PhD projects 2019
There are two routes currently available in the group:
- Fully funded 3.5/4 year studentships are available for 2019 through Doctoral Training Partnerships (DTPs), formally known as DTAs.
- A fully funded 3.5 year studentship is available for 2019 as part of an ERC research grant: ERC PhD Advert 2019 (PDF, 220kB)
Current projects in the group
Heavy Electron Compounds (Dr Friedemann)
Heavy Fermion compounds are made from rare earth elements and other metals. The rare earth elements contribute local magnetic moments which interact with conduction electrons. It is exactly this interaction which makes heavy fermion systems very interesting.
The local moments form composite quasiparticles that behave like electrons but with largely increased masses up to several thousand times the bare electron mass – thus the name heave fermions. They provide model systems for many questions central to solid state physics and beyond like high-temperature superconductivity, strong correlations, and correlated topology. They can be tuned in a very controlled way and thus allow detailed insight into strongly correlated behaviour.
Current projects investigate novel heavy fermion compounds with lower dimension and surface states. We employ measurements of the electrical resistance and Hall effect to study these materials and their electronic structure.
Email Dr Sven Friedemann for more information.
Sketch of a heavy electron with the local moment dressed by a sea of conduction electrons that slow down movement and hence make it appear heavy.
Quantum Electronic Order (Prof Hayden)
Strong correlations between electrons in solids can lead to some spectacular effects, perhaps the most astonishing of which is high temperature superconductivity. The field of correlated electron systems has been made rich and exciting by a series of experimental discoveries over the last two decades. In this project you will investigate electronic order and the associated collective excitations. The aim of the research is to explain physical properties of materials such as superconductivity, electronic nematic order or charge order by measuring the electronic correlations. The work involves neutron and x-ray scattering, laboratory measurements using high magnetic fields and low temperatures, crystal growth and theoretical modelling. We carry out experiments at synchrotrons and neutron facilities around the around the world.
Two examples of materials which we are presently working on are the large temperature superconductor YBa2Cu3O6+x where we have recently observed charge order  and the 4d oxide metals Sr2RuO4 and Sr2Ru3O7 (see Figure).
Email Stephen Hayden for more information.
crystal structure of cuprate superconductors
Field effect gating of correlated thin films and surfaces (Dr Bell)
Transistors have revolutionized the world of computers. For basic research they are also extremely powerful devices to study correlated materials which are sensitive to the electron density in the system. The electrostatic field effect used in transistors reversibly adds and removes electrons from a thin film or surface, and can tune the groundstate of the material. A recent trend is the use of ionic liquids as gates: the large capacitance of the induced Helmholtz layer and high breakdown electric fields means that they can dope far higher carrier densities than conventional gate materials. Several groups around the world have established this technique to make novel transistors, demonstrating control of superconductivity and magnetism at the surfaces of crystals or in thin films over a range that cannot be achieved by other means. We will use this technique to examine correlated materials, searching for new groundstates at field-effect doped surfaces.
Email Dr Chris Bell for more information.
High Pressure Studies on Superconductors (Dr Friedemann)
High pressure research provides vital information in the challenge to understand superconductors: It allows highly systematic studies tuning the properties of one sample without introducing disorder or breaking symmetries. As pressure is easily modelled our research offers decisive input for theoretical models.
This project aims at studying the new record superconductor H3S with a transition temperature of more than 200 K . A combination of magnetic and electrical measurements will be used to investigate the mechanism of high-temperature superconductivity. We will make use of novel pressure techniques like the gold patterned gemstone anvils in the picture for electrical transport studies. Measurements will be performed both at the HH Wills Laboratory in Bristol as well as at international high-field facilities.
Gold electrodes on a high-pressure anvil.
Pressure tuning high temperature superconductors
In order to uncover the mechanism for high temperature superconductivity in cuprate and other unconventional pairing materials, a necessary first step is to understand their normal state behaviour and the nature of any competing electronic ground states. In the cuprates, charge density wave order and the enigmatic pseudogap state are known competing phases, whereas in iron-based superconductors, both a nematic electronic state and a more conventional spin-density wave (antiferromagnetic) state are known. What is less clear is whether these phases are friend or foe. Do quantum critical fluctuations in these one or more of these phases give a boost to the superconducting transition temperature or does the tendency for these phases to form order rather depress superconductivity? A third option is that these competing phases may be simple too weak to have any effect and are thus irrelevant. The vast majority of studies that have attempted to uncover the nature of these phases and their link to superconductivity have used chemical doping as a tuning parameter to move the material across its phase diagram. In this project, we will rather use hydrostatic or uniaxial pressure to tune the materials. The advantage of the latter approach (pressure) is that by tuning a single sample the effects of chemically induced disorder can be avoided. Pressure also tunes the sample across the phase diagram along a different axis to chemical doping and thus can split apart phases which are approximately degenerate at a particular doping level.
The project will involve developing pressure cells for research in high magnetic fields (pulsed fields in Toulouse / Dresden /Los Alamos and DC field in Nijmegen and Tallahassee), and then using these to measure quantum oscillations and electrical transport properties. Other aspects will involve: single crystal sample growth, finite element analysis and electronic structure calculations.
Email Prof Antony Carrington for more information.
Oscillations in various properties give detailed clues about the electronic structure.
Superconducting gap structure of unconventional superconductors
The structure (anisotropy and symmetry) of the superconducting energy gap gives us unique insight into the interactions that lead to superconductivity. Getting accurate information about this structure requires a synthesis of several different techniques including bulk measurements: specific heat, magnetic penetration depth and spectroscopic techniques such as STM or angle resolved photoemission spectroscopy.
Here we will use several different techniques to elucidate the nature of the gap structure in iron-based and heavy fermion superconductors. One novel development in Bristol has been the exploitation of the non-linear Meissner effect, which manifests in the field dependence of the magnetic penetration depth. This has been used recently to get some novel insight into the gap structure of the iron-based superconductor KFe2As2 and the heavy fermion CeCoIn5. A goal of this project is to further exploit this technique to gain information on other materials which are of high interest in the field, such as FeSe, LaFePO or BaFe2(As1-xPx)2. These studies will be combined with other bulk techniques to gain a comprehensive picture on the same samples. The project will involve learning new lab based techniques, developing novel instrumentation, and also experiments at international high magnetic field facilities. The project could also involve numerical calculations of the thermodynamics of the quasiparticle excitations of these materials for a student with a particular interest in this area. The project may also be combined with quantum oscillation measurements of the normal state electronic structure of the same compounds.
Email Prof Antony Carrington for more information.
sketch of the d-wave gap around the Fermi surface
How to apply
For the DTP projects, contact the project supervisor.
Updated 20 February 2019