CORDIS

Superfluidity and magnetism are ubiquitous in Nature and characterize a wealth of interacting fermion systems: from Helium liquids to solid-state materials, up to nuclear and quark matter environments. From the interplay of these phenomena, two fundamental questions have been raised: Can superfluid pairing bear a mismatch in the two Fermi surfaces? Can a homogeneous fermion system become ferromagnetic via a zero-ranged interparticle repulsion?
Despite decades of interdisciplinary investigations, such questions have not got undisputed answers so far.
PoLiChroM aims at experimentally addressing these problems with a new model system composed of ultracold fermionic Chromium and Lithium atoms with resonant interactions. The two species will mimic electrons of different spins, or quarks of different colours, but exhibiting the high degree of control of an atomic quantum simulator.
In particular, two features make this system stand far beyond any other available one: the peculiar Chromium-Lithium mass ratio enables a resonant control of three-body elastic interactions on top of the usual two-body ones, together with an extraordinary suppression of atom recombination into paired states in the regime of strong interspecies repulsion.
The first property greatly enhances the observability of elusive polarized superfluid regimes, such as the Fulde-Ferrel-Larkin-Ovchinnikov phase, where pairs condense in nonzero momentum states.
The second makes such mixture a prime platform for the quantum simulation of Stoner’s model for itinerant ferromagnetism, whose study has been hindered in nowadays quantum gases experiments, where pairing instability plagues the formation of sizeable magnetic domains.
The PoLiChroM machine will employ high-resolution imaging of the system and state-of-the-art spectroscopy schemes for disclosing such exotic phases via a thorough investigation of the phase diagrams of Fermi-Fermi mixtures with attractive or repulsive interactions.
In particular, the project can be divided into five main items, the first two ones representing important preparatory steps for the implementation of the action: (i) Creation of a degenerate Fermi mixture of spin polarized Cr and Li atoms. (ii) Demonstration of selective Feshbach tuning of two-body Cr-Li interaction.
Once a two-species Fermi mixture with tunable interactions will be available in the lab, it will be employed to perform the following studies:
(iii) Investigation of resonant three-body forces and trimer states in Cr-Li mixtures.
(iv) Phase diagram of a degenerate two-fermion mixture with two- and three-body resonant interactions: Trimers, FFLO and polarized superfluids.
(v) Ferromagnetic phases in strongly repulsive long-lived mixtures of itinerant fermions.

Besides the last two major and most ambitious many-body investigations, the resonant control of the three-body interparticle interaction beyond the standard two-body one will open unexpected scenarios, further enriching the cross-disciplinary field of research of strongly-correlated fermions. If successful, PoLiChroM will provide new important hints into some fundamental many-body regimes, whose experimental investigation has been denied so far.