Nonisothermal activation: Nonequilibrium thermodynamics of metastable mesoscopic systems.

Abstract

A generalization is presented of Kramers's Brownian motion model in order to include nonisothermal activation events in noisy ``macroscopic'' systems (e.g., Josephson-junction devices). The theory is based on the thermodynamical significance of the effective potential for the pertinent degree of freedom (e.g., the magnetic flux) and heavily leans on notions from nonequilibrium thermodynamics. The thermodynamic potential generally depends on the specimen's local temperature (which may differ from the reservoir temperature), such that the entropy varies along the reaction coordinate. For thermally isolated activation events, the escape rate from the metastable state is determined by the entropy barrier, which is generically higher than the usual isothermal Gibbs (or Helmholtz) free-energy barrier. For systems with a small heat capacity, the theory predicts a substantial entropic barrier enhancement.