FeSe: What is the consensus on its gap structure? - Dr. Mahmoud Abdel-Hafiez
OCTOBER 9, 2016
By the in situ angle resolved photoemission spectroscopy (ARPES) measurements on an excellent quality of FeSe single crystal, an international joint team co-led by Dr. Mahmoud Abdel-Hafiez from HPSTAR, realized that the superconducting gap shows a pronounced twofold anisotropy around the elliptical hole pocket near Z (0, 0, π), with gap minima at the end points of its major axis. The study is published in the journal Phys. Rev. Lett. (DOI: 10.1103/PhysRevLett.117.157003 ).
“A basic property of any superconductor is its gap structure, which is related to the symmetry of its pairing state”, said Dr. Abdel-Hafiez. “In most Fe-based superconductors, both magnetic and nematic orders appear simultaneously near the superconducting state. These orders are believed to play critical roles in the superconductivity, and both spin-fluctuation-mediated and orbital-fluctuation-mediated superconducting pairing mechanisms have been proposed”.
Amongst iron-based superconductors, the simple material FeSe has attracted much attention because, when made in thin-film form, its superconductivity appears to persist to a critical temperature Tc ≃ 100 K. In bulk form, FeSe is unusual in that it undergoes the standard tetragonal-to-orthorhombic structural transition without the usual accompanying antiferromagnetic transition. This raises fundamental questions about the role of magnetism in causing superconductivity and nematicity.
The superconducting gap structure of FeSe and FeSe1−xSx has been under intensive debate, while no consensus has been reached on whether there are nodes or whether it has a nodeless multigap structure. Besides, although the vortex elongation in scanning tunneling spectroscopy (STS) suggests possible twofold gap symmetry, the band- and momentum-resolved gap structure is still unknown. ARPES studies on the superconducting gap structure is lacking due to the low Tc and small gap size.
To answer an open question on the superconducting gap, we report on an angle-resolved photoemission study on the superconducting gap structure in the nematic state. Our results show that the superconducting gap shows twofold anisotropy around the Z point (0, 0, π), and it is undetectable around the hole pocket near Γ (0, 0, 0) and the electron pockets at the zone corners (π, π, kz).
“These findings help to examine the previous controversial gap structures deduced from STS and thermodynamics measurements. The unique gap structure observed here cannot be reasonably fitted by most of the known theoretical models and their simple combinations, which suggests that the effects of nematicity on the superconductivity are substantial”, stated in the paper.
"The experimental results presented in this article will help the theorists currently developing the theory of one of the most enigmatic and intriguing quantum phenomena that finds more and more applications in modern technology", Mahmoud added.
Caption: Left: Superconducting gap size as a function of temperature fits to the Bardeen-Cooper-Schrieffer formula. Middle: Polar plot of the superconducting gap as a function of θ along the pockets α and α0. Right: Angular dependence of the superconducting gap on the α pocket and fitting results by cosine series.
导体的一个基本属性是其超导能隙结构，这直接关联着库伯对的配对对称性。在大多数铁基超导体中，磁性及向列相几乎同时在超导附近出现。这些现象在超导转变中扮演着重要的角色，目前人们提出自旋涨落调制及轨道涨落调制的两种超导配对机理。FeSe及FeSe1−xSx的超导结构虽然被人们广泛研究，但是其超导能隙中是否存在节点或是无节点的多带结构到目前为止一直没有一致的结论。另外，尽管扫描隧道显微镜和光谱得到的涡流延伸建议FeSe及FeSe1−xS可能存在双重能隙结构，人们对其能量和动量空间中的超导能隙结构也尚不清楚。由于低的超导转变温度及小的带隙使得角分辨光电子能谱的测量一直空缺。为了解决超导能隙问题，本研究以FeSe0.93S0.07为例，利用角分辨光电子能谱来研究向列相状态下的超导能隙，表明超导能隙在Z(0, 0, π)点附近具有各向异性,Γ (0, 0, 0) 点附近的空穴口袋和(π, π, kz)点的电子口袋无法探测到超导能隙。