Berlin 2012 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 8: Superconductivity: Fe-based Superconductors - 122 Part 1

TT 8.3: Talk

Monday, March 26, 2012, 15:30–15:45, H 2053

Spectroscopic study of transition metal impurities in iron pnictides — •Roberto Kraus, Valentina Bisogni, Luminita Harnagea, Saicharan Aswartham, Sabine Wuhrmehl, Bernd Büchner, and Jochen Geck — IFW Dresden, PF 270116, D-01171 Dresden, Germany

In Ba(Fe,TM)₂As₂, superconductivity emerges upon replacing Fe with heavier TM=Co or Ni. The prevalent interpretation of this effect is that the TM-impurities add electrons to the Fe-band and thereby induce superconductivity. However, this interpretation has been questioned recently based on theoretical grounds [1].

To clarify this we have performed valence band photoemission and Auger spectroscopy studies of single crystalline Ca(Fe,Co)₂As₂ and Ba(Fe,TM)₂As₂ with TM=Ni or Cu. The valence band photoemission data together with model calculations show that the Co and Ni impurity states largely overlap in energy with the Fe-bands, whereas the Cu impurity states do not. The LVV Auger lines measured at the L₃-absorption edge of the impurities unambiguously reveal a 3d⁸ final state for Ni and Cu.

This experimental result is incompatible with the naive assumption that Ni and Cu dope, respectively, 2 and 3 electrons into delocalized Fe-states. Rather our results reveal a much more complex situation and support the isovalent substitution of Fe by Ni and Cu.

[1] Wadati et al., PRL 105, 157004 (2010)