Bristol based primary supervisor: PhD projects for September 2026 entry

Coupling superconducting devices with light and sound

At present there are two key types of quantum technologies that use quite different materials and systems. Superconducting qubits provide the state-of-the-art technology exploited by companies like Google for computation. In parallel optics (especially telecom wavelengths) provides an ideal way to communicate securely over large distances. Combining these two types of quantum technology would allow for scalable quantum computing – which are currently heavily constrained by the cooling power of large cryostats – by allowing the coupling of many quantum computers together optically.

One possible way to achieve coupling between optical photons and superconducting devices is to exploit opto-acoustic transducers. These are well established devices which are used in filtering in modern mobile phones, and have high microwave to acoustic wave conversion efficiency. However, how these systems combine with superconductors is still an area of active research. At the fundamental level, there are a range of fascinating questions that can be addressed, such as: can we dynamically drive Cooper pairs in a Josephson junction using acoustic fields in the GHz regime?

This project’s broad remit will be creating superconducting devices which are coupled into acoustic resonators, with the ultimate aim of embedding them in opto-acoustic architectures at cryogenic temperatures.

Superconducting thin films and devices with actinides

Bristol hosts a range of thin film sputtering systems, unique in the UK, capable of creating high quality thin films of actinide materials (U and Th: compounds, alloys and heterostructures). There are a wide range of fascinating directions this project can move into, broadly in the area of creating and controlling novel types of superconducting thin films and devices using actinide materials, from fundamental materials science and physics, to device fabrication.

Examples include the growth of heavy fermion single crystal thin films (e.g. UGe₂ and UPt₃), which display a range of fascinating physics including unconventional superconductivity, quantum criticality and magnetism.

We are also interested in the fundamental physics of elemental uranium metal, which shows charge-density wave transitions and superconductivity – both of which we can tune with epitaxial strain, and substrate templating that can even control the crystal symmetry.

The strong spin-orbit coupling in uranium and thorium is also of interest for superconducting spintronic devices, where we combine superconducting materials with magnetic systems, and engineering novel superconducting order parameter symmetries on the nanoscale.

A fascinating example of this is using the heavy elements to aid the generation a novel odd-frequency, triplet s-wave superconducting state which can exist with ferromagnetic order, allowing us to develop new types of device architectures for disruptive superconducting electronics.

Tunnelling spectroscopy at high pressure in high-temperature hydride superconductors

Superconductivity under ambient conditions would unlock new technological capabilities, addressing major societal challenges such as reducing greenhouse gas emissions. The search for room-temperature superconductivity has progressed significantly following the discovery of several superhydride compounds, including H₃S [1] and LaH₁₀ [2], synthesised under ultra-high pressures. While near-room-temperature superconductivity has been independently confirmed in these materials by several research groups, including our own in Bristol [3], microscopic signatures of the superconducting state have so far been reported only in a single study of H₃S [4].

This experimental project aims to provide groundbreaking insights into record high-T_c materials such as LaH₁₀ and YH₉. Your work will (1) refine the understanding of the mechanisms driving high-temperature superconductivity in superhydrides and (2) guide the optimisation of strategies for discovering new superconductors that operate at ambient conditions. The high-quality experimental data and new theoretical understanding generated in this project will provide reliable inputs for artificial intelligence, particularly machine learning, enabling these tools to guide and accelerate the search for new superconductors [5]. This PhD project is closely aligned with the EPSRC research project led by Jonathan Buhot [6].

In this project, you will focus on measuring the superconducting gap of superhydrides by tunnelling spectroscopy [7]. To achieve this, you will build hydride superconducting tunnel junctions in Diamond Anvil Cells (DACs) at megabar pressures (see figure) by using thin-film methods [8,9]. You will also gain experience on transport and magnetic measurements, high-pressure techniques, and crystallography with x-ray diffraction experiments at synchrotrons (such as DLS, DESY, and ESRF).

You will work at the University of Bristol. Experimental work will be supervised by Jonathan Buhot and Sven Friedemann. You will also benefit from a day-to-day support of a postdoctoral research assistant.

[1] A. P. Drozdov et al. Nature 525, 73 (2015).
[2] A. P. Drozdov et al. Nature 569, 528 (2019).
[3] I. Osmond et al. Phys. Rev. B 105, L220502 (2022).
[4] F. Du et al. Nature 641, 619 (2025).
[5] J. B. Gibson et al. arXiv:2503.20005 (2025).
[6] EPSRC Open Fellowship (Grant No. EP/Z533555/1).
[7] E. L. Wolf, Principles of electron tunnelling spectroscopy (Oxford University Press, Oxford, 2012).
[8] J. Buhot et al. Phys. Rev. B 102, 104508 (2020).
[9] S. Cross et al. Phys. Rev. B 109, L020503 (2024)

Jonathan Buhot and his team recently found a method to build two tunnelling junctions in a Diamond Anvil Cell at a pressure of 1 million atmosphere.

Flux Noise Spectroscopy of Quantum Dynamics in the BKT Transition in 2D Superconductors

Crystalline monolayer superconductivity is now a forefront of research in both fundamental physics and prospective technology. But such aspirations are complicated by the Mermin Wagner theorem that continuous symmetries cannot be broken spontaneously at any finite temperature in 2D. Instead, Berezinsky, Kosterlitz and Thouless (BKT) posited that topological defects in the superconductive order parameter, i.e. the quantized vortices each sustaining one magnetic flux quantum, spontaneously proliferate as a dense strongly fluctuating fluid of vortex pairs with opposite polarity. Because understanding the quantum dynamics of monolayer superconductors has become pivotal to their technological utility, we propose to introduce flux-noise spectroscopy to studies of the spontaneous (BKT) vortex dynamics in crystalline 2D superconductivity.

Moreover, this is also a fundamental cutting-edge topic concerning the study of emergent gauge fields, in which analogues to electromagnetic fields arise spontaneously and fluctuate dynamically. Examples include quantum spin liquids; quantum spin-ices; and the quantum magnetic vortices generated spontaneously around the BKT transition. Despite emergent gauge fields’ long history, ubiquity, and widespread importance, from fundamental understanding through to potential applications, none has ever been unambiguously confirmed in experiment. This project aims, in parallel, to address this profound issue.

Motivated by recent theoretical proposals on how the BKT transition in 2D superconductors should be accompanied by a dramatic increase in the magnitude of fluctuations of the magnetic field (flux), the strategy of this project will be to use a combination of analytical and numerical theory (from the Bristol theory group) to make precise predictions in order to guide BKT flux noise exploration experiments (from the Oxford experimental group).

Contemporaneously, experimental advances at Oxford have vastly enhanced the sensitivity and bandwidth of flux noise spectroscopy e.g.  Nature 571, 234 (2019) ; PNAS 121, 2320384121 (2024) ; PNAS 122  e2422498122 (2024) ; arXiv 2408.00460  (2025); http://davis-group-quantum-matter-research.ie/Takahashi_270525.pdf, meaning that the flux noise spectrometers required for this project are immediately available. 

Strange Metals from Random Resistor Tessellations

High-temperature superconductors, and the mysterious ‘strange metals’ from which they form, feature significant disorder across a range of materials, leading some to suggest an un-known mechanism of self-tuning toward classical disorder. This project seeks to start from disorder in an attempt to capture as many strange metal phenomena as possible. 

Strange metals lie at the cutting edge of condensed matter physics. What began as a simple observation — resistivity increasing linearly with temperature — has evaded our understanding for decades, and threatens to undermine the most basic notions in the field, such as Fermi liquid theory. Despite strange metals’ highly-entangled quantum origins, a leading approach treats strange metallicity as an effect of classical disorder, modelled by random resistor networks. The idea is that islands of exotic matter are coupled by normal metal, and the geometries of the coupling pathways dictate the overall behaviour. Existing models place resistors on periodic square lattices for calculational simplicity, with disorder coming from random resistances along edges (the image shows the calculated voltages in a large grid). In this project we seek to make such models more realistic. One way in which we will do this is by then moving from periodic lattices to tessellations to more accurately capture local connections between islands in real materials such as cuprates. We will also investigate the statistics of the resulting current pathways.  

This theoretical project will involve a combination of analytics and numerics (developing original code in any language), as well as working closely with experimental groups. 

Studying high-temperature superconductors with quantum sensors

Over the last decade, new unconventional and conventional superconductors have been discovered [1,2]. Two key classes are the hydride and nickelates – both having superconducting states at high pressures. They offer new pathways to understand the optimal mechanism for superconductivity but also bring along new challenges. The main challenge is to understand sample variations. This calls for studies that compare spatial variations of the critical temperature with composition and structure. Understanding the mechanism for superconductivity requires studies of the superconducting and normal metal characteristics. One important quantity for this task is the lower critical field which can probe relations to quantum phase transitions [3]. Both tasks can elegantly be pursued using quantum sensors based on nitrogen-vacancy (NV) centres in diamond [4,5]. These atomic-scale defects are sensitive to local magnetic fields, making them probes of superconducting properties at the microscopic level. They can be read out with optical methods by shining a laser through the diamond anvil.

In your PhD work, NV centres will be used to map the superconducting transition temperature with micrometre spatial resolution. This information will be correlated with similar spatial resolution of composition and structure from synchrotron x-ray diffraction and Raman scattering as well as bulk transport measurements.

You will also use NV centres to study the lower critical field of clean superconducting samples which will give insight into the key characteristics and thus help understanding the mechanism. For instance, measurements of the critical field will reveal coupling to fluctuations near zero temperature magnetic transitions.

The project will benefit from close collaboration between Dr Sven Friedemann with many years of experience on high-pressure measurements and Dr Krishna Coimbatore Balram with many years of experience on NV centre quantum sensors. You will conduct work at Bristol to implement NV centres for high-pressure work and you will take part in complementary X-ray diffraction and other studies including at national and international synchrotron facilities.

[1] Wang, B. Y. et al. Annual Review of Condensed Matter Physics 15, 305–324 (2024).
[2] Pickard, C. J. et al. Annu. Rev. Condens. Matter Phys. 11, 57–76 (2020).
[3] Putzke, C. et al. Nature Communications 5, 5679 (2014).
[4] He, G. et al. Nature Communications 16, 8162 (2025).
[5] Hsieh, S. et al. Science 366, 1349 (2019).

Exploring Unconventional Pairing in Correlated and Topological Superconductors

The superconducting energy gap provides one of the most direct and powerful probes of the interactions that drive electron pairing and give rise to superconductivity. Determining its symmetry and structure is therefore essential for identifying the pairing mechanisms at play in unconventional superconductors and for advancing our understanding of these complex materials. A defining characteristic of many unconventional superconductors is a gap that is highly anisotropic in momentum space and may possess nodes, where the gap vanishes entirely. 

This project will employ two complementary techniques to probe the gap structure: measurements of the magnetic penetration depth and the electronic specific heat down to millikelvin temperatures. Both are sensitive to the low-energy quasiparticle density of states and can distinguish between fully gapped, anisotropic, and nodal superconductivity. 

The work will focus on recently discovered materials where unconventional pairing has been proposed but remains unresolved. Promising candidates include the kagome metals (AV₃Sb₅ and related 135 compounds), the spin-triplet candidate UTe₂, the quasi-one-dimensional Luttinger-liquid candidate Li_0.9Mo₆O₁₇, nickelate superconductors, and topological systems such as Sr_xBi₂Se₃. 

These measurements may be complemented by muon spin spectroscopy, controlled irradiation, and normal-state electronic transport studies to build a comprehensive understanding of the superconducting state. The project offers training in cryogenics, low-temperature experimental techniques, and data modelling, contributing to the wider effort to classify and understand unconventional superconductivity in correlated and topological materials. 

Further reading: 

[1] M. J. Grant, et al. “Superconducting energy gap structure of CsV₃Sb₅ from magnetic penetration depth measurements.” Journal of Physics: Condensed Matter 37.6 (2024): 065601.
[2] P. J. Hirschfeld. “Using gap symmetry and structure to reveal the pairing mechanism in Fe-based superconductors.” Comptes Rendus Physique 17.1-2 (2016): 197-231.

Synthesis of Superconducting COFs

The successful synthesis of a highly-conductive organic material in 1954¹ heralded a rush of research activity in this new field. It wasn’t long before a theoretical prediction was made to realize high temperature superconductors by organic chemical synthesis², although all subsequent experimental work across 60 years of effort failed to produce such a superconductor. A significant block to achieving this goal was the fact that in organic materials, long-range order which is critical for superconductivity, has proved extraordinarily difficult to achieve.

Work over the past 30 years on the creation of organic structures with long-range order however suggests a solution to this problem. Starting in 1995 with the synthesis of metal organic framework (MOF) materials³ (Nobel Prize in Chemistry 2025), structures that have long-range order and are entirely formed through covalent bonding, the covalently bonded framework (COF) materials allow the formation of organic materials with high crystallinity⁴. In March 2025, P-J Chen predicted that a COF comprised solely of carbon and nitrogen should have a superconducting T_c of ~ 85K⁵. If this could be synthesised and confirmed to be superconducitng, an entirely new field of superconducting materials would open up.

This project will synthesise and characterise COFs of the form suggested by Chen and also analogous COF materials. The molecules that the project will initially work with are functionalised conjugated polyaromatic hydrocarbons, but the choice of material will be driven by the results of characterisations. Our lab has demonstrated crystal engineering to control organic crystal growth, so we have a firm basis on which to pursue these materials^6-8.

References:
1. H. Akamatu, H. Inokuchi, Y. Matsunaga, Nature, 173, 168 (1954)
2. W. A. Little, Physical Review, 134, A1416 (1964)
3. O.M Yaghi and H. Li, JACS, 117, 10401 (1995)
4. A.P. Côté, et. al., Science, 310, 1166 (2005)
5. P.-J. Chen, Nano Lett., 25, 4087 (2025)
6. J. Potticary, et. al., Crystal Growth & Design, 20, 2877 (2020)
7. C. Hall, et. al., Cryst. Growth Des., 20, 6346 (2020)
8. J. Potticary, et. al., Nature Communications, 7, 11555 (2016)

Nano Lett. 2025, 25, 4087−4092.

Superconductor nanowires for the next generation of communication devices

In 2007 it was discovered that coherent terahertz (THz) radiation could be produced from the layered high temperature superconductor Bi2Sr2CaCu2O8+x (Bi-2212)1,2 (see figure). When a DC potential is applied across the junction, an AC current will flow. The superconducting energy gap limits the maximum voltage that can be sustained across a junction though, and so emission from a single junction is weak and unsuitable for any practical applications such as telecommunications and sensing, due to the larger power input required. This could be ameliorated however by using multiple, identical junctions all emitting radiation of the same frequency in phase, but nanofabrication of such an assembly is extremely difficult and intrinsically prone to overheating during operation. 

Bi-2212 nanowires would be the perfect solution to this problem, particularly if they have lengths upwards of a few micrometres and if the unit cell is orientated correctly so that the c-axis of the unit cell, which contains the intrinsic Josephson junctions, runs parallel to the length of the nanowire. This would allow THz radiation power in the 1000s of mW range to be produced for the first time, transforming the telecommunications and sensing industries. Unfortunately, this has not been possible as Bi-2212 nanowires did not exist. Fortunately, we have created them for the first time recently3. 

This project will therefore create a suite of nanowires of Bi-2212 via our recently-developed simple synthetic approach and then through nanofabrication in our laboratories at Oxford, characterise and develop them for use in powerful new THz devices4. 

References:

[1] L. Ozyuzer, et. al., Science., 318, 1291 (2007)
[2] R. Kleiner and H. Wang, J. Appl. Phys., 126, 171101 (2019)
[3] J. Potticary, et. al., Small Structures, 4, 2300087 (2023)
[4] A.W. Pang et. al., IEEE Micro. & Wireless Comp. Lett., 28, 669 (2018)

Model of the Josephson effect from intrinsic Josephson junctions found in Bi-2212 crystal structure. Blue and white layers represent the superconducting (CuO2) layers and insulating (Bi2O3, SrO) layers, respectively. Figure reproduced from Kleiner 2.

Exploring strange metallicity in unconventional superconductors using intense current pulses

In order to understand high-temperature superconductivity in cuprates, one must first understand the ‘strange metallic’ state from which superconductivity emerges. Key insights into the nature and phenomenology of a metal can be gained by studying its magnetoresistance (MR) and extracting information on the dominant scattering processes using Boltzmann transport theory. One of the most striking features of the cuprate strange metal, however, is its peculiar, quadrature (H²+T²) scaling form of the MR that is incompatible with Boltzmann theory involving any proposed form of scattering.

Unfortunately, the onset of superconductivity prevents this H/T scaling from being probed down to critical low temperature regime where disorder scattering dominates (see Fig. A). The goal of this project is to use intense current pulses (typical width ≈ 1 μs) to suppress the superconducting transition to lower field strengths (see Fig. B) and to investigate the form of the MR down to the lowest fields and temperatures possible. Fig. C shows results from our existing set-up.

In this project, we intend to use a focussed ion beam (FIB) to fabricate narrow channels of the relevant material and to confirm the extent of the quadrature scaling in cuprates and other quantum critical or strange metal candidates. Extension of the H/T scaling to limiting low-T regime would provide a stringent test for any future model of the magneto-transport of strange metals. More importantly, it would give compelling evidence that strange metallicity is associated with the charge dynamics of non-fermionic quasiparticles.

References: A) Ayres et al., Nature 595, 661 (2021) ; B) Grissonnanche et al, Nat. Commun. 5, 3280 (2014) ; C) Duffy et al., Instrum. Sci, Tech. doi.org/10.1080/10739149.2024.2388059 (2024).

Intrinsic topological superconductivity

Intrinsic topological superconductivity (ITS) is a quite extraordinary phase of electronic matter that stands at the frontier of modern quantum matter. Intrinsic topological superconductivity has been sought without success since the early 1960s. ITS promises both cutting-edge science and revolutionary quantum technology.

Key signatures for an ITS include the existence of odd-parity electron pairing Δₖ = −Δ₋ₖ, superconductive topological surface bands kₛ(E), and, when time-reversal symmetry (TRS) is broken, persistent chiral supercurrents with speed Vₛ(r) flowing along every surface. None of these characteristics has ever been detected.

Until recently, new candidate ITS materials have been discovered, and innovative atomic-scale visualization techniques designed to reveal the key characteristics of ITS have been developed.

You will exploit these exciting opportunities by using direct atomic-scale visualization of ITS fingerprints, such as odd-parity Δₖ, and/or superconductive topological surface bands kₛ(E), and/or persistent chiral supercurrents Vₛ(r). Thus, after decades of anticipation, your goal is to achieve detection of the fundamental characteristics of three-dimensional ITS.

The consequences will be sharp, clear provenance of which materials actually are ITS, distinct revelation of which ITS phenomena occur in nature, and eventual identification of the ITS that are most ideal for quantum technology.

You will focus on candidate materials 2M-WS₂, among others. You will pursue three scientific objectives: (1) Demonstrate the presence of odd-parity Δₖ topological surface band. (2) Momentum-space visualization of the topological surface band (TSB). (3)Detect a flowing chiral surface supercurrent when TRS is broken. Your ultimate goal will be to confirm the presence and explore the unprecedented physics of intrinsic topological superconductivity.

The equipment and the labs:
MINERVA and GEMINI are fourth-generation UHV, 14 Tesla, millikelvin STMs that are fully operational at the Beecroft Building, Oxford University. ATLAS is the dilution fridge based, 9 Tesla STM (on which our SJTM technique was originally developed) which is now under construction at the HH Wills Lab, Bristol University. All three STMs are used to study cleaved samples.

References:
[1] Science, 388, 938-944 (2025).
[2] Nature Physics, 21, 1555–1562 (2025)
[2] Nature 618, 921 2023.

Imaging Electron Correlations in Kagome Flat Band Superconductors at the Atomic Scale

The search for emergent novel states in superconductors with a kagome lattice is a wide-open frontier in condensed matter physics [1]. The interplay of an emergent flant band, frustrated geometry, electron correlations, and non-trivial topology paves the way for discovering new phases of quantum matter. Further advances in the research on kagome materials hinge on three key challenges:

• Precisely detecting electronic structures imposed by electron interactions.
• Identifying the fundamental origin of emergent quantum phases, such as charge-order and topological phases.
• Discovering new states and phases via new materials.

The key to addressing these challenges is to directly visualize these emergent quantum states at the atomic scale. Combining beyond state-of-the-art scanning tunnelling microscopy (STM) under magnetic fields and ultra-low temperatures [2-3], you will aim to detect the novel emergent quantum phases in kagome superconductors. Your focus will start from the electronic liquid crystal phases, then look into the topological phases, and finally investigate the mechanisms of the exotic charge density wave. You will explore the connections of these quantum states to other modern quantum matter such as Weyl semimetals, topological magnets, and superconductivity. These efforts will significantly expand our knowledge of quantum matter physics in recently discovered quantum materials, advancing the field of condensed matter physics.

This area represents an entirely new and blooming field of research [4], providing vast unexplored territories and an excellent opportunity to start a PhD. Identifying a topological quantum magnet could profoundly impact quantum sciences and future spintronics applications. This project aims to make new scientific contributions, entering one new research area in condensed matter physics and likely leading to high-quality publications. The project will also involve collaboration with other universities worldwide.