title: Meaning of temperature in different thermostatistical ensembles.
abstract: Depending on the exact experimental conditions, the thermodynamic properties of physical systems can be related to one or more thermostatistical ensembles. Here, we survey the notion of thermodynamic temperature in different statistical ensembles, focusing in particular on subtleties that arise when ensembles become non-equivalent. The 'mother' of all ensembles, the microcanonical ensemble, uses entropy and internal energy (the most fundamental, dynamically conserved quantity) to derive temperature as a secondary thermodynamic variable. Over the past century, some confusion has been caused by the fact that several competing microcanonical entropy definitions are used in the literature, most commonly the volume and surface entropies introduced by Gibbs. It can be proved, however, that only the volume entropy satisfies exactly the traditional form of the laws of thermodynamics for a broad class of physical systems, including all standard classical Hamiltonian systems, regardless of their size. This mathematically rigorous fact implies that negative 'absolute' temperatures and Carnot efficiencies more than 1 are not achievable within a standard thermodynamical framework. As an important offspring of microcanonical thermostatistics, we shall briefly consider the canonical ensemble and comment on the validity of the Boltzmann weight factor. We conclude by addressing open mathematical problems that arise for systems with discrete energy spectra.
authors.affiliation: Institute of Physics, University of Augsburg, Universitätstrasse 1, 86135 Augsburg, Germany Nanosystems Initiative Munich, Schellingstrasse 4, 89799 München, Germany hanggi@physik.uni-augsburg.de.
authors.forename: Peter
authors.identifier: http://orcid.org/0000-0001-6679-2563
authors.initials: P
authors.lastname: Hänggi
authors.affiliation: Exzellenzcluster Universe, Boltzmannstrasse 2, 85748 Garching, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Hilbert
authors.affiliation: Department of Mathematics, Massachusetts Institute of Technology, 77 Massachusetts Avenue E17-412, Cambridge, MA 02139-4307, USA.
authors.forename: Jörn
authors.identifier: http://orcid.org/0000-0001-8865-2369
authors.initials: J
authors.lastname: Dunkel
keywords: entropy; isolated systems; thermodynamic ensembles; weak and strong coupling