Crystal surface in iron-pnictide superconductors influences behaviour of electrons
Researchers from the University of Amsterdam and the Foundation for Fundamental Research on Matter have, together with colleagues from the University of Tampere (Finland) and the Leibniz Institute for Theoretical Solid State Physics in Dresden (Germany), shown that electrons in the new iron -pnictide superconductors (in this case BaFe2-xCoxAs2) respond to the presence of a crystal surface. This is apparent from both spectroscopic experiments and theoretical calculations. Accurate studies of the behaviour of electrons in a superconductor could give clues that would lead to the discovery of the mechanism underlying superconductivity. The researchers published their findings on 12 January 2011 in Physical Review Letters online.
Mulisch wrote: "It is about neither the depth nor the surface but the depth of the surface." In this research we have demonstrated that the unique layered structure of the iron-pnictides - in the much studied BaFe2As2 pnictide family - leads to a considerable disruption of the crystal grid. By accurately measuring the energy and impulse (the dispersion relationship) of electrons near this surface, Van Heumen and his colleagues were the first to experimentally characterise the surface states. This has major consequences for the interpretation of significant spectroscopic experiments on the electronic structure of these materials: the surface is 'deeper' than had been assumed to date.
Spectroscopic techniques
Two commonly used techniques to analyse superconductors are angle-resolved photoemission spectroscopy and scanning tunnelling microscopy. The first technique directly measures the dispersion relationship of electrons in a material, whereas the second technique gives access to the electron density at the atomic scale. Both techniques have clearly demonstrated their value in the study of unconventional superconductors such as the copper oxides and are being widely used to research the behaviour of the new iron-pnictide superconductors. As these experiments mainly yield information about the electronic states near a crystal surface, investigating to what extent the crystal surface influences the behaviour of the electrons is vital.
Technical background
The structure of the crystal surface is determined by diffraction of low-energy electrons. Using this approach the researchers discovered that the surface differs from the bulk as one of the atom layers had to be broken open. The free space that subsequently arises allows the atoms in the layers below to move away slightly from the fixed grid positions. Now they knew the real structure of the surface, the researchers could take the following step and calculate the effect of these disruptions on the electronic bands. Finally, the researchers completed the loop by comparing the theoretical calculations with their own angle-resolved photoemission measurements. This enabled them to identify the electronic bands that arose due to the disruptive potentials around the crystal surface.
This European collaboration between experiment and theory has provided a first step towards reliable spectroscopic measurements of this fascinating family of high-temperature superconductors.
Contact persons
Dr. Erik van Heumen
Van der Waals - Zeeman Institute
University of Amsterdam
Sciencepark 904
Telephone work: +31 (0)20 525 63 52
Prof.dr. Mark S. Golden
Van der Waals - Zeeman Institute
University of Amsterdam
Sciencepark 904
Telephone work: +31 (0)20 525 63 63
Prof.dr. J. van den Brink
Institute for Theoretical Solid State Physics
IFW Dresden
Helmholzstr. 20
01069 Dresden
Germany
Telephone work: +49 351 465 93 80