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Superconducting STM tips in high magnetic fields : A new atomic scale probe of the spin polarization of tunneling electrons - Markus Etzkorn - Mardi 10 septembre 2013 à 11 h

Markus Etzkorn, Max-Planck-Institute for Solid State Research, Stuttgart, Germany
Mardi 10 septembre 2013 à 11 h - Barre 22-32 - Salle 201 - 2e étage

Reducing dimensionality of objects leads to a multitude of interesting phenomena. For example superconductors can remain in their ordered state in magnetic fields much larger than their critical field when their dimensions are reduced. This leads to the interesting situation that the Cooper pairs are exposed to high magnetic fields. For superconductors with small spin orbit interaction this condition lead to a Zeeman splitting of the quasi-particle density of states that has been observed in planar tunnel junctions [1]. Under such circumstances a tunnel junction between a superconductor and a ferromagnet is an ideal probe to access the absolute value of the spin polarization of the ferromagnet at the Fermi energy [2]. We have transferred this concept from planar tunnel junctions to scanning tunneling microscopy using a superconducting tip. For vanadium tips, we indeed find that the tip apex, being confined, remains superconducting in external magnetic fields as high as 4 Tesla ! We observe a clear splitting of the quasi-particle density of states well described by a Zeeman term. We analyze the measured spectra in the framework of Maki´s theory [3]. I will show first experimental results obtained using such tips for spin polarized scanning tunneling microcopy on ferromagnetic structures. Since such measurements provide direct access to the spin polarization of the tunneling electrons in absolute scales, they can certainly complement established spin polarized STM techniques. For example the dependence of the measured spin polarization on the tip to sample distance reflects the decay of different electronic states into vacuum.

[1] R. Meservey, P.M. Tedrow and P. Fulde, Phys. Rev. Lett. 25 (1970) 1270.

[2] P.M. Tednow and R. Meservey, Phys. Rev. Lett. 26 (1971) 192.

[3] K. Maki, Prog. Theor. Phys. 32 (1964) 29.