Cambridge based primary supervisor: PhD projects for September 2026 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://link.aps.org/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.

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.

Figure 1. Proposed structure of the PSALM cable for fusion magnets

Figure 2. Basic structure of a Tokamak and the small-scale magnets to be made of PSALM

Enhancing the Microstructural Stability of REBCO HTS Tapes for Multilayer Deposition

High-Temperature Superconducting (HTS) tapes, particularly those made from REBCO, are in high demand for next-generation energy technologies due to their exceptional current density, compact form, and reduced cooling requirements. However, they face certain limitations, most notably high manufacturing complexity and AC losses, which affect practical applications. In addition, REBCO tapes often exhibit smaller grains, large grain boundaries, and numerous pores compared to thin films, leading to accelerated oxygen diffusion and reduced superconducting stability. The widely used Pulsed Laser Deposition (PLD) technique, which operates in elevated temperature zones, further restricts the fabrication of multilayer REBCO structures because of potential degradation of underlying layers. 

This project proposes to enhance the density and structural stability of REBCO tapes to withstand high-temperature deposition environments, enabling successful multilayer REBCO growth. The approach involves a controlled heat treatment process, where the tapes are preannealed in an argon atmosphere at 650–800°C, below the peritectic melting point of REBCO, to promote grain growth and reduce grain boundary density. Subsequent oxygen annealing step will be employed to restore the superconducting properties. The preannealing temperature will be optimized to avoid decomposition. The processed tapes will be used for deoxygenation studies and determine oxygen diffusion constants, providing data to refine the process. The resulting samples will be systematically characterized to analyse their microstructural evolution, superconducting performance, and structural integrity, with the goal of developing a reliable and scalable method for fabricating dense, thermally robust REBCO tapes suitable for multilayer applications.

Schematic diagram of oxygen out diffusion from grain boundaries and grains in REBCO (ACS Applied Electronic Materials 4.3 (2022): 1318-1326).

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 have been shown to manifest during 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, 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. 

Schematic of a SuperPower SCS4050 tape structure

AC susceptibility versus T profile for a GdYBCO SCS4050 tape – applied B only at 8 T

Resistance versus T profile for same GdYBCO SCS4050 tape – applied I and B at 8 T

Superconductivity: Next-Generation Power Delivery for Data Centres

This project explores the application of superconductors to reduce energy losses in data center power transmission. Superconductors exhibit zero electrical resistance when cooled below a critical temperature, making them highly efficient. With global data center 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:

1. 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.

1. 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.

• Economic Modelling and Feasibility Analysis

An economic model of the data center 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 center power transmission.

Figure 1. Energy use of global data centers

Figure 2. Multi-layer structure of the HTS DC cable

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://www.nature.com/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

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.

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 subsequnetly become trapped using a peak field of the order of that which is to be achieved when the usual pseudo-DC model of charging would require twich that field. Unfortunatly 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. 

Optimising the SDMG process for 10T and 10K applications in Superconducting Undulators

The SDMG (Single-Directional Melt Growth) process is a recently developed approach to growing bulk superconductors that offers high performance and high production yield. To date, around 10 papers have been published on SDMG growth, with a primary focus on the characterisation of grown bulks. In the proposed project, in collaboration with a leading company in the field, the aim is to focus on the optimisation of SDMG-grown bulk properties, most notably critical current density, for demanding applications.  

One exciting application of SDMG bulks is in superconducting undulators for particle physics, where the possibility of achieving high homogeneity and high field performance offers significant advantages over existing technology. However, such devices work in low temperatures (around 10 K) and high magnetic fields (up to 10 T). This is a regime rather different to that of most bulk superconductor applications and there is a need to tailor the materials to this exacting application. 

Due to the nature of melt processing, tailoring the microstructure is extraordinarily difficult, especially given that for the best properties it takes place on a scale of several millimetres (such as cracks or voids) down to nanosized additives or even atom-level substitutions. While the process of fully controlling microstructure on such a level is impossible, the SDMG process significantly increases the number of tunable growth parameters and can significantly aid in the introduction of nanosized pinning centres, which contribute the majority of critical current density at such conditions.  

This project will therefore involve developing synthesis techniques for the incorporation of pinning centres, materials characterisation and analysis to close the structure/properties relation loop.

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.

Quantum oscillatory phenomena in 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 transition metal compounds

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 by random search within the combinatorially large material space is ineffective. Instead, this project will implement a directed search loop that is based on our developing understanding of unconventional superconductivity and integrates (i) heuristic guiding principles, (ii) computational modelling, (iii) crystal growth, (iv) high pressure/low temperature measurement (see figure below). We will initially investigate the nature of superconductivity and the associated anomalous normal states in Ni-, Cr-, and Ru-based superconductors. We will then use these findings to search for superconductivity in related materials, gradually widening the search to other material families, such as Co-, V- and Mn-based materials.

Chemical tuning to access Fermi surface transformation in the cuprate superconductors.

Fermi surface studies have been key to understanding the mysterious parent state of the cuprate high Tc superconductors. Our group’s quantum oscillation measurements in the underdoped cuprate YBCO shed light on the ‘small’ Fermi surface out of which high Tc SC emerges in this family of materials [1-3]. An open 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]. 

Challenges in accessing the evolution from the underdoped to the overdoped side of the cuprate superconductors are many-fold. Only a few families of cuprate superconductors can be grown in single-crystal form of high enough quality to observe quantum oscillations – principally the ReBCO family of materials. However, these materials cannot be tuned slightly beyond optimal doping and into the overdoped state using chemical doping-means currently available i.e. for families of known rare-earths using maximal Oxygen doping and even including Calcium doping (figure below). Accessing the Fermi surface evolution from the underdoped to the overdoped regime in the cuprate superconductors has thus been precluded thus far. 

Our group has recently grown single-crystals using untried rare-earth substitutions in ReBCO, which have shown potential to access the previously unexplored overdoped regime. This PhD project will map for the first time an unexplored region in the phase diagram of the high Tc superconducting cuprates, by tracing the transformation of the Fermi surface across critical doping in a series of rare-earth cuprates ReBCO. 

The PhD student will make quantum oscillation measurements to study the Fermi surface evolution of the high Tc cuprate superconductors as a function of chemical doping across the critical doping in the high Tc cuprate superconductors. 

The project will thus explore a potentially new region of the phase diagram of the cuprate high Tc superconductors, in which the unconventional underdoped regime transforms to the Fermi liquid-like overdoped regime through a dramatic change in the underlying electronic structure. 

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)