Speaker: Jörg Fink, Leibniz Institute for Solid State and Materials Research Dresden
Time: November 16, 2015, 2:00 - 3:00
Unconventional/high temperature superconductivity (SC) is observed in heavy fermion systems, cuprates, molecular crystals, and ferropnictides close to a point in the phase diagram where, as a function of a control parameter such as pressure, chemical pressure, or doping, the antiferromagnetic order is suppressed. A widespread view is that at this point, which is called a quantum critical point, strong antiferromagnetic fluctuations are a candidate for the glue mediating superconductivity and that these fluctuations would also account for the strange normal state non-Fermi-liquid behavior as is visible in transport and thermal properties. Using angle-resolved photoemission spectroscopy (ARPES) we have studied the scattering rates and band dispersion of various iron pnictides and iron chalcogenides as a function of the control parameter. The detected scattering rates of all electron and hole pockets do not diverge at optimal doping, i.e., at the expected quantum critical point. This result is at variance with the above described scenario for quantum critical behavior. The scattering rates strongly differ for pockets having different orbital character, and are linear in energy, indicating marginal Fermi liquid behavior. The scattering rates for hole doped compounds are considerably larger than those of the electron doped systems, indicating a dependence on the Fe 3d count leading for a 3d5 configuration to a strongly correlated Hund’s metal. Near optimal doping the measurements also indicate a crossing of the top of hole or electron pockets, through the Fermi level which is related to Lifshitz transitions. Based on these experimental results together with calculations, we establish the following scenario which is different from the traditional view related to strong fluctuations at the quantum critical point: a co-action between a highly correlated electron liquid and a Lifshitz transition causes an anomalous band dispersion at the Fermi level which leads to a strong mass enhancement in the normal state, detected in the transport and thermal properties and to a small effective Fermi energy favoring a Bardeen-Cooper-Schrieffer – Bose-Einstein crossover state in the superconducting phase. The results can be generalized to other unconventional superconductors.