  We employ a generalized Dicke model to study theoretically the quantum
criticality of an extended two-level atomic ensemble interacting with a
single-mode quantized light field. Effective Hamiltonians are derived and
diagonalized to investigate numerically their eigenfrequencies for different
quantum phases in the system. Based on the analysis of the eigenfrequencies, an
intriguing quantum-phase transition from a normal phase to a superradiant phase
is revealed clearly, which is quite different from that observed with a
standard Dicke model.
