Graphene has long been regarded as one of the most promising materials for future electronics, but its relatively weak electron interactions have limited its potential for applications such as high-temperature superconductors. Now, researchers from Tohoku University have overcome a major obstacle by creating a stable version of the long-sought "boron graphene" on the surface of a three-dimensional crystal, revealing a new quantum state that could lead to more energy-efficient electronic devices.
The findings were published in Science Advances on July 2, 2026.
"We demonstrated a fundamentally new way of creating two-dimensional quantum materials," says Takafumi Sato of Tohoku University's Advanced Institute for Materials Research (WPI-AIMR). "Rather than attempting to produce an unstable free-standing sheet of boron atoms, we exposed a naturally occurring honeycomb boron layer that already exists within a stable three-dimensional crystal called LaRh₃B₂.
For years, scientists have been interested in borophene - a two-dimensional sheet of boron atoms - because its stronger electron interactions could produce exotic quantum phenomena not seen in graphene. However, borophene's ideal honeycomb structure is extremely unstable, making it almost impossible to manufacture.
Instead of trying to synthesize borophene directly, Sato and his colleagues took a different approach. They used the crystal structure of LaRh₃B₂, which naturally contains layers of boron atoms arranged in a honeycomb pattern. By exposing these layers at the crystal's surface, they created a stable two-dimensional electronic system with the properties of the elusive material.
Using angle-resolved photoemission spectroscopy (ARPES) at synchrotron radiation facilities, the team found an unusually high concentration of electrons near the material's Fermi level. This feature, known as a van Hove singularity, is important because it greatly strengthens interactions between electrons and can trigger unusual quantum behavior.
The researchers then combined these measurements with scanning tunneling microscopy and spectroscopy (STM/STS), which allowed them to observe the electrons in real space. Together, the two techniques showed that the electrons spontaneously aligned in one preferred direction, breaking the crystal's original six-fold symmetry and forming an "electronic nematic state" - a quantum state in which electrons behave similarly to molecules in a liquid crystal display.
"Instead of struggling to synthesize a fragile two-dimensional boron sheet from scratch, we looked inside a stable three-dimensional crystal that already contained a boron honeycomb lattice and exposed it on the material's surface," added Sato. "Observing this electronic liquid crystal state in a graphene-like material shows that carefully designing a material's electronic structure can unlock entirely new quantum phenomena."
A key aspect of the discovery was the combination of momentum-space and real-space imaging techniques. ARPES identified an electronic "hot spot" where the instability could emerge, while STM directly observed the resulting symmetry-breaking electronic pattern. By comparing the two sets of measurements, the researchers were able to explain how the electronic nematic state forms.
"Neither technique alone could have revealed the full picture," said Kosuke Nakayama, an assistant professor at Graduate School of Science. "By combining momentum-space information from ARPES with real-space observations from STM, we were able to connect the electronic instability with the emergence of the nematic state. This synergy was essential to understanding the physics behind this new quantum phase."
Because the crystal family used in this study allows many of its chemical elements to be substituted, researchers can readily adjust the number and behavior of electrons within the material. This flexibility provides a powerful platform for designing new quantum materials and could accelerate the development of next-generation superconductors and energy-saving quantum technologies.
- Publication Details:
Title: Realization of strongly correlated 2D honeycomb boron
Authors: Takemi Kato, Tomonori Nakamura, Kosuke Nakayama, Takumi Osumi, Seigo Souma, Asuka Honma, Alexandre Antezak, Pedro Rezende Gonçalves, Kiyohisa Tanaka, Miho Kitamura, Kenichi Ozawa, Koji Horiba, Hiroshi Kumigashira, Takashi Takahashi, Franck Fortuna, Andrés Felipe Santander-Syro, Rikio Settai, Yoshichika Onuki, Yoshinori Okada, and Takafumi Sato
Journal: Science Advances
