Mapping the Electronic Structure of Each Ingredient Oxide Layer of High-TcCuprate SuperconductorBi2Sr2<

Abstract

Understanding the mechanism of high transition temperature (Tc) superconductivity in cuprates has been hindered by the apparent complexity of their multilayered crystal structure. Using a cryogenic scanning tunneling microscopy, we report on layer-by-layer probing of the electronic structures of all ingredient planes (BiO, SrO, CuO2) of Bi2Sr2CaCu2O 8+δ superconductor prepared by argonion bombardment and annealing technique. We show that the well-known pseudogap (PG) feature observed by STM is inherently a property of the BiO planes and thus irrelevant directly to Cooper pairing. The SrO planes exhibit an unexpected Van Hove singularity near the Fermi level, while the CuO2 planes are exclusively characterized by a smaller gap inside the PG. The small gap becomes invisible near Tc, which we identify as the superconducting gap. The above results constitute severe constraints on any microscopic model for high Tc superconductivity in cuprates.PACS numbers: 74.72. Gh, 68.37.Ef, 74.50.+r, 74.25.Jb Superconductivity in perovskite-type layered cuprates [1], which thus far hold the record for the highest transition temperature (T c ), ranks among the most challenging and engaging problems in modern condensed matter physics. Despite nearly three decades' tremendous efforts of research all around the world, the key mechanism behind the Cooper pairing that lies at the heart of high-T c cuprate superconductors still remains puzzling. Many intriguing phenomena that intertwine with the occurrence of superconductivity have been discovered, leading to a very sophisticated phase diagram of cuprates [2]. These phenomena include the ubiquitous existence of various sorts of broken-symmetry states (e.g. charge density wave, spin density wave and electron nematicity) and the well-known pseudogap (PG) phenomenology, which has been considered a key finding in the research of cuprate superconductivity. A vast amount of experimental and theoretical studies have been devoted to understanding these phenomena themselves and their possible interplay with superconductivity, but so far most of which fell flat.From the view point of crystal structure, the cuprates consist of superconducting CuO 2 layers and charge reservoir building blocks (e.g. BiO/SrO in Bi 2 Sr 2 CaCu 2 O 8+δ (Bi-2212)) that stack alternatively along the crystallographic c-axis. In Bi-based cuprate superconductors, it is widely thought but empirical that the BiO and SrO block layers are insulating [3]. A considerable amount of surface-sensitive measurements, e.g. via angle-resolved photoemission spectroscopy (ARPES) [4] and scanning tunneling spectroscopy (STS) [5], have been conducted on the vacuum cleaved BiO planes of Bi-based cuprates and contributed largely to the cuprate PG data base. The measurements are generally assumed to reflect the superconducting properties of the CuO 2 planes, despite that they are located 4.5Å beneath the top BiO plane. Yet, such model has not been rigidly tested experimentally thus appears contentious, particularly regarding to the fact tha...

Nodeless pairing in superconducting copper-oxide monolayer films on Bi2Sr2CaCu2O8+

“…2a, we summarize our measurements of the averaged gap magnitudes, ∆ s and ∆ p , as a function of ∆ p,BiO simultaneously acquired on the exposed BiO surfaces. Here ∆ p,BiO and thus the hole carriers are delicately controlled by annealing the samples under ozone flux or UHV [3,4]. The larger energy gap, ∆ p , is found to scale linearly with ∆ p,BiO and bears striking quantitative correspondence in the magnitude (red dashes) and spatial variation (marked by the equal statistical uncertainty) to ∆ p,BiO , further stressing its nature as the gap in Bi-2212 substrates [13].…”

“…Because of asymmetric doping and a peculiar bonding configuration of CuO 2 on the inert BiO surface, we speculate that the total charge transfer is not sufficient to make the whole films superconducting, leading to the formation of two different regions. The situation may be improved by growing CuO 2 on the SrO surface of Bi-2212 [3,4].…”

Experimental observation of copper oxide layers in cuprates by scanning tunneling microscopy

“…同原子层进行直接研究 [47] . [48] 、角分 辨光电子能谱(Angle-Resolved Photoemission Spectroscopy, ARPES) [49] 等都是在BiO层上进行的, 在BiO和 SrO层是绝缘性的前提下 [50] [44] [66,67] . 这些能隙的大小也随空间而变化, 但明显小于(100)面 上的能隙 [47] .…”

“…关于铜氧化物超导体在费米面上出现能隙节点的 d波配对对称性, 已经被许多体或者表面敏感的实验技 术所研究, 其中包括表面敏感的扫描隧道显微镜 (Scanning Tunneling Microscopy, STM)和扫描隧道谱 (Scanning Tunneling Spectroscopy, STS)研究 [43] . 以 [43][44][45][46] .…”

Interrogating the superconductor Ca10(Pt4As8)(Fe2−xPtxAs2)5 Layer-by-layer

“…The layer-by-layer probing of these layered superconductors is extremely important as illustrated in the recent study of Bi 2 Sr 2 CaCu 2 O 8+δ , where it was found that “the well-known pseudogap feature observed by STM is inherently property of the BiO planes and thus irrelevant directly to Cooper pairing”19. By creating a fresh surface through single-crystal cleavage under ultra-high vacuum, one can study both structural (STM topography) and electronic properties (scanning tunneling spectroscopy (STS)) of the exposed surfaces.…”