Soutenance de thèse:
Collective behaviors in interacting spin systems
- Cécile Monthus, IPhT, CEA-Saclay, présidente
- Juan Garrahan, University of Notthingham, rapporteur
- Nicolas Laflorencie, LPT Université Paul Sabatier, rapporteur
- Cristiano Ciuti, MPQ Université Paris Diderot, examinateur
- Geoffrey Bodenhausen, Département de Chimie, ENS, examinateur
- Andrea De Luca, University of Oxford, invité.
- Alberto Rosso, LPTMS, Université Paris-Sud, directeur de thèse
Résumé: Dynamic nuclear polarization (DNP) is one of the most promising techniques towards a new generation of Magnetic Resonance Imaging (MRI). The idea is to use the Nuclear Magnetic Resonance (NMR) in other nuclei rather than the traditional hydrogen, such as carbon. For the carbon signal to be detected, one needs to enhance its spin polarization. In thermal equilibrium — at low temperature and high magnetic field — electron spins are far more polarized than any system of nuclear spins, which is due to their smaller mass. With the DNP technique we bring the system out-of- equilibrium irradiating it with microwaves. This triggers polarization transfer from the electron spins to the nuclear ones.
During my Ph.D, I have studied both analytically and numerically the competition between the dipolar interactions among electron spins (which can be tuned experimentally) and the disorder naturally present in the sample. I proposed two models to study DNP: a Heisenberg spin-chain and a system of free-fermions in the Anderson model. Two different regimes were found :
(I) For strongly interacting electron spins, the out-of-equilibrium steady state displays an effective thermodynamic behavior characterised by a very low spin temperature.
(II) In the weakly interacting regime, it is not possible to define a spin temperature, and it is associated to a many-body localized phase (or an Anderson-localized phase).
My research was focused on the properties of the two phases with respect to the performance of DNP, and I found it to be optimal at the transition between the two. This is a very important result that has been verified by recent experiments carried in École Normale Supérieure de Paris.