Scanning tunneling microscopy and spectroscopy on pnictide superconductors

Freek Massee - May 03 2010

Abstract

Using scanning tunneling microscopy (STM), we try to uncover (some of) the mysteries behind the superconductivity 
in the recently discovered iron based superconductors, the pnictides. First we investigate the cleavage surface 
of the '122' family of superconductors, which provides essential information for all surface sensitive techniques 
such as angle resolved photoemission (ARPES) and STM. Then we go on to show some of the surprising properties of 
the superconducting state: a strong spatial variation of the superconducting gap size on length scales of a few 
nanometers in the BaFe2-xCoxAs2 system. Since this seems at first glance difficult to reconcile with a very sharp 
transition temperature of less than 1 K, a pseudogap scenario, like has been observed in the cuprate high temperature 
superconductors, might also exist in the pnictides. However, temperature dependent spectroscopy excludes this 
possibility since all superconducting gaps of different size close at Tc. A different hypothesis is the presence of 
Co atoms in the superconducting Fe layers, which is essential for the system to become superconducting. 
Comparison to a different system without Co doping seems to confirm this idea.