基于最局域瓦尼尔函数的FeSe薄膜中电声耦合计算研究

Very recently, it is found that the monolayer FeSe thin film grown on SrTiO3 (001) shows signatures of superconducting transition at 65K. The angle-resolved photoemission spectroscopy suggests that the electrons in monolayer FeSe may have strong coupling with the specifically phonon modes of SrTiO3, due to interfacial effect. To our knowledge, there is no electron-phonon coupling calculations reported, because of the huge size of FeSe/SrTiO3 system and the necessity of using ultra-dense mesh points in momentum space to accurately describe electron and phonon scattering processes in the proximity of the Fermi surface. In this project, we intend to study the electron-phonon coupling in FeSe thin films based on maximally localized Wannier functions (MLWFs). Firstly, the electron-phonon matrix and phonon perturbation potential are calculated on a coarse mesh points from first-principle, and then the quantities at arbitrary electron and phonon wave vectors can be obtained using the unitary transformation in constructing MLWFs and the generalized Fourier interpolation. While preserving the accuracy of a full first-principles calculation, the whole procedure will give converged electron-phonon coupling constant and Eliashberg spectral function, which will help us to clarify what kind of role that electron-phonon coupling plays in FeSe thin film superconductors. By comparing the effect of different substrates on electron-phonon coupling in monolayer FeSe thin film, we will further search proper substrate materials which can modulate the Tc to a higher value.

最近实验观测到SrTiO3衬底上的单层FeSe薄膜展现出65K的超导特征，并指出由于界面效应FeSe薄膜中的电子可能与SrTiO3的特定声子模式存在很强的耦合。而电声耦合计算的前提是通过布里渊区的超密抽样来精确描述电声子在靠近费米面附近时的散射行为，因此目前尚未有理论研究给出FeSe/SrTiO3这一庞大体系的电声耦合常数。在本项研究中，我们拟采用最局域瓦尼尔函数来计算FeSe薄膜中的电声耦合。首先在布洛赫表象中用较为粗糙的k空间网格计算电声耦合矩阵元和声子微扰势，再利用构造最局域瓦尼尔函数的幺正变换和广义傅里叶插值在布洛赫表象中产生任意密集的声子波矢和电声矩阵元。在保留第一性原理计算精度的前提下，给出精确的电声耦合常数和Eliashberg谱函数，并回答电声耦合在单层FeSe薄膜的65K超导中扮演的作用。比较不同衬底对FeSe薄膜中电声耦合的影响，寻找可以将Tc调制到更高值的衬底材料。

外延生长于SrTiO3(001)衬底上的FeSe薄膜，其超导相变温度比体相的8 K超导温度高出约1个量级，并且对FeSe体相的重电子掺杂也可以给出48 K的超导温度。这些研究表明，电荷掺杂的FeSe基超导体Tc可以显著提升。那么重电子掺杂的FeSe基超导体的机理是否是电声子耦合呢？我们利用第一性原理计算和Wannier插值技术对这一问题进行了仔细研究。我们发现电荷掺杂虽然可以将FeSe体相的电声耦合常数提高为未掺杂时的1.24倍，但是Tc仅有0.04 mK。一方面说明了电子掺杂的FeSe中48 K超导电性不是电声耦合，另一方面也说明了单纯的电荷掺杂应该也不会将单层FeSe的Tc提高至100 K，所以SrTiO3衬底的作用是至关重要的。在完成这项研究的基础上，我们还开展了β-Fe4Se5中Fe空位有序情况下的多种反铁磁有序及其相互竞争、硼烯（二维硼的单层结构）的超导电性、钾掺杂菲的原子和电子结构、以及Ca6Li0.5Fe0.5Te2N3中单价铁离子的磁矩和自旋状态转变的第一性原理计算。