|Ph.D Student||Rinott Shahar|
|Subject||Tuning Across the BCS-BEC Crossover in Superconducting|
Fe(1+y)Se(x)Te(1-x): An Angle-Resolved
|Department||Department of Physics||Supervisor||Professor Amit Kanigel|
|Full Thesis text|
After over a century of superconductivity and thirty years of high temperature superconductivity, much is still unknown. One of the key questions is how to describe a system with strongly interacting electrons. The crossover from weak-coupling Bardeen-Cooper- Schrieffer (BCS) pairing to a Bose-Einstein condensate (BEC) of tightly bound pairs, as a function of the attractive interaction in Fermi systems, has long been of interest in the field of superconductivity as a possible scenario to answer this question. The crossover from BCS superconductivity to BEC is difficult to realize in quantum materials because, unlike in ultra cold atoms, one cannot tune the pairing interaction. Here we realize the BCS-BEC crossover in a nearly compensated semimetal Fe1ySexTe1-x by tuning the Fermi energy via chemical doping, which permits us to systematically change from 0.16 to 0.50, where is the superconducting (SC) gap. We use angle-resolved photoemission spectroscopy (ARPES) to measure the Fermi energy, the SC gap and characteristic changes in the SC state electronic dispersion as the system evolves from a BCS to a BEC regime. Our results raise important questions about the crossover in multi-band superconductors which go beyond those addressed in the context of cold atoms. We also show experimental and theoretical work regarding the orbital character of the electronic dispersion. New observations of the ARPES spectra in the superconducting state could lead to new insights on the superconducting mechanism in this Iron based superconductor.