  We develop a cluster expansion to show exponential decay of correlations for
quite general single scale spin systems, as they arise in lattice quantum field
theory and discretized functional integral representations for observables of
quantum statistical mechanics. We apply our results to: the small field
approximation to the coherent state correlation functions of the grand
canonical Bose gas at negative chemical potential, constructed by Balaban,
Feldman, Kn\"orrer and Trubowitz (2010); and to N component unbounded spin
systems with repulsive two body interaction and massive, possibly complex,
covariance. Our cluster expansion is derived by a single application of the
BKAR interpolation formula.
