By directly driving anisotropic changes in lattice parameter, uniaxial stress is a qualitatively different probe of correlated electron systems from hydrostatic stress. By using piezoelectric actuators to apply thestress, and with careful sample preparation, elastic strains in excess of 1% are routinely achievable. This is sufficient to drive strong qualitative changes in the electronic properties of many compounds. The typical correlated electron material is a highly complicated object, and the ability to place materials on continuous axes over which their properties are strongly tuned can provide much more information on the key processes in a material than study of the single fixed point represented by the unstressed compound alone.
In this seminar, I will discuss the unconventional superconductors Sr₂RuO₄ and YBa₂Cu₃O_6.67. Through strong uniaxial compression along its c axis, the main Fermi surface of Sr₂RuO₄ can be driven through two simultaneous topological transitions, changing from an electron-like to a hole-like Fermi surface. Thismirrors the evolution of the cuprate Fermi surface across the superconducting dome, however in a system where interactions are weaker and potentially easier to understand. In-plane uniaxial compression strongly influences both systems: Sr₂RuO₄ is driven through a strong peak in its superconducting critical temperature, while strong uniaxial stress applied to YBa₂Cu₃O_6.67 induces static 3D charge density wave order,which competes with and strongly suppresses the superconductivity.