Electronic structure of the ingredient planes of the cuprate superconductorBi2Sr2CuO6+δ: A comparison study withBi

Abstract

By means of low-temperature scanning tunneling microscopy, we report on the electronic structures of BiO and SrO planes of Bi2Sr2CuO 6+δ (Bi-2201) superconductor prepared by argon-ion bombardment and annealing. Depending on post annealing conditions, the BiO planes exhibit either a pseudogap (PG) with sharp coherence peaks and an anomalously large gap magnitude of 49 meV or van Hove singularity (VHS) near the Fermi level, while the SrO is always characteristic of a PG-like feature. This contrasts with Bi2Sr2CaCu2O 8+δ (Bi-2212) superconductor where VHS occurs solely on the SrO plane. We disclose the interstitial oxygen dopants (δ in the formulas) as a primary cause for the occurrence of VHS, which are located dominantly around the BiO and SrO planes, respectively, in Bi-2201 and Bi-2212. This is supported by the contrasting structural buckling amplitude of BiO and SrO planes in the two superconductors. Our findings provide solid evidence for the irrelevance of PG to the superconductivity in the two superconductors, as well as insights into why Bi-2212 can achieve a higher superconducting transition temperature than Bi-2201, and by implication, the mechanism of cuprate superconductivity.PACS numbers: 74.72. Gh, 68.37.Ef, 74.62.Dh, 74.25.Jb In high-transition temperature (T c ) cuprate superconductors, the maximum T c (T c, max ) varies substantially with the number (n) of CuO 2 planes in one unit cell, and peaks at n = 31 . In bismuth-based cuprates, for example, T c, max is approximately 34 K, 90 K, and 110 K for n = 1, 2, 3, respectively 2 . It has led to numerous competing proposals to explain this intriguing phenomenon, which include interlayer quantum tunneling of Cooper pairs 3-5 , intralayer hopping 6 , the energy level of apical oxygen 7 , magnetic exchange interactions 8,9 , and so on. Thus far, however, a consensus on which factor controls T c in cuprate superconductors has not yet been reached, partially due to a lack of knowledge about the detailed electronic properties outside the superconducting CuO 2 planes, which are anti-ferromagnetic insulators without doping. Indeed, it has long been hypothesized that outof-plane apical oxygen plays a primary role in determining the optimal T c of cuprate superconductors 7,10,11 . Identification of the out-of-plane effects are thus imperative to understanding T c, max and superconductivity mechanism in cuprates 12 , but extremely challenging because technically it demands nonstandard, profile-based preparation and imaging techniques.Cuprate superconductivity develops when the insulating CuO 2 planes are either electron or hole-doped by substitutional or interstitial chemical doping, e.g. excess oxygen dopants in the hole-doped cuprate superconductors. In addition to enabling superconductivity, the doping can lead to startling nanoscale electronic inhomogeneity and disordering 13 . The latter is usually detrimental to superconductivity 14 . However, its effect has been overemphasized over the past two decades 15-17 : the experimental efforts in minimizing thi...

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].…”

“…Upon chemical doping in the reservoir layers, rather than direct chemical doping in the CuO 2 layers, the resulting carriers transfer into the CuO 2 layers and boost superconductivity therein, resembling the HTS in single-unitcell FeSe films on SrTiO 3 [2]. By layer-by-layer removal with an Ar + bombardment technique, we could reveal respective quasiparticle (QP) excitations of the constituent planes of Bi-2212 [3] and Bi-2201 [4]. However, the CuO 2 surface obtained is too small to systematically investigate its superconducting property.…”

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

“…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] .…”

“…(a)-(d) 在超导真空500°C温度 下退火; (e)-(h) 在18000 Langmuir臭氧气氛以及450°C温度下退火; (i)-(l) 在另一个18000 Langmuir臭氧气氛以及450°C温度下 退火; (m)-(p)在超导真空400°C温度下退火. 图中黑色箭头指示了赝能隙的能量位置[45] Color online) Representative differential conductance dI/dV spectra on the exposed BiO(I), SrO(I), BiO(II), and BiO(I) constituent planes of the Ar + bombarded Bi-2201 single crystal, followed by a sequence of (a)-(d) UHV annealing at 500°C, (e)-(h) ozone exposure of 18000 Langmuir,(i)-(l) additional ozone exposure of 18000 Langmuir, and (m)-(p) UHV annealing at 400°C. Black arrows denote the energy positions of the PG.…”