Dresden 2009 – wissenschaftliches Programm

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TT: Fachverband Tiefe Temperaturen

TT 5: Postersession Superconductivity: Materials - Fabrication and Properties

TT 5.5: Poster

Montag, 23. März 2009, 13:00–16:45, P1A

Tunneling into Al doped MgB₂ thin films — •Rudolf Schneider, Alexander G. Zaitsev, and Jochen Geerk — Forschungszentrum Karlsruhe, Institut für Festkörperphysik, 76021 Karlsruhe

Superconducting thin films with composition Mg_1−xAl_xB₂ (0 ≤ x < 0.6) were prepared in situ by thermal sublimation of Mg combined with B rf and Al dc magnetron sputtering. The critical temperature, T_c, decreased linearly with a slope of -0.4 K per at% Al up to x ≈ 0.4. For 0.4 < x < 0.5 the formation of a plateau with a T_c ≈ 12 K was observed. The plateau-like effect might be due to the formation of the superstructure MgAlB₄ with ordered alternating Mg and Al planes separated by B planes. Quasiparticle tunneling measurements were performed on sandwich-type crossed-strip tunnel junctions with artificial aluminum oxide barriers. Differential conductance measurements at low voltage allowed the determination of the small energy gap on the Fermi surface π sheet. The π gap decreased linearly with decreasing T_c of the films in agreement with a band filling model. Conductance measurements in the phonon region enabled the determination of the Eliashberg function α²F for a low doping level x≈0.1 so far. Compared to the undoped MgB₂ a shift of the spectrum to higher energy was observed comparable to the renormalization of the phonon density of states measured on bulk samples with inelastic neutron scattering.