Exploring the ferromagnetic behaviour of a repulsive Fermi gas through spin dynamics

Year: 2017

Authors: Valtolina G., Scazza F., Amico A., Burchianti A., Recati A., Enss T., Inguscio M., Zaccanti M., Roati G.

Autors Affiliation: INO-CNR, Via Nello Carrara 1, 50019 Sesto Fiorentino, Italy; LENS and Dipartimento di Fisica e Astronomia, Università di Firenze, Via Nello Carrara 1,50019 Sesto Fiorentino, Italy; Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy; INO-CNR BEC Center and Dipartimento di Fisica, Università di Trento, 38123 Povo, Italy; Technische Universität München, James-Franck-Straße 1, 85748 Garching, Germany; Universität Heidelberg, Philosophenweg 19, 69120 Heidelberg, Germany

Abstract: Ferromagnetism is a manifestation of strong repulsive interactions between itinerant fermions in condensed matter. Whether short-ranged repulsion alone is sufficient to stabilize ferromagnetic correlations in the absence of other effects, such as peculiar band dispersions or orbital couplings, is, however, unclear. Here, we investigate ferromagnetism in the minimal framework of an ultracold Fermi gas with short-range repulsive interactions tuned via a Feshbach resonance. Whereas fermion pairing characterizes the ground state, our experiments provide signatures suggestive of a metastable Stoner-like ferromagnetic phase supported by strong repulsion in excited scattering states. We probe the collective spin response of a two-spin mixture engineered in a magnetic domain-wall-like configuration, and reveal a substantial increase of spin susceptibility while approaching a critical repulsion strength. Beyond this value, we observe the emergence of a time window of domain immiscibility, indicating the metastability of the initial ferromagnetic state. Our findings establish an important connection between dynamical and equilibrium properties of strongly correlated Fermi gases, pointing to the existence of a ferromagnetic instability.

Journal/Review: NATURE PHYSICS

Volume: 13 (7)      Pages from: 704  to: 710

More Information: We thank A. Morales and J. Seman for contributions in the early stage of the experiment, and G. Bertaina, G. M. Bruun, C. Di Castro, C. Fort, S. Giorgini, R. Grimm, W. Ketterle, P. Massignan, S. Pilati, R. Schmidt, W. Zwerger, M. Zwierlein and the LENS Quantum Gases group for many stimulating discussions. We thank H. Tajima and Y. Ohashi for providing us recent data of the lower branch spin susceptibility. This work was supported under European Research Council grants no. 307032 QuFerm2D, and no. 637738 PoLiChroM. A.R. acknowledges support from the Alexander von Humboldt Foundation. T.E. acknowledges the Physics Department, Sapienza University of Rome, for hospitality, and the Humboldt Foundation for financial support during part of this work.
KeyWords: Itinerant Ferromagnetism, Unitary Fermi Gas
DOI: 10.1038/NPHYS4108

Citations: 86
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