Collective Excitations at the FeSe(001) Surface and in Ultrathin FeSe Films

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FeSe is structurally the simplest Fe-based superconductor and exhibits superconductivity without doping. Although the material appears simple at first glance, still the individual role of structural, electronic and spin degrees of freedom in superconductivity remains elusive. More interestingly an ultrathin film of FeSe with a thickness of one unit cell grown on Nb-doped SrTiO3(001) shows a superconducting transition temperature which is more than an order of magnitude higher than the one of the bulk samples. The observation of such a high transition temperature has been attributed to the interface properties of the FeSe film in contact with the oxide substrate. This discovery has opened up new perspectives of materials design through the interface engineering. However, the underlying physical mechanism responsible for Cooper pairing and consequently such a high transition temperature in this system, compared to bulk FeSe, is highly under debate.In this project we would like to probe and investigate the frequency- (or energy-) and momentum-resolved collective excitations in ultrathin FeSe films grown on Nb-doped SrTiO3(001) and compare the results to those probed at the β–FeSe(001) surface. The full dispersion relation and lifetime of collective excitations shall be probed by means of spin-polarized high-resolution electron energy-loss spectroscopy along different symmetry directions and as a function of temperature, across the superconducting transition temperature. With this we would be able to gain valuable information regarding the role of different bosonic excitations in superconductivity.