Superconductivity in gallium-substitutedBa8Si46clathrates

Abstract

We report a joint experimental and theoretical investigation of superconductivity in Ga-substituted type-I silicon clathrates. We prepared samples of the general formula ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{46\ensuremath{-}x}{\mathrm{Ga}}_{x}$, with different values of $x$. We show that ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{40}{\mathrm{Ga}}_{6}$ is a bulk superconductor, with an onset at ${T}_{C}\ensuremath{\approx}3.3\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. For $x=10$ and higher, no superconductivity was observed down to $T=1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This represents a strong suppression of superconductivity with increasing Ga content, compared to ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{46}$ with ${T}_{C}\ensuremath{\approx}8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Suppression of superconductivity can be attributed primarily to a decrease in the density of states at the Fermi level, caused by a reduced integrity of the $s{p}^{3}$-hybridized networks as well as the lowering of carrier concentration. These results are corroborated by first-principles calculations, which show that Ga substitution results in a large decrease of the electronic density of states at the Fermi level, which explains the decreased superconducting critical temperature within the BCS framework. To further characterize the superconducting state, we carried out magnetic measurements showing ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{40}{\mathrm{Ga}}_{6}$ to be a type-II superconductor. The critical magnetic fields were measured to be ${H}_{C1}\ensuremath{\approx}35\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}$ and ${H}_{C2}\ensuremath{\approx}8.5\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$. We deduce the London penetration depth $\ensuremath{\lambda}\ensuremath{\approx}3700\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and the coherence length ${\ensuremath{\xi}}_{c}\ensuremath{\approx}200\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. Our estimate of the electron-phonon coupling reveals that ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{40}{\mathrm{Ga}}_{6}$ is a moderate phonon-mediated BCS superconductor.