Indirect evidence for the pseudogap
The electronic contribution to the specific heat capacity as a function of temperature for underdoped, optimally doped and overdoped samples of Y0.8Ca0.2Ba2Cu3O7x. For optimally doped (red line) and overdoped (green line) samples the heat capacity remains constant as the temperature is lowered, then peaks at the superconducting transition temperature, Tc, and rapidly approaches zero in the superconducting state. For underdoped samples (blue line), however, the heat capacity starts to fall well above Tc as the temperature is reduced, and there is only a small peak at Tc. This is indirect evidence for a pseudogap. (J W Loram et al. 1997 Physica C 282287 1405).