Theoretical and experimental study of the electronic structure of FeSe

This thesis aims to study the superconducting properties of the iron-based supercon- ductor FeSe by closely combining theoretical methods with data obtained from angle- resolved photoemission spectroscopy (ARPES).
In order to understand the unconventional superconducting state below Tc = 8 K, we first study the electronic structure of the normal state. Conventional ab-intio meth- ods can only provide qualitative information on the electronic structure of the iron- based superconductors. We overcome this limitation by optimising the hopping pa- rameters of a tight binding model directly to ARPES data of the tetragonal phase of FeSe at 100 K. This quantitatively accurate model of FeSe is then used to predict a large temperature dependence of the chemical potential within this system, which we confirm via ARPES measurements.
We then modify this tight binding model to account for the C4 symmetry breaking effect of the nematic phase of FeSe, which occurs below Ts = 90 K. By performing a detailed study of ARPES data measuring the nematic state, we determine an order parameter which can quantitatively account for the magnitude and symmetry of the band shifts observed as the material is cooled below Ts. We also find that whilst the- oretical models of FeSe suggest a Fermi surface that consists of one hole pocket and two electron pockets, only one of these electron pockets is detected below Ts within ARPES measurements of detwinned crystals. A similar phenomenon is also observed in NaFeAs. We then find additional evidence supporting this interpretation of FeSe by modelling Quasiparticle Interference experiments and comparing with experimental data.
We end by performing ARPES measurements on the superconducting gap of FeSe. We find a highly anisotropic gap structure which is qualitatively consistent with a pair- ing mechanism involving spin fluctuations. To support this interpretation, we model the momentum dependence of the superconducting gap using the ARPES-based tight binding model derived in the previous chapters. By using a model that only includes one hole pocket and one electron pocket, we obtain excellent agreement with the ex- perimental results.
This thesis consists of a complete study of the electronic structure of bulk FeSe and provides detailed information and insight into the tetragonal, nematic and super- conducting state of FeSe.