A team of physicists from the University of Birmingham and the University of Bath has identified a method to visualize, over a millionth of a billionth of a second, the initial quantum behavior of electrons on a surface.
The scanning tunneling microscope used to inject electrons into a silicon surface. Image credit: Michelle Tennison.
The scientists used a scanning tunneling microscope (STM) to inject electrons into a silicon surface, decorated with toluene molecules.
As the electrons propagated from the tip position across the surface, they induced the toluene molecules to react and ‘lift off’ from the surface.
By measuring the precise atomic positions from which molecules moved, they identified that electrons retain their initial trajectories, or quantum state, across the surface for the first 7 nm of travel, before they are disturbed and undergo random scattering like the ball in a pin-ball machine. In essence is a change from a quantum to a classical system.
The team’s findings, published in the journal Nature Communications, are a promising step towards being able to manipulate and control the quantum behavior of high-energy electrons — important for future high efficiency solar cells, and atomically engineered systems including proposed quantum computing devices.
“High-energy electrons are notoriously difficult to observe due to their short lifespan, about a millionth of a billionth of a second,” said senior author Dr. Peter Sloan, from the Department of Physics at the University of Bath.
“This visualization technique gives us a new level of understanding. We were surprised to find that the initial quantum trajectories stay intact for long enough for a single electron to ‘spread out’ over a disc 15 nm in diameter.”
“Quantum physics dictates that electrons behave as waves,” Dr. Sloan said.
“Just as a pebble dropped into a still pond forms concentric rings that propagate out, so during the initial 7 nm so does the high-energy electron.”
“The electron starts off as a tiny object less than a nanometer in diameter just after we inject it into the surface, then it calmly propagates out, getting bigger and bigger, by the time it’s disturbed (losing its pristine quantum nature) it reached the size of a series of rings 15 nm in diameter. That may seem small, but on the scale of atoms and molecules this is really a vast size.”
“These findings are, crucially, undertaken at room temperature,” said co-author Prof. Richard Palmer, from the Nanoscale Physics Research Laboratory at the University of Birmingham.
“They show that the quantum behavior of electrons which is easily accessible at close to absolute zero temperature (minus 273 degrees Celsius, or minus 459 degrees Fahrenheit) persist under the more balmy conditions of room temperature and over a large 15 nm scale.”
“These findings suggest future atomic-scale quantum devices could work without the need for a tank of liquid helium coolant.”
Now that the physicists have developed the method of visualizing quantum transport, the goal is to understand how to control and manipulate the wave function of the electron. This could be by injecting electrons through a cluster of metal atoms, or by manipulating the surfaces themselves to harness the quantum effects of electrons.
“The implications of being able to manipulate the behavior of high-energy electrons are far-reaching; from improving the efficiency of solar energy, to improving the targeting of radiotherapy for cancer treatment,” Prof. Palmer said.
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K.R. Rusimova et al. 2016. Initiating and imaging the coherent surface dynamics of charge carriers in real space. Nat. Commun. 7: 12839; doi: 10.1038/ncomms12839
