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TT: Tiefe Temperaturen

TT 18: Superconductivity: Borides, Borocarbides, Carbides, ...

TT 18.4: Vortrag

Dienstag, 28. März 2006, 15:45–16:00, HSZ 02

Specific heat of Mg(B_1−xC_x)₂: Two Superconducting Gaps — •Niels Oeschler^1,2, Robert A. Fisher¹, Norman E. Phillips¹, William E. Mickelson¹, and Alex Zettl¹ — ¹University of California, Berkeley, CA, USA — ²Max Planck Institute for Chemical Physics of Solids, Dresden, Germany

We present specific heat data of polycrystalline Mg(B_1−xC_x)₂ with x=0.1 and 0.2 from 1K to 35K in magnetic fields up to 9T.
Recently, MgB₂ was identified as a phonon-mediated two-gap superconductor with remarkably high transition temperature of 39K. The small gap and the large gap open at the same temperature due to interband coupling. Substituting carbon on the boron site has the effect of electron filling and inducing disorder. The superconducting transition temperature decreases monotonically from 39K for MgB₂ to 20K for Mg(B_0.8C_0.2)₂. The specific heat of both samples can be accurately fitted with two superconducting energy gaps using the extended α model. The amplitudes of the gaps are found to be larger than the BCS value for the large gap and smaller than the BCS value for the small gap. However, it is observed that the size of both gaps decreases upon increasing the carbon content.
The evolution of the gap sizes, the normal-state Sommerfeld coefficients, the Debye temperatures and the critical fields are compared to those of MgB₂.