ABSTRACT
A phase transition can be caused by varying two control variables, and the derivative of free energy corresponding to each of these control variables would show a discontinuity. For first order transitions, one should observe a discontinuity in the first derivatives of free energy, namely, volume, entropy, or magnetization. The Ehrenfest classification gives prime importance to the lowest derivative of free energy that shows a discontinuity at the phase transition. A phase transition can be caused by varying two control variables, and the derivative of free energy corresponding to each of these control variables would show a discontinuity. The investigation of vortex lattice melting provides a detailed look into how a first order transition was established experimentally, especially when there were conflicting theoretical models on the order of the transition. This was pursued by various groups, following both the Ehrenfest and the modern classifications.
