Principal Investigator Joern Dunkel
Over the past 60 years, a considerable number of theories and experiments have claimed the existence of negative absolute temperature in spin systems and ultracold quantum gases. This has led to speculation that ultracold gases may be dark-energy analogues and also suggests the feasibility of heat engines with efficiencies larger than one. Such negative temperature claims arise from the use of an entropy definition that is inconsistent both mathematically and thermodynamically. These conceptual deficiencies can be overcome if one adopts a microcanonical entropy functional originally derived by Gibbs. The resulting thermodynamic framework is mathematically self-consistent and implies that absolute temperature remains positive even for systems with a bounded spectrum.