Superconductivity—the magical phenomenon where electricity flows with zero resistance—has captivated scientists and engineers for more than a century. Imagine a world where power lines lose no energy, where magnetic levitation revolutionizes transport, and where quantum computers operate with unrivaled efficiency. That world hinges on mastering one of nature’s most elusive tricks: superconductivity at high temperatures. Now, a new study from researchers at the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC National Accelerator Laboratory has brought us one step closer to understanding this quantum enigma.
In a bold strike against long-standing theoretical assumptions, the research challenges the prevailing Hubbard model, a framework physicists have relied on for decades to decode the strange behavior of electrons in high-temperature superconductors, particularly in copper-based materials known as cuprates. The study, published in Physical Review Letters, reveals that even in simplified one-dimensional versions of cuprates, the Hubbard model falls short—significantly.
This is more than a theoretical squabble. It’s a pivotal turning point in the race to harness superconductivity for real-world technologies—and possibly, one day, at room temperature.
The Dream of High-Temperature Superconductors
Traditional superconductors require chilling to near absolute zero—just a few degrees above -273°C. But cuprates astonished scientists in the 1980s when they were found to superconduct at much warmer temperatures—still cold, but within reach of liquid nitrogen cooling systems, which are comparatively cheap and practical.
Cuprates remain the best-known class of high-temperature superconductors (HTS), with some operating as “warm” as -138°C. These copper-oxide compounds hint at the tantalizing possibility of creating superconductors that function at room temperature, a breakthrough that would revolutionize industries from energy to transportation to computing.
But despite nearly 40 years of research, scientists still don’t fully understand why cuprates superconduct at these relatively balmy temperatures. Unlike traditional superconductors, whose electron-pairing is elegantly described by the BCS theory, cuprates play by their own complex rules.
That’s where the Hubbard model entered the picture.
The Hubbard Model: A Hero—or a Red Herring?
The Hubbard model is a mathematical framework that attempts to capture how electrons behave in strongly correlated materials—systems where electrons don’t act independently but are tightly interlinked due to mutual repulsion and attraction.
For years, physicists hoped that this model would reveal the secret sauce of high-Tc superconductivity in cuprates. It was attractive because it offers a relatively simple way to describe the battle between an electron’s kinetic energy (its desire to roam freely) and the strong repulsion it feels from its negatively charged neighbors.
But there was a problem: while the model worked well in some scenarios, it couldn’t fully explain experimental results in actual cuprates, especially in two dimensions—where real superconductivity in these materials occurs.
So researchers at SLAC tried a clever approach: what if they simplified the problem down to a one-dimensional version of a cuprate? By stripping away complexity, could they finally test the Hubbard model’s predictions in a controlled way?
A Tale of Two Quantum Ghosts: Holons and Spinons
To test their idea, SLAC scientists created one-dimensional chains of cuprate atoms doped with oxygen. These synthetic quantum wires were then analyzed using advanced X-ray scattering techniques to observe the elusive behavior of two exotic quasi-particles: holons and spinons.
Let’s take a quick detour into the weird world of quantum mechanics. In typical physics, an electron is a single particle with mass, charge, and spin. But in certain quantum systems, those properties can “fractionalize”. This means the charge (holon) and spin (spinon) of an electron can behave like independent particles. It’s bizarre, but mathematically and experimentally well-supported.
In 2021, the SLAC team first observed holons in their 1D cuprate system. What they found was startling: the attraction between holons—standing in for electron charges—was ten times stronger than predicted by the Hubbard model. Something else was clearly at play, exerting an unseen force between charges.
Now, in their new study, the researchers turned their attention to spinons. If the mysterious force affected holons, it should also leave a mark on the spinons, which represent the magnetic moment of the electrons.
Cutting-Edge Experiments, Worldwide Collaboration
The new experiments were a marvel of international collaboration and precision physics. The team synthesized doped 1D cuprate chains at the Stanford Synchrotron Radiation Lightsource. They then used resonant inelastic X-ray scattering (RIXS)—a cutting-edge technique capable of probing the deepest properties of quantum materials—at the Diamond Light Source in the U.K. and the National Synchrotron Light Source II at Brookhaven National Laboratory in New York.
What they found was another nail in the coffin for the Hubbard model. The behavior of spinon pairs—akin to paired magnetic moments—was again inconsistent with the predictions of the model.
But when they modified the model to include an additional attractive force, the theory suddenly aligned much better with experimental data.
This suggests the presence of an additional force operating within the system—one the traditional Hubbard model completely overlooks.
So What’s Causing This Extra Attraction?
The million-dollar question now becomes: What is the nature of this mystery force that enhances electron attraction in cuprates?
Thomas Devereaux, a senior researcher at SLAC and Stanford who led the theoretical side of the study, has a compelling hypothesis. He believes the effect may be due to an interaction between electrons and the lattice vibrations, or phonons, of the crystal structure.
If that sounds familiar, it’s because phonon interactions are central to traditional BCS superconductivity. Could it be that, in the chaotic quantum environment of cuprates, a modified or previously overlooked electron-phonon interaction is also playing a key role?
If so, this opens a fascinating possibility: a hybrid theory that incorporates both strong electron correlations and phonon effects might finally explain the high-temperature behavior of cuprates.
Why This Matters for the Future
The implications of this research stretch far beyond academic interest. If we can identify and understand the mechanism that enables superconductivity at higher temperatures, scientists might design entirely new classes of materials that could operate at or near room temperature.
This would be revolutionary. Here’s a glimpse of what room-temperature superconductivity could unlock:
- Power grids that transmit electricity without loss, massively improving energy efficiency.
- Magnetic levitation for high-speed trains with zero friction.
- MRI machines that no longer require expensive cryogenic cooling.
- Quantum computers that remain coherent for longer, with dramatically improved stability and speed.
But to build such technologies, we need to understand the quantum underpinnings of superconductivity with surgical precision—and that means finding the right theory.
Beyond the Hubbard Horizon
The recent findings by the SLAC team mark a decisive shift in the quest to understand high-temperature superconductivity. The once-hopeful Hubbard model, while still a valuable tool, may no longer serve as the ultimate guide through the quantum labyrinth of cuprates.
As Wei-Sheng Lee, SLAC scientist and study co-lead, put it: “Our work has shown that the Hubbard model is inadequate to fully account for cuprate physics, even in a simple 1D system. If the model fails in 1D, we shouldn’t expect it to explain the more complex 2D systems where actual superconductivity occurs.”
The next phase in this journey will involve deeper exploration into the role of lattice dynamics, phonon interactions, and potentially entirely new mathematical frameworks. With advances in computational power, machine learning, and experimental techniques like RIXS, the scientific community is better equipped than ever to probe these mysteries.
The Superconducting Horizon
Superconductivity is more than just a curious quantum phenomenon—it’s a doorway into a future of technological marvels. The latest research from SIMES and SLAC isn’t just rewriting textbooks; it’s laying the groundwork for the superconducting technologies of tomorrow.
If we can understand what nature already does in cuprates, we might be able to replicate—and enhance—it in engineered materials. In doing so, we would not only unlock one of the greatest mysteries in condensed matter physics but also redefine the boundaries of what human technology can achieve.
The era of room-temperature superconductivity may not be here yet—but thanks to research like this, it just got a little closer.
Reference: Jiarui Li et al, Doping Dependence of 2-Spinon Excitations in the Doped 1D Cuprate Ba2CuO3+δ, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.146501. On arXiv: DOI: 10.48550/arxiv.2502.21316