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Dual Superfluidity in Two Dimensions

This thesis will explore dual superfluidity in two-dimensions. It will take place on a cold atom apparatus within the Fermi Gaz Team at LKB under the supervision of Tarik Yefsah and Christophe Salomon.  

The student will undertake tasks involving knowledge in optics, electronics, laser physics, atomic physics quantum physics, many-body physics, statistical physics and physics of phase transitions. The experiments will take place on the historical quantum gas machine of the Fermi Gas group operating with Lithium isotopes. It is located in the Physics Department of École Normale Supérieure.

Scientific Context

At sufficiently low temperature, a certain class of systems can become superfluid, which is a state of quantum matter where particles flow without friction. Superfluidity is the direct analog of superconductivity for electrically neutral particles, making it a subject of significant scientific interest. Both Bosonic and Fermionic fluids can enter the superfluid state when taken separately. But does superfluidity survive when bosons and fermions are put together and interact?

In the liquid Helium community, the search for such dual superfluidity has been a major quest for over 40 years, yet without success. The challenge in Helium 3 and Helium 4 mixtures resides in the fact that the system undergoes phase-separation when the concentration of Helium 3 exceeds 6%. Hence, if dual He3-He4 is to exist, it is predicted to occur at ultralow temperatures (on the order of micro-Kelvins), which challenges state-of-the art cryogenic techniques.

In dilute gases, Bose-Fermi mixtures have been produced with several different species over the past decade, however, without reaching dual superfluidity. In 2014, our group produced for the first time a mixture of Bose and Fermi gases that were both in the superfluid regime. Thanks to the tunability of interactions in quantum gases it was possible to prevent phase separation and reach double superfluidity for a mixture of Lithium 6 and 7 isotopes.

In this context, the question of dual superfluidity in two dimensions is a new qualitative turn that is awaited. In two dimensions, the superfluidity belongs to a different and richer paradigm than in 3D. As opposed to the latter case, the transition to the superfluid state is no longer driven by the breaking of continuous symmetry but by the binding of vortex and anti-vortex pairs. Furthermore, as vortices play the role of generic phase and density fluctuations in 2D, we expect them to play a dramatic role in the properties of a Bose-Fermi mixture and the dynamics of the phase transition. Indeed, vortices represent local trapping potentials where impurities can be trapped, which can prevent the binding of vortices. The interplay between Bose and Fermi statistics, inter-species interactions and vortices is expected to give rise to a rich collection of quantum phenomena.

Important note: Funding is not granted on this position. The selected candidates will be part of a joint application on a SIRTEQ PhD grant. The offer stands if the grant application is successful.