Study on superconductors at high temperature on the CASSIOPEE beamline

It is with this objective in mind that the "Novel States of Matter" group (LPS Orsay) and the Condensed State Physics Service (CEA, Saclay) are collaborating with the CASSIOPEE high resolution photoemission beamline group.

As part of their research, these groups have studied the electronic structure of two superconducting compounds at temperatures below 20K. These were two compounds obtained from BaFe₂As₂, which is magnetic below 140K. In the first compound, a small percentage of the iron atoms of BaFe₂As₂ are replaced by cobalt atoms, which have one more electron than the iron atoms. In the second compound it is ruthenium atoms that replace some of the iron atoms because, although ruthenium has the same number of electrons as iron in its valence band, they have a different electronic structure: the electrons are on a 3d layer for iron, but 4d for ruthenium.
Through these experiments, the researchers were able to confirm that the main effect of the cobalt atoms was to fill the valence band with electrons, while the ruthenium did not dope the compound.
Moreover, very significant differences were observed in their electronic structures, the Ru compound appearing much less correlated than the other two.
Both tested superconductors are therefore very different in relation to their electronic structure, which is rather surprising.
This finding should improve our understanding of the parameters required for stabilizing the superconductivity in this family of compounds, which still retains all its mysteries.

Comparison of the dispersion of hole pockets in three compounds from the iron pnictide family. While cobalt changes the position of the Fermi level without changing the slope of the dispersions in relation to BaFe₂As₂, they change completely in the ruthenium compound, which reflects a fundamental change in the electronic structure, characterized by weaker electronic correlations.

¹ – Strongly correlated electrons:
In solids, the valence band electrons of atoms can behave differently. This depends on the particular type of electronic orbitals that these electrons possess. Certain properties of the material will in turn depend on their behavior: in an insulator the electrons remain bound to their "original atom", in a metal conductor, electrons can move from one atom to another. In other cases observed in relation to d orbitals, the electrons move with difficulty, avoiding each other (they repel each other electrically) and they can only move in certain directions: they are then called strongly correlated electrons.

² - Cuprates : 
Compounds discovered in 1985 and whose superconducting properties - still unexplained today - occur at the highest temperatures observed to date: they reach 138 K, which is higher than the temperature of liquid nitrogen. Cuprates are based on combinations of copper and oxygen atoms with other elements, e.g: LBCO (lanthanum, barium, copper and oxygen compounds) and YBCO (yttrium, barium, copper and oxygen).

References:

Brouet, V., Marsi, M., Mansart, B., Nicolaou, A., Taleb-Ibrahimi, A., Le Fevre, P., Bertran, F., Rullier-Albenque, F., Forget, A., & Colson, D.
Nesting between hole and electron pockets in Ba(Fe1-xCox)2As2 (x=0–0.3) observed with angle-resolved photoemission.
Physical Review B, 2009, 80(16): art.n°165115

Brouet, V., Rullier-Albenque, F., Marsi, M., Mansart, B., Aichhorn, M., Biermann, S., Faure, J., Perfetti, L., Taleb-Ibrahimi, A., Le Fèvre, P., Bertran, F., Forget, A., & Colson, D.
Significant Reduction of Electronic Correlations upon Isovalent Ru Substitution of BaFe₂As₂.
Physical Review Letters, 2010, 105(8): art.n°087001