  The `system-plus-reservoir' (SPR) model is the most common and effective
approach to study quantum dissipative effects. Indeed, it makes quantization
possible by considering the whole energy-conserving system, while the
reservoir's degrees of freedom, assumed to be harmonic, can be `traced out' by
the path-integral technique, leading to a formulation that only includes the
system of interest. In the standard SPR model the environment is only coupled
with the system's coordinate and turns out to quench its quantum fluctuations.
However, there are physical systems coupled with an environment whose
`coordinates' and `momenta' can be completely interchangeable (e.g., magnets),
so an SPR coupling must symmetrically affect both canonical variables. In this
paper such a general environmental coupling is studied in the case of a
harmonic oscillator. It is found that quantum fluctuations are generally
enhanced by environmental coupling, with an unexpected nonmonotonic behavior.
This leads one to speculate about the possibility that spin-lattice coupling
could drive the 2D Heisenberg antiferromagnet to reach its quantum-critical
point.
