A density wave (DW) is a fundamental type of long-range order in quantum matter tied to self-organization into a crystalline structure. Although density waves are seen in a wide range of materials, such as metals, insulators, and superconductors, studying them has proven challenging, particularly when this order (the patterns of the wave’s particles) coexists with other types of organization, such as superfluidity, which allows particles to flow without resistance.
Scientists at EPFL have found a new way to create a “density wave” in an atomic gas. The findings could lead to a better understanding of the behavior of quantum matter.
Professor Jean-Philippe Brantut at EPFL said, “Cold atomic gases were well known in the past for the ability to ‘program’ the interactions between atoms. Our experiment doubles this ability!”
To study this interaction, Brantut, and his colleagues created a “unitary Fermi gas,” a thin gas of lithium atoms that has been cooled to incredibly low temperatures and in which atoms frequently clash.
The scientists next placed this gas in an optical cavity, a tool to keep light constrained in a small area for a long time. Two facing mirrors that reflect incoming light back and forth between them countless times make up optical cavities, which enable light particles, or photons, to accumulate inside the cavity.
In the study, the researchers employed the cavity to make the Fermi gas’s particles interact over great distances: no matter how far away it is from the first atom, the gas’ second atom will always absorb the photon that the first atom’s first emits and bounces off of it onto the mirrors. The atoms form a density wave pattern when an adequate amount of photons are collectively released and reabsorbed – readily controlled in the experiment.
Brantut said, “The combination of atoms colliding directly with each other in the Fermi gas, while simultaneously exchanging photons over a long distance, is a new type of matter where the interactions are extreme. We hope what we see there will improve our understanding of some of the most complex materials encountered in physics.”
- Victor Helson, Timo Zwettler, Farokh Mivehvar, Elvia Colella, Kevin Roux, Hideki Konishi, Helmut Ritsch, Jean-Philippe Brantut. Density-wave ordering in a unitary Fermi gas with photon-mediated interactions. Nature 24 May 2023. DOI: 10.1038/s41586-023-06018-3