Fermions with tunable interactions... In the Lithium lab we produce binary mixtures of 6Li Fermi gases to investigate fermionic superfluidity across the BEC-BCS crossover. Our main goal is to study two-dimensional fermionic superfluids.

We study the collective spin response and spin diffusion of an ultracold lithium Fermi gas artificially engineered in a fully ferromagnetic state, obtained by spatially segregating oppositely-oriented spins into two adjacent reservoirs. In this way, we show that strong repulsive interactions are sufficient to temporarily stabilize ferromagnetic correlations in a Fermi mixture. In particular, we reveal a substantial increase of the magnetic susceptibility of the gas while approaching the critical value of interaction. Correspondingly, we show that above the critical interaction a spin up-down domain wall can persist for a finite time, indicating the metastability of the ferromagnetic state. Such findings point to Stoner-like ferromagnetic instability driven only by short-range repulsion, and are consistent with our recent study of a repulsive Fermi liquid of polarons in strongly polarized Fermi gases.

G. Valtolina, et al.,
Exploring the ferromagnetic behaviour of a repulsive Fermi gas through spin dynamics
Nat. Phys. 13, 704 (2017)

Landau was first to suggest that collective excitations of particle ensembles could be treated as if they were particles themselves, with properties like momentum and mass. As such, these excitations are known as quasiparticles, and in contrast to free particles they possess a finite lifetime. In this study, we report on the investigation of a particular type of quasiparticle known as a Fermi polaron. This is a quantum impurity that is immersed in a Fermi sea and strongly interacts with it. In particular, Fermi polarons emerging from impurities repelling the surrounding particles are known as repulsive polarons. We could observe well-defined repulsive polarons even at very strong interactions, with impurities possessing the same mass as the surrounding particles. In such a system the existence of long-lived repulsive polarons was thus far debated. For this, we have spectroscopically probed an ultracold Fermi gas of lithium, where atoms in a specific internal spin state acted the role of the impurities interacting with a bath of atoms in another spin state. Our findings offer exciting prospects for studying many-body states that rely on repulsive interactions.

F. Scazza, et al.,
Repulsive Fermi Polarons in a Resonant Mixture of Ultracold 6Li Atoms
Phys. Rev. Lett. 118, 083602 (2017)

Shining a blue detuned thin (2 μm) barrier we produce a double-well potential, which creates a Josephson-like junction for fermionic superfluids. By varying the interactions we investigate the population and phase dynamics between the two wells, observing the Josephson effect across the BEC-BCS crossover.

G. Valtolina et al.,
Josephson effect in fermionic superfluids across the BEC-BCS crossover
Science 350, 1505 (2015)

Using the inverse Abel transform, the in-situ 3D density profile of a cloud of two-state 6Li atoms trapped in a potential with cylindrical symmetry can be derived. Following [M. Ku et al., Science 335, 563 (2012)] we reconstruct the equation of state of the system. In particular, local pressure and compressibility have been obtained for the unitary Fermi gas, revealing the superfluid transition.

The quest to develop new and efficient experimental schemes to produce large and highly degenerate fermionic samples is fundamental in the way of using them to study fermionic superfluidity with a high degree of control on properties as interaction strength and dimensionality. 6Li samples of two spin components offers a broad Feshbach resonance allowing a good tenability of the interactions and the ability to investigate superfluidity across the BEC-BCS crossover. On the other way, standard laser cooling configurations are not efficient due to the lack of sub-Doppler cooling mechanism on the D2 line. We use a gray molasses operating on the D1 atomic transition to produce degenerate quantum gases of 6Li with a large number of atoms. This sub-Doppler cooling phase allows us to lower the initial temperature of 109 atoms from 500 to 40 μK in 2 ms. We observe that D1 cooling remains effective into a high-intensity infrared dipole trap where two-state mixtures are evaporated to reach the degenerate regime. We produce molecular Bose-Einstein condensates of up to 5×105 molecules and weakly interacting degenerate Fermi gases of 7×105 atoms at T/TF

A. Burchianti et al.
Efficient all-optical production of large 6Li quantum gases using D1 gray-molasses cooling
Phys. Rev. A 90, 043408 (2014)

We aim at studying two-dimensional fermionic 6Li atoms across the BCS-BEC crossover. We plan to benefit of the recent advances in ultracold atomic systems, such as single-site addressability and the full control of the interparticle interactions. Tailoring arbitrary optical potentials will create the perfect environment for implementing quantum models. In particular, we want to characterize the superfluid phase by studying the interlayer tunneling, discriminating the coherent Josephson dynamics from the single-particle hopping. By adding disorder we will simulate the physics of granular superconductors, testing the robustness of the order parameter and the onset of metallic phases at higher temperatures.

Li people

Eleonora Lippi
Master student
Andrea Amico
PhD student
Francesco Scazza
Postdoc
Matteo Zaccanti
Scientific staff
Massimo Inguscio
Scientific staff
Giacomo Roati
Scientific staff
Former members:
Alessia Burchianti
Chiara Fort
Jorge Seman
Pedro Tavares
Giacomo Valtolina

Li contacts

For further information, request of material, job opportunities, please contact:

Giacomo Roati
(roati@lens.unifi.it)

Li funding