First direct view of an electron's short, speedy trip across a border

Phys.org ^ | February 11, 2019 | Glennda Chui, SLAC National Accelerator Laboratory

[ETL]

Electrons flowing across the boundary between two materials are the foundation of many key technologies, from flash memories to batteries and solar cells. Now researchers have directly observed and clocked these tiny cross-border movements for the first time, watching as electrons raced seven-tenths of a nanometer – about the width of seven hydrogen atoms – in 100 millionths of a billionth of a second.

Led by scientists at the Department of Energy's SLAC National Accelerator Laboratory and Stanford University, the team made these observations by measuring tiny bursts of electromagnetic waves given off by the traveling electrons – a phenomenon described more than a century ago by Maxwell's equations, but only now applied to this important measurement.

"To make something useful, generally you need to put different materials together and transfer charge or heat or light between them," said Eric Yue Ma, a postdoctoral researcher in the laboratory of SLAC/Stanford Professor Tony Heinz and lead author of a report in Science Advances.

"This opens up a new way to measure how charge – in this case, electrons and holes – travels across the abrupt interface between two materials," he said. "It doesn't just apply to layered materials. For instance, it can also be used to look at electrons flowing between a solid surface and molecules that are attached to it, or even, in principle, between a liquid and a solid."

Too short, too fast – or were they?

The materials used in this experiment are transition metal dichalcogenides, or TMDCs – an emerging class of semiconducting materials that consist of layers just a few atoms thick. There's been an explosion of interest in TMDCs over the past few years as scientists explore their fundamental properties and potential uses in nanoelectronics and photonics.

When two types of TMDC are stacked in alternating layers, electrons can flow from one layer to the next in a controllable way that people would like to harness for various applications.

But until now, researchers who wanted to observe and study that flow had only been able to do it indirectly, by probing the material before and after the electrons had moved. The distances involved were just too short, and the electron speeds too fast, for today's instruments to catch the flow of charge directly.

At least that's what they thought.

Maxwell leads the way

According to a famous set of equations named after physicist James Clerk Maxwell, pulses of current give off electromagnetic waves, which can vary from radio waves and microwaves to visible light and X-rays. In this case, the team realized that an electron's journey from one TMDC layer to another should generate blips of terahertz waves – which fall between microwaves and infrared light on the electromagnetic spectrum – and that those blips could be detected with today's state-of-the-art tools.

"People had probably thought of this before, but dismissed the idea because they thought there was no way you could measure the current from electrons traveling such a small distance in such a small amount of material," Ma said. "But if you do a back-of-the-envelope calculation, you see that if a current is really that fast you should be able to measure the emitted light, so we just tried."

Nudges from a laser

The researchers, all investigators with the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC, tested their idea on a TMDC material made of molybdenum disulfide and tungsten disulfide.

Working with SLAC/Stanford Professor Aaron Lindenberg, Ma and fellow postdoc Burak Guzelturk hit the material with ultrashort pulses of optical laser light to get the electrons moving and recorded the terahertz waves they gave off with a technique called time-domain terahertz emission spectroscopy. Those measurements not only revealed how far and fast the electric current traveled between layers, Ma said, but also the direction it traveled in. When the same two materials were stacked in reverse order, the current flowed in exactly the same way but in the opposite direction.

"With the demonstration of this new technique, many exciting problems can now be addressed," said Heinz, who led the team's investigation. "For example, rotating one of the two crystal layers with respect to the other is known to dramatically change the electronic and optical properties of the combined layers. This method will allow us to directly follow the rapid motion of electrons from one layer to the other and see how this motion is affected by the relative positioning of the atoms."

Explore further: Controlling charge flow by managing electron holes

More information: Eric Yue Ma et al. Recording interfacial currents on the subnanometer length and femtosecond time scale by terahertz emission, Science Advances (2019). DOI: 10.1126/sciadv.aau0073

Journal reference: Science Advances

Provided by: SLAC National Accelerator Laboratory

[Phillyred]

Undocumented electron.

[reed13k]

You wouldn’t happen to have a good book to recommend on the others would you? I’ve been looking for a decent one that covered them for some time but can only find additional work on the 4.

[PIF]

There, as far as I know, only a few copies of Maxwell's original work - all collectors' items. Most of the only line references omit Heaviside and present the four equations as Maxwell's (which they are not).

Some references I ran across:
James Clerk Maxwell. 1856. “On Faraday’s lines of force”. Transactions of the Cambridge Philosophical Society, vol. 10 (1856), pp. 27-83

James Clerk Maxwell. 1862. “On physical lines of force”. Philosophical Magazine Series 4, vol. 21 (1861), pp. 161-175, 281-291, 338-348; Philosophical Magazine Series 4, vol. 23 (1862), pp. 12-24, 85-95

James Clerk Maxwell. 1865. “A dynamical theory of the electromagnetic field”. Philosophical Transactions of the Royal Society, vol. 155 (1865), p. 459-512

This is the one I think you want to find:
James Clerk Maxwell. 1873. A Treatise on Electromagnetism. Oxford: Clarendon Press, 1873

Oliver Heaviside. 1894. Electrical Papers. New York and London: Macmillan & Co, 1894

[PIF]

Forgot to add that only in places obscure is anyone looking at the other 198 quaternions (as Maxwell called them). They are all field equations and there are few theoretical mathematicians working on them - if any.

Among the many reasons people avoid them is because they postulate the “impossible” like free energy for the ‘aether’ (see Nicola Tesla), collapsing distance - as in being able to step from one spot to another any where in the universe, instant communication over any distance, and so on.

Only ‘crackpots’ believe any of this is possible and so it is a career ending path and avoided (filed under ‘consensus thinking’); “we already know everything and it is just a matter of filling in the details”, is another way to put it - a popular science theology in the 1950-60s.

you can also try:
The Man Who Changed Everything
The Life of James Clerk Maxwell
by Basil Mahonr

and:
Oliver Heaviside
The Life, Work, and Times of an Electrical Genius of the Victorian Age
by Paul J Nahin

[reed13k]

Outstanding! Thank you!