  Spin dimer systems are a promising playground for the detailed study of
quantum phase transitions. Using the magnetic field as the tuning parameter it
is in principle possible to observe a crossover from the characteristic scaling
near critical points to the behavior of a finite temperature phase transition.
In this work we study two-dimensional coupled spin dimer systems by comparing
numerical quantum Monte Carlo simulations with analytical calculations of the
susceptibility, the magneto-caloric effect, and the helicity modulus. The
magneto-caloric behavior of the magnetization with temperature can be used to
determine the critical fields with high accuracy, but the critical scaling does
not show the expected logarithmic corrections. The zeros of the cooling rate
are an excellent indicator of the competition between quantum criticality and
vortex physics, but they are not directly associated with the quantum phase
transition or the finite temperature Berezinsky-Kosterlitz-Thouless transition.
The results give a unified picture of the full quantum and finite temperature
phase diagram.
