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

Periodic Reporting for period 4 - PoLiChroM (Superfluidity and ferromagnetism of unequal mass fermions with two- and three-body resonant interactions)

Reporting period: 2019-10-01 to 2020-09-30

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, which have not got undisputed answers so far: Can superfluid pairing bear a mismatch in the two Fermi surfaces? Can a homogeneous fermion system become ferromagnetic via a zero-ranged interparticle repulsion?
PoLiChroM aims at experimentally tackling these paradigmatic problems with model systems made of ultracold Fermi gases of chromium and lithium atoms. These mimic electrons of different spins, or quarks of different colors, within a novel, highly-controlled atomic quantum simulator.
Two peculiar features of our atomic system make it stand far beyond any other available one: the peculiar mass ratio of these two species 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. The second one makes this mixture emerge as a prime platform for realizing strongly repulsive Fermi gases, the paradigmatic framework for the so-called Stoner’s model for itinerant ferromagnetism.
The PoLiChroM plan comprises the construction of a new platform to produce the first chromium-lithium mixture worldwide, and the characterization of the collisional properties of this unexplored atomic system. The major scientific surveys of the project concern:
1. Resonant three-body forces and few-body cluster states in Cr-Li mixtures.
2. Unconventional superfluidity in degenerate Fermi mixtures with two- and three-body resonant interactions.
3. Ferromagnetic phases in repulsive Fermi gases.
At its completion, PoLiChroM succeeded in addressing a major part of its ambitious, cross-disciplinary pursuits, leading to important scientific breakthroughs.
PoLiChroM ends with a fully-operative experimental machine able to efficiently produce large chromium-lithium mixtures at ultralow temperatures. Such a novel quantum system provides a promising platform for a wealth of future studies, extending well beyond the time and topics covered by my proposal.
The experimental investigation carried throughout the project period have significantly advanced our understanding in the nature of repulsive Fermi gases, and demonstrated novel methods to probe the order parameter of strongly-interacting Fermi superfluids through Josephson transport measures.
PoLiChroM enabled exciting scientific and technological achievements, summarized in ten scientific papers already published in high-impact journals (1 Science, 1 Nature Physics, and 5 Physical Review Letters), plus four forthcoming ones.
The newly created Cr/Li team at CNR-INO could indeed successfully pursue a major part of the ambitious PoLiChroM plan within the project duration.
A prime effort of the PoLiChroM team focused on setting up a new cold-atom to produce the first chromium-lithium ultracold mixture worldwide. Nowadays, the experimental machine of the Cr/Li lab is able to produce samples of more than 10^5 chromium and 10^6 lithium ultracold fermionic atoms. Successful search for interspecies Feshbach resonances in this novel mixture guarantees the precise control over Cr-Li interactions. These achievements, subject of one recent Physical Review A publication, plus four ones currently in preparation, will guarantee the successful pursuit of resonant three-body forces and novel few-body cluster states (GOAL1) in chromium-lithium mixtures within the forthcoming year.
During the project period, we could thoroughly and successfully address GOAL3 and GOAL2 of PoLiChroM, namely the investigation of ferromagnetism in repulsive Fermi mixtures, and the search for exotic superfluidity in highly correlated ultracold Fermi mixtures.
By means of novel strategies only partially envisioned at the start of the project, PoLiChroM enabled to clarify the intricate many-body dynamics of repulsive Fermi mixtures. We unveiled the existence of a ferromagnetic instability, and gained important information about the interplay between ferromagnetism and pairing in these systems (GOAL3). Our surveys also disclosed an exotic heterogeneous phase, where spin-polarized atoms and pairs macroscopically coexist, arranged in spatially-separated micro-domains. Superfluidity in such an inhomogeneous system could not be demonstrated, yet, but this finding is crucially relevant in the context of GOAL2 of PoLiChroM. These studies, leading to five publications (including 1 Nature Physics and 3 Physical Review Letters), also allowed me to establish a strong collaboration with the Nobel Laureate Prof. W. Ketterle.
Upon developing an innovative theoretical framework, PoLiChroM allowed for a thorough characterization of resonantly interacting Fermi superfluids through Josephson transport measurements. Building on a theoretical model developed with Prof. W. Zwerger, together with Dr. G. Roati at CNR-INO we could measure the “current-voltage” characteristics of an atomic Josephson junction made of Fermi superfluids of lithium atoms. From the Josephson supercurrent measure, we could extract the superfluid order parameter throughout the BCS-BEC crossover. These surveys, highly relevant for PoLiChroM GOAL2, yielded four high-impact publications, including 1 Science and 2 Phys. Rev. Lett.
PoLiChroM was born as a highly-innovative proposal within the cold atom physics research field, and indeed scientific progresses beyond the state of the art have been achieved by the Cr/Li team during the project period.
The novel Cr-Li mixture already appears to outperform all other mass-imbalanced Fermi systems nowadays available. The efficient production of very large Cr-Li samples in the ultracold regime, and the discovery of well-isolated Feshbach resonances at extremely convenient fields, definitely enable us to overcome the difficulties encountered in all other Fermi mixtures realized so far.
The thorough investigation of the many-body properties of repulsive Fermi gases has provided fundamental new insights on such cumbersome many-body systems. Our studies of repulsive Fermi polarons enabled to gain crucial information about the highly-debated nature of repulsive Fermi liquids. The disclosure of a ferromagnetic instability in such a system certainly represents a breakthrough highly relevant for a wide community.
Unveiling a novel heterogeneous phase of correlated fermions and pairs will certainly boost novel theoretical studies in the context of polarized superfluidity, in and out the quantum gases community.
The experimental and theoretical methods to realize, probe and interpret superfluid currents in an atomic Josephson junction represent a significant step forward in the understanding of crossover Fermi superfluids, and of the phenomenon of pair condensation.