The Superconducting Materials research group focusses on relationships between processing, microstructure and properties of a wide variety of superconducting materials. A major part of our current research activity is associated with the Oxford Centre for Applied Superconductivity (CfAS) and involves working closely with local industrial partners to address materials challenges in the superconducting magnet industry, such as superconducting joints and MgB2 bulk materials. We have a growing activity in studying radiation damage in high temperature superconductors for nuclear fusion magnets. In addition, we specialise in the processing and characterisation of thin films, and have a new research interest in novel superconducting materials for quantum device applications . On the more fundamental side, we use advanced microstructural characterisation techniques including High-Resolution Electron Backscatter Diffraction (HR-EBSD), Magnetic Force Microscopy (MFM) and synchrotron microscopy (e.g nanoARPES, PEEM) to study homogeneity and phase separation in single crystals of novel Fe-based superconductors. Understanding the interplay between magnetism and superconductivity in these materials is thought to be of crucial importance for discovering the elusive mechanism for high-temperature superconductivity.
Evolution of the Fermi surface of the nematic superconductors FeSe1-xSx
npj Quantum Materials
We investigate the evolution of the Fermi surfaces and electronic
interactions across the nematic phase transition in single crystals of
FeSe1-xSx using Shubnikov-de Haas oscillations in high magnetic fields up to 45
tesla in the low temperature regime. The unusually small and strongly elongated
Fermi surface of FeSe increases monotonically with chemical pressure, x, due to
the suppression of the in-plane anisotropy except for the smallest orbit which
suffers a Lifshitz-like transition once nematicity disappears. Even outside the
nematic phase the Fermi surface continues to increase, in stark contrast to the
reconstructed Fermi surface detected in FeSe under applied external pressure.
We detect signatures of orbital-dependent quasiparticle mass renomalization
suppressed for those orbits with dominant dxz=yz character, but unusually
enhanced for those orbits with dominant dxy character. The lack of enhanced
superconductivity outside the nematic phase in FeSe1-xSx suggest that
nematicity may not play the essential role in enhancing Tc in these systems.
Nanoscale Stoichiometric Analysis of a High-Temperature Superconductor by Atom Probe Tomography.
The functional properties of the high-temperature superconductor Y1Ba2Cu3O7-δ (Y-123) are closely correlated to the exact stoichiometry and oxygen content. Exceeding the critical value of 1 oxygen vacancy for every five unit cells (δ>0.2, which translates to a 1.5 at% deviation from the nominal oxygen stoichiometry of Y7.7Ba15.3Cu23O54-δ ) is sufficient to alter the superconducting properties. Stoichiometry at the nanometer scale, particularly of oxygen and other lighter elements, is extremely difficult to quantify in complex functional ceramics by most currently available analytical techniques. The present study is an analysis and optimization of the experimental conditions required to quantify the local nanoscale stoichiometry of single crystal yttrium barium copper oxide (YBCO) samples in three dimensions by atom probe tomography (APT). APT analysis required systematic exploration of a wide range of data acquisition and processing conditions to calibrate the measurements. Laser pulse energy, ion identification, and the choice of range widths were all found to influence composition measurements. The final composition obtained from melt-grown crystals with optimized superconducting properties was Y7.9Ba10.4Cu24.4O57.2.
Atom probe tomography, YBCO, stoichiometry, superconductor
Strain in epitaxial MnSi films on Si(111) in the thick film limit studied by polarization-dependent extended x-ray absorption fine structure