TRIUMF uses subatomic particles as probes of materials structure at the Centre for Molecular and Materials Science (CMMS). The chief techniques are μSR and β-NMR.
Ever since the invention of the microscope, scientists have been peering deeper and deeper into the world around us. During the past century, new and better techniques for looking inside materials have been found, including X-ray diffraction, electron microscopes and neutron scattering. Scientists at TRIUMF are using another technique to examine materials, called μSR (pronounced “mew-ess-are”), which has become a unique and powerful probe to peer into and gain a deeper understanding of what goes on inside materials like semiconductors, magnets and superconductors.
In the acronym μSR, the “μ” stands for “muon” (μ is the greek letter “mu”), “S” is for “spin”, and “R” can stand for a number of words, including but not limited to rotation, relaxation, resonance or research. The acronym is meant to draw attention to the analogy between μSR and the more commonly know techniques of NMR (nuclear magnetic resonance) and ESR (electron spin resonance). Whereas NMR utilizes the spins of ordinary atomic nuclei, μSR is a collection of methods that use the muon’s spin to examine structural and dynamical processes in bulk materials on an atomic scale.
The broadest application of the μSR technique is as a magnetic probe. Beams of positive muons are created with their spins lined up in the same direction. When these beams are shot into a material, the muons’ spins precess (wobble like a top) around the local magnetic fields in the material. The unstable muons soon decay into positrons; since these antielectrons tend to be emitted in the direction of the muons’ spin, μSR scientists can examine how the internal magnetic fields of different materials have affected the muons’ spins by observing the directions in which the positrons are emitted.
β detected NMR is an exotic form of nuclear magnetic resonance (NMR) in which the nuclear spin precession signal is detected through the beta decay of a radioactive nucleus. It takes advantage of the new world class rare-isotope beam facility (ISAC) at TRIUMF. We have recently developed a beam of low energy hyperpolarized radioactive nuclei for applications in condensed matter using an optical polarization scheme. The nuclear method of detection along with the high degree of spin polarization means that β-NMR at ISAC is about 10 orders of magnitude more sensitive than a conventional NMR experiment. The beam line and associated instruments are unique in the world and open a new window into the magnetic and electronic properties of ultrathin films, nanostrucures and interfaces. The central question to be studied is how do the local electronic and magnetic properties near an interface or surface of new materials (e.g,. a high Tc superconductor) differ from those of the bulk?