Scientists at the Tokyo Institute of Science have discovered stable “special fermionic superfluidity” – a new quantum phase of matter in which special singularities called special points are an intrinsic property of the state itself. The work was published in the journal Physical Review Letters (PRL).
The authors' research is devoted to open quantum systems, in which particle loss and asymmetry in the direction of their motion can occur. Such systems are described by non-Hermitian models in which the usual laws of quantum mechanics are violated. Special points appear in them – states in which the corresponding energy levels and quantum states simultaneously merge. Previously, it was believed that such points appear only at the boundaries of phase transitions, such as during the destruction of the superfluid state.
Physicists have shown that this is not so. By studying a non-Hermitian version of the gravitational Hubbard model with violation of pairwise spin consistency, they discovered a stable superfluid phase in which the peculiarities exist forever. According to project leader and associate professor Akihisa Koga, this phase is characterized not only by a finite order parameter but also by special points created in momentum space, essentially distinguishing it from previously known non-Hermitian superfluid states.
The key mechanism turns out to be a special form of dissipation, in which particles with opposite spins prefer to move in different directions. Instead of destroying the Cooper pairs, this mismatched spin stabilizes the new state. Calculations have shown that the geometry of the lattice plays a decisive role: on a square lattice, special superfluidity occurs even when the attraction is arbitrarily weak, while on a cubic lattice, strong dissipation can destroy it. Furthermore, in three-dimensional space, particular points form lines and in two dimensions, isolated points.
The authors emphasize that the new phase is potentially feasible in experiments with ultracold atomic gases such as lithium-6 or potassium-40, where particle loss can be precisely controlled. According to Akihisa Koga, this discovery opens a new direction in the study of strongly correlated non-equilibrium quantum matter and shows that dispersion may not be the enemy but a tool for the formation of new quantum states.
Before that, people knew for the first time what really happened inside a black hole.
