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TT: Fachverband Tiefe Temperaturen

TT 54: SC: Heterostructures, Andreev Scattering, Proximity Effect

TT 54.1: Talk

Thursday, March 17, 2011, 15:30–15:45, HSZ 301

Observation of Andreev bound states at spin-active interfaces — •Florian Hübler^1,2, Michael J. Wolf¹, Detlef Beckmann¹, and Hilbert von Löhneysen^2,3 — ¹Karlsruher Institut für Technologie, Institut für Nanotechnologie — ²KIT, Institut für Festkörperphysik — ³KIT, Physikalisches Institut

Spin-active interfaces, for instance between superconductors and ferromagnets are characterized by spin-dependent transmission amplitudes as well as phase shifts[1]. The relative phase shift between spin-up and -down, the so-called spin-mixing angle θ_s, is an important parameter for the triplet proximity effect. So far, little quantitative information is available about θ_s. Recent theoretical models indicate that θ_s may actually be quite large in structures with ultra-thin tunnel barriers[2]. One of the consequences of finite θ_s in S/F hybrids is the presence of bound states at the interface[3], at energies which have a particularly simple relation to θ_s, є=± Δ cos(θ_s/2). We report here the experimental observation of Andreev bound states induced by spin-mixing in Al/Al2O3/Fe structures. From the energy of the bound states, we can determine θ_s with great accuracy. As predicted theoretically, we find examples of θ_s ≈ Π, but also a significant spread from contact to contact, even within the same fabrication batch. The large spread indicates that spin mixing in real structures depends in a subtle way on details of the interface, and is possibly controlled by defects.
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