skutterudite Structure

New germanium-platinum compounds with the filled-skutterudite crystal structure were synthesized. The crystal structure and composition were investigated by x-ray diffraction and microprobe analysis. Magnetic susceptibility, specific heat, and electrical resistivity measurements evidence superconductivity in LaPt4Ge12 and PrPt4Ge12 below 8.3 K. The parameters of the normal and superconducting states were established. Strong coupling and a crystal electric field singlet ground state is found for the Pr compound. Electronic structure calculations show a large density of states at the Fermi level. Similar behavior with lower Tc was observed for SrPt4Ge12 and BaPt4Ge12.

Application of spark plasma sintering to the fabrication of binary and ternary skutterudites

Abstract Spark plasma sintering (SPS) synthesis of CoSb3 and LaFe4Sb12 from ball-milled elemental metal powders has been performed obtaining samples with high relative densities (∼99%). Fracture behaviour, the microstructure and mechanical properties were studied. From ultrasound and nano-indentation measurements, Young’s moduli of 148 GPa for CoSb3 and 141 GPa for LaFe4Sb12 were deduced. SPS has been also applied to compact alkali metal and alkaline-earth metal iron antimonides with filled skutterudite structure.

Magnetism of ternary compounds RE6Fe13X; RE=Pr, Nd; X=Cu, Ag, Au, Zn, Cd, and Hg

Novel compounds RE6Fe13X (X=Cu, Ag, Au, Zn, Cd, Hg) have been synthesized by arc melting (Cu, Ag, Au) or by reaction sintering (Zn, Cd, Hg), followed by heat treatment at 550 or 600 °C up to 350 h in evacuated silica capsules. From room temperature x‐ray powder diffraction analyses the alloys were found to crystallize with the ordered Nd6Fe13Si type. The Curie point of all the compounds is found to lie around room temperature. Due to their low magnetization values a ferrimagnetic coupling within the Fe sublattice is suggested. A canted RE‐spin structure cannot be ruled out. 57Fe Mossbauer spectra were recorded from the samples with X=Ag and Au. Additionally, 197Au spectra were taken from the two Au‐containing compounds. The 57Fe spectra are composed of various subpatterns, originating from the different Fe sites and were found to be essentially unaffected by the specific element RE and X. In all cases under investigation the spectra could only be fitted under the assumption of an easy axis of magnetizatio...

Magnetic and transport properties of rare-earth-based half-Heusler phases RPdBi: Prospective systems for topological quantum phenomena

RPdBi (R = Er, Ho, Gd, Dy, Y, Nd) compounds were studied by means of x-ray diffraction, magnetic susceptibility, electrical resistivity, magnetoresistivity, thermoelectric power and Hall effect measurements, performed in the temperature range 1.5-300 K and in magnetic fields up to 12 T. These ternaries, except diamagnetic YPdBi, exhibit localized magnetism of

Evidence for time-reversal symmetry breaking in superconducting PrPt4Ge12

R^{3+}

On surface Raman scattering and luminescence radiation in boron carbide

ions, and order antiferromagnetically at low temperatures (

Valence fluctuations of europium in the boride Eu4Pd29+xB8

T_{N}