Minimal coupling in oscillator models of quantum dissipation
  The dissipative harmonic oscillator has two representations. In the first representation the central oscillator couples with its position to an oscillator bath. In the second one it couples with its momentum to the bath. Both representations are related by a unitary transformation. If the central oscillator couples with its position and momentum to two independent baths, no such unitary transformation exists. We discuss two possible models of this type and their physical relevance. 
Heiner Kohler, Fernando Sols
cond-mat.stat-mech quant-ph