Superconductivity, a phenomenon where materials conduct electricity with zero resistance, has long been a holy grail in physics. Imagine a world where electrical power could travel without losses, where energy efficiency is maximized, and where the limits of technology are stretched to new heights. Superconductivity at room temperature, or even at more easily achievable temperatures, has the potential to revolutionize industries from energy storage to transportation.
Yet, for decades, researchers have faced the monumental challenge of finding materials that can superconduct at temperatures above the freezing point of water. Most superconductors need to be cooled to extremely low temperatures—often hundreds of degrees below zero. But there has been a breakthrough, and this new discovery might bring us one step closer to making high-temperature superconductivity a reality.
In 2024, a team of researchers, building on previous work, unlocked a crucial piece of the puzzle. They revealed why certain high-temperature superconductors have a greater ability to carry current without resistance, even at higher temperatures. This discovery, published in Physical Review Letters, centers on the cuprate family of superconductors—a class of materials that has long been the focus of research for their promise to operate at temperatures far higher than their metallic counterparts.
Understanding Cuprates: The Role of Copper-Oxygen Planes
At the heart of this discovery lies the cuprates, a family of copper-oxide materials that have been the focus of scientists for years. These materials have provided some of the highest superconducting temperatures ever recorded, but the mechanisms at play are far from fully understood.
What makes cuprates so special? They are unique because of their crystal structures, which include copper-oxygen (CuO₂) planes. These planes are crucial for their superconducting properties. Cuprates are typically ceramic materials, which makes them both fascinating and challenging to study.
Among the various cuprates, two notable candidates have stood out: Bi₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) and HgBa₂Ca₂Cu₃O₈+δ (Hg1223). The former has been easier to study, even though it superconducts at a temperature of 110 Kelvin (−163°C), while the latter, Hg1223, surpasses it with a superconducting temperature of 134 K. However, despite Hg1223’s higher performance, researchers initially favored Bi2223 for practical reasons—it’s simpler to manipulate into uniform, measurable samples, making it an ideal subject for early studies.
The New Frontier: Growing the Perfect Crystals
In 2024, a breakthrough came from researchers in Japan who discovered how to grow better-quality crystals of a cuprate with superconducting properties similar to those of Hg1223. This new material, (Hg,Re)1223, is a modified version of Hg1223 where some of the mercury atoms are replaced with rhenium atoms (Re). The substitution of mercury with rhenium helped stabilize the crystal, making it much easier to study and analyze.
The resulting material was found to have a critical superconducting temperature of 130 K—slightly lower than Hg1223, but still a significant leap in terms of achievable temperatures for ambient-pressure superconductivity. The team, led by Masafumi Horio at the University of Tokyo, began analyzing these new crystals using a cutting-edge technique called angle-resolved photoemission spectroscopy (ARPES), which enabled them to peer into the electronic properties of the material at an unprecedented level of detail.
ARPES: Unveiling the Secrets of Superconductivity
ARPES is a powerful experimental method used to map a material’s electronic structure. It works by shining ultraviolet or X-ray light onto a sample and observing how electrons are ejected from it. These emitted electrons carry valuable information about the material’s electronic band structure—essentially, how electrons behave within the material and how they interact with each other.
The technique reveals the “binding energy” of the electrons, which allows scientists to determine the energy levels of electron pairs—also known as Cooper pairs—that are responsible for superconductivity. These Cooper pairs travel without resistance, which is what makes superconductors so extraordinary. In simple terms, ARPES gives researchers a clear map of the material’s electronic landscape, showing them how the electrons are arranged and how they contribute to the material’s superconducting properties.
What Horio and his colleagues discovered was a key difference between the superconducting gaps in different layers of the (Hg,Re)1223 cuprate. The superconducting gap refers to the energy difference between the lowest energy states that electrons can occupy and the next available energy states. A wider superconducting gap correlates with higher superconducting temperatures, as it prevents smaller excitations (like electron scattering) that could interfere with superconductivity.
Unraveling the Mystery of the Outer Layers
Through their ARPES analysis, the researchers found that the superconducting gap in the inner copper-oxide (CuO₂) layer of (Hg,Re)1223 was almost identical to that in Bi2223—around 62–63 millielectronvolts (meV). This confirmed that the inner layers of these materials play a critical role in their superconducting properties. In Bi2223, this layer is responsible for its high critical temperature of 110 K.
However, the outer layers of (Hg,Re)1223 revealed a surprising twist. The superconducting gap in these layers was much wider—57 meV for the outermost layer compared to only 43 meV in Bi2223. This increase in the superconducting gap in the outer layers of (Hg,Re)1223 led to an increase in the material’s critical temperature, pushing it up to 130 K. This result suggests that it’s not just the inner layer but also the outer layers of the cuprate that contribute to its higher performance at superconducting temperatures.
The researchers’ findings suggest that the outer layers in the Hg-based cuprates, like (Hg,Re)1223, may play a crucial role in achieving higher superconducting temperatures at ambient pressure. Previously, the focus had been on the strong pairing mechanisms in the inner CuO₂ layers, but this new work suggests that the pairing energy in the outer layers is just as important.
The Road Ahead: Fine-Tuning Superconductivity
With this new understanding of how the outer layers contribute to superconductivity, researchers are now in a stronger position to refine their approach to creating high-temperature superconductors. The enhanced pairing energy in the outer layers of (Hg,Re)1223 could be key to unlocking the potential for even higher superconducting temperatures. This discovery opens up new avenues for investigating the finer details of cuprate superconductors, including the role of electron-phonon coupling and the interactions between different layers within the crystal.
This type of detailed analysis, made possible by ARPES, will allow scientists to better understand the parameters that govern superconductivity in cuprates. By fine-tuning these parameters, researchers hope to unlock the secret to achieving room-temperature superconductivity—an ultimate goal that could revolutionize power grids, transportation, and countless other fields.
A Glimpse into the Future
The discovery made by Horio and his team is just one piece of the puzzle, but it brings us closer to a future where superconductivity can be harnessed at practical temperatures. By exploring the intricate details of cuprate crystals and understanding the role of their inner and outer layers, scientists are one step closer to making room-temperature superconductivity a reality.
As the research continues, we can expect more breakthroughs, each one pushing the boundaries of what we know about high-temperature superconductivity. The potential applications are vast: Imagine a world where energy is transmitted with zero loss, where magnetic levitation is used in everyday transportation, and where the power of quantum computing is harnessed at scale. The possibilities are endless, and it all begins with the fundamental understanding of how these materials work at the atomic and electronic levels.
As we stand on the precipice of a new era in materials science, the question is no longer “If” we will achieve room-temperature superconductivity, but “When.” And thanks to the work of scientists like Horio and his colleagues, the answer may be closer than we think.
More information: M. Horio et al, Enhanced Superconducting Gap in the Outer CuO2 Plane of the Trilayer Cuprate (Hg, Re)Ba2Ca2Cu3O8+δ, Physical Review Letters (2025). DOI: 10.1103/p4c3-t34b. On arXiv: DOI: 10.48550/arxiv.2506.08763
