Magneto-caloric effects, quantum critical points, and the   Berezinsky-Kosterlitz-Thouless transition in 2D coupled spin dimer systems
  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. 
Dominik Strassel, Peter Kopietz, and Sebastian Eggert
cond-mat.str-el cond-mat.stat-mech