Adriana Moreo – February 10, 2017

Iron Pnictides: A New Piece in the High Tc Superconductivity Puzzle

Adriana Moreo
Department of Physics and Astronomy,
The University of Tennessee
Materials Science and Technology Division,
Oak Ridge National Laboratory
PSB 160/161, 4:00-5:00pm

Abstract: During most of the XX century superconductivity was observed in some metals at the very low temperatures achieved with liquid Helium. Below a critical temperature Tc electrons overcome their Coulomb repulsion thanks to an attraction created by the distortions of the ionic lattice and form Cooper pairs that can move without resistance. The efforts to raise Tc were unsuccessful until the discovery of the high Tc superconducting cuprates in 1986. This family of materials are magnetic ceramic insulators that become superconductors with Tc’s that in many cases can be achieved with liquid Nitrogen, when electrons or holes are doped into them. However, the mechanism that produces the electron pairing in the cuprates still remains a puzzle. The interaction between the electrons and the lattice does not seem to be sufficient and it is believed that magnetism plays a role. The discovery of a new family of high Tc superconductors, the iron pnictides, in 2006 provided a new trove of data. While many similarities with the cuprates were found, such as the need to introduce electrons or holes in a magnetic parent compound to develop superconductivity, there are important differences as well. The parent compounds are poor metals, rather than insulators, the magnetic order is not the same as the one in the cuprates, and, in addition to the magnetism, it appears that the orbital degrees of freedom are active. This poses extra challenges, but it also creates the need to develop novel experimental and theoretical approaches to deal with the added complexity. Novel approaches to the study of the iron pnictides based mostly on computational methods will be presented and comparison with experiments will be made.  The focus will be on the interaction among electronic, magnetic, orbital, and lattice degrees of freedom and the complex phases that they produce, such as nematic states [1], and collinear and bicollinear magnetic orders [2].

References:

[1] S. Liang, C. Bishop, A. Moreo, and E. Dagotto, Phys. Rev. B 92, 104512 (2015).
[2] C. Bishop, A. Moreo, and E. Dagotto, Phys. Rev. Lett. 117, 117201 (2016).