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Atomic Collapse Observed in Graphene

Left: Building a supercritical nucleus from Ca dimers (STM topograph). Right: Non-relativistic electron orbiting a subcritical nucleus (top) and ultra-relativistic electron orbiting a supercritical nucleus (bottom).
Berkeley Lab scientists have experimentally observed “atomic collapse” for the first time, confirming decades-old predictions and providing important insights for future graphene devices.

For more than 70 years, theorists have predicted that electrons in atoms with very large nuclei should behave completely differently than those in regular atoms. Instead of making stable orbits around the nuclei, these ultra-relativistic electrons should spiral into and out of the nuclear region and have some chance to escape — a phenomenon that has come to be known as atomic collapse.

Confirmation of atomic collapse has proved elusive because of the difficulty in making nuclei with nearly twice as many protons as the largest known element. To get around this problem the researchers took advantage of the special properties of graphene, in which the nuclear charge threshold for atomic collapse is much lower. The researchers assembled a cluster of charged calcium atoms on graphene using a scanning-tunneling microscope (STM), with pairs of Ca atoms playing the same role that protons play in regular atomic nuclei.

Using an STM, the researchers directly imaged how electrons behaved around the artificial nuclei as they increased the nuclear charge to the supercritical limit, thereby observing the signature of atomic collapse in good agreement with theory.

In addition to confirming basic relativistic quantum mechanics predictions, this exotic phenomenon will help illuminate the role of defects and dopants in future graphene devices.

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“Observing Atomic Collapse Resonances in Artificial Nuclei on Graphene.” Y. Wang, D. Wong, A.V. Shytov, V.W. Brar, S. Choi, Q. Wu, H.-Z. Tsai, W. Regan, A. Zettl, R.K. Kawakami, S.G. Louie, L.S. Levitov, M.F. Crommie, Science 1234320, 2013. doi:10.1126/science.1234320