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 am a Reader 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

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.

03/2021: Royal Society support for our work
The UK’s Royal Society has awarded our group funding for a collaborative project with Cheng Lu from the China University of Geosciences, to investigate new hydrogen storage materials based on metal alloy host matrices.

01/2021: Superconductivity in dense lead hydrides
Superconducting polyhydrides are the best superconductors known to mankind. We have re-visited the formation and properties of lead polyhydrides, PbHn, under pressure, and discovered that a new composition, PbH6, previously not considered for any group-IV hydrides, will form first under pressure. The superconducting Tc can reach 180K for PbH8 – placing lead hydrides amongst the most promising p-block metal hydrides. The work has just been published in Phys. Rev. B.

01/2021: Disintegration of water at planetary pressures
Water is a major component of icy planets’ interiors. In mixtures with other important species, such as methane or ammonia, it can change in unexpected ways. We had previously predicted that water molecules in a water-ammonia mixture would completely disintegrate under pressure, leaving behind an ionic solid instead of a hydrogen-bonded molecular crystal. Now, Raman spectroscopy measurements have detected the fingerprints of this ionic phase in high-pressure experiments. The work, led by Eugene Gregoryanz’ group, has just been published in Phys. Rev. Lett.

01/2021: Cooperative diffusion “pre-melting” in compressed calcium
Melting is not necessarily an all-or-nothing deal. In certain circumstances, a melted state can coexist with a solid state. One such example in high-pressure scenarios we previously studied is “chain melting” where 1D chains of atoms in a crystalline lattice “melt” while the surrounding atoms remain fixed. Now, using simulations based on quantum mechanics and machine learning, we show that even the simplest possible crystal structure can support chain melting, as the simple cubic phase of calcium exhibits a thermally excited regime of cooperative diffusion. The work, led by Jian Sun, is now published in Phys. Rev. X.

11/2020: Thermal excitations in dense weakly interacting matter
Methane is a good approximation of a noble gas element: basically spherical, inert, not forming bonds with other atomic or molecular species. This changes under pressure, and in a study led by Jian Sun and now published in Natl. Sci. Rev., we investigated thermally excited states in dense helium-methane mixtures, revealing a hierarchy of states from the solid to the fluid regime.

10/2020: Rare earth hydride superconductivity
Rare earth polyhydrides under pressure are amongst the best superconductors known. We have now shown, in a study published in Phys. Rev. B, that a ‘second island’ of high-Tc superconductivity exists in the late lanthanide hydrides, with superconducting Tc around 100 K in YbH10 and LuH8.

10/2020: High-energy density materials from molecular mixtures
Polymeric nitrogen is a promising high-energy density material. Pressure is needed to transform nitrogen from a molecular N2 crystal to macromolecular (Nm) or polymeric form. Here, in a computational study led by Feng Peng and published in Phys. Rev. Materials, we show that mixing N2 with methane, CH4, can result in molecular CxNyHz compounds with high energy density at lower pressures than required for pure nitrogen.

10/2020: Structural and electronic transition in topological matter
Tin telluride, SnTe, is a proposed topological crystalline insulator. Real samples are always non-stoichiometric and undergo a phase transition at low temperatures that breaks topological protection of several electronic states. Here, in a study published in Phys. Rev. B and led by experiments by Chris O’Neill and Andrew Huxley, we show the impact of this transition on SnTe’s Fermi surface, and how the transition can be suppressed by pressure.