An Ultra Low-Density Liquid

An ultra low-density liquid, some 10¹³ times thinner than water, might form inside Bose-Einstein condensates under the action of the "Efimov effect," a quantum phenomenon in which the atoms in the cloud attract each other when considered two at a time but repel each other when considered three at a time. In such an Efimov cloud the atoms would be some 20 times farther apart that in a BEC, which is itself pretty sparse---a million times thinner than air. And yet this new type of condensate would not be a gas but a liquid!

According to Aurel Bulgac of the University of Washington (bulgac@phys.washington.edu, 206-685-2988), the exquisite coordination of atoms in an Efimov condensation would allow it to be self-bound (the constraining magnetic fields used to keep a BEC from drifting apart would be unnecessary); moreover, it would be neither compressible nor dilutable. This extraordinary quantum liquid---the smallest density condensed matter system yet proposed---could probably only be formed at much colder temperatures than are now available in BEC experiments. Bulgac proposes that Efimov droplets made from boson atoms be called "boselets." The fermion version would be "fermilets." (Aurel Bulgac, Physical Review Letters, 29 July 2002.)