A recent study found that the Hubbard model failed to accurately predict the behavior of a simplified one-dimensional cuprate system. According to scientists at SLAC, this suggests the model is unlikely to fully account for high-temperature superconductivity in two-dimensional cuprates.
Superconductivity—the ability of materials to conduct electricity without energy loss—holds immense promise for transformative technologies, from ultra-efficient power grids to advanced quantum devices. Cuprates, a family of copper-oxide materials, have been central to this research because they exhibit superconductivity at relatively high temperatures compared to traditional superconductors.
“Understanding what causes high-temperature superconductivity in cuprates is a decades-long puzzle,” said Jiarui Li, a SLAC postdoctoral researcher and lead author of the study. “We’re building on the work of many great scientists here at SLAC and Stanford, and I’m excited to help push the frontiers of this research.”
Cuprates and the Hope for Practical Superconductors
While traditional superconductivity typically requires temperatures close to absolute zero (-273°C or -459°F), cuprates maintain their superconductivity at temperatures up to -138°C (-216°F). This relatively higher operating temperature—still cold, but significantly above liquid nitrogen’s boiling point—makes them attractive candidates for practical technological applications.
Understanding the mechanisms that enable superconductivity at these temperatures could open the door to materials that superconduct even closer to room temperature.
In conventional metals like mercury and lead, electron pairing—a key requirement for superconductivity—is explained by the Nobel Prize-winning BCS theory. However, the unique electronic structure of cuprates requires a different theoretical explanation. Early hopes centered on the Hubbard model, a framework designed to describe strong electron interactions. Yet experimental validation of the model has proven elusive, largely due to the complexities of cuprate materials and the mathematical challenges involved.
A One-Dimensional Approach Yields New Clues
In 2021, SLAC researchers simplified the problem by examining cuprate behavior in one dimension rather than two. They created 1D chains of doped cuprate atoms and used X-ray techniques to study holons—quasiparticles representing an electron’s charge. Their analysis revealed that the attraction between neighboring electrons was ten times stronger than the Hubbard model predicted, pointing to an additional attractive force not accounted for by the model.
Seeking further confirmation, the researchers turned their attention to another electron property: spin. Using resonant inelastic X-ray scattering at the Diamond Light Source in the U.K. and the National Synchrotron Light Source II at Brookhaven National Laboratory, they studied the behavior of spinons—quasiparticles representing an electron’s spin—in doped 1D cuprate chains synthesized at the Stanford Synchrotron Radiation Lightsource.
Once again, the Hubbard model fell short. However, when researchers incorporated the additional attractive force previously observed, their theoretical predictions better matched experimental results.
“Our work shows that the Hubbard model is inadequate to fully account for cuprate physics, even in a simple 1D system,” said Wei-Sheng Lee, a SLAC staff scientist, and Zhi-Xun Shen, a professor at SLAC and Stanford and co-principal investigator of the study. “If the model already fails at the 1D level, it is unlikely to fully capture the more complex 2D behavior where high-temperature superconductivity occurs.”
The Search for the Missing Piece
The next challenge is to determine the source of this additional attractive force. Thomas Devereaux, a SLAC and Stanford professor and SIMES investigator who supervised the theoretical work, suggests that interactions between electrons and phonons—vibrations within the lattice structure—may be responsible. Further experimentation will be needed to test this hypothesis.
Reference:
Jiarui Li et al., “Doping Dependence of 2-Spinon Excitations in the Doped 1D Cuprate Ba₂CuO₃₊δ,” Physical Review Letters, April 8, 2025. DOI: 10.1103/PhysRevLett.134.146501
Funding:
This research was supported in part by the DOE Office of Science. The Stanford Synchrotron Radiation Lightsource at SLAC and the National Synchrotron Light Source II at Brookhaven National Laboratory are DOE Office of Science user facilities. Li also received funding from SLAC’s Laboratory Directed Research and Development Program.
Source: Scitech Daily
