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

FeSe是结构上最简单的铁基超导体，在没有掺杂的情况下表现出超导性。虽然这种材料乍一看很简单，但结构、电子和自旋自由度在超导性中的单独作用仍然难以捉摸。更有趣的是，在Nb掺杂的SrTiO 3（001）上生长的具有一个晶胞厚度的FeSe的超导薄膜显示出比体样品高一个数量级以上的超导转变温度。观察到这样高的转变温度归因于与氧化物衬底接触的FeSe膜的界面性质。这一发现为界面工程材料设计开辟了新的视角。然而，与本体FeSe相比，在该系统中负责库珀配对和因此如此高的转变温度的潜在物理机制，在这个项目中，我们想探索和调查的频率-（或能量-）和动量分辨的集体激发，并将结果与在β-FeSe（001）上探测的结果进行比较面应通过自旋极化高分辨率电子能量损失谱沿着不同对称方向探测集体激发的全部色散关系和寿命，并作为温度的函数，跨越超导转变温度。有了这个，我们将能够获得有关不同玻色子激发在超导性中的作用的有价值的信息。

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.