title: Thermal phase transitions for Dicke-type models in the ultrastrong-coupling limit.
abstract: We consider the Dicke model in the ultrastrong-coupling limit to investigate thermal phase transitions and their precursors at finite particle numbers N for bosonic and fermionic systems. We derive partition functions with degeneracy factors that account for the number of configurations and derive explicit expressions for the Landau free energy. This allows us to discuss the difference between the original Dicke (fermionic) and the bosonic case. We find a crossover between these two cases that shows up, for example, in the specific heat.
authors.affiliation: Instituto de Física Teórica, UNESP-São Paulo State University, Caixa Postal 70532-2, 01156-970 São Paulo, SP, Brazil.
authors.forename: M Aparicio
authors.initials: MA
authors.lastname: Alcalde
authors.forename: M
authors.initials: M
authors.lastname: Bucher
authors.forename: C
authors.initials: C
authors.lastname: Emary
authors.forename: T
authors.initials: T
authors.lastname: Brandes
mesh_terms: D003198:Computer Simulation; D006358:Hot Temperature; D008956:Models, Chemical; D008958:Models, Molecular; D044367:Phase Transition