Abstract
We study the expansions for the off-diagonal thermal kernel of quantum systems with a nonuniform, time-independent magnetic field in physically relevant parameters. In particular, the high-temperature expansion and a new heavy-mass expansion is discussed. We provide recursion relations and a diagrammatic method to calculate the coefficients in these expansions. We compare these expansions in detail with the semiclassical expansion derived previously [Phys. Rev. A 30, 2024 (1984)]. As an illustration we present the first few coefficients explicitly.