Perspective on muon-spin rotation/relaxation under hydrostatic pressure

Pressure, together with temperature, electric, and magnetic fields, alters the system and allows for the investigation of the fundamental prop- erties of matter. Under applied pressure, the interatomic distances shrink, which modifies the interactions between atoms and may lead to the appearance of new (sometimes exotic) physical properties, such as pressure-induced phase transitions; quantum critical points; new structural, magnetic, and/or superconducting states; and changes of the temperature evolution and symmetry of the order parameters.

Muon-spin rotation/relaxation (μSR) has proven to be a powerful technique in elucidating the magnetic and superconducting responses of various materials under extreme conditions. At present, μSR experiments may be performed in high magnetic field up to 9 T, temperatures down to ≃10–15 mK, and hydrostatic pressure up to ≃2.8 GPa. In this perspective, the requirements for μSR experiments under pressure, the existing high-pressure muon facility at the Paul Scherrer Institute (Switzerland), and selected experimental results obtained by μSR under pressure are discussed.