Physicists describe a fractional Fermi sea — ScienceDaily

Physicists have developed a theoretical framework for a new quantum state — a fractional Fermi sea. In theoretical calculations, ultracold cesium atoms in this state can be transferred into a high-energy but ordered configuration. The study was conducted by the group of Hanns-Christoph Nägerl in collaboration with theoretical physicist Alvise Bastianello of CNRS and Paris Dauphine University, ScienceDaily reports.

A cycle between attraction and repulsion

The work was published in the journal Physical Review Letters. In their calculations, the researchers considered ultracold cesium atoms in a one-dimensional system, where their motion is restricted to one direction. They modeled a cyclic change in interactions between atoms — from strong repulsion to strong attraction.

The calculations showed that the system can be transferred from its ground state, meaning the state with the lowest energy, into a highly excited state without losing its internal order. The authors called this configuration a fractional Fermi sea because, according to their description, its particles obey a modified rule for filling quantum states.

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Hidden order in an excited state

The new state is characterized by pronounced Friedel oscillations — wave-like changes associated with the mutual influence of quantum particles. The calculations also indicated unusual patterns in the weakening of correlations between particles as distance increases, even with comparatively weak repulsive interaction.

These features differ from the predictions of Tomonaga-Luttinger liquid theory, which is traditionally used to describe interacting quantum particles in one-dimensional systems. Hanns-Christoph Nägerl noted that the state is highly excited but not random: its hidden order is manifested through correlations.

The researchers also raised the question of the nature of unusual collective excitations in the system. Nägerl suggested that such quasiparticles may in the future be called “superfermions.” The current publication forms a theoretical basis for fractional Fermi seas, while a related study of their experimental implementation using quantum simulation is under review.