Quantum Critical superconductors at high pressures

When a second order phase transition is suppressed to zero temperature, fluctuations between the ordered and non-ordered state are no longer thermal excitations but rather quantum in nature. In metals, this gives rise to completely new phenomena including deviations from the standard Landau-Fermi liquid behaviour. In addition, unconventional superconductivity is often observed around quantum critical points. Thus, quantum criticality remains a topical and timely research area for a wide variety of materials.
In cuprate and chalcogenide superconductors, the link of superconductivity to quantum criticality remains controversial. In cuprate superconductors theoretical proposals suggest quantum criticality from the suppression of the pseudogap phase, charge density wave order, or superconductivity itself. In chalcogenide materials quantum criticality and emergent superconductivity is associated with the suppression of charge density wave order, nematicity, or spin-density wave order. At the same time, alternative theories propose a complete absence of coherent electronic states in cuprates. This project will use electronic transport measurements and quantum oscillation studies as decisive probes to detect the relation of quantum criticality and superconductivity and will probe whether coherent electronic states persist as the materials are tuned across the quantum critical point.
High pressure studies will be used to study clean samples in a systematic approach. Sven Friedemann has developed high-pressure techniques for a host of new measurements. This includes SQUID magnetometry as well as quantum oscillation and electrical transport measurements at international high-magnetic-field facilities, both beyond 10 GPa . The student will be based almost entirely in Bristol for the first half of the project and will be trained in high-pressure methods. During this time, he will conduct lab-based measurement on NbSe2 and TiSe2 chalcogenide superconductors. In preparation for the 2nd half of the project, he will grow crystals of HgBa2CuO4. The 2nd half will include periods of several months based at Radboud University and at international facilities. This part of the project will focus on facility-based studies of quantum oscillations and electronic transport in HgBa2CuO4 and FeSeS.