Electronic Properties of novel FeAs-based superconductors.
- Universidade de Évora(líder)
- Instituto Superior Técnico(parceiro)
- Universidade do Porto - Faculdade de Ciências(parceiro)
The discovery of iron pnictide superconductors is very recent (the first paper appeared in January 2008) and has spurred enormous interest, quickly becoming a hot topic, with several preprints appearing almost every day on the arXiv.com, authored by research groups all over the world.
In this Project, we propose to study the properties of these high temperature superconductors. The critical temperatures attained are the second highest and an effort has been made to establish similarities and differences to the cuprate superconductors. While the band structure is fairly well known, with a Fermi surface comprising four sheets, the experimental evidence for the pairing symmetry is not yet clear. Thus, a hot debate is going on among experimentalists and theorists as to the unconventional nature of the gap function, with basically two different scenarios in dispute.
We will focus on the possible origin and effects of the various pairing symmetries, attempt to shed light on the relation between the various classes of iron pnictides and their different pairing symmetries, and on the unexpected higher temperatures observed when the system contains rare earths. We will study these materials' response to imposed currents, magnetic fields (to induce vortices) and disorder. We will systematically study the phase diagrams of the various materials. The problem bears some similarity to the case of heavy fermions but the latter are well known to be strongly correlated systems. As mentioned above, the novel superconductors do not seem to be strongly correlated, which may suggest that the topology of the phase diagrams may not be directly associated to strong correlations. This is an important technical point because the description of the systems will most likely be under better control. As in the case of the cuprates and heavy fermions, the vicinity of the superconducting phase to a magnetic phase suggests the importance of magnetic fluctuations as a mechanism for the (repulsive) pairing interaction. However, contrary to cuprates where magnetism is expected to occur between localized magnetic moments, in the pnictides the magnetism seems to have an itinerant character. A comparative study of the role of impurities in the iron pnictides and the cuprates will be made, with emphasis in their effects on the magnetic and superconducting temperatures.
Objectives, activities and expected/achieved results
The main points to be clarified about the pnictides and which we are going to address:
1- Clarification of the pairing symmetry
2- Effort towards the identification of the mechanism of superconductivity
3- Richness of properties due to multiband structure of these materials
4- Identification of a possible quantum critical point separating antiferromagnetism and superconductivity in some of these materials.
5- The role of impurities (dopants) in suppression of Néel temperature and enhancement of superconducting critical temperature.
The methods we shall use are:
- quantum-field theory methods (Feynman diagrams, functional integration)
- Green's functions for impurity disorder (self-energy beyond Born approximation)
- computer simulations using Bogoliubov-de Gennes equations
- renormalization group
- numerical solution of mean-field equations (non-linear systems of equations)
Regarding item 1, experiments seem to show that both nodal and nodeless gap functions are observed in different classes of the pnictides. We intend to analyze transport properties and, in particular, Andreev reflection to further extend the results for the various gap symmetries.
The outcome of Item 2 is harder to predict since it involves similar efforts to those carried out for over 23 years in the context of the cuprates of even longer for the heavy fermions. In this point we will take into account the nature of the itinerant magnetism as the glue that leads to the pairing in these materials, extending previous suggestions by other authors.
We will pay special attention to issues of competition/coexistence of the various phases. We will also pay particular attention to the rare earth substituted materials. Finally, the role of dopant impurities in the destruction of antiferromagnetic order and subsequent enhancement of superconducting temperature. Impurity resonant states will be searched for, as well as their effects on the electronic spectrum.