Innovative technique reveals that leaping atoms remember where they have been

Researchers at the University of Oxford have used a new technique to measure the speed of charged particles (ions) on the fastest timescales, revealing new insights into the fundamental processes of transport. These include the first demonstration that a flow of atoms or ions has ‘memory’. The results are published in the journal The nature.

The laser setup used in the experiment is in the laboratory of Dr. Matthias Hoffmann at the Stanford Linear Accelerator (SLAC) National Laboratory in California. Photo credit: Dr. Andrey Polityev.

Whether charging a battery or pouring water, the flow of matter is one of the most fundamental processes in the universe. But a surprising amount is unknown about how this happens at the atomic scale. Understanding it better can help us solve a wide range of problems, including developing the materials needed for tomorrow’s technologies.

In the new study, a team of Oxford-based researchers Department of Finance And the Stanford Linear Accelerator (SLAC) National Laboratory in California made the surprising discovery that the motion of individual ions can be influenced by their recent past. In other words, there is a ‘memory effect’. This means that history matters on a microscopic scale: what a particle did one moment ago can affect what it does next.

Until now, this has been extremely difficult to observe because such an effect is imperceptible by simple observation. To test whether ion motion has memory, something unusual must be introduced: perturb the system, and then see how the perturbation dissipates.

Senior author Professor Saiful Islam (Department of Materials, University of Oxford) said: ‘To use a visual analogy, such an experiment is like throwing a stone into a pond to see how far the ripples travel. But to see the flow of atoms, the rock in our study must be pulsed with light. Using light, we have captured the movement of ions on the fastest time scale ever, revealing the connection between the individual motion of atoms and the macroscopic flow.’ used battery materials as a model system for investigation. When a battery is charged, an applied force physically moves many ions from one electrode to another. The multitude of random motions of individual ions collectively add up to a net motion similar to liquid flow. What was not known was whether this overall flux was influenced by memory effects acting on individual ions. For example, do the ions bounce back after forming atom-sized hoops, or do they flow smoothly and randomly?

To capture this, the team used a technique called pump-probe spectroscopy, which uses sharp, intense pulses of light to both stimulate and measure the movement of ions. . Such nonlinear optical methods are commonly used to study electronic phenomena in applications ranging from solar cells to superconductivity, but this was the first time it measured ionic motions without involving electrons. was used for

Lead author Dr Andrey Polityev (Department of Materials, University of Oxford, and formerly SLAC National Lab) said: ‘We found an interesting phenomenon, which occurred shortly after the ion motions we directly stimulated. . Ions repel: if we push them to the left, they preferentially flip to the right later. It resembles a viscous substance that shakes quickly and then slowly relaxes – like honey. This means that when we pushed the ions with light, we knew something about what they would do next.’

The researchers were only able to observe such an effect for a very short period of time, a few trillionths of a second, but expect it to increase as the sensitivity of the measurement technique improves. Follow-up research aims to take advantage of this new understanding to make faster and more accurate predictions of how far materials can transfer charge to batteries, and to engineer new types of computing devices that are more efficient. Will work fast.

According to the researchers, quantifying this memory effect will help predict the transport properties of potential new materials for better batteries that we need to develop electric vehicles. However, the results have implications for all technologies in which atoms flow or move, whether in solids or fluids, including neuromorphic computing, desalination, and others.

Dr Polityev added: ‘Apart from the implications for material discovery, this work refutes the notion that what we see at the macroscopic level – transport that appears to be memory-free – is directly replicated at the atomic level. is done The difference between these scales makes our lives so complicated because of the memory effect, but we have now shown that it is possible to measure and quantify it.’

The study ‘Memory Persistence in Ionic Conduction by Nonlinear Optics’ is published. The nature.

Source: University of Oxford

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