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TT: Fachverband Tiefe Temperaturen

TT 8: Superconductivity - Poster Session

TT 8.20: Poster

Montag, 26. März 2007, 14:00–17:45, Poster A

Superconductivity and electron-phonon coupling in doped MgB₂ and related compounds — •Vivien Petzold¹, Klaus Koepernik^1,2, and Helge Rosner¹ — ¹MPI CPfS Dresden, Germany — ²IFW Dresden, Germany

Recently, substitutions on the Mg site in MgB₂, e.g., Mg_1−xSc_xB₂, Mg_1−x(AlLi)_xB₂ were investigated intensively. For achievable doping levels, Mg_1−xSc_xB₂ shows only very small structural changes but clear changes in the electronic structure, whereas AlLi doping affects the lattice parameters but has almost no influence on the electronic structure. Our theoretical approach comprises different approximations in the framework of band structure calculations: the rigid band and virtual crystal method as well as supercell calculations and coherent potential approximation. We show that the latter two lead to consistent results with respect to lattice expansion and electronic properties. We show that lattice effects are of minor importance. Concluding that the B 2p σ states remain the most relevant subsystem with regard to superconductivity, we calculated the electron phonon coupling constant λ and the critical temperature T_c. In contrast, for ZrB₂ as a typical representative of transition metal diborides TB₂ we find the sp²(B)-d(T) hybridization to be crucial. Comparing calculated and measured angle dependent dHvA-data we show that: (i) LDA provides an excellent description of the electronic structure of TB₂. (ii) The electron phonon coupling is too small to expect superconductivity above a few mK for the stoichiometric compounds.
The Emmy-Noether program is acknowledged for financial support.