Cooper Pairs Dance in a Disk

Physicists like to play with dimensions and hunt for surprises in 1D and 2D worlds. In solids, for example, reducing the number of dimensions—say, by making a crystal whose electrons move only in a plane—disrupts the natural tendency of matter to order at low temperatures. This hindrance to order comes from thermal fluctuations, which are stronger in 1D and 2D systems than in 3D systems. But in the 1970s, Berezinskii, Kosterlitz, and Thouless (BKT) discovered a mechanism that, in certain 2D systems, tames fluctuations [1, 2] and allows “quasi”-long-range order, in which correlations fall off much more slowly than in a disordered phase. Now, experiments with cold fermionic atoms in a disk-shaped trap explore the effect of the BKT mechanism on the superfluid phase transition of paired-fermions (Cooper pairs), a system of potential interest to understanding the physics of high-temperature superconductors [3, 4]. In their experiments, Martin Ries and colleagues at the University of Heidelberg, Germany, cool the paired atoms to low temperatures and utilize specialized techniques to determine the onset temperature T_c for superfluidity, a frictionless phase of neutral particles analogous to superconductivity of electrons.