Emergence of novel electronic states in 5d transition metal oxides

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics

Abstract

In the fascinating search for materials which display new phenomena, including materials with potential for exploitation in technological applications, the spotlight has recently fallen on the role of the spin-orbit interaction. This is a relativistic interaction which couples the electron's intrinsic spin to its orbital angular momentum. Normally, the effect of the spin-orbit interaction on the electronic properties of materials is minimal and can be taken into account by treating it as a weak perturbation.

However, the spin-orbit interaction increases rapidly with atomic number Z (in proportional to Z to the fourth power), and for heavier atoms, especially third row transition-metal atoms with unfilled 5d orbitals, it is comparable in strength with the other interactions that determine the electronic properties of materials. The resulting competition between interactions of similar strength is predicted to result in new emergent states of matter in 5d compounds which have never been observed before.

In this project we shall synthesise and investigate the fundamental properties of several families of 5d transition-metal oxides. Intense theoretical activity over the last few years has produced a number of predictions for these materials. One predictions is of phases called topological Mott insulators which owe their stability to an interesting topological property of the electron bands caused by the strong spin-orbit interaction. Other predictions include, thermally-driven metal-insulator transitions, unconventional high temperature superconductivity, and so-called spin-liquid states which have as-yet undiscovered emergent excitations called Majorana fermions.

5d transition metal oxides have been relatively unexplored from an experimental point of view, largely because of the difficulty to make good quality single crystal samples. Our proposal is designed to integrate all of the necessary components required for a successful research programme. A concerted effort will be made to prepare materials of interest in the form of pure, high-quality, single-crystals. Complete characterisation of their electrical, thermal and magnetic properties will be undertaken using equipment in our home laboratories. Armed with the best possible samples we will exploit spectacular recent advances in synchrotron X-ray techniques to probe the novel spin-orbital states predicted to exist in 5d oxides. This work will be performed partly in collaboration with colleagues at large-scale facilities such as the Diamond Light Source in Oxfordshire, and the European Synchrotron Radiation Facility in Grenoble. Finally, we shall work closely with theorists to develop a comprehensive understanding of the novel electronic states we discover.

Publications

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Boseggia S (2012) On the magnetic structure of Sr3Ir2O7: an x-ray resonant scattering study. in Journal of physics. Condensed matter : an Institute of Physics journal

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Donnerer C (2019) Selective probing of magnetic order on Tb and Ir sites in stuffed TbIrO using resonant x-ray scattering. in Journal of physics. Condensed matter : an Institute of Physics journal

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Moreschini L (2014) Bilayer splitting and wave functions symmetry in Sr 3 Ir 2 O 7 in Physical Review B

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Moretti Sala M (2015) Evidence of quantum dimer excitations in Sr 3 Ir 2 O 7 in Physical Review B

 
Description We investigated the physical properties of several iridium and osmium oxides which exhibit unusual electronic phenomena, and provided definitive answers to a number of key questions:

1. We have established the relevance and robustness of strong spin-orbit coupling in the ground state of iridium in the formation of the Mott insulating phase in several iridium oxide compounds in the series (Sr/Ba)n+1IrnO3n+1.
2. This work has also served to illuminate the unusual bond-directional form of the magnetic interactions in these classes of material arising from the spin and orbitally entangled nature of the ground state.
3. Stability of the spin-orbit Mott insulator has been explored with respect to doping and pressure revealing, among other things, pressure-induced metallisation and the collapse of the Mott gap and emergence of a nodal Fermi liquid in lightly doped Sr2IrO4.
3. We have identified the magnetic structure in a member of the R2Ir2O7 family which is thought to be a realization of a special electronic state known as a Weyl semi-metal.
4. We have provided the first definitive experimental proof of a Slater transition (a metal insulator transition driven by magnetic order) more than 50 years after the original prediction.
5. We have discovered how a ferroelectric-like transition can occur in a metal.
6. We have also found the first example of a metal in which magnetic order causes a loss of a centre of symmetry, i.e. a metallic analogue of a multiferroic material.
Exploitation Route Further experimental and theoretical investigations.
Sectors Other

URL http://xray.physics.ox.ac.uk/Boothroyd/
 
Description A state-of-the-art optical floating-zone furnace for crystal growth at high pressures
Amount £893,916 (GBP)
Funding ID EP/R024278/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 03/2018 
End 09/2020
 
Description Standard grant
Amount £488,109 (GBP)
Funding ID EP/N034872/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 11/2016 
End 10/2019
 
Description IoP Beijing 
Organisation Chinese Academy of Sciences
Department Institute of Physics
Country China 
Sector Academic/University 
PI Contribution Experimental measurements by neutron and x-ray scattering
Collaborator Contribution Supply of high quality single crystals
Impact A growing list of publications
Start Year 2016
 
Description NIMS 
Organisation National Institute for Materials Sciences
Country Japan 
Sector Academic/University 
PI Contribution I have performed neutron and x-ray diffraction measurements to assist the research of partners at NIMS. I shall host a scientist from NIMS as part of an exchange agreement.
Collaborator Contribution Partners at NIMS have provided samples made by specialised high pressure methods. They have also performed characterisation measurements and electronic structure calculations. I was invited to NIMS for one week as part of a visitor programme,
Impact A number of scientific papers have resulted.
Start Year 2013
 
Description UCL-LCN 
Organisation University College London
Department London Centre for Nanotechnology
Country United Kingdom 
Sector Academic/University 
PI Contribution My group prepared single crystal samples and performed neutron scattering measurements.
Collaborator Contribution The LCN group under Professor Des McMorrow perfermed synchrotron X-ray scattering experiments on the crystals prepared in Oxford.
Impact Publications
Start Year 2012