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 recently shown that it is even possible to use µSR to probe three body correlations in the bulk of the vortex glass phase.

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 ISIS Scientific Highlight 2003).

References

Superconductivity: Resistance is Useless,
'Neutrons and Magnetism', ILL Publication, Bob Cubitt and Charles Dewhurst (2004).

Field-induced transition from hexagonal to square vortex lattice in La_1.83Sr_0.17CaCuO₄,
ILL Annual Report, ILL Scientific Highlights (2002).

Direct Observation of the Flux-Line Vortex Glass in La_0.1Sr_0.8CaCuO₄,
ILL Scientific Highlight, ILL Annual Report (2004).

The study of magnetic field distributions in thin film superconducting samples,
Science Highlights, ISIS Annual Report (2003).

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

Square vortex lattice at anomalously low magnetic fields in electron-doped Nd_1.85Ce_0.15CuO₄ ,
R. Gilardi, J. Mesot, S.P. Brown, E.M. Forgan, A. Drew, S. L. Lee, R. Cubitt, C.D. Dewhurst, T. Uefuji, K. Yamada, Phys. Rev. Lett. 93( 217001) (2004).

Direct observation of the flux-line vortex glass phase in a type II superconductor,
U. Divakar, A.J. Drew, S.L. Lee, R.Gilardi, J. Mesot, F.Y. Ogrin,D. Charalambous, E.M. Forgan, G.I. Menon,C.D. Dewhurst, C. Baines, Phys. Rev. Lett. 92(23) 237004 (2004).

Direct Evidence for an Intrinsic Square Vortex Lattice in the Overdoped High-Tc Superconductor La_0.83Sr_0.17CuO_4+d , R. Gilardi J. Mesot, A. Drew, U. Divakar, S. L. Lee, E. M. Forgan, O. Zaharko, K.Conder, V. K. Aswal, C. D. Dewhurst, R. Cubitt, N. Momono, and M. Oda, Phy. Rev. Lett. 88 217003 (2002)