Condensed Matter Seminar

Andrey Chubukov
University of Wisconsin - Madison

Tuesday, April 7, 2009; 4:00 pm in JFB 334

Back to the iron age: the pairing symmetry in Fe-pnictides

I will share with the audience the excitement in condensed-matter community about the discovery of a new class of superconsuctors - Fe-pnictides, with T_c as high as 55 K. The phase diagram of Fe-pnictides is quite similar to that for high-temperature cuprates, and this fueled early speculations that the physics should be similar. However, Fe-pnictides differ from the cuprates in one important aspect - these systems are metals even without doping. I will review recent experiments on three different families of Fe-pnictides, and discuss various theoretical proposals for the pairing mechanism and the symmetry of the pairing gap.

More specifically, I will discuss the interaction Hamiltonian, the nesting, the RG flow of the couplings at energies above and below the Fermi energy, and the interplay between SDW magnetism, superconductivity and charge orbital order. I will argue that SDW antiferromagnetism wins at zero doping but looses to superconductivity upon doping. I will show that the most likely symmetry of the superconducting gap is the extended s-wave gap, which, however, can be with or without nodes, depending on the parameters.