High-temperature cuprate superconductors based on conducting planes of copper-oxide can have transitions temperatures up to about 130 K.  They have potential applications in energy, healthcare, and fundamental science. However, the fundamental mechanism responsible for the superconductivity is poorly understood.  Superconductivity derives from a pairing of conduction electrons and in the cuprates these conduction electrons show unconventional behaviour [1] in the normal state at temperatures higher than the superconducting transition temperature Tc.  For example, a "Planckian" behaviour is found where the resistivity is proportional to the absolute temperature over a wide range up to 600 K.

A promising method to understand the collective properties of the conduction electrons is to measure their collective spin and charge dynamics [2].  We know from studies of low temperature superconductors that superconductivity in intimately connected to the low-energy collective excitations.   The present project is to grow single crystals of cuprate superconductors and measure their spin and charge excitations using resonant inelastic x-ray scattering (RIXS), inelastic neutron scattering (INS) and other scattering techniques.  Experiments will be performed at world-leading facilities such as the Diamond light source and the ISIS spallation neutron source.  The work involves laboratory-based work together with data analysis and theoretical modelling of data.

Contact: Professor Stephen Hayden (S.Hayden@bristol.ac.uk) for further details.

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[1] S. A. Hartnoll and A. P. Mackenzie, Colloquium: Planckian Dissipation in Metals, Rev. Mod. Phys. 94, 041002 (2022).

[2] M. Zhu, D. J. Voneshen, S. Raymond, O. J. Lipscombe, C. C. Tam, and S. M. Hayden, Spin Fluctuations Associated with the Collapse of the Pseudogap in a Cuprate Superconductor, Nat. Phys. 19, 99 (2023).

Understanding the complex behaviour of electrons in solids is an immensely difficult task, but one which is hugely important for future technological exploitation.

You would develop and apply cutting-edge electronic structure methods which go beyond the standard density functional theory approach, focusing on dynamical mean field theory and the so-called GW approximation [1,2]. You will make use of Bristol’s high-performance computing capability. Most importantly, a key part of the project will be to test those calculations against state-of-the-art experiments by calculating quantities which can be experimentally determined.

On the experimental side, the project will take advantage of Bristol’s unique NanoESCA facility for probing electronic structure (http://www.bristol.ac.uk/physics/facilities/nanoesca/) using photoemission. In addition, we will use Compton scattering to provide access to the (many body) ground-state electronic wavefunction, and also the Fermi surface [3]. The Fermi surface is one of the most important concepts in the physics of metals. Its shape is important for understanding their properties and behaviours and measuring it is a challenging but important task. The group is currently working on a diverse range of materials, including topological superconductors, high-entropy alloys, half-metallic ferromagnets, (topological) Kondo insulators and correlated oxides, all materials with particularly desirable properties to underpin future technologies.

You will have the opportunity of operating in an environment where experimental work is supported and inspired by electronic structure theory developed and executed within the group. You would be expected to take a leading role in the theoretical endeavours.

The project would suit someone who has a strong interest in computational theoretical condensed matter physics but would like to maintain a very close connection to experiment.  As part of this project, you might also be involved with experiments at local, national and international facilities (such as SPring-8 in Japan).

Contact Professor Stephen Dugdale (s.b.dugdale@bristol.ac.uk) for further details or apply here : http://www.bristol.ac.uk/study/postgraduate/apply/

Figure: One of the electron Fermi surface sheets obtained from a Compton experiment on the high-entropy alloy NiFeCoCr. The colours show how the coherence length (in angstrom) extracted from the experiment varies across the Fermi surface. The wireframe box shows the first Brillouin zone. Further details can be found in [4].

[1] A.D.N. James et al., Phys. Rev. B 103 035106 (2021)
[2] E. I. Harris-Lee et al.,Phys. Rev. B 103 235144 (2021)
[3] S.B. Dugdale, Physica Scripta 91 053009 (2016)
[4] H.C. Robarts, et al. : Physical Review Letters 124 046402 (2020).

c & R. Springell

Bulk crystals can display heavy fermion behaviour, where the effective mass of mobile carriers can be up to 1000 times larger than the bare electron mass due to electron-electron interactions. Many of these materials are U-based compounds, such as UGe₂ and UPt₃, and they display a range of fascinating physics including unconventional superconductivity, quantum criticality and magnetism [1, 2]. The ability to grown such materials as thin films opens up a range of interesting possibilities: (i) we can explore the effect of dimensionality by tuning the film thickness; (ii) apply compressive and tensile strains using different crystalline substrates to tune the emergent physics; (iii) we can create more complex structures such as superlattices and device architectures to interrogate the system in novel ways. While there have been a few studies in these directions [3-5] there is a vast range of opportunities open to explore

In Bristol we have a thin film sputtering system, unique in the UK, capable of growing compounds of uranium in high quality single crystal form [6,7]. In the coming 18 months this system will be upgraded to a National Nuclear User Facility with added capabilities. This project will leverage this equipment to investigate the low temperature properties of various U-based compounds in the search for novel tuning parameters to control heavy fermion behaviour, superconductivity and magnetism in these materials.

Contact Dr. Chris Bell (christopher.bell@bristol.ac.uk) for further details or apply here : http://www.bristol.ac.uk/study/postgraduate/apply/

Figure: Schematic of tuning heavy fermion thin films with epitaxial strain (horizontal axis) and finite size (diagonal axis).

[1] Saxena et al., Nature 406, 587 (2000)

[2] Joynt & Taillefer, Rev. Mod. Phys. 74, 235 (2002)

[3] Jourdan et al., Nature 398, 47 (1999)

[4] Jourdan et al., Phys. Rev. Lett. 93, 097001 (2004)

[5] Mizukami et al., Nat. Phys. 7, 849 (2011)

[6] Bright et al., Thin Solid Films 661, 71 (2018)

[7] Bao et al., Phys. Rev. B‏ 88,134426 (2013)

Supervisor: Dr. Chris Bell (christopher.bell@bristol.ac.uk) 

Uranium, the heaviest naturally occurring element, has a complex structural phase diagram with an orthorhombic structure at ambient conditions, and a body-centred cubic (bcc) phase at higher temperatures. Electronically U shows superconducting character and a range of unique charge-density wave (CDW) transitions. In thin film form, epitaxial strain and lattice matching with the substrate has been used to control the properties [1]. We have been able to stabilise the crystal structure in the bcc phase, and a completely novel hexagonal close-packed (hcp) phase has be created [2]. In the latter case dynamic instabilities may also be present adding more intrigue into the stability of the system. The structural malleability of uranium impacts the spin-orbit strength, changes the CDW transition, how the superconductivity changes is still unknown.

This project will investigate these fundamental questions, and examine low temperature properties and electronic structures of various uranium crystals. It will leverage the range of thin film sputtering systems in Bristol, unique in the UK [3], which can create high quality thin films and heterostructures of uranium and other materials.

[1] Springell et al., Adv. in Phys. https://doi.org/10.1080/00018732.2023.2230292 (2023)
[2] Nicholls et al., Phys. Rev. Mater. 6, 103407 (2022)
[3] https://nnuf-farms.bristol.ac.uk/

Contact Dr. Chris Bell (christopher.bell@bristol.ac.uk) for further details or apply here : http://www.bristol.ac.uk/study/postgraduate/apply/

The project connects the Quantum and Soft Matter Theme, with the Interface Analysis centre, part of the Materials and Devices Theme, so there many staff and other PhD students who can support you, and give advice about your project. The Graduate School within the School of Physics is there to support you locally too and monitor your progress. The Bristol Doctoral College supports PhD students centrally within the University with a range of online and in-person training events, and career advice.‌

Project: This project will focus on experimental studies of novel superconductors including nickel-oxides like La3Ni2O7 and hydrogen compounds like H3S [1–3]. Discoveries in 2018 and 2023 show superconductivity operating up to 260 K in hydrogen and nickel-oxide compounds at high pressure. During the PhD you will characterise the superconducting and normal state. This work will provide clues for the mechanism of superconductivity and will help the search for new superconductors with higher Tc and at lower pressure.

Superconductivity has the prospect to advance energy, health care, and computing, but currently is limited to very low temperatures. Finding new superconductors that operate at higher temperature will unlock the full potential, e.g., in medical imaging and nuclear fusion.

PhD work: You will learn how to conduct high-resolution electrical, magnetic, thermodynamic, and spectroscopic measurements. You will use diamond-anvil cells to generate pressures up to 2 million bar. Experiments will be carried out at the University of Bristol and at facilities like the European High Magnetic Field laboratory and the Diamond Light Source synchrotron. You will be part of a very active group with many related projects on high-temperature superconductivity in different materials and many friendly and motivated PhDs, postdocs, and research staff.

Please contact Dr. Sven Friedemann (Sven.Friedemann@bristol.ac.uk) or visit http://www.bristol.ac.uk/physics/research/qsm/postgrad/ for more details.

Figure: Discovery of record superconductors. Inset shows sample in diamond anvil cell at 200,000 bar pressure with electrodes used for measurements.

Today, superconductivity is widely recognised as a pivotal player in the frontier development of quantum computation. The definitive theory of superconductivity was published by Nobel laureates Bardeen, Cooper and Schrieffer in 1957. Their idea was that two electrons (single-particle states) pair to form a boson, which can later condense at the lowest temperature. The ‘BCS theory’ proved to be remarkably successful in explaining the properties of almost all known superconductors. Over time, however, a number of superconducting materials have emerged that appear to challenge the BCS template. Significantly, their superconducting properties appear, in many respects, to be superior. 

Intriguingly, this new class of superconducting materials also host a metallic (resistive) state that fails to conform to the standard models of metallic behaviour. At high temperatures, the electron mean free path diminishes to a fraction of the interatomic distance while at low temperatures, the electrical resistivity varies linearly with temperature, not quadratically as expected. The fact that the electronic states in these systems are so unconventional has led to suggestions that the superconducting condensate may emerge from the incoherent, rather than the coherent part of the electron self-energy. We call this alternative paradigm 'un-particle superconductivity' and marks the transition from maximal to minimal dissipation.   

The goal of this project is to explore the viability of un-particle superconductivity in candidate materials. Researchers from Queen’s University in Belfast will seek to develop the theoretical framework for the pairing of electronic states formed from the incoherent part of the electron spectral function, while the experimental team at Bristol, of which you would be part, will test the resulting predictions with precise measurements of their superfluid and normal carrier densities. Fulfillment of these research goals could lead to a new paradigm for (high temperature) superconductivity, one far-removed from the original BCS template.

Contact Professor Nigel Hussey (nigel.hussey@bristol.ac.uk) for further details or apply here : http://www.bristol.ac.uk/study/postgraduate/apply/

Dr. Wang's research group from the Physics Department of the University of Bristol, UK, is recruiting full scholarship doctoral students for the autumn of 2024. The research group's focus is on experimental condensed matter physics, primarily involving macroscopic quantum phenomena, quantum matter such as high-temperature superconductors, topological superconductors, heavy fermions and other strongly correlated systems. The main research methods include ultra-low temperature strong magnetic field Scanning Tunneling Microscopy (STM), Scanned Josephson Tunneling Microscopy (SJTM), and Scanned Andreev Tunneling Microscopy (SATM). Using these techniques, we will search for new quantum states in unconventional superconductors and topological quantum matter.

Seeking dedicated PhD candidates in quantum physics with a focus on bulk topological superconductors (TSCs). TSCs, with their unique properties, have the potential to revolutionize qubit stability in quantum computing, paving the way for more efficient quantum technology paradigms. In this project, we aim to visualize the topologically protected quantum states at atomic-scale, laying bare the electronic structures of topological quantum matter. Using the state-of-the-art millikelvin scanning tunneling microscopy (mK-STM), our focus will be on uranium-based heavy fermions and other unconventional superconductors, which have shown promise as topological superconductor candidates. The novelty of this research project lies in the Scanned Andreev Tunneling Microscopy (SATM), enabling visualization of topological surface states at nearly absolute zero temperatures. These quantum states could have profound impact on quantum sciences.

As the PhD candidate on this project, you would develop the newest generation of a quantum microscope to visualize unconventional superconductors at cryogenic temperatures. You will develop new data analysis algorithms and experimental techniques. This is a truly cutting-edge project with very significant and profound impact in academic research, intended to provide deeper understanding to quantum science and technology, and leading to high quality publications.

If you are interested to work in a stimulating environment and are curious in exploring the physical phenomena in quantum matter and low temperature physics, we would be happy to talk to you. You would be registered at the University of Bristol and join the Quantum and Soft Matter Theme of the School of Physics, with Dr. Shuqiu Wang (https://www.physics.ox.ac.uk/our-people/shiquiwang ). This project will be collaborating with University of Oxford.

To submit an application, visit the following FindAPhD page. For additional information, please contact Dr Shuqiu Wang (shuqiucwang@gmail.com ). For general information on Physics at Bristol and the Graduate School, please contact phyiscs-pg@bristol.ac.uk.

Glasses are used in a wide range of technological applications for example, in windows, fibre optic cables, lasers and sensors. However, many applications are limited by the difficulty in forming the glasses in the first place and avoiding the introduction impurities in  their fabrication. In this project you will apply newly developed acoustic levitation methods to produce novel glasses for future applications.

 A wide range of technical glasses are formed from high melting temperature oxides such as SiO₂, Al₂O₃, and  Ga₂O₃ for which their morphology, their phase composition and their purities are of importance. When fabricating such glasses in crucibles contamination from the crucible and impurities is difficult to avoid. In this Ph.D. project you will use containerless processing techniques, in particular acoustic levitation and laser heating methods, to fabricate high-purity oxide spheres and to characterise their thermal, physical, chemical and optical properties. Acoustic levitation will be accomplished using our recently developed acoustic levitation system that is capable for levitating objects with densities in excess of 10 g∙cm^-3 with radii up to ~ 1 mm.  In this way levitated materials can be heated to temperatures in excess of 2500 K using a CO₂ laser heating system and rapidly quenched under containerless conditions. The PhD student will actively participate in the improvement of the existing acoustic levitation prototypes. This project is in partnership with Johnson Matthey PLC. A particular interest from Johnson Matthey is to utilise the levitation system for characterising droplet/particle drying and specifically both the kinetics and the morphology evolution of laser heated droplets/particles. This will provide fundamental insight into spray drying processes which are intensively used in the manufacturing of many JM products.

Contact Dr. Adrian Barnes (A.C.Barnes@bristol.ac.uk) for further details or apply here : http://www.bristol.ac.uk/study/postgraduate/apply/

Figure : (a) CO₂ laser heating setup with the “MightyLev” ultrasonic levitator. (b) MightyLev levitating a hot sample of laser heated glass. (c) A simulation of the acoustic field in MightyLev.