Phase transitions constitute a very general concept in physics, describing a change of material properties due to the variation of an external control parameter. For example, liquid water turns into solid ice when the temperature falls below the freezing point. This concept is also applicable to many-body quantum systems, where distinct quantum phases with unique properties can be observed by adjusting a control parameter. Conventionally restricted to ground states, similar quantum phase transitions are nowadays also predicted for nonzero excitation energies ("excited-state quantum phase transitions").
A team around Bernd Meyer-Hoppe and Fabian Anders has now presented the first experimental mapping of a phase diagram in such excited-state systems. This facilitates the structuring of the huge number of possible energy states of a quantum system into just a few excited-state phases with distinct properties. They have published their findings as cover article in the current issue of the journal Physical Review Letters.
Original publication
Excited-State Phase Diagram of a Ferromagnetic Quantum Gas
B. Meyer-Hoppe, F. Anders, P. Feldmann, L. Santos, and C. Klempt
Phys. Rev. Lett. 131, 243402
https://doi.org/10.1103/PhysRevLett.131.243402
