Superconductivity as seen by muons.

In type II superconductors the internal field distribution in the vortex state (London penetration depth and coherence length) is measured directly by µSR [pdf]. The muon is also a sensitive probe of spontaneous magnetic fields, used extensively to determine magnetic phase diagrams and to look for anomalous magnetic fields predicted in exotic theories of high Tc superconductors.

More details:

There are significant gaps in our understanding of this unusual phase of matter especially in unconventional superconductors for which the description of the vortex structure is a subject if great controversy. The µSR technique provides a sensitive local probe of the spatially inhomogenious magnetic field associated with the vortex state. For the case of a regular vortex lattice the magnetic penetration depth and the coherence length can be simultaneously extracted from the measured internal field distribution.

The penetration length is directly related to the density of of superconducting carriers in the material and measurements of its variation with temperature magnetic field and impurity level can provide essential information on the symmetry of the order parameter. The coherence length measured with µSR is the length scale of spatial variations of the order parameter with a vortex core. Our µSR shows that taht measurements of the fundamental length scales are fairly robust with respect to the details of how the field distribution is modeled.

There are many other important applications of µSR to the study of superconductivity that don't involve measurements on the vortex state, such as study of the antiferromagnetic or spin-glass phase, measurement of relaxation rates associated with phase transitions, and detection of spontaneous internal magnetic field in unconventional superconductors.

If you would like to know more about our superconductivity research please follow this link.

µSR and Fullerene.

Similar studies are proceeding on fullerene compounds but with a twist: we use a muonium atom (Mu+e-) as a probe of molecular dynamics and electronic structure. Roughly speaking, the unpaired electron on the muon acts as an amplifier, greatly increasing sensitivity to quasiparticle excitations.

If you would like to know more about our fullerenes research please follow this link.

µSR and Semiconductors and Semimetals.

In semiconductors our results on electronic structure of muonium give the best available information on the structure of isolated hydrogen in semiconductors. We are currently investigating charge and spin dynamics of muonium in intrinsic and doped semiconductors, to learn more about dynamics and reactivity of hydrogen in such material.

More details on µSR in semimetals:

The main purpose of the measurements in semimetals (e.g. graphite or antimony)
is to study how the low carrier concentration and the gapless spectrum of excitations affect local properties of a simple impurity such as a positive muon and to compare this behaviour
with current theories.

The theoretical interest in this problem is driven by the fascinating cooperative many-body
phenomena (also known as Kondo effect) involved in the screening of such an impurity. Despite the
remarkable success achieved in theory, the experimental investigation
of this problem by using conventional magnetic resonance techniques is
virtually impossible. Here the major difficulty is to find an
appropriate technique that has the required sensitivity without requiring a
large number of impurities to be present in the sample. Muon spin
rotation/relaxation is an ideal technique to investigate this problem
since it has the required sensitivity -- typically only one muon is in
the sample at a time. Also the positive muon represents a simple point
charge disturbance to the system.

If you would like to know more about our research on semimetals please follow this link.

Quantum Diffusion.

Our experiments on quantum diffusion of muonium are the clearest example of a crossover between classical hopping and quantum tunneling expected for motion of light interstitial atoms. We are particularly interested in how tunneling is influenced by the spectrum of quasiparticle excitations associated with the lattice, qualitatively different in metals, insulators and superconductors.

If you would like to know more about our superconductivity research please follow this link.