Novel Properties of Electrons Near Intefaces Studied with muSR and beta-NMR

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All electronic, magnetic and structural properties are altered near an interface between two materials (or a surface) due to the broken translational symmetry and delocalized nature of electrons in which the character of the electronic wavefunction varies as a function of distance from the boundary. In some cases these changes are small and confined to a few atomic layers while in other situations, the collective behaviour is modified over many nanometers or even microns. Giant magneto-resistance in magnetic multilayers is one notable example which had a dramatic impact on magnetic storage. Only a few experimental methods are capable of probing local magnetic and electronic properties in a depth resolved manner. We have developed one of them at TRIUMF called low energy beta-detected nuclear magnetic resonance (beta-NMR). Another closely related method is low energy muon spin rotation/relaxation, which has been developed at the Paul Scherrer Institute (PSI) in Switzerland. Both methods will be used to explore the coherent properties of electrons near interfaces and within heterostructures in order to address important problems in condensed matter physics. This includes the magnetic/electronic properties of the SrTiO3/LaAlO3 interface, the electromagnetic response of topological insulators and the depth dependence of the superconducting and magnetic order parameter in highly correlated systems. Our main objective is to explore/control the electronic/magnetic properties near the interfaces and surfaces of quantum materials. The results will be used to test theoretical predictions and search for new phenomena. This is turn will help create a more unified picture of how electrons organize themselves into coherent states of matter at low temperatures and how they respond to electric and magnetic fields near interfaces and within heterostructures. The potential impact comes from the expectation that future devices will increasingly utilize on the properties of electrons at interfaces and within heterostructures. We will use newly developed techniques of low energy muSR and low energy beta-NMR, which have similar principles but provide complementary information. The magnetic moment of the muon (or radioactive nucleus) acts as a probe of the local magnetic/electronic environment. Conventional muon spin rotation and beta-NMR were invented in 1957 along with the discovery of parity violation in weak interactions. However, the particular variants used here are relatively new and still developing. The key points are that unlike conventional NMR, the signals in beta-NMR and muSR are independent of sample size and can be monitored as a function depth on a nm length scale. Consequently both methods are ideally suited to studies of thin films and heterostructures. They are both forms of magnetic resonance but provide complementary information. The muon is a sensitive probe of static or quasi static magnetic fields whereas 8Li is much more sensitive to slow electronic relaxation processes (e.g.Korringa relaxation in a metal) due to the much longer lifetime (1.2s), compared to 2.2 microseconds for the muon.