  We present a new generalized Dicke model, an impurity-doped Dicke model
(IDDM), by the use of an impurity-doped cavity-Bose-Einstein condensate. It is
shown that the impurity atom can induce Dicke quantum phase transition (QPT)
from the normal phase to superradiant phase at a critic value of the impurity
population. It is found that the IDDM exhibits continuous Dicke QPT with an
infinite number of critical points, which is significantly different from that
observed in the standard Dicke model with only one critical point. It is
revealed that the impurity-induced Dicke QPT can happen in an arbitrary
coupling regime of the cavity field and atoms while the Dicke QPT in the
standard Dicke model occurs only in the strong coupling regime of the cavity
field and atoms. This opens a way to observe the Dicke QPT in the intermediate
and even weak coupling regime of the cavity field and atoms.
