Unconventional superfluidity in Bose-Fermi Mixtures

Pairing between fermions that attract each other, reveal itself to the macroscopic world in the form of superfluidity. Since the discovery of fermionic superfluidity, intense search has been going on to find various unconventional forms of fermion pairing as well as to increase the transition temperature. Here, we show that a two dimensional mixture of single-component fermions and dipolar bosons allows to reach experimentally feasible superfluid transition temperatures for non-standard pairing symmetries. Excitations in these superfluids are anyonic and their statistics depends on the order of their permutations, i.e is non-Abelian. Our results provide for the first time an example of a highly tunable system which exhibits various kind of pairing symmetry and high transition temperature. Additionally, they provide a playground to observe anyonic excitations and their braiding properties.

💡 Research Summary

This theoretical paper proposes a novel platform for achieving high-temperature unconventional superfluidity and non-Abelian anyons using ultracold atomic gases. The system consists of a two-dimensional mixture of single-component (spinless) fermions and bosons with significant dipole-dipole interactions (e.g., Cr-52 atoms or polar molecules).

The core mechanism relies on the boson-mediated interaction between fermions. The dipolar bosons form a condensate whose excitation spectrum develops a “roton” minimum at a finite momentum. As the boson density increases, bringing the roton energy close to zero, the effective mass of the bosons at that momentum diverges. This dramatically amplifies the induced interaction between fermions that exchange momentum near the roton wavevector. Integrating out the bosonic degrees of freedom yields a momentum-dependent effective attraction between fermions.

By projecting this interaction onto different angular momentum channels (characterized by quantum number ’m’), the study demonstrates that the system is highly tunable. The dominant pairing channel can be switched by adjusting the fermionic dimensionality parameter (η, related to the transverse confinement) and the dipolar interaction strength (g_3d). Specifically, p-wave (m=1) pairing dominates near the 3D limit, h-wave (m=5) near η~0.6, and f-wave (m=3) at even lower dimensionality. Crucially, the interaction strength in these channels can reach the strong-coupling regime (λ_m > 1), allowing the superfluid critical temperature (T_c) to approach the Fermi temperature (T_F). This represents an improvement of up to five orders of magnitude compared to previous proposals with contact-interacting Bose-Fermi mixtures.

The paper further analyzes the chiral superfluid states that break time-reversal symmetry (e.g., p_x+ip_y). It shows that vortices in these odd-wave superfluids host zero-energy Majorana fermion modes bound to their cores. The spatial form of these modes depends on the pairing symmetry. When multiple well-separated vortices are present, the Majorana modes obey non-Abelian anyonic statistics, meaning their quantum state transformation upon exchange depends on the order of exchanges. This makes the system a potential playground for studying non-Abelian braiding and topological quantum computation.

The authors suggest experimental realizations using quantum degenerate mixtures like Chromium and Potassium, or heteronuclear polar molecules. In summary, this work provides the first proposal of a highly tunable, experimentally feasible cold-atom system capable of hosting a variety of high-T_c unconventional superfluid phases with non-Abelian excitations, bridging fundamental condensed matter physics and quantum information science.