A wide variety of properties such as half-metallicity is found among Heusler compounds. In order to separate intrinsic and extrinsic properties, high quality single crystals are required. Here, we report on differently grown crystals of the half-metallic ferromagnet ${\text{Co}}_2\text{FeSi}$ . All crystals show excellent ordering, resulting in outstanding electrical behavior with low residual resistivity and high residual-resistivity-ratio. All ${\text{Co}}_2\text{FeSi}$ crystals show a plateau in the resistivity below 50 K, which might point to half-metallic ferromagnetism. The cross-over from this unusual to more conventional transport ($T^2$ dependence) around 50 K indicates the onset of spin flip scattering and thus is indispensable for understanding the strong temperature dependence of ${\text{Co}}_2\text{FeSi}$ tunneling magnetoresistance-devices.

Single crystals of Ba1$-${}xNaxFe2As2 with x=0, 0.25, 0.35, 0.4 were grown using a self-flux high temperature solution growth technique. The superconducting and normal state properties were studied by temperature dependent magnetic susceptibility, electrical resistivity, and specific heat, revealing that the magnetic and structural transition is rapidly suppressed upon Na substitution at the Ba site in BaFe2As2, giving rise to superconductivity. A superconducting transition as high as 34 K is reached for a Na content of x=0.4. The positive Hall coefficient confirms that the substitution of Ba by Na results in hole doping similar to the substitution of Ba by K. Angle resolved photoemission spectroscopy was performed on all Ba1$-${}xNaxFe2As2 crystals. The Fermi surface of hole-doped Ba1$-${}xNaxFe2As2 is to a large extent the same as the Fermi surface found for the K-doped sister compounds, suggesting a similar impact of the substitution of Ba by either K or Na on the~electronic band dispersion at the Fermi level.

${\text{Mn}}_2\text{NiGa}$ is reported to be a shape memory material with a martensite–austenite phase transition. Temperature dependent measurements of the transport and magnetic properties reveal the martensitic transition close to room temperature with a thermal hysteresis of about 27 K. The electronic structure of the valence band in both phases was studied by hard x-ray photoelectron spectroscopy. The results clearly indicate that strong changes in the electronic structure appear at the phase transition.

Angle-resolved photoemission spectroscopy is used to study the band dispersion and the quasiparticle scattering rates in two ferropnictide systems. We find the scattering rate for any given band to depend linearly on energy but to be independent of the control parameter. We demonstrate that the linear energy dependence gives rise to a weakly dispersing band with a strong mass enhancement when the band maximum crosses the chemical potential. The resulting small effective Fermi energy favors a BCS [J. Bardeen et al., Phys. Rev. 108, 1175 (1957)] –Bose-Einstein [S. N. Bose, Z. Phys. 26, 178 (1924)] crossover state in the superconducting phase.

Downsizing well-established materials to the nanoscale is a key route to novel functionalities, in particular if different functionalities are merged in hybrid nanomaterials. Hybrid carbon-based hierarchical nanostructures are particularly promising for electrochemical energy storage since they combine benefits of nanosize effects, enhanced electrical conductivity and integrity of bulk materials. We show that endohedral multiwalled carbon nanotubes (CNT) encapsulating high-capacity (here: conversion and alloying) electrode materials have a high potential for use in anode materials for lithium-ion batteries (LIB). There are two essential characteristics of filled CNT relevant for application in electrochemical energy storage: (1) rigid hollow cavities of the CNT provide upper limits for nanoparticles in their inner cavities which are both separated from the fillings of other CNT and protected against degradation. In particular, the CNT shells resist strong volume changes of encapsulates in response to electrochemical cycling, which in conventional conversion and alloying materials hinders application in energy storage devices. (2) Carbon mantles ensure electrical contact to the active material as they are unaffected by potential cracks of the encapsulate and form a stable conductive network in the electrode compound. Our studies confirm that encapsulates are electrochemically active and can achieve full theoretical reversible capacity. The results imply that encapsulating nanostructures inside CNT can provide a route to new high-performance nanocomposite anode materials for LIB.