1930’s theory on atomic collapse proved

Nieuws | de redactie
19 maart 2013 | Atomic collapse, a phenomenon first predicted in the 1930s but never observed, has now been seen for the first time in an “artificial nucleus” simulated on a sheet of graphene. This discovery by MIT and Berkeley could pave the way for new kinds of graphene-based electronic devices.

The discovery has been done by a team of scientists mainly from MIT and Berkeley. Leonid Levitov, physics professor at MIT and a co-author of the paper, says this work follows up on an early success of quantum mechanics that showed why matter is stable. It detailed how the positive charge of an atomic nucleus, and the negative charge of its surrounding electrons, balance each other out, and thereby preventing the atom from collapsing. 

Natural and man-made atoms are too light

But those early calculations also showed that this balance should break down above a certain point, to be more specific when the charge of the nucleus is more than 137. According to theory, in such super-charged atomic nuclei, the electrons should collapse into the nucleus, where they would then eject their antimatter opposites, positrons, which would spiral outward and away. Later refinements of the theory raised the threshold number from 137 to 170, but the underlying principle remained: “Such atoms were expected to collapse by grabbing an electron from vacuum, pulling it onto the nucleus and recharging,” Levitov says.

Since the known natural and man-made elements only reach an atomic number of 118, the prediction has been hard to demonstrate experimentally. Physicists have tried to demonstrate atomic collapse in particle accelerators by taking two heavy nuclei, such as those of uranium atoms (atomic number 92) and smashing them together, Levitov explains. “These experiments have been tried for decades,” he says, but no clear-cut evidence of collapse has been found.

Mimicking an atomic nucleus

In this research atoms are sitting on a sheet of graphene that exactly mimics the properties of atomic nuclei, and which can be manipulated to recreate and observe complex atomic phenomena. The key is that while electrons move through graphene as though they were massless, even though they actually do have mass. This means that their motion is 300 times slower than that of true massless particles. As a result, the expected phenomenon of collapse should take place at one-three-hundredth the normal nuclear charge which puts it in reach of experimental observations.

To simulate atomic nuclei, the researchers used pairs of calcium atoms on the graphene surface.  The researchers were able to manipulate these pairs, which are called dimers, on the surface using the probe tip of a scanning tunneling microscope. As soon as three dimers were pushed close together, the surrounding field of electrons showed a specific spectrum of resonances that precisely matched the predictions from the 1930’s of atomic collapse. The observed resonances persisted in a four-dimer and five-dimer artificial nucleus. These results provide “a clear case for collapse,” Levitov says. “We are certain this is correct.”

Paving the road for graphene-based electronics

Though the initial motivation for the work was the desire to prove a long-accepted theory about the quantum-mechanical behavior of atoms, the work has more than just theoretical relevance. As researchers around the world race to create electronic devices on graphene these findings, and the techniques developed to produce them, could provide important insights into graphene’s behavior, Levitov says.

The delicate sensitivity of artificial atoms on a graphene surface makes them incredibly responsive to surrounding conditions, which could lead to new detectors for trace chemicals or biomarkers, he suggests. The technique will continue to be used to probe different configurations of artificial atoms to probe factors important to basic physics and chemistry.

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