Unconventional Superfluids

Solid, liquid, gas and plasma. These are the “classical” phases of matter, but nature allows for a much richer variety of “quantum” phases of matter with remarkable and often technologically useful properties. In superfluids and superconductors, a macroscopic number of identical particles are all in an indistinguishable “macroscopic” quantum state. Currents in these materials flow around obstacles without any resistance because  scattering off an obstacle gives information about the location of a particle, and this cannot happen unless there is sufficient energy/momentum to “localize” the particle. This phenomenon breaks down when temperatures get too high, the magnetic fields get too strong, or the current gets too large, and there is tremendous motivation to raise these limits as high as possible. Unfortunately, we still do not fully understand the physical mechanisms at work in the high-critical-temperature superconductors that have already been discovered, and we are struggling to understand the response of some superconductors to applied magnetic fields.

The difficulty in understanding these systems comes partly from the fact that quantum mechanics allows much more complicated and subtle ways for particles to organize them selves than predicted by “classical” physics. We have good mathematical tools for handling simple quantum systems, and some quantum many-body systems can be handled using approximate techniques, but for many important classes of systems (those with strong/nonlocal correlations, multiple interacting particle species, or disorder) our current theoretical and computational tools are clearly not sufficient. Even the best supercomputers cannot keep track of all the quantum degrees of freedom in a modest-sized physical system, and we need more insight to guide the development of new approaches to quantum many-body problems.

One of the best prospects for making progress on this front is to create highly idealized and controllable models of quantum many-body systems in laboratory experiments. Ultracold atomic gases are one of the best platforms currently available for such an effort, because of the incredible degree of control that is possible in these laboratory systems. We plan to create and study a critical test case in quantum many-body physics: an “unconventional’ magnetized superfluid called the Fulde-Ferrell Larkin-Ovchinnikov (FFLO) state. This state was predicted to exist in the 1960’s, and is believed to play an important role in diverse physical systems including certain poorly-understood (heavy-Fermion) superconductors, and “color superconductivity” in dense quark matter. Decades of experimental efforts have only produced indirect evidence for its existence; in the experiments we plan to conduct we hope to directly observe and study it in detail for the first time.