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High-pressure physics

Piling on the pressure

The machine that houses the world's largest laser, and which stands in for the starship Enterprise's warp core in the film Star Trek Into Darkness, has compressed diamond to the density of lead. See Letter p.330

The stars and planets we can see in the night sky were formed by strong gravitational forces that crushed their constituent atoms tightly together at immense pressures. How, on Earth, can we figure out what effect this force has had on the inside of these distant and inaccessible objects? We are confident about the physics that operates in these stars and planets, but what about their chemistry? Predictions abound, but hard experimental data are desperately needed. On page 330 of this issue, Smith et al.1 present the results of groundbreaking experiments on the compression of carbon diamond up to a pressure similar to that at the centre of Saturn.

The machine used to perform the experiments, the US National Ignition Facility (NIF), is unique (Fig. 1). It houses the world's largest laser, which can be focused onto a millimetre-scale target held at the centre of a 10-metre aluminium sphere. It certainly looks the part: indeed, it stood in for the starship Enterprise's warp core in the movie Star Trek Into Darkness. The NIF's primary mission is to study inertially confined nuclear fusion2, but a portion of the laser 'shots' have been allocated to fundamental science — from laboratory astrophysics to plasma physics and planetary science.

Figure 1: The National Ignition Facility machine.
figure1

Smith et al.1 have used this machine to compress diamond up to a record pressure of 5 × 1012 pascals.

The new NIF experiments have succeeded in compressing diamond up to a pressure of 5 terapascals (5 × 1012 Pa) — 14 times the pressure at the centre of the Earth. In addition to their brute power, the laser pulses can be exquisitely manipulated, allowing the pressure in the target to be increased in a precisely controlled manner known as dynamic ramped compression. Dynamic compression can generate enormous pressures far beyond those accessible in static experiments that use, for example, diamond anvil cells3. A crucial aspect of the current set-up is that the use of ramped compression reduces the dissipative heating of the sample. Ramped compressions can explore materials at conditions similar to those encountered deep within large planets, whereas compressions using shock waves generally lead to higher temperatures.

The discovery of multiple planets beyond our Solar System, many of which are much larger than Jupiter and Saturn, has led to a dramatic change in our picture of the Universe. Understanding the make-up and evolution of these exoplanets requires the development of theoretical models, which depend on the pressure–density equations of state of the most likely planetary materials4. Until now, these equations of state have largely been determined by extrapolating from terrestrial data.

Extrapolation is a perilous activity. Theoretical calculations of terapascal-pressure phase transitions in, for example, aluminium (which is used in high-pressure dynamical experiments as a standard material with well-understood properties) predict that it will transform from a close-packed structure to a complicated non-close-packed structure at terapascal pressures. At pressures higher than those investigated by the authors, some of the valence electrons of carbon are expected to move away from the nucleus and play the part of the fluorine anions in ionic calcium fluoride, with the calcium sites being occupied by carbon cations5. All of this suggests that the structures adopted at terapascal pressures may be surprising and far from simple6.

Simple quantum-mechanical theories, such as the Thomas–Fermi–Dirac theory for very hot dense matter, and more sophisticated quantum algorithms, including both the path-integral Monte Carlo method for 'warm' dense matter and density functional theory for condensed phases, have been shown to provide largely consistent descriptions in the pressure–density regions where their applicability overlaps7. For the pressures and densities probed by the current experiments, a series of phase transitions is predicted to occur in which carbon becomes denser than in its diamond form. Interestingly, the experiments did not detect any of these phase transitions, which may have been smoothed out, or deferred, through some as-yet-unknown mechanism. Overall, however, the agreement between results from density-functional-theory calculations and the experiment is good, so the theory is likely to be on a solid footing.

The authors are confident that their carefully designed dynamic ramped compression has achieved temperatures that are similar to those inside planets. Although the temperatures generated in their experiments can be inferred through theoretical predictions, Smith et al. cannot directly measure the actual temperatures. In addition, it is not currently possible to use their methods to determine crystal structures at terapascal pressures. These are exciting challenges for the future. Important progress has been made in this direction8, and there is hope that laser-driven dynamic compression, coupled with free-electron lasers, will provide diagnostic snapshots of structures and their dynamics.

Planets form over many millions of years, whereas the reported dynamic ramped compression procedure is over in a flash. It is not clear whether these experiments, despite reaching relevant temperatures and pressures, are able to closely model the largely equilibrated, dense rocks and ices existing within giant planets. However, the brevity of the experiments does have an advantage. Just as nanotechnology has been a gift to theoreticians, allowing meaningful computations of manageable numbers of atoms, the short experimental timescales actually make the behaviour of compressed atoms easier to model in dynamical simulations. Through mutual benchmarking and the testing of predictions, we expect that experiment and theory will together improve our understanding of matter under extreme compression.

A final note of perspective. Although the pressures and densities probed in the current experiments are immense, nature is even more ambitious. The giant exoplanets are a stepping stone to the stars, where petapascal pressures (1 petapascal is 1015 Pa) are reached. The predictions of rich terapascal-pressure physics should caution against assumptions of simple structures. Indeed, a recent theoretical study9 anticipates a complex metallurgy for the crusts of neutron stars. Over to the experimenters!

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Correspondence to Chris J. Pickard.

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Pickard, C., Needs, R. Piling on the pressure. Nature 511, 294–295 (2014). https://doi.org/10.1038/511294a

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