Superconductivity in Isolated Single Copper Oxygen Plane

One of the central questions in cuprate superconductivity is if superconductivity can exist in an isolated single CuO$2$ plane without any interlayer coupling. There have been numerous experimental efforts to answer this question, but it still has not been clearly resolved. Here we present a heterostructure system with an isolated half-unit-cell La${2-x}$Sr$_x$CuO$_4$ which has a single CuO$_2$ plane. Using in-situ angle-resolved photoemission spectroscopy, we measured the electronic and gap structures of a single CuO$_2$ plane. We observed a \textit{d}-wave-like gap which closes somewhat above the bulk T$_c$. Moreover, almost identical gap properties are seen for both single CuO$_2$ plane and bulk. These observations lead us to the conclusion that the d-wave superconductivity of cuprates also exists in a single CuO$_2$ plane. Our results demonstrate that cuprate superconductivity is essentially a two-dimensional phenomenon and provide a platform to study cuprate superconductivity in a purely two-dimensional system.

💡 Research Summary

The paper addresses a long‑standing question in cuprate physics: can superconductivity exist in an isolated single CuO₂ plane without any interlayer coupling? To answer this, the authors engineer a heterostructure consisting of a half‑unit‑cell La₂₋ₓSrₓCuO₄ (LSCO) grown on an insulating LaSrAlO₄ (LSAO) substrate. The stack, from top to bottom, comprises an LSAO capping layer, a monolayer of nominally undoped La₂CuO₄ (LCO), an LSAO buffer, and a heavily doped LSCO conducting layer that provides a macroscopic charge reservoir for ARPES measurements. Because Sr from the buffer intermixes into the LCO layer, the topmost CuO₂ plane is effectively doped to x≈0.26, placing it in the overdoped superconducting dome.

High‑resolution scanning transmission electron microscopy (HAADF‑STEM) and energy‑dispersive X‑ray spectroscopy (EDX) confirm that a single Cu atom layer is sandwiched between Al‑containing layers, establishing the presence of a true single CuO₂ plane. The authors then perform in‑situ angle‑resolved photoemission spectroscopy (ARPES) on this monolayer system, which avoids the connectivity problems that plague transport measurements on ultrathin films.

The ARPES Fermi‑surface maps of the monolayer and a 30‑layer (≈15 unit‑cells) LSCO reference sample both display a 4 × 4 reconstruction, leading to folded bands. By unfolding the reconstruction, the underlying band structure matches that of bulk LSCO. Tight‑binding fits yield hole concentrations of 0.26 (monolayer) and 0.27 (30‑layer), confirming comparable doping levels.

Momentum‑dependent spectra along nodal to antinodal cuts reveal a leading‑edge shift that increases smoothly from node to antinode, characteristic of a d‑wave superconducting gap. Symmetrized energy‑distribution curves (EDCs) show a clear gap opening away from the node, with a maximum size of ~10 meV. Temperature‑dependent measurements at an antinodal momentum show the gap persisting up to 40–80 K, notably higher than the bulk Tc≈35 K measured by resistivity for the 30‑layer film. The same temperature range is observed for the monolayer, indicating that the gap is not a simple pseudogap but likely reflects superconducting pairing fluctuations or pre‑formed pairs, a phenomenon widely reported in overdoped cuprates.

The authors fit the symmetrized EDCs with a Norman self‑energy model, extracting gap values that follow the (cos kₓ − cos kᵧ)/2 d‑wave form. Both the angular dependence and the temperature evolution of the gap in the monolayer mirror those of the bulk‑like 30‑layer sample. Minor non‑zero gap values at the node or at high temperature are attributed to fitting artifacts and instrumental resolution limits.

These observations lead to the central conclusion: a single CuO₂ plane can host d‑wave superconductivity, and its gap magnitude and temperature behavior are essentially identical to those of a multilayer cuprate. Consequently, interlayer coupling is not a prerequisite for high‑temperature superconductivity in LSCO; the phenomenon is fundamentally two‑dimensional. This challenges theories that rely on interlayer tunneling and supports models emphasizing in‑plane electronic correlations, such as spin‑fluctuation mediated pairing.

Beyond its conceptual impact, the work provides a new experimental platform: a pristine, epitaxial single CuO₂ plane that can be probed by surface‑sensitive techniques without the complications of transport connectivity. Future studies can exploit this platform to investigate quantum criticality, interface‑induced phenomena, and the interplay of superconductivity with other ordered states in a strictly two‑dimensional setting.