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 Ba8 Si46-x Gax, with different values of x. We show that Ba8 Si40 Ga6 is a bulk superconductor, with an onset at TC 3.3 K. For x=10 and higher, no superconductivity was observed down to T=1.8 K. This represents a strong suppression of superconductivity with increasing Ga content, compared to Ba8 Si46 with TC 8 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 p3 -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 Ba8 Si40 Ga6 to be a type-II superconductor. The critical magnetic fields were measured to be HC1 35 Oe and HC2 8.5 kOe. We deduce the London penetration depth 3700 and the coherence length c 200 . Our estimate of the electron-phonon coupling reveals that Ba8 Si40 Ga6 is a moderate phonon-mediated BCS superconductor. 2007 The American Physical Society.