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Superfluidity and ferromagnetism of unequal mass fermions with two- and three-body resonant interactions

Periodic Reporting for period 3 - PoLiChroM (Superfluidity and ferromagnetism of unequal mass fermions with two- and three-body resonant interactions)

Reporting period: 2018-04-01 to 2019-09-30

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.
During the first 18 months, the newly created PoLiChroM team has organized a completely new lab-space at INO-CNR to host a new generation machine devoted to the production of the first lithium-chromium Fermi mixture worldwide.
Within this initial period, the PoLiChroM project has benefited by the scientific and technical skills of the PI (Matteo Zaccanti), two members of the INO-CNR senior staff (Marco Fattori and Giacomo Roati), one project INO-CNR researcher (Andreas Trenkwalder), one junior Post-doc (Francesco Scazza) and one technician (Andrea Aglietti). PoLiChroM also benefited by the participation on a voluntary basis of one PhD student (Elettra Neri) and two undergraduate students (Francesco Di Noia and Antonio Cosco) from the Physics Department, University of Florence.
Aside organizing a new lab space well suited for an ultracold gas experiment, the major efforts of the PoLiChroM team have been devoted to designing and setting up a new experimental machine, as well as to purchasing most of the required equipment and consumables.
Dr. Zaccanti and Dr. Scazza, with the contribution of the PhD student E. Neri, focused on designing the vacuum set-up, inside which Lithium and Chromium atoms will be cooled, trapped, and manipulated. Parallel to this, the project of Feshbach, MOT and Zeeman slower coils has been carried on. The design of the apparatus was completed in June 2016, and it consists of a science chamber plus two independent vacuum lines for each species, each of which comprises a Zeeman slower, connecting the main chamber to an effusion cell.
Dr. Trenkwalder, with also the help of Dr. Fattori and Aglietti and the contribution of the two undergraduate students, focused on the realization of a home-made laser, able to deliver up to 800 mW of light at 425 nm.
This source has been designed, mounted and already successfully tested, and it delivers the main cooling light of Chromium atoms.
As the Lithium laser lights are concerned, Dr. Roati, with the contribution of E. Neri, has built the optical setup, which employs four commercial laser sources, and that has been finalized at the end of 2016.
The control program and electronic devices to run the experimental cycle, though requiring still some upgrades, have been tested and installed in the lab.

The second 18 months of PoLiChroM (month 19-36) have seen a partial rearrangement of the team, composed within this period by the PI and two CNR project researchers (Dr. A. Trenkwalder and Dr. M. Jag), and also by the Florence University students Dr. E. Neri (PhD) and M. Seminara (Master) engaged on a voluntary basis on the project.
Within this period the whole vacuum setup has been assembled and successfully baked, in order to attain ultra-high vacuum conditions within the science chamber. This task has been mainly carried out by Dr. Trenkwalder and M. Seminara.
The optical setups needed to laser cool both Cr and Li atomic species have been finalized, and the magnetic field coils needed for the manipulation of the cold and ultracold mixtures have been assembled, characterized and installed on the main experimental apparatus. These tasks have been carried out by Dr. Jag, Dr. Neri and the PI.
Furthermore, the high power laser sources required to implement an optical dipole trap for successive evaporative and sympathetic cooling steps have been purchased and tested, while their dedicated optical setup has been designed (mainly by the PI).
Within the very last months of this period, PoLiChroM has been able to obtain its first lithium and chromium atomic clouds in the cold regime, trapped in a magneto-optical trap (MOT). Furthermore and very importantly, the PoLiChroM team has been able to obtain the very first Cr-Li double species MOT worldwide.
This achievement represents the successful reach of the first milestone of the PoLiChroM project, namely the realization of a novel cold lithium-chromium mixture. The attainment of the ultracold, degenerate regime with such a new bina
PoLiChroM is a higly-innovative and potentially revolutionary project within the research field of cold atom physics. It aims to create a new Fermi mixture in the ultracold regime, combining the well-known Lithium species with the poorly explored Chromium one. The few- and many-body properties of such a heteronuclear system are largely unknown: they were never experimentally probed and they are poorly studied even in theory.
However, on very general grounds it can be argued that such a Cr-Li mixture has the potential to disclose unprecedented novel few- and many-body quantum phases, and it might allow to investigate elusive superfluid and magnetic regimes predicted decades ago but not observed in any physical system thus far.
As such, each step of PoLiChroM intrinsically represents a significant progress beyond the state of the art, already at this early-intermediate stage.
For instance, even though within the first 18 months no milestone was planned, the PoLiChroM team has built up a high-power laser set-up at 425nm, whose features positively compare with the ones of state-of-the-art commercial products.
The survey on itinerant ferromagnetism in repulsive Fermi mixtures, already successfully pursued by the team on Li gases within the second and third year of the project, represents a major advance in the understanding of a phenomenon which encompasses the interdisciplinary field of research of highly-correlated fermionic matter.
The realization of the first cold lithium-chromium mixture worldwide, attained within the first 36 months of the project, paves the way to tackle in the near future the ambitious major studies of PoLiChroM. Their success will provide a crucial step forward in our understanding of fundamental phenomena and regimes that are ubiquitous in contemporary physics, and which also lay at the heart of a variety of everyday-life devices.
Overview of the core of the PoLiChroM machine and images of Li, Cr, and Li-Cr MOTs.