PhD projects 2020

Projects available in our group for 2020 are listed below. Please get in touch with Dr Amalia Coldea (amalia.coldea@physics.ox.ac.uk) for further details.
We welcome applications from enthusiastic students who are excited and challenged by understanding the rich novel physical phenomena displayed by novel quantum materials.

1. Developing electronic tunable devices of thin flakes of iron-based superconductors

This project is to explore electronic and topological behaviour of quasi-two dimensional devices based on thin flakes of highly crystalline superconducting iron-based chalcogenides as well as Dirac and Weyl semimetals. The project will involve device preparation and a suite of physical properties measurements to study their electronic properties using high magnetic field and low temperatures. The aim is to search for quantum phenomena as well as for signature of topological matter in these highly tunable quantum material devices. The project will be hosted by the recently funded Oxford Centre for Applied Superconductivity (CfAS) in the Department of Physics. The student will investigate the phase diagrams of novel superconducting thin flake devices under different extreme conditions of high magnetic field, strain and pressure. Experiments using advanced techniques for transport will be performed using high magnetic field facilities available in Oxford and elsewhere. A suitable candidate needs to have a good understanding of condensed matter physics and good computing skills as well the ability to work well in an experimental team.

2. Tuning electronic ground states and superconductivity of iron-based superconductors under extreme experimental conditions

Applied hydrostatic pressure is a unique tuning parameter to study the characteristics of a nematic quantum critical point in the absence of long-range magnetic order in a single material and to gives access to the electronic structure and correlations of new magnetic and structural phases. FeSe is an unique superconductor that show a nematic electronic phase in which absence of magnetism at ambient pressure. However, a magnetic phase is stabilized at high pressure and superconductivity is enhanced four-fold. By combining the chemical pressure with the hydrostatic pressure in the series FeSe1-xSx, it is possible to separate the nematic and magnetic phases. This project will aim to understand the evolution of the complex Fermi surfaces and electronic interactions across the nematic phase transitions using applied hydrostatic pressure in different iron-based superconductors. High magnetic field and low temperatures will be used to access directly the Fermi surface by detecting quantum oscillations in different ground states tuned by applied hydrostatic pressure. A suitable candidate needs to have a good understanding of condensed matter physics and good experimental and computational skills as well the ability to work well in an experimental team.

3. Strain-Tuning of the Electronic Structure of Novel Quantum Materials Using Angle Resolved Photoemission Spectroscopy

Superconductivity is a unique state of matter with significant potential for practical applications due to the ability of electrons to form Cooper pairs that travel without dissipations in these materials. The electronic structure in the superconducting materials is strongly affected by the presence of certain interactions due to the lattice, magnetic, charge or orbital fluctuations. Iron-based superconductors are a new class of exciting material that offer a new prospective on the important routes towards high-temperature superconductivity. This project focuses on establishing experimentally the key ingredients that govern the electronic structure and the superconducting state found in different classes of iron-based superconductors in the presence of competing ground states. The project will use various external parameters that modify the electronic structure such as chemical pressure, in-situ electron doping and applied strain to build up a comprehensive picture of superconductivity towards high-temperature regimes. Experiments will be performed both at the Diamond Light Source and at the University of Oxford by complementing state-of-the art angle resolved photoemission spectroscopy and magnetotransport studies as well as theoretical first-principle calculations. A suitable candidate needs to have a good understanding of condensed matter physics and good computing skills as well the ability to work in an experimental team. This project will be joinly supervised by Dr Amalia Coldea and Dr Timur Kim at the Diamond Light Source.

4. Exploring the electronic structure and superconductivity of quantum materials under strain

This project will investigate the response of quantum materials, especially iron-based superconductors, to applied lattice strain. The fundamental reason to do this is that in these materials there are many electronic processes that occur simultaneously, which can be difficult to disentangle when looking at the unstressed system alone. One can gain much more information by understanding how elastic lattice distortion, applied through hydrostatic, biaxial, or uniaxial stress, affects the electronic properties of a material. For example, lattice strain may be used to suppress an electronic instability, or to enhance one or more of the processes in the material. One major area of interest in the iron-based compounds, which is likely to be a key part of this project, is electronic nematicity, a form of electronic order which breaks rotational but not translational symmetries. Nematicity may play an important role in high-temperature superconductivity. Uniaxial stress can be used to increase nematic polarisation in a material, by adding to the orthorhombic lattice distortion associated with nematic order.

Measurements to be performed may include quantum oscillations in high magnetic fields of up to 45T and potentially angle-resolved photoemission at the Diamond Light Source. The project also involves the development of high-precision, piezoelectric-driven stress apparatus, for example of a miniaturised stress cell that fits into a high-field magnet and therefore is suitable for students who want to develop their engineering skills along with their scientific skills. A suitable candidate should have a good understanding of condensed matter physics, good experimental and computing skills, and an ability to work well in an experimental team. This project will be jointly supervised by Dr Amalia Coldea at Oxford University, and by Dr Clifford Hicks and Prof Andrew Mackenzie at the Max Planck Institute for Chemical Physics of Solids, in Dresden, Germany. The student will be expected to spend approximately half of the time in Oxford and half in Dresden.

5. Revealing topological signatures in the electronic behaviour of bulk quantum materials with Dirac dispersion

This is an experimental project combining electronic transport and quantum oscillations to detect unusual signatures of the manifestation of topology in single crystals of quantum materials with Dirac dispersions. The student will perform a series of studies in high magnetic fields and at low temperatures to search for evidence of non-trivial Berry phases and low temperature quantum transport. Studies will be also extended under applied pressure and strain to identify proximity to new toplogical superconducting phase. The work will be combined with first-principle band structure calculations to compare with experiments and disentangle trivial from non-trivial effects. A suitable candidate needs to have a good understanding of condensed matter physics and good computing skills as well the ability to work well in an experimental team.

Details about the application process can be found here and concerning the available scholarships can be found here.