The discovery of high-temperature superconductivity in iron pnictides in 2008 opened a new era in the study of superconductors. Since then superconductivity has been observed in several families of both pnictides and chalcogenides. Similar to the case of cuprates, the other family of high-temperature superconductors known, these are layered materials in which superconductivity appears when a magnetic phase is suppressed. But contrary to the former, which are Mott insulators, magnetic pnictides and chalcogenides are generally metallic, what has raised questions on the strength of correlations. The mechanism for the superconductivity is not clear yet, as coupling to conventional phonons cannot explain the high values of the critical temperatures. Magnetic or orbital fluctuations have been proposed instead.
The electronic properties are controlled by the Fe-d orbitals. Several bands cross the Fermi level. It is believed that the five d-orbitals have to be included in their description. The effect of interactions is largely controlled by Hund’s coupling. Correspondingly, they are termed Hund’s metals. Iron superconductors are very interesting systems to study the interplay between magnetic, orbital and lattice degrees of freedom. The coupling between the lattice and magnetism is anomalous and phonons have been proposed to enhance orbital fluctuations.
Different spectroscopic and thermodynamic phenomenology is found in different materials, partly associated to their multi-band behavior. The magnetic order changes among families and includes stripe order in pnictides (antiferromagnetic along one direction and ferromagnetic along the other one), and double stripe or block order in chalcogenides. Stripe order is generally preceded by a structural transition and many anisotropic properties. This has led to suggestions of nematicity or orbital order in these materials.
Our work has been mostly concentrated in understanding the nature of the correlations and stripe order in pnictides, its competition with other magnetic orders, the anisotropic transport, and spectroscopic properties and their coupling to the lattice.
Magnetic interactions in iron superconductors: A review.
E. Bascones, B. Valenzuela, and M.J. Calderón. arXiv:1503.04223.
Electronic correlations in multiorbital systems and iron superconductors. L. Fanfarillo and E. Bascones. arXiv:1501.04607
Correlation, doping and interband eﬀects on the optical conductivity of iron superconductors.
M. J. Calderón, L. de’Medici, B. Valenzuela, and E. Bascones. Phys. Rev. B 90, 115128 (2014). arXiv:1407.6935
Coupling of the A1g-As phonon to magnetism in iron pnictides.
N. García-Martínez, B. Valenzuela, S. Ciuchi, E. Cappelluti, M.J. Calderón and E. Bascones. Phys. Rev. B 88, 165106 (2013), arXiv:1307.7065.
Magnetic interactions in iron superconductors studied with a five-orbital model within the Hartree-Fock and Heisenberg approximations.
M.J. Calderón, G. León, B. Valenzuela and E. Bascones. Phys. Rev. B 86, 104514 (2012), arXiv:1107.2279.