Structure and superconductivity of hydrogen and hydrides under extreme pressure
Lead Research Organisation:
University of Cambridge
Department Name: Physics
Abstract
The first stage of this project involves testing new density functionals purpose-built for solid hydrogen under 100-500 GPa pressure, with the goal of establishing whether semi-local density functional theory (DFT) is able to accurately describe this system. This is an ongoing piece of work, running alongside the development of our in-house anharmonic phonon code by another member of our group. The end goal of this work is a comparison of our calculated anharmonic pressure-temperature phase diagram with signatures observed in experiment.
A further aspect of this project is the study of hydrides under high pressure. Since the hydrogen atoms in these systems are "chemically pre-compressed" by the presence of the other atoms, they can exhibit the exciting predicted properties of metallic hydrogen at pressures which are more easily accessible to experiment. Of these properties, we are particularly interested in conventional (phonon-mediated) superconductivity. This part of the project involves studying various high-pressure hydride systems using ab initio random structure searching (AIRSS), density functional perturbation theory (DFPT) and Migdal-Eliashberg theory to predict the stability, crystal structure, phase behaviour and superconducting properties of these systems from first principles. These theoretical predictions can be compared directly to current and future experiments. We are also interested in attempting to construct low-cost proxies for electron-phonon coupling strength and in searching for superconducting hydrides which can operate in lower pressure regions.
A further aspect of this project is the study of hydrides under high pressure. Since the hydrogen atoms in these systems are "chemically pre-compressed" by the presence of the other atoms, they can exhibit the exciting predicted properties of metallic hydrogen at pressures which are more easily accessible to experiment. Of these properties, we are particularly interested in conventional (phonon-mediated) superconductivity. This part of the project involves studying various high-pressure hydride systems using ab initio random structure searching (AIRSS), density functional perturbation theory (DFPT) and Migdal-Eliashberg theory to predict the stability, crystal structure, phase behaviour and superconducting properties of these systems from first principles. These theoretical predictions can be compared directly to current and future experiments. We are also interested in attempting to construct low-cost proxies for electron-phonon coupling strength and in searching for superconducting hydrides which can operate in lower pressure regions.
Organisations
People |
ORCID iD |
| Richard Needs (Primary Supervisor) | |
| Alice Shipley (Student) |
Studentship Projects
| Project Reference | Relationship | Related To | Start | End | Student Name |
|---|---|---|---|---|---|
| EP/N509620/1 | 01/10/2016 | 30/09/2022 | |||
| 1948686 | Studentship | EP/N509620/1 | 01/10/2017 | 31/03/2021 | Alice Shipley |
| Description | My work involves modelling the structural and vibrational properties of materials from a theoretical/computational perspective. Work of this type is typically done using density functional theory (DFT). I typically study materials under very high pressure and materials containing light atoms (such as hydrogen) which exhibit a large degree of anharmonicity in their vibrations. The first stage of my project involves testing new density functionals purpose-built for solid hydrogen under 100-500 GPa pressure, with the goal of establishing whether semi-local DFT is able to accurately describe this system. This is an ongoing piece of work, but has so far shown results which are very similar to an existing density functional (BLYP), meaning that we may have reached the limit of the descriptive power of semi-local DFT in this system. A further aspect of this project is the study of high-pressure hydrides. Since the hydrogen atoms in these systems are "chemically pre-compressed" by the presence of the other atoms, they can exhibit the exciting predicted properties of metallic hydrogen at pressures which are more easily accessible to experiment. Of these properties, we are particularly interested in conventional (phonon-mediated) superconductivity. This part of the project involves studying various high-pressure hydride systems using ab initio random structure searching (AIRSS), density functional perturbation theory (DFPT) and Migdal-Eliashberg theory to predict the stability, crystal structure, phase behaviour and superconducting properties of these systems from first principles. In a recently submitted paper of ours, we predict a new metastable hexagonal phase of the current record-holding superconducting material LaH10, which may explain the experimental observation of hexagonal impurities in cubic samples. We also make predictions about the sister system of YH10, the results of which can be compared to (and used to interpret) the results of future experiments. We have also worked on attempting to find hydrides which exhibit superconductivity at lower pressures, utilising machine learning methods to do so. The main result of this work was the prediction of two new binary hydrides which extend the frontier of known superconducting materials in pressure-temperature space towards ambient conditions. This second paper has also been recently submitted. |
| Exploitation Route | The predictions we make in our work regarding the structure, phase diagram and superconducting properties of high-pressure hydrides can be directly compared to current and future experiments to aid understanding of these difficult systems. Our successful use of machine learning in the context of informing structure search directions demonstrates that these methods can be used to selectively search for materials possessing certain desirable properties and this could be put into use by others in similar situations. My ongoing project has also been running alongside the development of an anharmonic phonon code by another member of our group and has lead to improvements in this software. |
| Sectors | Other |