Matter under the extreme pressures found inside planets and stars behaves in unexpected and exotic ways. The pressures at the center of the Earth are experimentally accessible inside diamond anvil cells, and the pressures in the cores of giant planets such as Jupiter and Saturn can be reached with shock wave experiments. But higher pressures, such as those found inside stars, are well beyond the reach of our laboratory tools, and the only way in which we can study the behaviour of atoms in stars is by solving the equations of quantum mechanics.
In our research, we investigate the physics and chemistry of matter under extreme pressures using quantum mechanics. As an example, we have studied the phase diagram of hydrogen, the most abundant element in the Universe, to explain the properties of the molecular solid phases that are observed experimentally [ Nat. Commun. 6, 7794 ], and to predict that it will become a metal at pressures just above those found at the core of the Earth [ Phys. Rev. Lett. 112, 165501 ]. We have also studied helium, the second most abundant element in the Universe, to show that under terapascal pressures it becomes a metal [ Phys. Rev. Lett. 112, 055504 ]. Our helium work has demonstrated that the atmospheres of white dwarf stars are worse heat conductors than previously believed, which has profound implications for our current estimates of the ages of these stars.
Topology has emerged as a new tool to classify and understand phases of matter. Materials with nontrivial topology carry currents that cannot be stopped by impurities, exhibit electromagnetic effects beyond the standard Maxwell equations, and provide realisations of particles such as Weyl fermions that had so far only been theorised in particle physics. Potential applications are numerous, and one of the most exciting is the use of topology for the construction of robust quantum computers. From a theoretical point of view, the study of topological materials is interesting because it typically requires the use of quantum mechanics with the inclusion of relativistic effects.
In our research, we investigate the microscopic properties of topological materials. As an example, we have studied how temperature can induce phase transitions between trivial and topological insulators via the coupling of electrons to lattice vibrations [ Phys. Rev. Lett. 117, 226801 ], demonstrating how to design materials with topological effects that persist up to room temperature, a necessary condition for technological applications. This work has been highlighted in a Synopsis in Physics: Topological insulators feel the heat. Another example is the proposal of a new type of functional material, which we call an antiferroelectric topological insulator, in which an electric field can be used to control the topological order [ Phys. Rev. Lett. 119, 036802 ].
The development of sustainable energy sources is one of the grand challenges of modern society. Novel materials drive our techonological advances, for example in the areas of batteries, solar power, or nuclear fusion. The combination of quantum mechanics with powerful supercomputers allows us to design these materials at the atomic level in a virtual laboratory, reducing the costs and accelerating the process.
In our research, we study the atomic scale of the complex materials that will enable the technological advances of the sustainable future. For example, we have shown how to predict the effects of temperature variations on the operation of solar cells based on hybrid perovskite materials [ J. Phys. Chem. Lett. 7, 5247 ]. We have also worked with experimentalists on novel alloys that can be used to build jet engines that work at higher temperatures and therefore exhibit higher efficiencies [ Intermetallics 48, 62 ].
Quantum mechanics is the physical theory that allows us to describe the microscopic world of electrons and nuclei in atoms. Although it was developed almost a century ago, we only know exact solutions to the equations of quantum mechanics for the simplest of systems. Using modern supercomputers and smart approximations, it is becoming possible to study increasingly complex quantum systems.
In our research, we work on developing new methods to understand and solve the equations of quantum mechanics to be able to study the physics problems we are interested in. As an example, we have proposed novel ways to study the interaction between electrons and atomic nuclei that are computationally advantageous [ Phys. Rev. B 92, 184301 and Phys. Rev. B 93, 014302 ]. This has allowed us to, amongst other things, include relativistic effects in the calculations of topological insulators, and study complex systems such as the perovskite materials of interest in solar cells.