Physicists See Electron Bound States in the Continuum for First Time
Physicists have observed electronic bound states in the continuum (BICs) for the first time, in a two-dimensional metal. The work from Columbia University, Brookhaven National Laboratory and Indiana University Bloomington appeared on October 22 in Science. A preprint was posted earlier on arXiv (2410.19227).
What a bound state in the continuum is
Normally a state whose energy falls inside a continuum of available states can leak into it and spread out. A BIC does not: it stays localised even though it has the same energy as the continuum. Such states are well known in optics; this is the first observation in an electronic system, according to Physics World.
How they found it
The team used scanning tunnelling microscopy and spectroscopy on Pd5AlI2, a two-dimensional van der Waals metal with a decorated checkerboard lattice, and combined the measurements with density functional theory and simple models. They identified two defect-induced electronic states that stay localised inside the metallic continuum. Two nearby defects showed evidence of hybridisation.
The calculations explain the effect. Interference between electron hops gives a quasi-flat band of localised states that are orthogonal to the continuum. Atomic defects shift these states into the continuum and form robust, symmetry-protected BICs. According to Physics World, the states sit above random palladium vacancies, are isolated from the continuum and were visible only because the microscope tip's disturbance lets electrons move in and out.
"Usually hybridization would be suppressed by a spectral gap, but here it's suppressed by quantum interference." — Raquel Queiroz, Columbia University, who developed the models
Alexandru Georgescu of Indiana University offered an analogy: in an insulator a defect state is like a glass of water that needs energy to get out, while here the "water" forms a wave that interferes with itself and does not travel.
Limits and what comes next
The result is fundamental and concerns one specific material; applications such as simulating many-body physics or catalysis are speculative. The BICs formed at vacancies that already existed at random sites. "There's no reason why one couldn't create these on demand," said Abhay Pasupathy of Columbia, who led the experiments, and the team is refining the technique to place them at chosen sites. Boubacar Kanté of UC Berkeley, who demonstrated the first optical BIC laser and was not involved, called the work "very interesting" and said it "shows the universality of wave phenomena". More in our Science section.