Evolution of the structural and electronic properties of molecular fullerides with change in valence state
Despite the presence of strong correlations, the newly synthesized fulleride (box) continues to behave like a weak metal, by passing the transition to an insulating state and continuing to conduct electrons, even when reduced to cryogenic temperatures. Other materials are shown for comparison.
Image Credit: Osaka Metropolitan University
Scientific Frontline: Extended "At a Glance" Summary: Ytterbium Cesium Fulleride (\(\text{Yb}_2\text{CsC}_{60}\))
The Core Concept: Ytterbium cesium fulleride (\(\text{Yb}_2\text{CsC}_{60}\)) is a newly synthesized, all-carbon molecular system that continuously conducts electrons and maintains a robust metallic state, even when subjected to extreme cryogenic temperatures.
Key Distinction/Mechanism: While typical strongly correlated materials undergo a Mott metal-insulator transition—where interacting electrons become localized and turn the material into an insulator—\(\text{Yb}_2\text{CsC}_{60}\) possesses an unusual \(C_{60}\) valency of 5-. This near-filled electron band structure leaves a single "hole," allowing the quantum effect known as Hund's coupling to keep the electrons highly mobile within their p-orbitals. This mimics the electronic behavior typically restricted to transition metal d-orbitals.
Major Frameworks/Components:
- Mott Metal-Insulator Transition: A state change where conducting metals transform into insulators due to suppressed electron freedom, which is uniquely bypassed by this fulleride material.
- Hund's Coupling: A quantum mechanical effect governing how electrons populate orbitals and align their spins. Counterintuitively, this coupling preserves electron mobility in this specific single-hole state rather than trapping the electrons.
- p-Orbital vs. d-Orbital Parity: The discovery establishes an unexpected physical and electronic parallel between light-element molecular fullerides (p-orbitals) and well-documented transition metal systems (d-orbitals).
Orthorhombic Molecular Structure: The structural foundation of the newly synthesized \(\text{Yb}_2\text{CsC}_{60}\) compound that enables the rare 5- valence state.
Branch of Science: Condensed Matter Physics, Materials Science, and Quantum Chemistry.
Future Application: The molecular framework provides pathways toward discovering unconventional superconductivity in related systems, directly influencing the development of next-generation energy infrastructure, advanced electronic components, and transformative quantum technologies.
Why It Matters: By proving that correlated molecular p-electron systems can mirror the complex electronic behaviors of transition-metal d-electron materials, this breakthrough bridges a critical gap in fundamental physics. It broadens the foundational understanding of strongly correlated quantum matter and expands the architectural possibilities for advanced materials.
Molecular systems based on all-carbon fullerides mirror established electronic behavior of transition-metal materials
An international team whose research was coordinated by Osaka Metropolitan University (OMU) has reported the survival of metallic behavior in the strongly correlated molecular material ytterbium cesium fulleride (\(\text{Yb}_2\text{CsC}_{60}\)). The electrons in the newly synthesized material remained mobile and continued to conduct electricity even at the lowest temperatures studied, despite strong electron interactions that would normally be expected to force the material into an insulating state.
In materials such as metals, electrons move freely, allowing the materials to conduct electricity. However, as interactions between electrons become stronger, freedom of motion can be suppressed. Under these conditions, materials undergo a phenomenon known as a Mott metal-insulator transition, in which they change from a conducting metal into an insulating state where electrons become effectively immobile.
However, the group’s newly synthesized \(\text{Yb}_2\text{CsC}_{60}\) compound was special, as the localizing electron interactions were overcome, allowing the material to maintain its metallic state.
“The synthesis and availability of the new fulleride material were key,” said Keisuke Matsui of the Graduate School of Engineering at OMU.
To make this discovery, an international team—including the Institute Jozef Stefan (IJS) in Slovenia, the National Institute of Standards and Technology (NIST) in the United States, and the Aristotle University of Thessaloniki (AUT) in Greece—investigated the structural and electronic properties of the new compound. In their material, \(C_{60}\) had a valency of 5−, an uncommon feature that implied the existence of a single hole in the occupation of the triply degenerate lowest unoccupied molecular orbitals.
In many strongly correlated materials, strong interactions between electrons normally trap the electrons in place and turn the material into an insulator. This effect is often strengthened by a quantum effect known as Hund’s coupling when the electronic bands are half-full. According to Hund’s rule, electrons spread out across different orbitals with aligned spins before pairing up.
However, in the newly synthesized fulleride, the electronic bands are almost completely filled except for a single missing electron, or “hole.” In this case, Hund’s coupling instead helped the electrons remain mobile, allowing the material to retain its metallic behavior despite strong electron interactions.
This phenomenon has been well documented in transition-metal compounds in which the active electrons reside in d-orbitals. But it has remained virtually unexplored for “light-element” molecular systems, including fullerides, in which the electrons occupy p-orbitals.
The team discovered that the properties of their synthesized p-orbital fulleride material mirrored those of its d-orbital counterparts.
“We were all excited to see that our predictions were proven correct,” said Professor Yoshiki Kubota of the Graduate School of Science at OMU. “The Mott transition was suppressed, and the robust metallic state survived even when the compound was exposed to cryogenic temperatures.”
“These findings were due to the new material, which allowed us to pursue an exhaustive series of experimental measurements combined with theoretical calculations,” said Kosmas Prassides, a professor at IJS and a visiting researcher at OMU. “This allowed us to assert that electronic correlations and the competition with Hund’s coupling follow the behavior seen in the well-studied transition-metal compounds.”
The group believes its research into strongly correlated materials could aid transformative technologies. Previous fundamental discoveries in quantum mechanics eventually enabled semiconductors and computers, whereas superconductivity research contributed to technologies such as MRI systems.
They hope that follow-up research to understand how electrons collectively behave in molecular materials like \(\text{Yb}_2\text{CsC}_{60}\) will ultimately influence future electronics, energy systems, and quantum technologies.
“In this experimental work, we found unexpected similarities between two major classes of quantum materials: specifically, correlated molecular p-electron systems and transition-metal d-electron materials,” explained Professor Denis Arcon from IJS. “This provides new insights into fundamental concepts such as Hund’s coupling and strongly correlated quantum matter.”
“The newly synthesized orthorhombic fulleride may also open pathways toward discovering unconventional superconductivity in related molecular systems,” he added.
Funding: This work was financially supported by Grants-in-Aid for Scientific Research (JSPS KAKENHI grant numbers JP21H01907, JP22K18693, and JP23KJ1843), JST SPRING (grant number JPMJSP2139), the Slovenian Research Agency (grant numbers P1-0125 and J1-3007), and NIST (CMB in whole, and partial support for RAK).
Published in journal: Nature Communications
Title: Survival of the metallic state in a single-hole multiband p-orbital molecular system
Authors: Keisuke Matsui, Ryan A. Klein, Naoya Yoshikane, John Arvanitidis, Matjaž Gomilšek, Urh Klopčič, Shogo Kawaguchi, Hitoshi Yamaoka, Nozomu Hiraoka, Hirofumi Ishii, Qiang Zhang, Shigeo Mori, Hiroki Ishibashi, Yoshiki Kubota, Craig M. Brown, Denis Arčon, and Kosmas Prassides
Source/Credit: Osaka Metropolitan University
Edited by: Scientific Frontline
Reference Number: phy061426_01
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