Bound states in the continuum (BICs) – static waves that remain isolated from propagating waves at similar energies – have been observed in electronic systems for the first time. This is analogous to a well-established phenomenon in optics, which has been exploited in lasers, sensors and directional antennas. The work was done by researchers in the US who believe that the electronic BICs could be useful for simulating exotic many-body physics and potentially even in fields like catalysis.
In the canonical electronic band structure model of materials, electrons in the valence band are localized, with insufficient energy to move freely. The conduction band, meanwhile, comprises higher-energy, larger orbitals more distant from the atomic nuclei. These hybridize throughout the material, so an electron that enters these is effectively free. The band gap in a semiconductor is defined by the energy required to excite an electron from the valence band to the conduction band. In a metal, the valence and conduction band energies overlap, creating a continuum of allowed energy levels.
However, the Schrödinger equation also allows bound electronic states to occur when electrons are confined not by energetic potential wells but by the inherent properties of the wavefunction. These bound states in the continuum (BICs) can therefore sit within the gapless band structure of a metal’s free electrons without coupling to them. In 2025, researchers at Columbia University in New York City developed the novel 2D metal Pd5AlI2 and showed that it displayed some features consistent with frustrated electron movement.
Tetragonal checkerboard
Now, researchers at Columbia and elsewhere have explained these features in terms of BICs and have verified this experimentally by using scanning tunnelling microscopy. The material’s crystal structure comprises a tetragonal checkerboard lattice with a relatively high density of defects, and the bound states form at these defects.
“Usually hybridization would be suppressed by a spectral gap, but here it’s suppressed by quantum interference,” says Columbia’s Raquel Queiroz, who developed the models. “All the different ways that [electrons] could jump out cancel each other out and interfere destructively.”
Team member Alexandru Georgescu of the University of Indiana offers an analogy to the way electrons are trapped in insulators. “In an insulator, a defect state is like a glass of water: the water would need some energy to get out,” he explains. “In a metal, it should be able to go anywhere because [the surface] is essentially flat. But in this material, you pour the water, it forms this nice wave that interferes with itself, and it doesn’t travel.”
The team used scanning tunnelling microscopy and scanning tunnelling spectroscopy, combined with density functional theory, to work out the locations and energies of the bound states. They occurred above stochastic palladium vacancies in the metal lattice, they found; they were extremely well isolated from the continuum states; and the researchers could only see them because of the disturbance to the band structure induced by the scanning tunnelling microscope tip, which allowed electrons to move in and out.
Easily engineered
When two defects occurred close together, the researchers saw evidence of hybridization between them. This could produce the kinds of many-body effects that have been seen in work with optical BICs, the researchers believe, in which structured surfaces are inherently easier to engineer. If successful, this could allow them to model phenomena such as superconductivity.
Optical supercavity drives tiny and efficient laser
The researchers are now refining their technique to allow them to place BICs at pre-determined sites. “There’s no reason why one couldn’t create these on demand,” says Columbia University’s Abhay Pasupathy, who headed the experimental work. “There’s fairly easy ways to make a bunch of holes in things and one could imagine seeing what happens to the electronic structure.”
The work could even be useful in catalysis, the researchers believe: “In general having places where electrons can go and attach themselves to under-coordinated sites is what catalysis is about, and I can imagine that if you have a particular kind of bound [transition] state then this kind of catalysis could enhance that reaction,” says Pasupathy.
Boubacar Kanté of University of California, Berkeley, whose group demonstrated the first optical BIC laser, says that the work is “very interesting” and “shows the universality of wave phenomena”. He notes that the concept of electronic systems were the first in which BICs were theoretically proposed back in 1929 by John von Neumann and Eugene Wigner, but the oscillating electromagnetic potential was too tricky to produce. “The fact that these bound states in the continuum are now coming back too electronics is very interesting,” he says. “It may be a new way to engineer quantum devices in the future, but this is a fundamental work,” he says.
The research is described in Science.
