Flying pigs help explain electron transport

Imagine a world where impossible events – such as pigs flying – are not only probable, but actually have a direct effect on the technologies we use.

Since the dawn of the digital age, our computing power has exponentially increased. This is largely due to a continuous and significant decrease in the size of electronic components: namely transistors. Recently, traditional techniques for shrinking transistors have been approaching a minimum size limit, prompting a need for new methods to make smaller electronic components.

At such small sizes, electron transport is governed by quantum physics, which is fundamentally different from the everyday world we experience. Unlike the macroscopic (visible) world where physical events must follow the classic laws of physics, on the quantum level events that violate our macroscopic intuition occur with a finite probability.

An exciting area of research in quantum transport is molecular electronics, which aims to create electronic devices on the nanoscale – a million times smaller than a millimetre – and to harness quantum effects in the construction of devices.

At James Cook University (JCU), a research team has been investigating theoretical aspects of molecular electronics in order to better understand the behaviour of quantum transport.

JCU PhD student Samuel Rudge is researching how quantum phenomena affect waiting times in electron transport, and how to use these waiting times to further characterise electron behaviour at short time scales.

“When you start trying to miniaturise technology to as small as you can possibly go – which is at a molecular level – you run into some problems,” Samuel said. “First, you will observe fluctuations in the electric current, which is not something we necessarily want, and secondly, transport through the molecular system may display behaviour that is macroscopically impossible.”

Samuel’s work is entirely theoretical, and as such it relies heavily on modelling assumptions.

“We try to model the experimental setup used in molecular electronics,” Samuel said. “There are two macroscopic gold electrodes, which act as electron reservoirs. One is held at a high voltage and the other at a low voltage, so that electrons are encouraged to flow from the high energy electrode to the low energy electrode, causing electric current.

“Sandwiched between is the molecular system of interest, whether it be a molecule, chain of molecules, or some other structure. As electrons move through the molecular system they may display the quantum phenomena that we are interested in, and this may affect the electric current.”

Schematic of a molecular junction. A molecule is coupled to two macroscopic metal electrodes. The electrodes are held at different voltages, so that electrons are encouraged to flow from left to right across the system and induce electric current. Image: Julian Lawn

When Samuel mentions “quantum phenomena”, he is referring to the macroscopically impossible events that can only occur at such small sizes. Consequently, they are hard to visualise without macroscopic analogies.

“A large number of air molecules spontaneously condensing under a pig and pushing it up into the sky would be thermodynamically impossible in our everyday world, but such violations of macroscopic thermodynamics could occur if the pig were small enough.”

So far, his work has shown that for certain molecular systems, the presence of quantum phenomena does in fact affect the waiting times, and that the strength of the relationship depends on the molecular system. He stresses, however, that there remains experimental difficulty in measuring waiting times between these quantum phenomena.

“You can’t actually observe what’s happening inside the molecule because then you will stop the impossible events from happening… there is a fundamental limit on the amount of information that you can extract from a measurement on a quantum event,” Samuel said.

“Currently, to calculate waiting times, experimentalists measure tiny blips in the electric current. By recording fluctuations in the current they are able to infer the waiting times, although this method would struggle to incorporate quantum events.”

Despite these experimental difficulties, Samuel is confident that the field of molecular electronics will have an impact on future technologies.

“People see this as a way of the future, and that would be the broad context of where my research sits. It is a tiny part of making computer devices faster, more efficient and smaller.”

Samuel, who is supported by an Australian Government Research Training Program Stipend, and his supervisor Associate Professor Daniel Kosov have just released a paper titled, ‘Waiting times between electron cotunnelings’, which looks at how impossible events on a microscopic level are affected by electrons interacting with each other.

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