Hydrogen under extreme pressures: crystals, fusion and planets

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• Graeme Ackland, Edinburgh
• Thursday 30 June 2016, 14:00-15:00
• Theory Library.

If you have a question about this talk, please contact Mike Gunn.

In this talk I will describe the behaviour of hydrogen under extreme pressures, in particular the quest to create metallic hydrogen on earth. Along the way, I will discuss some fundamental concepts in quantum mechanics, thermodynamics and the joys of relating theory to experiment.

Metallic hydrogen is the only plausible candidate for the source of the magnetic fields of giant planets. The difference between a plasma, and fluid metallic hydrogen is theoretically moot, so one can argue that it has been created in magnetic confinement fusion devices (tokamaks), in H-bombs and in shock wave experiments. Despite numerous predictions and false reports, nobody has yet created it in crystalline form.

Theory and simulation of hydrogen combines many aspects of theoretical solid state physics. The electrons can, in the known phases be well described by standard methods such as density functional theory. However, the proton is light enough that it too should be treated as a quantum object. In the molecule the protons must be treated as an indistinguishable pair: this leads to significant differences between fermionic hydrogen H2 and bosonic deuterium D2. At room temperature, the zero point energy of solid hydrogen is an order of magnitude greater than the classical thermal energy, which in turn is another order of magnitude above the quantum thermal energy. At pressure, the de Broglie wavelength approaches the interatomic spacing suggesting the existence of a quantum superfluid phase, while the high frequency of the phonons suggests possible room-temperature superconductivity. Despite this, simulations using classical protons have given remarkably accurate predictions for known temperature-driven phase transitions. There is a strong cancellation of errors between classical configurational entropy, and the quantum uncertainty.

Diamond anvil cells (DAC) recently generated pressures above 400GPa, accessing conditions where the mechanical work of compression equals the chemical bonding energy. Most elements undergo dramatic structural changes in this regime, and rival predictions for hydrogen included molecular and atomic metals, superfluidity, superconductivity and one-dimensional melting. Yet when the new crystal phase IV was discovered in 2011, it was none of these things: it was a totally unexpected complex molecular insulator. At these conditions experimental data is sparse: we must exploit it to the fullest extent, yet previous theoretical work has concentrated on routine density functional theory (DFT) simulation producing unmeasurable predictions. I will discuss the theoretical problem of finding free energies, and demonstrate theoretical methods to extract Raman frequencies and linewidths from simulation. This requires a thorough re-examination of the quantum scattering processes in the framework of DFT , including the interaction timescale and in metals, and a full quantum treatment of indistinguishable nuclei. Although there is well established theory for Raman scattering from free rotors or harmonic oscillators, understanding coupled modes is limited. Moreover, the Raman process involves coupling to the polarizability of a sample, which should be zero in a metal. Nevertheless, in experiments Raman signals can be clearly seen in metals!

This talk is part of the Theoretical Physics Seminars series.

This talk is included in these lists:

• Bham Talks
• Theoretical Physics Seminars
• Theory Library
• What's on in Physics?

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