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R. G. Herb Condensed Matter Seminar
The Zeeman Effect in Confined Superconductors: A new probe of absolute spin polarization on the atomic scale
Date: Thursday, October 2nd
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Christian Ast, Max Planck Institute for Solid State Research
Abstract: When a dimensionally confined superconductor is subjected to a magnetic field, the compensating currents are suppressed such that superconductivity persists to much higher magnetic fields than in the bulk. This effect has been observed first in planar tunnel junctions by Meservey, Tedrow and Fulde [1]. The resulting Zeeman splitting of the quasiparticle density of states can be exploited as a probe for the absolute spin polarization of a spin polarized tunnelling current [2]. We have transferred this concept to the scanning tunnelling microscope (STM) to probe the spin polarization of the tunnelling current on the atomic scale by using a superconducting tip. As the superconducting tip carries no magnetic moment itself, the information in the spin polarized tunnelling current can be attributed to the magnetic properties of the sample. This is a decisive advantage over other spin-polarized STM techniques. First measurements on magnetic cobalt islands on a Cu(111) substrate are in excellent agreement with literature. Measuring the spin polarization of the tunnelling current as a function of tip-island distance, we find that the spin polarization increases by 65% when the distance is increased by only 2.3Å. This can be attributed to the different exponential decay of majority and minority states into the vacuum. This means that we can exploit the vacuum barrier as a tunable spin filter.

References
[1] R. Meservey et al., Phys. Rev. Lett. 25, 1270 (1970).
[2] P. M. Tedrow and R. Meservey, Phys. Rev. Lett. 26, 192 (1971).
Host: Franz Himpsel
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