Phase Transitions and Thermal Hysteresis

Many transitions trace a different path on the way up than on the way down. Heat and cool the sample and watch the operating point move around the loop — this is why capturing the full story needs a stage that does both.

20 °C
Interactive plot of a first-order phase transition with thermal hysteresis: a property such as electrical resistance is high in the low-temperature phase and low in the high-temperature phase. On heating the change happens at a higher temperature than on cooling, so the heating and cooling branches enclose a hysteresis loop.
Temperature
20 °C
Phase
Insulating
Resistance (norm.)
1.00
The physics. A first-order transition needs to nucleate a new phase, which costs energy, so it overshoots: the sample superheats before switching on the way up and supercools before switching back on the way down. The gap between the two switching temperatures is the hysteresis width. The classic example is vanadium dioxide (VO₂), whose metal–insulator transition near 68 °C drops the resistance by orders of magnitude and shows exactly this loop.