Magnetic quantum criticality and superconductivity of Fe-based compounds

Unconventional and high-temperature superconductivity is often found in materials near continuous magnetic phase transitions. Associated divergences in the materials’ magnetic susceptibility are connected with an enhancement of spin fluctuations that can enable the formation of Cooper pairs. Many Fe-based superconductors show this proximity of high-temperature superconductivity and magnetic phase transitions. In addition, their phase diagrams often feature structural phase transitions. We have been exploring structural effects on the magnetic and superconducting phase diagrams by pressure tuning of selected 122 compounds using thermodynamic, magnetic, and synchrotron diffraction measurements in collaboration with Diamond Light Source. 

We are also studying other Fe based compounds, in which the magnetic fluctuations that are potentially so important in the formation of unconventional superconductivity can be studied in isolation. Magnetic fluctuations near a continuous ferromagnetic phase transitions typically lead to an instability of the associated low-temperature quantum critical point. In NbFe2, we are exploring the masking of its ferromagnetic quantum critical point by an emergent spin-density-wave phase using unpolarised and polarised neutron diffraction and spectroscopy.

Novel states of electronic matter

Strong electronic correlations can cause the emergence of novel electronic matter. Examples include nematicity, skyrmion phases, the spin liquid, topological insulating or superconducting states. A long-standing mystery in this area has been the nature of the so-called 'hidden order' phase in the heavy-fermion compound URu2Si2. The onset of the hidden order phase is signalled by a large peak in the specific heat. Assuming antiferromagnetic order the ordered moment is expected to be 0.4 B per U atom. However, only a sample dependent tiny average moment of less than 0.04 B per U atom has been found in experiment. If tuned by hydrostatic pressure, URu2Si2 maintains its peak in the specific heat but develops the expected size of magnetic order. Using the technique of neutron Larmor diffraction we have mapped out the phase diagram of URu2Si2 and shown that any residual magnetic moment in the hidden-order phase can be understood to arise from a small volume fraction in the antiferromagnetic state due to local lattice distortions. This reinforces the notion that the hidden-order state is a much more exotic new state of electronic matter and we have been investigating its low energy excitations using neutron spectroscopy. 

Tuning Correlated Electron Systems from Magnetism to Unconventional Superconductivity

In this project you will explore the rich phase diagrams of strongly correlated electron systems. You will investigate the ferromagnetic quantum critical region, where the emergence of spin-density wave order has been observed. You will also tune antiferromagnetic interactions to search for new phases of electronic matter including unconventional and high-temperature superconductivity. The project will involve the exploration of selected Fe-based compounds. To achieve a high success rate in our search for correlated-electron phenomena, we tune candidate materials by applying high pressure and by changing the chemical composition. The combination of electrical transport, susceptibility and magnetisation measurements at Royal Holloway with neutron and X-ray scattering experiments at large facilities will allow you to collect comprehensive results during your PhD project. 

Contact person : Dr Philipp Niklowitz

Polarised neutron spectroscopy of energy materials

Neutron spectroscopy can provide deep insight into the physics underpinning technologically relevant phenomena. The spin, energies and wavelengths of thermal neutrons make them ideal for the study of heat carrying quasiparticles, diffusing ions and spin waves. However, in an experiment on “real” materials, these signals can overlap, making it difficult to interpret results. By exploiting the spin of the neutron these different components can be separated and compared to molecular dynamics simulations to gain deep understanding of the underlying transport mechanisms. Experiments will be performed using the LET spectrometer at ISIS, where the recently installed polarised neutron capability will be employed to study diffusion of both real particles and emergent quasiparticles in fuel cell and battery materials using inelastic neutron scattering. It is planed that complementary measurements will be performed at the ILL in Grenoble, France and at J-PARC in Japan. The student will also develop code to relate first-principles molecular dynamics simulations to experiment. The studentship is fully funded, and this covers the standard tax-free UKRI stipend and fees for UK and EU nationals for three and a half years.

Contact person : Professor Jon Goff