Superconductivity in novel Ge-based skutterudites: {Sr,Ba}Pt_4Ge_{12}

Combining experiments and ab initio models we report on $\rm SrPt_4Ge_{12}$ and $\rm BaPt_4Ge_{12}$ as members of a novel class of superconducting skutterudites, where Sr or Ba atoms stabilize a framework entirely formed by Ge-atoms. Below $T_c=5.35$ K, and 5.10 K for $\rm BaPt_4Ge_{12}$ and $\rm SrPt_4Ge_{12}$, respectively, electron-phonon coupled superconductivity emerges, ascribed to intrinsic features of the Pt-Ge framework, where Ge-$p$ states dominate the electronic structure at the Fermi energy.Comment: 4 pages, 4 figures, accepted for publication in PR

Superconductivity in the complex metallic alloy phase ß-Al3Mg2

Transport and thermodynamic properties were studied for the complex metallic compound beta-Al3Mg2, composed of 925 atoms per unit cell. beta-Al3Mg2 exhibits bulk superconductivity below T-c=0.87 K. An exponential temperature dependence of the specific heat well below T-c indicates BCS-like behavior with a nodeless gap of width Delta(BCS)(0)approximate to 1.5 K. The coherence length is derived as xi(0)=4.85x10(-8) m and the Ginzburg Landau parameter kappa(GL)approximate to 13, characterizing beta-Al3Mg2 as a type II superconductor. Superconductivity in beta-Al3Mg2 occurs in the absence of inversion symmetry of the crystal. Surprisingly, in spite of the rather complex crystal structure of beta-Al3Mg2, physical properties turn out to be quite simple

Magnetic ordering in HoB12 below and above TN

We present results of neutron scattering experiments on frustrated antiferromagnet HoB12 above and below Ne el temperature TN. Diffuse scattering patterns indicate that above TN 7.4K pronounced correlations between neighboring magnetic moments of Ho ions appear, similar to one dimensional 1D magnets. Moreover, the diffuse scattering patterns show a symmetry reduction from fcc to simple cubic. Analogous behavior in three dimensional 3 D systems is not known, although it was predicted by theory. Results below TN and in applied magnetic field, on the other hand, reveal three amplitude modulated incommensurate magnetic structures in this compound. The role of various interactions leading to this behavior above and below TN is being discussed. Additional attention is paid also to r T resistivity dependencies in various magnetic fields close to and above the quantum critical point. PACS 72.15. v; 75.50.Ee; 78.90. t Keywords Rare earth compound; Magnetic structure; Neutron scattering; Electrical resistivit

Superconductivity in the complex metallic alloy beta-Al3Mg2

Based on low temperature electrical resistivity and specific heat measurements, we have shown that beta-Al3Mg2 undergoes a phase transition into a superconducting ground state at Tc=0.87 K. Microscopically, superconductivity can be understood in terms of the phonon-mediated BCS model. An exponential behavior of the specific heat well below Tc implies a nodeless superconducting gap in the electronic density of states, of the order of 1.6 K. The initial slope of the upper critical field is deduced to be about −0.2 T/K, while an extrapolation T→0 yields mu0Hc2 about 0.14 T. The limiting pair breaking mechanism seems to be orbital pair breaking, as concluded from the model of Werthamer et al. Superconductivity in beta-Al3Mg2 occurs in a crystal environment without inversion symmetry. Broken inversion symmetry has a distinct influence on the superconducting phase, which usually relies on the formation of pairs of electrons in degenerate states with opposite momentum. The availability of such states is normally guaranteed by time reversal and inversion symmetries. The absence of inversion symmetry would favor a strong antisymmetric spin-orbit coupling and, as a consequence, a mixture of spin-singlet and spintriplet pairs in the superconducting condensate can be expected. The small values of the upper critical field, however, seem to exclude a substantial portion of spin-triplet pairs in the condensate. Moreover, the lightweight elements Al and Mg may be responsible for only a minimal spin-orbit coupling in beta-Al3Mg2; hence, the spin-singlet condensate dominates. Additionally, the very complex crystal structure is supposed to smooth the effect of the missing inversion symmetry. A rather conventional superconductivity seems to appear, which also follows from the agreement of the upper critical field with Werthamer’s model. Presently, only a small number of superconductors without inversion symmetry have been found. Although the crystal structure of beta-Al3Mg2 appears to be rather complicated, the various physical quantities derived in both the superconducting and the normal state region turn out to be simple. In the first approximation, some of these quantities even look like a balanced superposition of pure Al and Mg. The latter follows from macroscopic measurements such as the specific heat and microscopic data like those derived from NMR as well

Ternary clathrates Ba-Zn-Ge: phase equilibria, crystal chemistry and physical properties

The formation, phase relations, crystal chemistry and physical properties were investigated for the solid solution Ba8ZnxGe46−x−yy deriving from binary clathrate Ba8Ge433 with a solubility limit of 8 Zn atoms per formula unit at 800 ◦C ( is a vacancy). Single-crystal x-ray data throughout the homogeneity region confirm the clathrate type I structure with cubic primitive space group type Pm¯3n. Temperature-dependent x-ray spectra as well as heat capacity define a lowlying, almost localized, phonon branch, whereas neutron spectroscopy indicates a phonon mode with significant correlations. The transport properties are strongly determined by the Ge/Zn ratio in the framework of the structure. Increasing Zn content drives the system towards a metal-to-insulator transition; for example, Ba8Zn2.1Ge41.52.4 shows metallic behaviour at low temperatures, whilst at high temperatures semiconducting features become obvious. A model based on a gap of the electronic density of states slightly above the Fermi energy was able to explain the temperature dependences of the transport properties. The thermal conductivity exhibits a pronounced low-temperature maximum, dominated by the lattice contribution, while at higher temperatures the electronic part gains weight. Zn-rich compositions reveal attractive Seebeck coefficients approaching −180 μV K−1 at 700 K

Superconductivity and magnetism in MPt4Ge12, M = Ca, Ba, Sr, Eu

X-ray powder data for Ba0.8Ca0.2Pt4Ge 12 and X-ray single crystal data for EuPt4Ge12 define cubic body-centered symmetry consistent with novel Ge-based skutterudites SrPt4Ge12 and BaPt4Ge12 (space group Im3). Structural and electron microprobe analysis investigations evidence a complete filling of the icosahedral cages without large atomic displacement parameters. Ba0.8Ca0.2Pt4Ge12 exhibits phonon-mediated superconductivity at Tc = 5.2 K. Density functional theory (DFT) calculations (LDA+U) carried out for EuPt 4Ge12 proved that Eu guest atoms strongly stabilize the compound in which the calculated density of states around the Fermi energy essentially consists of hybridized Ge 4p-like and Pt 5d-like states. Low temperature resistivity studies evidence magnetic ordering at Tm ? 1.7 K. Susceptibility measurements reveal a divalent state for europium, in excellent agreement with the DFT calculations