Atomic movies reveal 'ultimate spring'

Normally, when a metal is crushed suddenly, as during an impact, it deforms and buckles, with the atoms re-arranging themselves in a complex way to take up the deformed shape – and usually only small pressures allow a metal to 'bounce back' like a spring. However, by subjecting a piece of copper to a pressure equivalent to nearly a quarter of that of the centre of the earth within just a few tens of trillionths of second, the team found the atoms had no time to re-arrange and instead acted just like a perfect spring right up to the theoretical breaking point of crystalline matter – where the atoms would spontaneously slide over each other.  

The team's results, published in Science, pinpoint the precise breaking point when the extreme pressures began to permanently deform the copper structure, or lattice, so it could no longer retain its perfect spring-like response. It also confirmed the accuracy of complex computer simulations that model the behaviour of hundreds of millions of atoms within micron-sized samples of material. Such simulations are used to design stronger, more durable materials – such as shielding for satellites to withstand high-speed pelting by space debris – but they have been hard to test in the lab because of the tiny samples and short timescales involved. Movies like these will help researchers create new kinds of materials and test the strength of existing ones.

Watch a movie showing how shock impacts on copper here.

Unravelling the scattered X-ray signals to determine how the atoms moved, and understanding the 'ultimate spring' response, required computer simulations performed by Oxford University physicists Professor Justin Wark and Dr Andrew Higginbotham.

'We were particularly pleased that when we made our X-ray movies on the computer, they exactly matched the experimental ones, confirming that we have a very good understanding of how this material reacts to such an extreme event,' said Dr Andrew Higginbotham of Oxford University's Department of Physics.

'Being able to track in real time the motion of atoms subjected to nearly a million atmospheres of pressure in a few tens of trillionths of a second is quite an achievement: the LCLS X-ray laser at SLAC is a remarkable machine that is revolutionising many areas of science,' said Professor Justin Wark of Oxford University's Department of Physics.

In this experiment, researchers shocked a layer of copper about one thousandth of a millimetre thick with optical laser pulses, and then probed the copper's lattice with ultra-bright X-ray pulses. They compiled the X-ray images, each with a shutter speed less than a tenth of a trillionth of a second, into atomic-scale movies that detail how the lattice responded at various times after the shock, including the moment the copper reached its ultimate breaking point.

'The results enable a number of materials experiments that can be compared to simulations at the same scales,' said Dr Despina Milathianaki, a staff scientist at SLAC's Linac Coherent Light Source (LCLS) who led the experiment. 'This and future experiments, designed to provide a direct comparison with simulations, will help us to accurately predict the strength of materials in extreme conditions.'

The same collaborative research team – composed of scientists from SLAC, the University of Oxford, Stanford University and Lawrence Livermore National Laboratory – also shocked other metals, including iron and titanium, and is analysing the data obtained from those samples. Follow-up research scheduled at LCLS in March seeks to extend the research to additional materials and to enlist other X-ray scattering techniques, which may provide more details about the origins of the damage in the lattice.

SLAC's LCLS is the world's most powerful X-ray free-electron laser. Located in Menlo Park, California, SLAC is operated by Stanford University for the US Department of Energy’s Office of Science. The Oxford research was supported by the UK's Engineering and Physical Sciences Research Council.