  We investigate the relationship between ground-state (zero-temperature)
quantum phase transitions in systems with variable Hamiltonian parameters and
classical (temperature-driven) phase transitions in standard thermodynamics. An
analogy is found between (i) phase-transitional distributions of the
ground-state related branch points of quantum Hamiltonians in the complex
parameter plane and (ii) distributions of zeros of classical partition
functions in complex temperatures. Our approach properly describes the first-
and second-order quantum phase transitions in the interacting boson model and
can be generalized to finite temperatures.
