Glassy systems are fascinating objects which have always attracted a lot of attention from the scientific community. Spins glasses are maybe one of the most well-known examples of such systems : they consist in spins randomly distributed in a metallic matrix. Due to alternating sign of the RKKY interaction with the distance between two spins, the spin glass can be viewed as a set of spins which interact with a random interaction. The very simple question concerning spin glasses is then : what is the fundamental state of such a system ?? If the question may sound naive, the answer is actually very subtle. Different models try to describe this system, and a lot of theoretical effort has been put on this problem over the last decades of the 20’s century. In particular, the mean field description of the spin glass leads to a fundamental state which actually consists in several degenerated configurations of the spins. The energy landscape presents several minima, each minimum corresponding to a different spin configuration. This landscape can be mapped on to a hierarchical tree, each leave on the tree representing a spin configuration.
This mean field model, although very elegant, this model suffers from the fact that there is no experimental confirmation of its validity so far. The point is that using "global" measurements (susceptibility, specific heat...) one can only access the average property of the system, but it is impossible to distinguish the different spin configurations. In other words, it is not possible to prove that upon several cooling down, the systems ends up in a different spin configuration. This problem could be overcome using Mesoscopic Physics : in a quantum coherent conductor, the fluctuations of the conductance with the magnetic flux is directly related to the microscopic configuration of the scatterers. This has been proved in the case of configurational disorder (position of the atoms). Recently, we have shown that in a spin glass, these fluctuations of conductance can be directly related to the magnetic configuration of the spins. Using this tool, we should be able to access the microscopic configuration of the spins, and thus probe directly the topology of the phase space of a frozen spin glass.
This activity is thus at the intersection of two different fields of the Condensed Matter Physics, namely the physics of spin glasses and the Mesoscopic Physics, two activities which have always been at the heart of the activities of the lab. The theoretical side of this subject is mainly done at the "École Normale Supérieure de Lyon" by the group of David Carpentier, with whom we have a strong relationship.
Publications récentes :
Remanence effects in the electrical resistivity of spin glasses Thibaut Capron, A. Perrat-Mabilon, Christophe Peaucelle, Tristan Meunier, David Carpentier, Laurent P. Lévy, Christopher Bäuerle, Laurent Saminadayar EPL, 93 27001 (2011)
Contact : Laurent Saminadayar