Several angle resolved photoemission spectroscopy (ARPES) studies reveal a poorly nested Fermi surface of LiFeAs, far away from a spin density wave instability, and clear-cut superconducting gap anisotropies. On the other hand a very different, more
nested Fermi surface and dissimilar gap anisotropies have been obtained from quasiparticle interference (QPI) data, which were interpreted as arising from intraband scattering within hole-like bands. Here we show that this ARPES-QPI paradox is completely resolved by interband scattering between the hole-like bands. The resolution follows from an excellent agreement between experimental quasiparticle scattering data and T-matrix QPI calculations (based on experimental band structure data), which allows disentangling interband and intraband scattering processes.
The Nernst effect has recently proven a sensitive probe for detecting unusual normal state properties of unconventional superconductors. In particular, it may sensitively detect Fermi surface reconstructions which are connected to a charge or spin de
nsity wave (SDW) ordered state, and even fluctuating forms of such a state. Here we summarize recent results for the Nernst effect of the iron pnictide superconductor $rm LaO_{1-x}F_xFeAs$, whose ground state evolves upon doping from an itinerant SDW to a superconducting state, and the cuprate superconductor $rm La_{1.8-x}Eu_{0.2}Sr_xCuO_4$ which exhibits static stripe order as a ground state competing with the superconductivity. In $rm LaO_{1-x}F_xFeAs$, the SDW order leads to a huge Nernst response, which allows to detect even fluctuating SDW precursors at superconducting doping levels where long range SDW order is suppressed. This is in contrast to the impact of stripe order on the normal state Nernst effect in $rm La_{1.8-x}Eu_{0.2}Sr_xCuO_4$. Here, though signatures of the stripe order are detectable in the temperature dependence of the Nernst coefficient, its overall temperature dependence is very similar to that of $rm La_{2-x}Sr_xCuO_4$, where stripe order is absent. The anomalies which are induced by the stripe order are very subtle and the enhancement of the Nernst response due to static stripe order in $rm La_{1.8-x}Eu_{0.2}Sr_xCuO_4$ as compared to that of the pseudogap phase in $rm La_{2-x}Sr_xCuO_4$, if any, is very small.
