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

FeSe是结构最简单的铁基超导体，在不掺杂的情况下具有超导性。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，在掺nb的SrTiO3（001）上生长的厚度为一个单位电池的超薄FeSe薄膜，其超导转变温度比主体样品高出一个数量级以上。观察到如此高的转变温度归因于与氧化物衬底接触的FeSe薄膜的界面特性。这一发现为界面工程材料设计开辟了新的视角。然而，与大块FeSe相比，导致Cooper配对的潜在物理机制以及该系统中如此高的转变温度仍存在很大争议。在这个项目中，我们想要探测和研究在nb掺杂SrTiO3（001）上生长的超薄FeSe薄膜中的频率（或能量）和动量分辨集体激发，并将结果与在β-FeSe（001）表面探测的结果进行比较。利用自旋极化高分辨率电子能量损失谱沿着不同的对称方向和作为温度的函数，跨越超导转变温度，探测集体激发的全色散关系和寿命。有了这个，我们将能够获得关于不同玻色子激发在超导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.