Ultrathin Bismuth Film on High-Temperature Cuprate Superconductor Bi₂Sr₂CaCu₂O_8+δ as a Candidate of a Topological Superconductor

Abstract

One of the key challenges in condensedmatter physics is to establish a topological superconductor that hosts exotic Majorana fermions. Although various heterostructures consisting of conventional BCS (Bardeen−Cooper−Schrieffer) superconductors as well as doped topological insulators were intensively investigated, no conclusive evidence for Majorana fermions has been provided. This is mainly because of their very low superconducting transition temperatures (T c ) and small superconducting-gap magnitude. Here, we report a possible realization of topological superconductivity at very high temperatures in a hybrid of Bi(110) ultrathin film and copper oxide superconductor Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212). Using angle-resolved photoemission spectroscopy and scanning tunneling microscopy, we found that three-bilayer-thick Bi(110) on Bi2212 exhibits a proximity-effect-induced s-wave energy gap as large as 7.5 meV which persists up to T c of Bi2212 (85 K). The small Fermi energy and strong spin−orbit coupling of Bi( 110), together with the large pairing gap and high T c , make this system a prime candidate for exploring stable Majorana fermions at very high temperatures.

Origin of Suppression of Proximity‐Induced Superconductivity in Bi/Bi2Sr2CaCu2O8+δ Heterostructures

“…We do not observe any leading edge shift of EDCs with temperature, indicating an absence of proximity effect in the present film. This is in sharp contrast to the recent study by Shimamura et al, [24] who have reported a gap opening in the Bi films of similar thickness. We might expect that the thicker the film, the less chance for inducing the pairing at the surface of the film, as the proximity effect is generally limited by the c-axis coherence length.…”

“…In summary, we have studied the electronic structure of Bi films of various thicknesses on Bi2212. Our results show no evidence of a proximity-induced gap opening in the film-derived states, in contrast to the recent study by Shimamura et al [24] By depositing thinner films and accessing both the substrate and overlayer Bi-states, we discover the fundamental reason for the suppression of proximity effect. Both ARPES and core-level results demonstrate that the Bi deposition modifies the doping level, severely reducing the hole doping and causing the loss of superconductivity at the film-substrate interface.…”

“…[29] The dispersion of states along theΓ-M for the present film is shown in Figure 1c. A comparison with the reported theoretical calculations [24,25,29] suggests that the observed states correspond to the 2 and 3 BL Bi(110) films.…”

“…In case of thin films, it is know that the hexagonal phase of bismuth Bi(111) is preferred over the Bi(110) phase for the thicker films. [24] The Bi(110) structure is stabilized in the thinner films, while the transition to Bi(111) occurs at a critical thickness that is dictated by the substrate. [25] In the present study, the film thickness was ≤ 9 Å, with only the Bi(110) structure observed.…”

“…Theoretical calculations also suggest that the s-wave gap claimed on Bi 2 Se 3 /Bi2212 system by Wang et al [18] may not be purely superconducting in origin. [23] In a recent experimental study, Shimamura et al [24] suggest that the TI is not required for the interface TSC and that the latter may be realized even in Bi/Bi22212 heterostructures by inducing the pairing in the Rashba-spin-orbit-coupled states of Bi. By combining ARPES and STM study, they concluded that the observed gap in the Bi-states is due to the proximity induced superconductivity.…”