Thermodynamic signature for the phase transition at the pseudogap temperature in underdoped YBCO (6.56)

The physics of the pseudogap, and its connection to the strange metal phase remain poorly understood. The outstanding problem is whether the apparent crossover between these two regimes is a thermodynamic phase boundary. We performed high precision resonant ultrasound spectroscopy measurement on de-twinned monocrystals of underdoped YBCO (6.56) in a broad temperature range up to 300 K. We find a compelling thermodynamic signature for the phase transition at the pseudogap temperature T = 270 K.