Superconductivity in gallium-substituted Ba8Si46 clathrates
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We report a joint experimental and theoretical investigation of superconductivity in Ga-substituted type-I silicon clathrates. We prepared samples of the general formula Ba8Si46−xGax, with different values of x. We show that Ba8Si40Ga6 is a bulk superconductor, with an onset at TC≈3.3K. For x=10 and higher, no superconductivity was observed down to T=1.8K. This represents a strong suppression of superconductivity with increasing Ga content, compared to Ba8Si46 with TC≈8K. 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 sp3-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 Ba8Si40Ga6 to be a type-II superconductor. The critical magnetic fields were measured to be HC1≈35Oe and HC2≈8.5kOe. We deduce the London penetration depth λ≈3700Å and the coherence length ξc≈200Å. Our estimate of the electron-phonon coupling reveals that Ba8Si40Ga6 is a moderate phonon-mediated BCS superconductor.