Today my atomic manipulation group at Bath, in collaboration with the University of Birmingham, have published a nice article in Nature Communications: "Initiating and imaging the coherent surface dynamics of charge carriers in real space." Below is a copy of the official University of Bath press release.
The work is the the 3rd paper in a series starting in 2010 that outlines the life of an electron from hopping off a scanning tunnelling microscope tip onto a surface (PRL 2010), then undergoing some pristine quantum mechanics (the new paper, NatComms 2016), before scattering and forming a quasi-equilibrium system (NatComms 2015) and finally causing a single molecule to detach from the surface (Nanotechnology 2016 in press).
The work published today was primarily the PhD project of Kristina Rusimova (pictured) a URS funded PhD student, with input from the previous student Dr Duncan Lock and a pair of MPhy project students Nicola Bannister and Patrick Harrison (pictured below). Nicola and Patrick liked doing research so much they are now doing PhDs at Bath (CMP CDT) and Birmingham respectively. The work was funded by the EPSRC, the University of Bath and several generous donations from my collaborator Prof Richard Palmer at the University of Birmingham.
Scientists have identified a method to visualise, over a millionth of a billionth of a second, the initial quantum behaviour of electrons on a surface. The findings are a promising step towards being able to manipulate and control the quantum behaviour of high energy, or ‘hot’, electrons – important for future high efficiency solar cells, and atomically engineered systems including proposed quantum computing devices.
A team from the Department of Physics at the University of Bath, working with colleagues at the University of Birmingham, used a Scanning Tunnelling Microscope 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, the team identified that electrons retain their initial trajectories, or quantum state, across the surface for the first seven nanometers of travel, before they are disturbed and undergo random scattering like the ball in a pin-ball machine. In essence changing from a quantum to a classical system.
Dr Peter Sloan, from the University of Bath (in picture), said: “Hot electrons are notoriously difficult to observe due to their short lifespan, about a millionth of a billionth of a second. This visualisation 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 nanometers in diameter. “
“Quantum physics dictates that electrons behave as waves. Just as a pebble dropped into a still pond forms concentric rings that propagate out, so during the initial 7 nm so does the hot electron. The electron starts off as a tiny object less than a nanometer 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 (click on picture for animation). That may seem small, but on the scale of atoms and molecules this is really a vast size.”
Professor Richard Palmer, from the University of Birmingham, explained: “These findings are, crucially, undertaken at room temperature. They show that the quantum behaviour of electrons which is easily accessible at close to absolute zero temperature (-273°C!) persist under the more balmy conditions of room temperature and over a large 15 nanometre scale. These findings suggest future atomic-scale quantum devices could work without the need for a tank of liquid helium coolant.”
Now that the team have developed the method of visualising quantum transport, the goal is to understand how to control and manipulate the initial quantum state of the electron. As Prof Palmer put it: “The implications of being able to manipulate the behaviour of hot electrons are far-reaching; from improving the efficiency of solar energy, to improving the targeting of radiotherapy for cancer treatment.”
The research is published in Nature Communications.
The theoretical modelling was done in collaboration with Dr Simon Crampin at Bath. The experiments were primarily performed by PhD student Kristina Rusimova), with a subset run as a project for undergraduates. Nicola Bannister, who is now starting at physics PhD at University of Bath said: “The MPhys undergraduate project was my first introduction into academic research. It was a fantastic experience and is the reason I chose to continue my academic career by doing a PhD.”
Here I wish to play a bit with the "Twin Paradox" of Special Relativity. [*note, it ain't a paradox]. For this you'll need to be happy with (1) time-dilation, (2) length-contraction, (3) Lorentz velocity transforms and (4) straight lines. I like to draw things, so in the video I construct 3 space-time diagrams for the various reference frames of the problem. A pdf version of the 3 space-time diagrams can be found here: Sloan_Twin_SPACETIME so you can see the final versions.
The subtext to this is really it's a chance to put into practice some of the fundamental properties of Special Relativity outside their usual undergraduate use of simple problems. The logic I follow is hopefully step-by-step simple, but the picture we build up will be complicated. At the end the paradox will melt away, in fact it was never there in the first place.