Oxford scientists have observed an extraordinary effect at the atomic scale: the movement of ions can retain a “memory” of what happened moments earlier. The discovery challenges a simple assumption about how matter flows and could help scientists develop better batteries and new computing technologies

Whether water flows through a pipe or electrical charge moves through a battery, transport is one of the most fundamental processes in nature.

At the scale of individual atoms, however, scientists still have much to learn about what actually happens.

Researchers from the University of Oxford’s Department of Materials, working with scientists at SLAC National Accelerator Laboratory in California, have now measured the movement of charged atoms — ions — on extremely short timescales.

They discovered something unexpected.

An ion’s next movement can be influenced by its recent past.

The researchers describe this as a “memory effect.”

Atoms that remember

The word “memory” should not be taken literally: atoms do not think or store memories as a brain does.

Instead, it means that atomic movement is correlated over time.

If scientists disturb an ion and cause it to move in a particular direction, its subsequent movement is not immediately completely random.

For a tiny fraction of a second, information about the earlier movement remains detectable.

Lead researcher Dr Andrey Poletayev explained what the team observed: after ions were pushed in one direction, they showed a preference for moving back in the opposite direction.

Like moving honey

The researchers compared the behaviour to rapidly disturbing a viscous substance such as honey.

Push it suddenly, and it does not instantly return to completely random movement. Its previous motion influences the way it relaxes.

Something comparable happens with the ions.

The effect lasts only a few trillionths of a second.

But on the atomic timescale, that is long enough to measure — provided scientists have instruments fast enough to see it.

Using light to watch atoms move

To observe the phenomenon, the researchers used a technique known as pump-probe spectroscopy.

Extremely rapid and intense pulses of light were used both to disturb the ions and to measure their response.

Professor Saiful Islam, senior author of the study, compared the experiment to throwing a stone into a pond and observing how the resulting waves spread.

In this case, however, the “stone” is a pulse of light and the researchers are watching atoms rather than water.

The technique allowed the team to connect the microscopic movement of individual ions with the macroscopic flow of matter.

Why batteries matter

The researchers used a battery material as their experimental system.

When a battery charges, enormous numbers of ions move from one electrode towards another.

At the macroscopic level this appears as a smooth overall flow of charge. But that flow is actually the combined result of vast numbers of individual atomic movements.

Until now, it was difficult to determine whether these tiny movements were completely independent or whether one movement influenced the next.

The new experiments demonstrate that, for a brief period, they are not independent.

The ions retain a measurable correlation with their previous movement.

From microscopic behaviour to better materials

Understanding this phenomenon could have practical consequences.

Scientists searching for better battery materials need to predict how efficiently ions will move through a particular substance.

If models assume that every atomic movement is independent when it is actually influenced by previous movements, those models may miss part of the underlying physics.

Measuring and quantifying the memory effect could therefore help researchers make more accurate predictions about new materials before manufacturing them.

That could eventually contribute to the development of batteries that transport charge more efficiently — particularly important as electric vehicles and renewable-energy storage increase demand for improved battery technologies.

Beyond batteries

The implications extend further.

Ion transport is important in many technologies and natural processes.

Oxford’s researchers point to potential relevance for areas including neuromorphic computing, which attempts to build computer systems inspired by the architecture of the brain, as well as desalination technologies and other systems in which atoms or ions move through solids or liquids.

The measurement technique itself may also become increasingly powerful.

As its sensitivity improves, researchers expect to be able to observe memory effects over longer timescales and in a wider variety of materials.

The microscopic world is different

Perhaps the most fundamental result is conceptual.

At the scale we experience in everyday life, many transport processes appear to have no memory.

Water flows. Electrical charge moves. Heat spreads.

We can often describe those processes without keeping track of the precise history of every individual particle involved.

But the Oxford experiment shows that this macroscopic simplicity does not necessarily exist at the atomic level.

There, history can matter.

An ion makes a movement.

For a few trillionths of a second, that movement leaves a trace in what happens next.

And scientists can now see it.

Source: Based on “Innovative technique reveals that leaping atoms remember where they have been,” published by the University of Oxford on 15 February 2024. The research was conducted by scientists from Oxford’s Department of Materials and SLAC National Accelerator Laboratory. The underlying study, “The persistence of memory in ionic conduction probed by nonlinear optics,” was published in Nature.