SNU researchers solve 40-year physics mystery, revealing high-temperature superconductivity mechanism through “thermal decoupling”

A research team from the High-Temperature Superconductivity Research Group at Seoul National University, led by Prof. Gun-Do Lee, Research Professor at the Research Institute of Advanced Materials, has successfully explained the fundamental origin of high-temperature superconductivity—an unresolved question in physics for nearly 40 years—through a novel concept called “thermal decoupling.”

The study provides a quantitative explanation for a wide range of experimental results that could not be understood under traditional electron-centered theories, and has been recognized as presenting a new paradigm in superconductivity research.

These findings were published online on October 27 in Materials Today Physics (Impact Factor 9.7), an international academic journal in the field of materials physics, under the title “Thermal Decoupling in High-Tc Cuprate Superconductors.”

Superconductivity—the state in which electric current flows without resistance—was first discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes. In 1957, American physicists John Bardeen, Leon Cooper, and Robert Schrieffer developed the BCS (Bardeen–Cooper–Schrieffer) theory, explaining the mechanism behind superconductivity, for which they later received the Nobel Prize in Physics. However, this theory only applied to materials at temperatures below roughly –250°C (around 25 K).

In 1986, Bednorz and Müller at IBM Zurich Research Laboratory discovered that cuprate oxides exhibited superconductivity even at –240°C, and subsequent research found superconductivity at –140°C under atmospheric pressure. Since then, physicists around the world have been grappling with the fundamental question: “Why does superconductivity occur at such high temperatures?”

Professor Lee’s team at SNU determined that previous approaches over the past four decades had failed because they focused solely on the electronic properties of materials. Instead, the team turned its attention to the thermal properties of layered high-temperature superconductors. They found that most high-temperature superconducting materials have a layered (two-dimensional) crystal structure, with each layer composed of different elements, resulting in weak interlayer coupling. Remarkably, they discovered that below about –70°C (≈200 K), the flow of heat between layers is severed, leading to a phenomenon they termed “thermal decoupling.”

In particular, the team found that the CuO₂ layers—composed of copper and oxygen, where superconductivity actually occurs—maintain a low effective temperature inside YBCO (yttrium barium copper oxide) and thus satisfy the BCS theory conditions. However, experimental measurements typically reflect the higher surface temperature of the barium–oxygen layers, explaining why past experimental data appeared inconsistent with theoretical predictions. The key factor responsible for this temperature difference was identified as alkaline earth metals such as barium (Ba), which modulate ionic bonding between layers and block heat transfer.

* Alkaline earth metals: Elements belonging to Group 2 of the periodic table.

Moreover, theoretical calculations showed that once this temperature separation effect is accounted for, long-standing puzzles—including the linear temperature dependence of resistivity, the Uemura relation, the superconducting dome, and the reduced isotope effect—can all be quantitatively explained within a single unified framework.

* Linear-T resistivity: In conventional metals, electrical resistance before the superconducting transition follows a quadratic temperature dependence as predicted by Fermi liquid theory. In contrast, almost all high-temperature superconductors show resistance that varies linearly with temperature.

* Uemura relation: The critical temperature of high-Tc superconductors is proportional to their Fermi temperature.

* Superconducting dome: The critical temperature of high-Tc materials increases with carrier doping and then decreases again, forming a dome-like dependence.

* Reduced isotope effect: According to BCS theory, the superconducting critical temperature in low-Tc materials is inversely proportional to the square root of the isotope mass. In high-Tc superconductors, this effect is dramatically reduced.

By uncovering the mechanism behind high-temperature superconductivity—a long-standing challenge in condensed matter physics—this study is expected to accelerate innovation beyond semiconductor-based electronics, paving the way for quantum devices, power transmission, quantum computing, magnetic levitation, and energy storage technologies. The team has already filed a patent based on the core ideas for developing superconductors near room-temperature, and plans to begin machine learning–based exploration of new high-Tc superconducting materials in the near future.

Prof. Gun-Do Lee commented, “This study introduces a new physical paradigm that goes beyond the conventional concept of thermal equilibrium. Like Einstein’s theory of relativity or Planck’s quantum theory in their early days, it may provoke intense debate—but that is precisely what drives physics forward. We also expect experimental verification of thermal decoupling to emerge soon.”

Dr. Sungwoo Lee, the first author who performed the key theoretical calculations at SNU’s Research Institute of Advanced Materials, will extend this research to explore whether the thermal decoupling mechanism also applies to other high-Tc materials such as magic-angle twisted bilayer graphene and Kagome superconductors.

This work was supported by the “Future Promising Convergence Technology Pioneer” National Science Challenge Project of the Ministry of Science and ICT (MSIT) and the National Research Foundation of Korea (NRF), as well as by the KISTI Supercomputing Center’s Grand Challenge Research Project.

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