New Emergent Quantum States of Matter at High Pressure

Lead Research Organisation: University of Edinburgh
Department Name: Sch of Physics and Astronomy

Abstract

At ambient conditions, the light alkali metals Li, Na and K are nearly free electron (NFE) metals. But rather than becoming MORE free-electron like when compressed, these metals undergo transitions to unusual and complex structural and electronic forms as a result of density-driven changes in the interactions of the ions and electrons. While such behaviour is expected in all high-density matter, the physics is most evident in the alkali metals due to their NFE behaviour at ambient conditions, and their very high compressibilities. They thus offer a unique insight into the behaviour of all other metals at very high densities. We will exploit our team's expertise in experimental high-pressure physics to create solid and fluid alkali metals at unprecedented densities, and then determine their structural behaviour using x-ray diffraction techniques at synchrotrons, x-ray free electron lasers, and high-energy laser facilities. We will then use electronic structure and quantum-molecular-dynamics calculations to understand the physics behind the observed behaviour, and thereby develop new understanding and improved predictive capabilities in the behaviour of matter at ultra-high densities.

Planned Impact

Knowledge Impact: Developing a detailed knowledge of how simple metals and their melts actually behave at very high compression, and then developing the computational tools to model and understand that behaviour, will generate impact both in the immediate scientific areas of condensed matter physics and high energy-density physics, and also in other areas where high-density matter is prevalent, such as planetary science and inertial confinement fusion (ICF). The new knowledge provided by our research will enable the behaviour of high-density matter to be computed more accurately at conditions even more extreme than we plan to attain here. Economic and societal impacts will derive from the knowledge outcomes of the project, and from the computational methods which will be developed and implemented in standard software packages.

By deepening our understanding of the physical properties of matter at high pressure, we will provide standardized computational tools for calculating electronic and structural properties at high pressures. These tools will help other researchers to understand why high-density materials behave as they do, and make it possible to identify materials with specific desirable properties and even to design new materials.

People and Training: The scale of this project provides an excellent opportunity to train the next generation of researchers in a variety of state-of-the-art x-ray and laser experimental techniques, as well as computation and theoretical modelling. This importance of this training aspect has been recognised by our Project Collaborator AWE who have funded two CASE studentships for the project. Each student and postdoc involved in the research will benefit from the diverse range of experimental and computational techniques we will employ, making them highly employable at new facilities such as ESRF-EBS and European-XFEL, and also at government laboratories, including AWE.

Outreach: The project will impact on society through outreach activities aimed at explaining the exciting magnetic and electronic phenomena found in materials which have correlated electrons and dense electrons. Members of the public, high school students and prospective university students will be able to attend our Science Festivals and Open Days or participate in workshops aimed at de-mystifying the fundamental science in the project.

Publications

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McMahon M (2019) Structure and magnetism of collapsed lanthanide elements in Physical Review B

 
Description We have successfully developed our own toroidal diamond anvils, enabling us to obtain samples pressures in excess of 400 GPa. We have successfully applied the new anvils to the study of lanthanide metals and alkali metals. These studies have been supplemented by studies using "normal" diamond anvils to 300 GPa, both an numerous synchrotrons and at the PAL-XFEL and the European-XFEL.
Exploitation Route The toroidal anvil technique is available to others if they have the FIB required to machine them. The new developments we have made at the XFELs are as part of a large international collaboration, and we expect the techniques we develop to be used by all researchers using the same facilities in the future.
Sectors Aerospace, Defence and Marine

 
Description The initial results of this award have resulted in the award of am AWE-funded CASE top-up to an EPSRC studentship to start in 2019. The latest results from the XFEL experiments are of great interest to AWE, and the PI is working with them to ensure that they remain up to date on developments. The results of our research will also form party of the science case for a UK-FEL, to be submitted in mid-2020.
First Year Of Impact 2019
Sector Aerospace, Defence and Marine
Impact Types Cultural

 
Description CASE Award: Dynamic States in Diamond Anvil Cells
Amount £24,541 (GBP)
Organisation Atomic Weapons Establishment 
Sector Private
Country United Kingdom
Start 09/2019 
End 08/2023
 
Title Focussed Ion Beam Milling in Diamond 
Description In order to create toroidal diamond anvil cells, we have developed techniques to modify a diamond culet's diameter using focussed ion beam (FIB) milling. A beam of ions, usually Ga, is incident on the diamond surface. If the energy absorbed by the surface atoms during this bombardment exceeds the surface binding energy, the atoms are sputtered and ejected from the diamond. The transfer of ion momentum and energy to the surface occurs either through ion-electron interactions which results in ionized atomic cores and the ejection of electrons and radiation from the surface, or ion-atom interaction which can displace or sputter atomic cores. By scanning the ion beam across a solid surface, intricate patterns can be milled including circular tores. There are several parameters influencing the milling process such as incidence angle, scan pattern, dwell time, and beam current to name a few. 
Type Of Material Technology assay or reagent 
Provided To Others? Yes  
Impact The development of in-house FIB milling facilities has greatly accelerated our toroidal DAC programme, and enabled us to deploy these diamond anvils at synchrotron beam lines in Germany and the USA. 
 
Title Toroidal Diamond Anvil Cells 
Description Ever since its inception, the diamond anvil cell (DAC) has been an important tool for research at high pressures. At its core, the DAC consists of two opposing diamond anvils mounted on seats containing an aperture to allow an X-ray to penetrate the apparatus. The material under consideration is placed between the anvils along with a pressure transmitting medium a pressure calibrant before the entire pressure chamber is compressed by applying a force on the seats. A central gasket placed between the anvils provides lateral support to the pressure chamber and prevents the sample from extruding between the anvils. When placed in front of an X-ray beam the resulting diffraction pattern depends intimately on the material's crystal structure, allowing for analysis of the unit cell's response to pressure and revealing pressure-induced phase changes. We have modified the standard DAC by sculpting a circular tore into the diamond culet using focussed ion beam milling. This has several effects: it decreases the surface area of the culet, increasing the pressure for a constant applied force; it increases radial support for the gasket and sample, mitigating sample extrusion past the culet and containing it at ultra-high pressures; and it decreases the effect that 'cupping' has, the phenomenon of culet edges bending towards each other as the culet becomes increasingly concave under pressure, by milling away part of the culet edge. 
Type Of Material Technology assay or reagent 
Year Produced 2018 
Provided To Others? Yes  
Impact Presently, this development has enabled us to extend the pressure range accessible to our group from 320 GPa to 390 GPa. We are working to extend it further, at least until the compression stress limit of diamond at 420 GPa. 
 
Description AWE 
Organisation Atomic Weapons Establishment
Country United Kingdom 
Sector Private 
PI Contribution We have participated in joint experiments at the Orion laser
Collaborator Contribution They have participated in beamtime They have constructed targets They have awarded a fellowship to the PI
Impact Joint publication subnitted
Start Year 2011
 
Description LLNL 
Organisation Lawrence Livermore National Laboratory
Country United States 
Sector Public 
PI Contribution We have collaborated with LLNL on experiments at LCLS and on the JANUS laser. We have submitted successful beamtime applications with them to Omega and NIF
Collaborator Contribution They have helped twith target preparation, hand-on help during experiments, and advise on data analysis and simulations
Impact Successful beam time applications Publications