µSR Research at the Paul Scherrer Institute

Research at the Laboratory for Muon-Spin Spectroscopy (LMU) uses positive and (occasionally) negative muons (µ+, µ-) as local magnetic probes in matter. The experimental techniques referred to as µSR (for Muon-Spin Rotation, Relaxation, Resonance or Research) are universally applicable since the muons available at meson factories such as the PSI proton accelerator complex can be implanted in any material. Moreover, due to parity violation in the pion decay, muons are emitted with perfect spin polarisation, providing µSR with a great advantage: whereas eg NMR and ESR rely upon a thermal equilibrium spin polarisation, µSR begins with a perfectly polarised probe regardless of the conditions of the sample.

The property of the muon that makes the spin polarisation observable is its decay into an energetic positron (positive muons) or electron (negative muons) which, again due to parity non-conservation, is emitted preferentially along the direction of the muon spin, thus carrying information on the µ-spin polarisation out of the investigated material.

The muon is a very sensitive probe of both static and dynamic magnetic properties of materials: due to its mean lifetime of 2.2 µs and a gyromagnetic ratio of 2pi·135.5 MHz/T, the accessible magnetic fields and widths of field distributions range from ~10 µT to several Tesla, and the time scales for dynamic properties from pico- to milliseconds.

As a 'light isotope' of the proton (muon rest mass = 1/9 of the proton rest mass) the µ+ can form the hydrogen-like 'exotic' atom Muonium (Mu = µ+ e-) which may substitute for hydrogen in insulators and organic materials, providing a very sensitive spin label.

At the LMU, solid-state physicists, chemists and materials scientists from PSI and abroad use muons to investigate fundamental and technologically relevant aspects of structural, magnetic and electronic phenomena in magnets, superconductors, semiconductors and insulators. Samples range from pure elements to inorganic and organic compounds and molecular systems.

Our laboratory maintains and actively develops a µSR User Facility, presently consisting of six different spectrometers covering a wide range of techniques and applications. In 2010, more than 150 research proposals of groups from PSI, Swiss universities and from abroad have been active, using roughly 70% of the total beam time allocated to approved experiments at the target M and E beam lines. About 350 scientists from different countries are involved in the µSR proposals.

Two unique extensions to µSR have been developed at PSI. First, Low Energy Muons, which can be implanted at very small and controllable depths below the surface of a sample (a few to a few hundred nanometers), allow all the advantages of µSR to be applied to thin samples and multilayered structures, near surfaces and as a function of implantation depth on a nanometer scale.

The second important development is a fast-switching electrostatic deflector, able to extract single muons from a continuous beam upon request ("MORE") from a spectrometer. Routinely available, this provides unique frequency resolution and increases measurable relaxation times to milliseconds at the full time resolution (1ns) of our spectrometers.

The user programs involve a large variety of topics in condensed matter research. The majority of the proposals is devoted to magnetism and superconductivity. Effort is put on the study of new materials such as high spin molecules, low dimensional magnetic systems, organic superconductors, conducting polymers, liquid crystals and novel solar cell materials.