Contact

Office 2604
School of Physics and Astronomy
James Clerk Maxwell Building
Peter Guthrie Tait Road
The University of Edinburgh
Edinburgh, EH9 3FD
Phone +44 131 650 5824
Email a.hermann@ed.ac.uk

About Me

I hold the Chair of Computational Physics in the School of Physics and Astronomy at the University of Edinburgh. My research is in the field of computational materials science: using first-principles, parameter-free computing methods to predict and understand properties of materials – such as their stability, elastic, electronic, and optical attributes.

As member of the Centre for Science at Extreme Conditions, a part of my research focuses on the occurrence of new, interesting phases of various materials under conditions of extreme compression and high temperatures.

Join the Group

Students interested to join the group for a PhD are always welcome. Research projects can be found on the School’s web pages, together with information on the application process.
Potential undergraduate research projects are listed on the School’s wiki page (requires login).

News

06/2022: Dimerization in hot dense lithium
Lithium, a prototypical simple metal at ambient conditions, was famously predicted to form dimers at high pressures. This sparked a paradigm shift, with the realisation that high-pressure physics does not necessarily end up with simple close packing of atoms but instead quite complex structures. The dimerised lithium structures never materialised – until now, as we found hints of their formation in a high-temperature phase around lithium’s melting point minimum. The work, in close collaboration with Prof Sun from Nanjing University, is now published in Phys. Rev. B.

05/2022: Quantum solids vs sphere packings: methane-hydrogen mixtures
When methane and hydrogen mix under pressure, they form a series of compounds. We have now revised the 25-year old phase diagram of those compounds, based on painstaking experimental diffraction and spectroscopy work headlined by Dr Howie, accompanied by our electronic structure calculations. In the process we established the material that stores the most amount of molecular hydrogen by weight, and another that features the highest molecular vibrational frequency in any compound. Results now published in Phys. Rev. Lett., and featured in APS Physics.

04/2022: Dr Conway
Lewis has successfully defended his thesis entitled “Elements of Life under Pressure: Chemistry and Crystal Structures of H-C-N-O Compounds in Planetary Interiors”. Many congratulations Lewis, and best of luck in your next role, at the University of Cambridge.

03/2022: Elastic response of subduction slabs
Oceanic tectonic plates are ‘wet’, they carry water into the deep Earth in subduction slabs that are pushed below continental plates. The water is stored inside minerals, initially (at low depths) in clay-based materials. Talc is a prototypical water carrier and its elastic response under pressure is crucial for explaining seismological observations in subduction slabs. Our ab initio calculations of talc’s elastic constants, just published in Geosci. Front., show it has anomalous features that could explain seismological features of subduction systems.

01/2022: New sulfur oxides help explain volcanic paradox
Volcanic eruptions emit more sulfur than should be stored in the magma. The ‘sulfur excess’ paradox could be explained by hitherto unknown sulfur reservoirs in Earth’s deep mantle. Here, in work led by Dr Wang from Jilin University and just published in Sci. Bull., we report a new sulfur oxide, S3O4, to become stable at lower mantle pressure conditions, and which could therefore act as the missing sulfur reservoir.

01/2022: Understanding ammonia-water mixtures
The ammonia-water phase diagram is very rich, with at least three different stoichiometric compositions, each with numerous phases found at different pressure-temperature conditions. These compounds have the potential to significantly influence the interior structure of icy moons and planets, as well as provide hints at potential gas hydrates that can have technological implications, yet many of them are unsolved. Extremely difficult neutron diffraction experiments led by Dr Loveday and his group, supported by our calculations, now identified a promising structural candidate for ammonia monohydrate IV, as reported in Crystals.

12/2021: Ternary metal polyhydrides
Clean energy production and storage are key components to make societies carbon neutral and limit man-made climate change. Hydrogen is an ideal fuel source for cars, engines, or heating, because its consumption only produces water. Ideal hydrogen storage materials are lightweight, cheap, and have large hydrogen storage capacity by weight. We studied potential magnesium-based hydrogen storage materials that form ternary M1-M2-H metal hydrides, and uncovered a new hydrogen-rich material with promising low dehydrogenation temperature. The work has been published in Phys. Rev. B.

12/2021: Non-fermi liquid behaviour within a magnetically ordered state
In metals, identical Fermions in three dimensions invariably form a Fermi liquid or superconducting state at low temperature, except at quantum critical points separating differently ordered states. In a long-standing collaboration with the Huxley group we have studied the uranium compound UAu2 and shown that it breaks the above paradigm, hosting a robust non-Fermi liquid within a magnetically ordered state. An accompanying charge density wave suggests this mechanism may involve charge degrees of freedom. Our results have just been published in Proc. Natl. Acad. Sci. Non-Fermi liquid behavior untethered from a quantum critical point in a clean metal has only clearly been seen in the magnetically partial-ordered phase of MnSi at high pressure and the potential quantum critical phase of magnetically unordered β-YbAlB4. The mechanism for UAu2 must be different, suggesting such states may exist more widely.

09/2021: Room-temperature superionicity in dense HCl
What happens to hydrogen bonds at high pressure? Different systems have different answers: molecular autoionization (ammonia), bond symmetrization (water) – but what next? We explored the post-symmetrization bonding regime of hydrochloric acid, HCl, and found evidence for proton disorder and room-temperature superionicity above 1 Mbar. A fruitful collaboration, bringing together painstaking and thorough experimental work led by Dr Dalladay-Simpson from HPSTAR with our detailed molecular dynamics simulations, resulted in a paper just published in Science Advances.

08/2021: Minimal model for anomalous melting lines
Many elements, for instance hydrogen and the alkali metals, exhibit a melting point maximum followed by pronounced drops in the melting temperature upon increasing pressure. What are the minimum requirements for a model system to exhibit these features? We show, in a paper in collaboration with ICTP Trieste and Xi’an University just published in Phys. Rev. B, that a thermodynamic two-state model with some non-linearity is necessary and sufficient to reproduce such phase diagrams. While the melting point maximum is caused by a solid-solid transition at much higher pressures, its location is nowhere near the liquid-liquid transition or crossover.

08/2021: Orbital order manipulation by pressure
Calcium ruthenate, Ca2RuO4, is a textbook layered perovskite material and exhibits a range of intriguing properties related to its strongly correlated electronic structure. Under pressure, unusually, the spacing between its layers expands. We have now shown that this behaviour is due to changes in the orbital structure of the Ru atoms, which correlates with RuO6 octahedral shape changes, and the subsequent lattice expansion. Harry Keen has led this work, which just got published in Phys. Rev. B.

06/2021: Metallic molecular mixtures of hydrogen sulfide
Hydrogen sulfide, H2S, is one of the best conventional superconductors we know – at megabar pressures. A recent experiment on carbonaceous sulfur hydride even found room temperature superconductivity. Here, in a project led by former group visitor Dr Xiao-Feng Li, we investigated related nitric sulfur hydrides, and demonstrated that a wide variety of stable compounds can form, some of which are metallic and superconducting. Published now in Comms. Chem.

05/2021: Dr Keen
Harry has successfully defended his thesis entitled “Coupling of Crystal, Electronic, and Magnetic Structures in Quantum Materials”. Many congratulations, Harry! Watch this space for several publications from Harry’s work still to come.

05/2021: Free electron to electride crossover in dense potassium
Potassium, like the other alkali metals, forms close-packed crystal structures. However, at some elevated pressure, their liquid phases are denser than the crystals. How can that be? We studied this in a collaboration with ICTP Trieste and Xi’an University researchers, with results just published in Nat. Phys. Via a sequence of electronic structure calculations and classical molecular dynamics using a machine-learned interatomic potential, we could show that a liquid crossover from free electron-like to electride-like behaviour is responsible for this behaviour.

05/2021: Ammonium fluoride vs ice
Ammonium fluoride, NH4F, in many ways behaves like an ionic analogue to water ice – with its tetrahedral hydrogen bond networks that mimic known ice phases. How far does this analogy go, and how does it break down? Lewis Conway performed an extensive set of calculations to answer these questions, now published in J. Chem. Phys. He finds that at high pressures, NH4F departs drastically from ice-like behaviour; but at much lower pressures, it holds intriguing promise to act as host framework to small guest species, like water does in gas hydrates.

04/2021: H-C-N-O chemical space under pressure
The elements H, C, N, and O are amongst the most abundant elements in the universe. In proto-planetary systems they form molecular ices – H2O, CH4, and NH3. Once these ices agglomerate, they can form icy planets and moons. In these bodies’ interiors, matter is under extreme pressure and temperature conditions, and the ices are expected to disintegrate and react to form entirely new compounds. In this work, just published in Proc. Natl. Acad. Sci., we have performed the first unbiased survey of exactly what types of compounds would form, why they form, and what happens to the molecular ices.