Ba_(0.6)K_(0.4)Fe_(2)As_(2)的精细电子结构和超导能隙

Genuine electronic structure and superconducting gap structure in (Ba_(0.6)K_(0.4)Fe_(2)As_(2)FeAs superconductor

精细的电子结构和超导能隙是理解铁基超导体超导机理的前提和基础,然而,即使对于被广泛研究的最佳掺杂的Ba_(0.6)K_(0.4)Fe_(2)As_(2),其电子结构和超导能隙仍然存在不少争议.为了解决这些争议,本文利用高分辨氦灯和激光角分辨光电子能谱对最佳掺杂的Ba_(0.6)K_(0.4)Fe_(2)As_(2)进行了详细的研究,确定了r点和M点的精细电子结构和超导能隙.超导态和正常态下在M点附近作者观测到了一个小的电子型能带。和一个M型的能带δ,直接提取的超导能隙大小是5.5 meV,远远小于之前所有的ARPES测量结果.另外,作者直接观测到了超导态下和正常态下都存在r点和M点能带的复制,确定了超导态下r点附近平带的起源,是由M点e能带复制到r点的能带和p能带的超导回弯2部分组成.这些实验结果解决了关于最佳掺杂(Ba_(0.6)Ka_(0.4))Fe_(2)As_(2)的电子结构和超导能隙的一系列争议,这对于检验并建立理论来理解铁基超导体的超导机理提供了关键的信息.

The electronic structure and superconducting gap structure are prerequisites to establish microscopic theories in understanding the superconductivity mechanism of iron-based superconductors.However,even for the most extensively studied optimally-doped Ba_(0.6)K_(0.4)Fe_(2)As_(2),there remain outstanding controversies on its electronic structure and superconducting gap structure.Here we resolve these issues by carrying out high-resolution angle-resolved photoemission spectroscopy(ARPES)measurements on the optimally-doped Ba_(0.6)K_(0.4)Fe_(2)As_(2)superconductor using both Helium lamp and laser light sources.Our results indicate the‘‘flat band"feature observed around the Brillouin zone center in the superconducting state originates from the combined effect of the superconductivity-induced band back-bending and the folding of a band from the zone corner to the center.We found direct evidence of the band folding between the zone corner and the center in both the normal and superconducting state.Our resolution of the origin of the flat band makes it possible to assign the three hole-like bands around the zone center and determine their superconducting gap correctly.Around the zone corner,we observe a tiny electronlike band and an M-shaped band simultaneously in both the normal and superconducting states.The obtained gap size for the bands around the zone corner(~5.5 meV)is significantly smaller than all the previous ARPES measurements.Our results establish a new superconducting gap structure around the zone corner and resolve a number of prominent controversies concerning the electronic structure and superconducting gap structure in the optimally-doped Ba_(0.6)K_(0.4)Fe_(2)As_(2).They provide new insights in examining and establishing theories in understanding superconductivity mechanism in iron-based superconductors.