Supervisor: Dr. Paul Freeman
Ideal charge-stripe order in 2 dimensional layered crystal structrue of La5/3Sr1/3CoO4, where the charge-stripes are shaded grey [3]
Charge Ordering
The magnetism of charge ordered materials will be investigated using the techniques of neutron and x-ray scattering. Understanding the magnetism of charge ordered phases is important to determine its role in effects such as high temperature cuprate superconductivity, and develop the theory of magnetism in materials with strong electron correlations.
High temperature superconductivity is believed to originate from magnetic interactions. Since the discovery of cuprate high temperature superconductivity great advances have been made in our understanding of strongly correlated electron systems and quantum magnets, at the same time as ground breaking advancements of experimental techniques have been made. Ever since the observation of charge-stripes in a specific hole doped cuprate, there has been a great debate on the roll of charge-stripes in high temperature superconductivity[1]; cause or competing phase? A key to resolving this question may come from studying model charge-stripe ordered materials such as the non-superconducting insulators La2-xSrxCoO4 and La2-xSrxNiO4+d[2-4].. Recently our neutron scattering studies of the magnetism of La5/3Sr1/3CoO4 have shown striking similarities to the magnetism of the cuprates, that can be explained with a disordered charge-stripe phase[2,3].
Projects on the magnetism of charge ordered materials, such as La2-xSrxCoO4 and La2-xSrxNiO4+d , will explore the magnetic and charge ordered structures as well as their complex excitation spectrums. These projects will be guided by both recent advances in scientific understanding of these materials, and the experimental instrumentation used to study them.
Experiments will be proposed at world leading research facilities on world class instruments. This could include research based on data taken at neutron sources such as the Insitut Laue-Langevin in France, the ISIS neutron facility in Oxfordshire, SINQ at the Paul Scherrer Institut in Switzerland, as well as x-ray sources such as the ESRF in France. Projects could potentially be in collaboration with international research groups at other universities. These projects could additionally be supported by laboratory based measurements carried out at the university, that will maximise our understanding of the materials to maximise the use of central laboratory facilities.
Quantum Magnetism:
Quantum magnets are magnetic materials in which quantum effects govern the behaviour of materials, typically at low temperatures where quantum fluctuations dominate over thermal fluctuations. In research projects on quantum magnets we seek to materials in which the magnetism can no longer be explained by individual atomic contributions but as coming from collective behaviour, or in materials in which quantum fluctuations supress magnetic ordering. Like the studies of strongly correlated materials such as charge ordered magnets, these projects are based of experiments at international research facilities using techniques such as neutron scattering, supported by laboratory based investigations. Current projects are interested in model S=1/2 systems, and quantum magnets with large spin-orbit interactions such as in our previous studies [5], aiming to uncover collective phenomena and new fundamental insights into quantum mechanics for future application.
[1] J. M. Tranquada, et. al., Nature 375, 561 (1995).
[2] A. T. Boothroyd, P. Babkevich, D. Prabhakaran & P. G. Freeman Nature 471, 341–344 (2011).
[3] P. Babkevich, P. G. Freeman, M. Enderle, D. Prabhakaran & A. T. Boothroyd. Nature Communications 7 11632 (2017).
[4] P. G. Freeman, S. R. Giblin, M. Skoulatos, R. A. Mole & D. Prabhakaran, Sci. Rep. 9, 14468 (2019)
[5] A. Nag, et. al., Phys. Rev. Lett. 116, 097205 (2016).
The hourglass shaped magnetic excitation spectrum of charge ordered La5/3Sr1/3CoO4 [2]