Mesoscopic physics is at the interface of classical and quantum physics. It is a field of great fundamental and applied potential. The study of macroscopic quantum phenomena in magnetism made recently a link between magnetism and mesoscopic physics.

The new " tool of magnetism " is extremely powerful, mainly for two reasons:
(i) this is a field relatively well understood, an theoretical works in magnetism often start from first principles and;
(ii) it provides a quasi-infinite reservoir of systems to be studied.

This project, in which the series of rare earth ions will be used as a complement to the studies of large magnetic molecules, gives a good illustration of the power of magnetism as a tool for the study of mesoscopic physics.

This project intends to work on new problems in quantum magnetism. Although we are interested in many of them, we will concentrate this project on the physics of 2-level spin systems in the presence of dissipation (V15 molecules, other low spin molecules, and rare earth ions).
In particular we hope to be able to understand why that large molecules with low spins are not in equilibrium, when submitted to a sweeping magnetic field. This is also be the case for large molecules of quantum spin S=1/2 (in V15). Preliminary experiments performed by the Grenoble team indicate that both spin-phonons and spin-hyperfine transitions, contribute to reach the equilibrium. However this is not enough and a hole should be burned in both distributions. We believe this physics is generic of quantum systems with dissipation. The effect of an increasing concentration of conduction electrons on quantum tunnelling (taking seriously in consideration the Yahn-Teller effect) will also be studied to reach the spin-glass limit.

More precisely, we hope to see how
- the effect of the effect of environment on the 2-level quantum spin system, give a quantum interpretation of magneto-caloric effect. The relationship between magneto-caloric effect and hole burning might then be generalized to other cases of mesoscopic physics.
- the Yahn-Teller effect minimizes the total energy of a quantum system to open a tunnelling gap;
- increasing the concentration of conduction electrons should kill tunnelling. We would like to see that in details;
- increasing the concentration of ions should increase progressively couplings to the spin bath, and at some point destroy tunnelling. However the effect of transverse fields should reverse the situation and quantum spin glasses could be studied.

Big molecules (already known and new ones) as well as rare earth ions will be synthesized and characterized and their quantum properties studied in details using new developed techniques (microSQUID and microHall magnetometry, Megagauss experiments) by the Florence and Grenoble teams. The most appropriated theoretical models to describe in particular, the molecular bistability and quantum phenomena in mesoscopic magnets and in diluted rare-earth systems with half and integer spins will be developed by NIS groups (magnetocaloric effect and its quantum interpretation, Yahn-Teller effect on tunnelling). Finally new mathematical models for computer simulations of the studied materials will tentatively be developed.