Quantum Interfaces studied with low energy β-NMR and μ-SR

All electronic, magnetic and structural properties of a material are altered near an interface between two materials due to the broken translational symmetry and the influence that one material can have another. In some cases these changes are small and confined to just a few atomic layers while in other situations changes in collective behaviour are profound and extend over many nanometers or even microns.

Giant magnetoresistance in magnetic multilayers and the profound impact this discovery has had on magnetic storage is one notable example. There are only a few experimental methods which are capable of probing local properties in a depth resolved manner. We have developed one of these in Canada at TRIUMF called depth-resolved beta-detected NMR (nuclear magnetic resonance). The only other similar method is low energy muon spin rotation/relaxation which can only be performed at the Paul Scherrer Institute (PSI) in Switzerland. The two methods are the same in principle but provide complementary information, due to the different time scales of the radioactive probe in each case. The muon has a mean lifetime of 2.2 microsec whereas a radioactive nucleus such as 8Li has a mean lifetime of 1.2 s. In both cases the observed quantity is the time evolution of the muon (or nuclear) polarization, which is broadcast through the emission of a high energy beta particle from the radioactive probe. Both may be regarded as a special form of magnetic resonance.

Both are ideally suited to studies in thin films and interfaces because they require roughly 10 orders of magnitude fewer spins than conventional NMR. Our goal is to explore electronic properties of interfaces of quantum materials (where electrons exhibit coherent collective effects such as superconductivity) using newly developed nuclear methods.