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 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
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
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.