Single Atomic Layer Shows Superconductivity, Bringing Lossless Power Closer to Reality

Researchers have long sought to determine whether superconductivity can occur within a single copper-oxygen plane, independent of interactions between layers. Youngdo Kim, Byeongjun Gil, and Sehoon Kim, from the Department of Physics and Astronomy at Seoul National University, alongside Yeonjae Lee, Donghan Kim, Jaeung Lee et al., now present compelling evidence for this phenomenon using a novel heterostructure containing an isolated half-unit-cell LaSrCuO layer. Their application of in-situ angle-resolved photoemission spectroscopy reveals a characteristic gap structure within this single plane, closely mirroring that of bulk cuprates. This finding significantly advances our understanding of high-temperature superconductivity, demonstrating its fundamentally two-dimensional nature and establishing a new avenue for investigating the mechanisms driving this complex state of matter.

Superconductivity confirmed within a single cuprate plane establishes two-dimensional behaviour

Scientists have definitively demonstrated that superconductivity can exist within a single copper oxide plane, resolving a long-standing question in the field of cuprate materials. This breakthrough establishes that the phenomenon of high-temperature superconductivity is fundamentally two-dimensional, rather than reliant on interlayer interactions.
Researchers achieved this by fabricating a novel heterostructure containing an isolated half-unit-cell layer of La₂₋ₓSrₓCuO₄, effectively creating a single CuO₂ plane devoid of coupling with adjacent layers. Using in-situ angle-resolved photoemission spectroscopy, they meticulously mapped the electronic and gap structures of this singular plane.

The study reveals a d-wave-like gap in the electronic structure, which closes slightly above the bulk critical temperature, Tc. Significantly, the observed gap properties are almost identical to those found in bulk cuprate materials, indicating that the essential mechanisms driving superconductivity are present even in this isolated two-dimensional system.

These findings provide compelling evidence that cuprate superconductivity does not require interlayer coupling to emerge and function. This work overcomes decades of challenges in isolating a single CuO₂ plane, previously hindered by connectivity issues in ultrathin films and the difficulty of separating interlayer effects from intrinsic superconducting properties.

By employing a carefully engineered heterostructure consisting of La₂₋ₓSrₓCuO₄, LaSrAlO₄, and La₂CuO₄, the team created a stable and high-quality monolayer system suitable for detailed spectroscopic analysis. High-angle annular dark field scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed the structural integrity and composition of the fabricated heterostructure.

The ability to study superconductivity in a purely two-dimensional system opens new avenues for understanding the underlying physics of cuprates and provides a platform for exploring novel superconducting materials and devices. This research not only clarifies the fundamental nature of high-temperature superconductivity but also paves the way for designing and engineering future superconducting technologies with enhanced performance and functionality.

Heterostructure fabrication and photoemission spectroscopy of monolayer cuprates

In-situ angle-resolved photoemission spectroscopy served as the primary technique to investigate the electronic and gap structures of a single CuO₂ plane. The research focused on a heterostructure comprising a monolayer of La₂CuO₄ atop a LaSrAlO₄ buffer layer, with a La₂₋ₓSrₓCuO₄ conducting layer beneath.

This specific configuration was engineered to isolate a single CuO₂ plane for analysis, addressing a long-standing question in cuprate superconductivity regarding interlayer coupling. The heterostructure design compensated for photoelectron escape, ensuring accurate measurements of the monolayer system.

Fabrication began with the growth of La₂₋ₓSrₓCuO₄, chosen for its single CuO₂ plane per layer and suitability for heterostructure engineering. LaSrAlO₄ was selected as both the insulating layer and substrate to decouple electronic structures and maintain structural integrity. To prevent excessive doping from strontium diffusion, La₂CuO₄, devoid of strontium, was used as the topmost monolayer, positioning the hole doping within the superconducting dome.

This layered structure, LSAO, LCO, LSAO, and LSCO, was meticulously constructed to achieve a high-quality monolayer system. Confirmation of the heterostructure’s quality involved high-angle annular dark field scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy. HAADF-STEM imaging revealed well-aligned layers of LSAO, LCO, LSAO, and LSCO, demonstrating a high degree of structural order across a large area.

EDX imaging further validated the arrangement, clearly showing a single layer of copper atoms sandwiched between aluminium layers, confirming the successful growth of an epitaxial, high-quality single CuO₂ plane. The ARPES measurements then focused on the electronic structure of this topmost LCO monolayer, as the insulating LSAO layer exhibited negligible spectral weight near the Fermi level.

Superconductivity emerges in isolated lanthanum strontium copper oxide planes

Researchers demonstrated the existence of superconductivity within a single copper oxide plane in a lanthanum strontium copper oxide heterostructure. Angle-resolved photoemission spectroscopy measurements revealed a d-wave-like gap closing slightly above the bulk transition temperature, Tc. Gap properties observed in the single copper oxide plane were almost identical to those found in bulk material, confirming superconductivity can occur in this reduced dimensionality.

The study utilized a heterostructure comprising a half-unit-cell La2−xSrxCuO4 layer, grown on a LaSrAlO4 insulating substrate, to isolate a single CuO2 plane. High-angle annular dark field scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed the structural integrity of the fabricated heterostructure.

The design incorporated a lanthanum copper oxide layer to control hole doping and position it within the superconducting dome. ARPES measurements focused on the electronic structure of the topmost lanthanum copper oxide monolayer, effectively isolating the signal from the insulating lanthanum strontium aluminate buffer layer below.

The observed d-wave superconducting gap provides the first direct evidence of superconductivity in a single CuO2 plane. These results establish that cuprate superconductivity is fundamentally a two-dimensional phenomenon, offering a new platform for investigating the mechanisms of high-temperature superconductivity in a purely two-dimensional system.

Two-dimensional superconductivity confirmed in isolated cuprate planes

Researchers have demonstrated the existence of superconductivity within a single copper oxide plane in cuprate materials. Using a specifically engineered heterostructure containing an isolated layer of lanthanum strontium copper oxide, they employed angle-resolved photoemission spectroscopy to examine the electronic characteristics and energy gap of this single plane.

Observations revealed a d-wave-like energy gap that closes at a temperature slightly above the bulk superconducting transition temperature. Crucially, the properties of this gap were found to be almost identical in both the single plane and the bulk material, indicating that superconductivity can indeed occur within an isolated CuO₂ plane.

These findings establish that cuprate superconductivity is fundamentally a two-dimensional phenomenon, as it does not require interlayer coupling to emerge. This research provides a novel platform for investigating the mechanisms of cuprate superconductivity in a purely two-dimensional system. The authors acknowledge that previous attempts to achieve single-plane superconductivity were hampered by challenges in transport measurements, particularly issues with connectivity in ultrathin films.

Angle-resolved photoemission spectroscopy overcomes these limitations by probing the electronic structure directly, without relying on global electrical conductivity. Future research may focus on further characterising the properties of this isolated superconducting plane and exploring the potential for manipulating and enhancing superconductivity in these two-dimensional systems.