Antiferromagnetism of spin 1 boson : experimental study with ultracold sodium atoms
Tuesday 31 January 2012 at 3pm
Room "Rémy Lemaire" K 223 (1st floor) Building K, Institut Néel / CNRS
Fabrice GERBIER (Laboratoire Kastler Brossel, Paris)
In this talk, I will present a set of experiments done at Laboratoire Kastler Brossel (ENS Paris) on the magnetic properties of ultracold bosons cooled well below the Bose-Einstein condensation temperature. After a brief introduction to ultracold gases, I will present an experimental study of the phase diagram of spin-1 bosons with antiferromagnetic interactions, highlighting the similarities and differences with the best known antiferromagnetic system, spin 1/2 on a lattice with antiferromagnetic Heisenberg interactions. I will discuss in details the behavior of the system for small magnetic fields and magnetizations, where anomalously large fluctuations are observed. We show they can be explained by collective spin fluctuations, that would vanish in the thermodynamic limit but are important due to the small size (~3000 atoms) of the samples we study. This illustrates on a particular example how fluctuations in small systems are effective to counter the general mechanism of spontaneous symmetry breaking.