Researchers observe localized electron states inside a metal
The team calls this the first observation of electronic bound states in the continuum and is working to place these confined states at chosen sites.
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The team calls this the first observation of electronic bound states in the continuum and is working to place these confined states at chosen sites.
Researchers at Columbia University and other institutions have observed two electronic states that remain confined near defects in the two-dimensional metal Pd5AlI2. Their energies lie within the metal’s conducting continuum—the range of energies available to electrons that can move through the material.
The researchers describe the finding as the first observation of electronic “bound states in the continuum” (BICs), in Physics World’s 27 October report. Such states were predicted for electrons in 1929, but analogous states involving light were established earlier and have been used in lasers, sensors and directional antennas.
In 2025, Columbia researchers developed Pd5AlI2 and found signs that electron movement was being frustrated. The new work explains those features through BICs. The 2026 Science paper is titled “Electronic bound states in the continuum in a two-dimensional metal.”
An electron can ordinarily be confined by an energy barrier or by a gap in the available conducting states, as in a semiconductor. A BIC instead stays localized even when conducting states exist at the same energy. Its confinement comes from the properties of its quantum wave, rather than an energy barrier.
The team used scanning tunnelling microscopy to locate the states and scanning tunnelling spectroscopy to probe their energies. Together with density functional theory, a method for calculating electronic structure, the measurements placed the states above palladium vacancies—sites where palladium atoms were missing from the lattice. These vacancies occurred randomly.
Columbia’s Abhay Pasupathy led the experimental work, while Raquel Queiroz developed the models. Alexandru Georgescu of the University of Indiana was also a team member. Queiroz’s explanation connects the material’s checkerboard-like atomic lattice to destructive quantum interference: the different routes by which an electron could leave a defect cancel one another, suppressing mixing with the conducting states.
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That cancellation is the modelling explanation for the measured localization. The researchers say the states were so strongly isolated that the microscope could detect them only because its tip disturbed the material’s electronic structure, allowing electrons to enter and leave. The measurement therefore involved disturbing the isolation it was probing.
Analysis
When two defects lay close together, the researchers saw evidence of hybridization, meaning that their bound states mixed. The team believes deliberately coupled BICs could let researchers study collective behaviour involving many interacting electrons, potentially including simulations of superconductivity. That proposed use goes beyond the observed mixing between nearby defects.
Pasupathy also suggests a possible use in catalysis, where a material helps a chemical reaction proceed. He points to sites where atoms have fewer neighbours and electrons can attach, and proposes that a suitable bound transition state during a reaction could enhance it. This remains a proposed mechanism, rather than a demonstrated reaction enhancement.
Boubacar Kanté of the University of California, Berkeley, whose group demonstrated the first optical BIC laser, says the result shows that wave phenomena work across different physical systems. He describes it as fundamental research that might eventually offer a way to engineer quantum devices.
The team is refining its technique to place BICs at predetermined sites instead of relying on randomly occurring vacancies. Pasupathy says creating them on demand should be possible, which would allow controlled tests of how their electronic states interact.
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