Superconductivity and Magnetism in Silicon and Germanium Clathrates

Abstract

Clathrates are materials containing closed polyhedral cages stacked to form crystalline frameworks. With Si, Ge, and Sn atoms populating these frameworks, a wide variety of electronic and vibrational properties can be produced in these materials, by substitution upon framework sites or through incorporation of ions in cage-center positions. Commonly-formed structures include the type I, type II, and chiral clathrate types, whose properties will be described here. Ba 8 Si 46 with the type

NMR study of Ba8Cu5SixGe41−x clathrate semiconductors

“…The synthesis involved melting the pure-elements, followed by annealing and hot pressing. Powder X-ray diffraction (XRD) and analysis provided full structural details of the samples, and dispersive X-ray spectroscopy was measured to determine the compositions of the samples (nominal x = 0, 3,6,10,13,18,25,35,41) ) and thereby lead to reduced carrier concentration. The previous results also showed that, while the lattice constants of the compounds decease linearly with increasing Si content, some internal parameters such as the Si occupation at the 24k site show nonlinear behavior.…”

“…6,[8][9][10][11] There have been many efforts to study Ba 8 Ga 16 Ge 30 and similar ternary semiconducting clathrates since they can have large thermoelectric figures of merit, [3][4][5] ZT E 1.…”

“…1,2 Among them intermetallic clathrates have gained great attention due to high thermoelectric efficiency [3][4][5][6][7] and other interesting physical properties such as glass-like thermal conductivity, superconductivity, and magnetism. 6,[8][9][10][11] There have been many efforts to study Ba 8 Ga 16 Ge 30 and similar ternary semiconducting clathrates since they can have large thermoelectric figures of merit, [3][4][5] ZT E 1.…”

“…NMR shifts can be particularly sensitive to hybridization effects in the conduction and valence bands, and we find that with x increasing through the x = 18 composition there is very little change in such behavior, however approaching Ba 8 Cu 5 Si 41 the behavior changes rapidly as the framework becomes more metallic as observed both by Cu and Ba NMR. compositions of the samples (nominal x = 0, 3,6,10,13,18,25,35,41) as reported in ref. 16.…”

Magnetic moment of Eu-dopant and superconductivity in Eu-substitutional Ba8Al5Si41 clathrates

“…Many research groups are engaged in the research and development of this superconducting material. Any substitutions of Si on the framework or Ba in the center of the cage can profoundly affect superconductivity [11][12]. The destructive effect on superconductivity generally comes from several reasons: (i) the decrease in density of state at Fermi-level (N(E F )), (ii) a lowering of the carrier concentration, (iii) a breakage of the integrity of the sp 3 -hybridized network, (iv) the magnetic breakdown of paired electrons, and (v) the red shift of phonon dispersion spectra, which resulting in the depression of electron-phonon coupling.…”