Electron-phonon coupling in conventional and unconventional superconductors: Density functional theory vs. experiment.
Detailed comparison of density functional theory calculations with experiment can yield a wealth of information about both conventional and unconventional physics in superconductors and other metals with strong interactions. These interactions lead to experimentally observable phonon renormalization as well as features in the electronic dispersions near the Fermi surface in the form of kinks. The most clearly observable phonon renormalization occurs as broadening and/or softening of certain phonons at specific wave vectors. It appears in all conventional superconductors with high Tcs as well as in cuprate superconductors. The phonon effects can be well explained by the DFT in conventional metals as will be demonstrated on the example of YNi2B2C as well as on Cr. The DFT fails completely in the cuprates in explaining the renormalization of phonons as well as the photoemission kink. I will present new circumstantial evidence that the two are related. In the pnictide superconductors the situation is completely different. They show a very unusual coupling of phonons to the magnetic degrees of freedom, which is not well understood. I will discuss very recent results on CaFe2As2 and BaFe2As2.