Study of giant proximity effect and critical opalescence in magnetic materials
The observation in recent years of giantproximity effects in materials made of high temperature superconductors has been an exciting development for both fundamental and applied research. The proximity effect is a very general phenomenon by which an ordered state of a given material (e.g. magnetism or superconductivity) “leaks” into an adjacent one over some finite length scale. It was believed to be well understood. However, the unusually long-ranged proximity effects observed in junctions made of different cuprate perovskites have challenged our understanding of this problem. The behaviour observed in these systems is known as the giant proximity effect; it is believed to be a consequence of many-body phenomena, which are of scientific and technological importance in the areas of high temperature superconductivity and colossal magnetoresistence.
In order to understand these surprisingly long-ranged proximity effects, it is necessary to take into account the nature of the phase transition involved. In the case of superconductivity, the description of the giant proximity effect requires a more general formulation taking into account fluctuations near a second order transition . The increased length scale of the superconducting order parameter in the non-superconducting material can thus be understood as a quantum version of critical opalescence – analogous to the behaviour of water near its liquid-vapour critical point. Indeed, critical opalescence is a property common to all second order phase transitions. As the transition into a ferromagnetic state is often second order in nature, the mechanism giving rise to giant proximity effects in a system with different superconducting materials should also apply to magnetic systems. It is this magnetic version of the giant proximity effect that will be studied.
The research project
The project will involve the use of neutron reflectivity and µSR techniques, as well as laboratory -based sample characterisation measurements. Preliminary work using these methods has already been carried out to demonstrate the feasibility of the project. The present collaboration involves the Diamond Light Source (Silvia Ramos) and ISIS' large scale structures group (Timothy Charlton). Samples are being grown at the MIT (Jagadesh Moodera) and the ISIS Theory Group (Jorge Quintanilla) is providing theoretical support. The phenomena described above will be initially studied in semiconducting ferromagnets (EuS, EuO) in order to avoid the effects of Friedel oscillations (RKKY interaction) due to the conduction electrons. At a later stage, the research can be extended to systems made of ferromagnetic metals and hence test the generality of the effect.
 J. Quintanilla, K. Capelle and L. N. Oliveira, Phys. Rev. Lett. 90, 089703 (2003).
For further information contact Professor Jon Goff.