One of the most challenging tasks of solid-state physics today is to understand the mechanism for superconductivity in cuprates. These materials, which have a relatively high critical temperature Tc, are based on doped CuO2 planes. Since at zero doping they are antiferromagnets, several theories ascribe their superconductivity to holes interacting via a magnetic medium. Yet the phenomenon of superconductivity begins at doping levels in which magnetism almost disappears, and therefore there is no clear evidence relating the two.
High spin molecules (HSM) are molecules consisting of ions coupled by ferromagnetic or antiferromagnetic interaction; these molecules crystallize in a lattice where neighboring molecules are very well separated, yielding at low temperatures (temperatures lower than the magnetic interaction between ions) molecules that behave like noninteracting giant spins.
This research project is funded by the United States Israel Binational Science Foundation. The project name is Degenerate Magnets: A Study by Local Probes, and it duration is 1998-2001. The principal investigators are: A. Keren (Technion), Y. J. Uemura and G. M. Luke (Columbia Univ.).
Traditionally, the spin lattice relaxation time of a muon in a magnetic environment, also known as T1, is interpreted using the concept of spin correlation time. However, in some situations, no correlation time could be assigned to the spins, and a different interpretation of T1 is required. Our group recently developed such an interpretation. As a test case spin glasses where chosen since the theoretical expectation is for a time scale invariant spin evolution over a few decades. Indeed, the experiments show that the spin correlation function is best described by a product of a power law and a cut-off functions and that the slowing down spin fluctuations are manifested essentially by the temperature dependence of the power law term.