Superconductivity—a phenomenon where certain substances can transmit electric current without losing energy—carries significant potential for developing innovations such as efficient power transmission lines and sophisticated quantum equipment.
A publication in Physical Review Letters Research conducted by scientists at the Stanford Institute for Materials and Energy Sciences (SIMES), located within the Department of Energy's SLAC National Accelerator Laboratory, has illuminated a long-standing enigma in the field of superconductivity: specifically, how high-temperature superconductivity operates in materials known as cuprates.
Building upon findings from an earlier SLAC study, this research offers additional proof that the Hubbard model—which is considered the primary theory for explaining strong electron correlations in quantum materials—falls short of accurately depicting electron behavior in cuprates, even when examining simpler, one-dimensional setups.
"Understanding what causes high-temperature superconductivity in cuprates is a decades-long problem, and we're building on the work of many great scientists here at SLAC and Stanford," said Jiarui Li, a SLAC postdoc and lead author of the study. "As a postdoc, I'm excited to continue pushing the frontiers of this research."
Generally, superconductivity happens when materials are cooled close to absolute zero, which is around -273 degrees Celsius or -459 degrees Fahrenheit. However, specific compounds called cuprates—which contain copper oxide—are capable of maintaining this state even at higher temperatures up to -138 degrees Celsius or -216.4 degrees Fahrenheit—a temperature considerably warmer compared to absolute zero yet still quite chilly. Notably, these conditions allow them to function about 100 degrees Fahrenheit above the boiling point of liquid nitrogen, thus enabling more widespread technological uses due to easier cooling methods.
Grasping the processes that enable superconductivity at these relatively elevated temperatures might be crucial for creating future technologies and designing novel materials capable of superconducting at even greater temperatures, preferably approaching room temperature.
Researchers understand that superconductivity happens when electrons pair up inside a substance, forming what are referred to as Cooper pairs. This pairing mechanism in materials like mercury and lead can be described using the BCS theory, which earned a Nobel Prize for explaining this phenomenon. However, the electron arrangement in compounds known as cuprates differs significantly from those found in conventional metals and necessitates an alternative theoretical framework to elucidate the process through which superconducting electrons couple together.
At first, researchers believed that the Hubbard model, known for its capability to illustrate robust electron interactions, could potentially elucidate the mechanism behind high-temperature superconductivity in cuprates. However, this hypothesis remained unsupported, and attempts at empirical confirmation have been fraught with difficulty. The inherent complexities of both cuprate compounds and the intricate mathematics involved in the Hubbard model render precise computational modeling using today’s technology and methods quite challenging.
In 2021, SLAC researchers found A method to streamline the investigation involves looking at cuprates in one dimension instead of two dimensions. For the first time, researchers formed a single row of cuprate atoms infused with oxygen and utilized X-rays to examine how holons—the quasi-particles symbolizing an electron’s charge—behave. The findings indicated that the pull between adjacent electrons was significantly more potent than what the Hubbard model anticipated, being about tenfold greater. This discrepancy implies the presence of an extra attractive interaction overlooked by the Hubbard model.
Scientists discovered that if this force existed, it would imprint distinctive marks on another key characteristic of electrons called spin. To tackle the issue from a different perspective, they created a novel experiment aimed at providing deeper understanding of how pairs of spinons behave. Similar to holons, which are quasi-particles representing aspects of an electron’s attributes, spinons also act as such entities but specifically relate to an electron’s spin, whereas holons pertain to its charge.
The research group created a one-dimensional specimen of doped cuprate chains at the Stanford Synchrotron Radiation Lightsource located at SLAC, after which they analyzed it through resonant inelastic X-ray scattering conducted at the Diamond Light Source in the UK as well as at the National Synchrotron Light Source II situated at Brookhaven National Laboratory.
Again, their study of spinon pair behaviors revealed that the Hubbard model failed to precisely forecast electron actions. Nevertheless, upon incorporating an additional attractive force identical to what was used in previous experiments into their computations, the results matched their observed data much better.
Our research indicates that the Hubbard model falls short of explaining cuprate physics comprehensively, even when applied to a straightforward one-dimensional setup. Given this limitation at the simpler one-dimensional stage, it’s unlikely that the model would accurately represent phenomena in the more intricate two-dimensional systems where high-temperature superconductivity takes place in cuprates,” stated Wei-Sheng Lee, a SLAC staff scientist, along with Zhi-Xun Shen, a SLAC and Stanford professor who served as co-principal investigators for the investigation.
The current query revolves around identifying the mechanism responsible for the extra attractive force. According to Thomas Devereaux, a SLAC and Stanford professor along with being a SIMES investigator who oversaw the theoretical aspects of this research, he believes this phenomenon might be attributed to an interaction between electrons and vibrations called phonons within the lattice framework that holds the cuprates together. More experiments will have to be conducted to explore this hypothesis further, noted Devereaux.
More information: Jiarui Li and colleagues explored the impact of doping levels on two-spinon excitations within the doped one-dimensional cuprate material known as Barium Copper Oxide. 2 CuO 3+δ , Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.146501 . On arXiv : DOI: 10.48550/arxiv.2502.21316
Provided by SLAC National Accelerator Laboratory
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