Quantum Critical Superconductivity

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Summary: High temperature superconductors have the potential to have a significant impact in industry from increasing efficiency of electrical power transmission to generating stronger magnetic fields in fusion reactors. However, they will only be of widespread use when we are able to work with materials that are superconductors at room temperature and ambient (or just very low) pressure. A main part of solving this problem is to better understand the fundamental physics behind this superconducting state in the unconventional superconductors such that we can improve the ability to find new ones. It has been theorised that superconductivity in these materials is enhanced by the presence of quantum critical fluctuations often from other competing magnetic states. In this project I will be investigating the presence of quantum critical points (second order phase transitions supressed to 0K by a tuning parameter) in these superconducting materials in order to ascertain the impact they have on the superconducting state. Quantum critical points and the structure of the superconducting gap itself can be investigated by measuring the variation of the magnetic penetration depth in the superconducting state. I will be measuring this parameter using a Tunnel Diode Oscillator which relates a change in frequency in the oscillator circuit to a change in penetration depth. Tracking how this parameter varies with temperature can provide insight into the nature of the superconducting gap (whether it is nodal), but I will also be investigating how the penetration depth varies with a change in interaction strength. To do this I will be tuning the applied pressure using a cylindrical piston cell and tracking how both the superconducting critical temperature, and the magnetic penetration depth vary with pressure. I 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 enhancing of the superconducting state (increase of critical temperature). I aim to employ this method on a range of high Tc superconductors but specifically focussing on cuprates. The goal will then be to either confirm or reject a link between quantum critical points and enhanced superconductivity. The TDO technique for measuring penetration depth has been used extensively by the group, as has use of piston pressure cells. The combining of these two techniques, however, is a new development that will enable the construction of pressure phase diagrams with penetration depth and Tc which will hopefully increase our understanding of the mechanisms behind high Tc superconductivity in cuprates. This project falls within the EPSRC superconductivity research area. It is 50% funded by the EPSRC and 50% by Bristol Department of Physics Potter Endowment fund.