UNIVERSALITY OF DISSIPATIVE 2-STATE SYSTEMS

The static properties of a dissipative two-state system are studied by perturbation, mapping to the quantum sine-Gordon model, and a variational approach. We find that its properties at zero temperature depend not only on the spectral density J(omega) is similar to SIGMA(l)g(l)2-delta(omega-omega(l)), but also on the explicit form of the coupling strength g(l) is similar omega(l)lambda (omega(l) is the phonon frequency). For weak coupling (lambda greater-than-or equal-to 1), the effect of displacement of the phonon state is dominant; while for strong coupling (lambda greater-than-or-equal-to 1), one must take account of both displacement and deformation. Displaced squeezed states are proposed as ground states of the bath under strong coupling, and we show that the localization transition of the Ohmic dissipation occurs at alpha(c) = 3 - 2-lambda instead of alpha(c) = 1 as for the weak-coupling case.