Abstract
We explore the stability of the phase separation phenomenon in few-fermion spin-1/2 systems confined in a double-well potential. It is shown that within the SU(2) symmetric case, where the total spin is conserved, the phase separation cannot be fully stabilized. An interaction regime characterized by metastable phase separation emerges for intermediate interactions which is inherently related with ferromagnetic spin–spin correlations emerging within each of the wells. The breaking of the SU(2) symmetry crucially affects the stability properties of the system as the phase separated state can be stabilized even for weak magnetic potential gradients. Our results imply an intricate relation between the phenomena of phase separation and ferromagnetism that lies beyond the view of the Stoner instability.
Highlights
Understanding the properties of itinerant magnetism has been a long-standing problem in condensed matter physics [1, 2]
The emergence of ferromagnetism in systems of spatially delocalized short-range repulsively interacting spinor fermions has been historically qualitatively understood in the framework of the Stoner instability [5]
By employing pump-probe spectroscopy the emergence of short-range two-body anti-correlations in the repulsive Fermi-gas supporting some sort of ferromagnetic order has been revealed [13], while the possibility of macroscopic phase separation has been ruled out. These experimental evidences indicate that the relation between phase separation and magnetism might be more intricate and involved than it appears within the framework of the Stoner instability manifested within the Hartree-Fock theory
Summary
Understanding the properties of itinerant magnetism has been a long-standing problem in condensed matter physics [1, 2]. By employing pump-probe spectroscopy the emergence of short-range two-body anti-correlations in the repulsive Fermi-gas supporting some sort of ferromagnetic order has been revealed [13], while the possibility of macroscopic phase separation has been ruled out These experimental evidences indicate that the relation between phase separation and magnetism might be more intricate and involved than it appears within the framework of the Stoner instability manifested within the Hartree-Fock theory. The metastability of the phase separated state is shown to be inherently connected with the appearance of a quasi-degenerate manifold of eigenstates characterized by intra-well ferromagnetic correlations of both wells but a varying total spin The occurrence of this manifold is attributed to the ferromagnetic Hund exchange interactions [39,40,41] emanating within each well of the DW setup. Appendix C provides the derivation of the Anderson effective kinetic exchange interaction for our DW setup and Appendix D describes the employed numerical approach, namely the ML-MCTDHX method
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