In type II superconductors magnetic flux penetrates the sample in the form of vortices of quantised flux. In the simplest case these form an ordered hexagonal lattice, but in exotic superconductors, such as the high-Tc cuprate materials, a whole zoo of vortex phases exist, including vortex glasses and vortex liquids. These phases of vortex matter are an interesting experimental testing ground for theories of statistical physics.

In our research we use a combination of muon spin rotation(µSR) and small-angle neutron scattering (SANS) to provide a very powerful approach to the study of vortex matter in superconductors. SANS provides information on the vortex lattice symmetry and long range order, and may also be used to probe two body correlations of vortices in disordered systems. µSR provides information on the local arrangement of vortices and is particularly useful in disordered and glassy systems. We have shown that it is even possible to use µSR to probe three body correlations in the bulk of the vortex glass phase. Recently we have used this to reveal a novel glass phase in anisotropic superconductors.

We use another technique, polarised neutron reflectivity (PNR) to study mesoscopic vortex physics in thin film superconductors. The increased rôle of the surface in such systems can lead to vortex behaviour which is quite different from that found in the bulk (see Drew et al. (2009)).

In collaoration with the group of Simon Bending at the University of Bath, we have also recently undertaken a number of studies of the oxide superconductor Sr₂RuO₄, widely believed to have a chiral p-wave order parameter. Here we combine µSR with scanning Hall probe measurements to investigate a number of properties, including the search for spontaneous edge states arising due to the chiral order parameter. We have also gained convincing evidence for the existence of asemi-Meissner state thought to arise from the multi-band nature of the superconductivity.

References

Search for spontaneous edge currents and vortex imaging in Sr2RuO4 mesostructures,
Curran, P., Bending, S. J., Desoky, W., Gibbs, A. S., Lee, S. L. and Mackenzie, A. K.,Phys. Rev. B 89 144504 (2014).

Muon-spin rotation measurements of the vortex state in Sr2RuO4: Type-1.5 superconductivity, vortex clustering, and a crossover from a triangular to a square vortex lattice,
S. J. Ray, A. S. Gibbs, S. J. Bending, P. J. Curran, E. Babaev, C. Baines, A. P. Mackenzie, and S. L. Lee, Phys. Rev. B 89 094504 (2014).

Measurement of the spatial extent of inverse proximity in a Py/Nb/Py superconducting trilayer using low-energy muon-spin rotation,
M. G. Flokstra ,S. J. Ray, S. J. Lister, J. Aarts, H. Luetkens, T. Prokscha, A. Suter, E. Morenzoni, and S. L. Lee,
Phys. Rev. B 89 054510 (2014).

Muon-Spin Rotation Measurements of an Unusual Vortex-Glass Phase in the Layered Superconductor Bi_2.15Sr_1.85CaCu₂O_8+d
D. O. G. Heron, S. J. Ray, S. J. Lister, C. M. Aegerter, H. Keller, P. H. Kes, G. I. Menon, and S. L. Lee, Phys. Rev. Lett. 110, 107004 (2013)

Vortex imaging and vortex lattice transitions in superconducting Sr₂RuO₄ single crystals
P. J. Curran, V. V. Khotkevych, S. J. Bending, A. S. Gibbs, S. L. Lee, and A. P. Mackenzie, Phys. Rev. B 84, 104507 (2011).

Suppression of the antinodal coherence of superconducting (Bi,Pb)₂(Sr,La)₂CuO_6+d as revealed by muon spin rotation and angle-resolved photoemission
R. Khasanov, Takeshi Kondo, M. Bendele, Yoichiro Hamaya, A. Kaminski, S. L. Lee, S. J. Ray, and Tsunehiro Takeuchi
Phys. Rev. B 82, 020511 (2010)

Using spin-polarized neutron reflectivity to probe mesoscopic vortex states in a Pb thin-film superconductor
A. J. Drew, M. W. Wisemayer, D. O. G. Heron, S. Lister, S. L. Lee, A. Potenza, C. H. Marrows, R. M. Dalgliesh, T. R. Charlton, and S. Langridge
Phys. Rev. B 80, 134510 (2009)

Zero-field superfluid density in a d-wave superconductor evaluated from muon-spin-rotation experiments in the vortex state
R. Khasanov, Takeshi Kondo, S. Straessle, D. O. G. Heron, A. Kaminski, H. Keller, S. L. Lee, and Tsunehiro Takeuchi
Phys. Rev. B 79, 180507 (2009)

Evidence for a Competition between the Superconducting State and the Pseudogap State of (Bi,Pb)₂(Sr,La)₂CuO_6+d from Muon Spin Rotation Experiments
R. Khasanov, Takeshi Kondo, S. Strässle, D. O. G. Heron, A. Kaminski, H. Keller, S. L. Lee, and Tsunehiro Takeuchi, Phys. Rev. Lett. 101, 227002 (2008)

Muons as Local Probes of Three-Body Correlations in the Mixed State of Type-II Superconductors
G. I. Menon, A. Drew, U. K. Divakar, S. L. Lee, R. Gilardi, J. Mesot, F. Y. Ogrin, D. Charalambous, E. M. Forgan, N. Momono, M. Oda, C. Dewhurst, and C. Baines
Phys. Rev. Lett. 97, 177004 (2006)